Exploring the Potential of Metal Xanthate Precursors for

Exploring the Potential of Metal

Xanthate Precursors for the Synthesis of

Doped and Quaternary Metal Sulfides

A Thesis Submitted to the University of Manchester for

the Degree of Doctor of Philosophy in the Faculty of

Science and Engineering

2020

Abdulaziz M. Alanazi

Department of Chemistry

1

Table of contents

Table of contents ............................................................................................................................... 1

List of Figures .................................................................................................................................... 5

List of Tables ................................................................................................................................... 11

Abstract ............................................................................................................................................ 13

Declaration....................................................................................................................................... 14

Copyright Statement ....................................................................................................................... 15

Acknowledgment ............................................................................................................................. 16

Abbreviations .................................................................................................................................. 17

Chapter 1. Introduction .................................................................................................................. 19

1.1. Classification of solids ........................................................................................................... 19

1.2. Semiconductors ...................................................................................................................... 19

1.3. Intrinsic and extrinsic semiconductors ................................................................................... 20

1.3.1. n-type doping .................................................................................................................. 21

1.3.2. p-type doping .................................................................................................................. 22

1.3.3. p-n junction ..................................................................................................................... 23

1.4. Direct and indirect semiconductors ........................................................................................ 23

1.5. The semiconductor bandgap .................................................................................................. 25

1.6. Classification and applications of semiconductors ................................................................ 29

1.7. Nanoparticle materials ........................................................................................................... 30

1.8. Nanocrystals of semiconductors ............................................................................................ 31

1.8.1. Compound semiconductors ............................................................................................. 32

1.9. Transition metal chalcogenide semiconductors ..................................................................... 33

1.9.1. Binary TMCs .................................................................................................................. 35

1.9.2. Ternary TMCs ................................................................................................................. 39

1.9.3. Quaternary TMCs ........................................................................................................... 40

1.10. Doping ................................................................................................................................. 44

1.11. Synthesis of nanoparticle semiconductors ........................................................................... 48

1.11.1. Hot injection method ..................................................................................................... 51

1.11.2. The solvent-less thermolysis ......................................................................................... 53

2

1.12. Synthesis of thin films .......................................................................................................... 54

1.12.1. Spin coating method...................................................................................................... 55

1.12.2. The doctor blade method ............................................................................................... 56

1.13. Single source precursors (SSPs) .......................................................................................... 57

1.14. SSPs for metal sulfide nanostructures .................................................................................. 58

1.14.1. Xanthates: a general introduction .................................................................................. 59

1.15. Aims and objectives ............................................................................................................. 68

1.16. References ............................................................................................................................ 69

Chapter 2. Instruments section ...................................................................................................... 86

2.1. Measurement Methodologies ................................................................................................. 86

2.2. Elemental analysis ................................................................................................................. 86

2.3. Thermogravimetric analysis (TGA) ....................................................................................... 86

2.4. X-Ray crystallography ........................................................................................................... 87

2.5. Powder X-ray Diffraction (p-XRD) ....................................................................................... 87

2.6. Raman Spectroscopy .............................................................................................................. 89

2.7. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) .... 90

2.8. UV/Vis spectroscopy ............................................................................................................. 92

2.9. Magnetic measurements ......................................................................................................... 92

2.10. Reference ............................................................................................................................. 93

Chapter 3. Structural investigations of α-MnS nanocrystals and thin films synthesised from

single source precursors by hot injection, scalable solvent-less and doctor blade routes ......... 94

3.1. Introduction ............................................................................................................................ 94

3.2. Author distribution ................................................................................................................. 95

3.3. References .............................................................................................................................. 95

3.4. Structural investigations of α-MnS nanocrystals and thin films synthesised from single

source precursors by hot injection, scalable solvent-less and doctor blade routes ........................ 97

3.4.1. Abstract ........................................................................................................................... 97

3.4.2. Introduction ..................................................................................................................... 98

3.4.3. Experimental ................................................................................................................. 100

3.4.4. Results and discussion .................................................................................................. 104

3.4.5. Conclusion .................................................................................................................... 121

3

3.4.6. Acknowledgements ....................................................................................................... 122

3.4.7. References ..................................................................................................................... 123

3.4.8. Supporting Information ................................................................................................. 127

Chapter 4. The influence of single precursor on manganese incorporation into Mn-doped PbS

(Pb1-xMnxS) nanoparticles by solvent-less thermolysis. ............................................................. 139

4.1. Introduction .......................................................................................................................... 139

4.2. Author distribution ............................................................................................................... 140

4.3. References ............................................................................................................................ 140

4.4. The influence of single precursor on manganese incorporation into Mn-doped PbS (Pb1-

xMnxS) nanoparticles by solvent-less thermolysis. ..................................................................... 141

4.4.1. Abstract ......................................................................................................................... 141

4.4.2. Introduction ................................................................................................................... 141

4.4.3. Experimental ................................................................................................................. 143

4.4.4. Results and discussion .................................................................................................. 145

4.4.5. Conclusion .................................................................................................................... 154


4.4.7. References ..................................................................................................................... 155


Chapter 5. Effects of annealing temperature on the structural and optical properties of

CMTS (Cu2MnSnS4) nanoparticle prepared by solvent-less thermolysis ................................ 165

5.1. Introduction .......................................................................................................................... 165


5.3. References ............................................................................................................................ 166

5.4. Effects of annealing temperature on the structural and optical properties of CMTS

(Cu2MnSnS4) nanoparticle prepared by solvent-less thermolysis ............................................... 167

5.4.1. Abstract ......................................................................................................................... 167

5.4.2. Introduction ................................................................................................................... 168

5.4.3. Experimental ................................................................................................................. 170

5.4.4. Results and dissections .................................................................................................. 174

5.4.5. Conclusions ................................................................................................................... 184


5.4.7. References ..................................................................................................................... 185


Chapter 6. A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4)

4

powders as potential solar absorber materials ........................................................................... 196

6.1. Introduction .......................................................................................................................... 196


6.3. Citation ................................................................................................................................. 197

6.4. References ............................................................................................................................ 197

6.5. Manuscript 1: A molecular precursor route to quaternary chalcogenide CFTS (Cu2FeSnS4)

powders as potential solar absorber materials ............................................................................. 199

6.5.1. Abstract ......................................................................................................................... 199

6.5.2. Introduction ................................................................................................................... 200

6.5.3. Materials and experimental ........................................................................................... 203

6.5.4. Result and discussion .................................................................................................... 206

6.5.5. Conclusions ................................................................................................................... 218


6.5.7. References ..................................................................................................................... 218


Chapter 7. Conclusion and Future Work ................................................................................... 229

7.1. Conclusion ........................................................................................................................... 229

7.2. Future work .......................................................................................................................... 233

7.3. References ............................................................................................................................ 234

Final Word Count: 43524

5

List of Figures

Figure 1. 1. Donor levels produced by n-type doping ...................................................................... 22

Figure 1. 2. Acceptor levels produced by p-type doping ................................................................. 23

Figure 1. 3. Schematic diagram illustrating the direct and indirect band gap of a semiconductor.46

..........................................................................................................................................................24

Figure 1. 4. Band structures of bulk, nanoparticles and molecule.60 ............................................... 32

Figure 1. 5. Schematic representation of the structure development tree for the formation of binary,

ternary and multinary semiconductors starting from a II–VI parent compound.65 ............................ 33

Figure 1. 6. The crystalline structures of cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD 01-

089-4952), metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288) and

hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-040-

1289). Color code: Mn, violet; S, yellow.129 ..................................................................................... 38

Figure 1. 7. Unit cell representations of Cu2FeSnS4; (a) the Stannite type structure a = 5.449 Å; c =

10.726 Å, α. β and γ= 90, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α. β

and γ= 90 ICDD: 0005843.1 .............................................................................................................. 43

Figure 1. 8. setup of hot injection method.236 ................................................................................... 52

Figure 1. 9. setup of solvent-less thermolysis.243 ............................................................................. 53

Figure 1. 10. Basic diagram of the spin coating technique. 267 ......................................................... 55

Figure 1. 11. Schematic picture of doctor blade coating process for thin film deposition.270 .......... 56

Figure 1. 12. Some common ligands used in single source precursors to prepare metal sulfides. ... 58

Figure 1. 13. Synthesis of alkali metal xanthates ............................................................................. 61

Figure 1. 14. Bimolecular synthesis of potassium ethyl xanthate .................................................... 62

Figure 1. 15. Coordination behaviour of xanthate ligands. (A): monodentate; (B) isobidentate; (C)

anisobidentate; (D) and (E): bimetallic bridging through sulfur; (F) and (G): bridging to metal

through oxygen. ................................................................................................................................ 63

Figure 1. 16. The hydrolysis of dissolved the xanthate in water ...................................................... 63

Figure 1. 17. The complex of metal xanthate, where M(II) = different metals and R an alkyl group.

..........................................................................................................................................................64

Figure 2. 1. Schematic representation of Bragg’s Diffraction. ......................................................... 88

Figure 2. 2. Top; Energy level diagram of stimulated Raman scattering, down; Raman spectrum

showing the relative intensities of the different scattering processes ................................................ 90

Figure 3. 1. The crystalline structures of (a) cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD

01-089-4952), (b) metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-

6

1288) and (c) hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å

(ICDD 00-040-1289). Colour code: Mn, violet; S, yellow.41 .......................................................... 100

Figure 3. 2. The molecular structures of the manganese xanthates. [Mn(S2COMe)2.TMEDA] (1),

[Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),

[Mn(S2COnPen)2.TMEDA] (5), [Mn(S2COnHex)2.TMEDA] (6) and [Mn(S2COnOct)2.TMEDA] (7).

H atoms are omitted for clarity. Violet = Mn, yellow = S, red = O, blue= N, grey = C. ............. 107

Figure 3. 3. Thermogravimetric analysis (TGA) profiles of complexes (1-6) and inset picture for

complexes (5 and 6). ....................................................................................................................... 108

Figure 3. 4. P-XRD patterns of MnS prepared at 250 °C via hot injection from precursors 1-

6.The standard pattern ( black sticks) is cubic α–MnS (ICDD No. 03-065-0891).56 ............. 110

Figure 3. 5. Representative secondary electron SEM images (10 kV) of MnS samples prepared

using precursor (a-f) (1-6) prepared by hot injection thermolysis at 250 °C, taken at magnification

of 1µm ............................................................................................................................................. 111

Figure 3. 6. P-XRD patterns of MnS prepared at 350 °C via solvent-less thermolysis of precursors

(1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-065-0891).56 .......... 113

Figure 3. 7. Representative secondary electron SEM images (10 kV) of MnS samples using

precursor (a-f) (1-6) prepared by a solvent-less thermolysis at 350 °C .......................................... 114

Figure 3. 8. P-XRD patterns of MnS thin films prepared at 350 °C Deposition by the doctor blade

method from precursor (1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No.

03-065-0891).56 ...............................................................................................................................116

Figure 3. 9. Representative secondary electron SEM images (10 kV) of MnS thin films using

precursor (a-f) (1-6) deposited by the Doctor Blade method at 350 °C .......................................... 117

Figure 3. 10. X-band EPR spectrum of -MnS NCs obtained from complex 2 ............................ 118

Figure 3. 11. Thermal dependence of the magnetisation for -MnS NCs obtained from complex 2,

measured in zero-field cooled (ZFC) (red circles) and field-cooled (FC) (black squares) regimes,

with the difference MFC-MZFC plotted in blue. Insert: Plot of –d(MFC-MZFC)/dT for the same

nanocrystals..................................................................................................................................... 119

Figure 3. 12. Plot of 1/ versus temperature for -MnS NCs obtained from complex 2, measured

in zero-field cooled (ZFC) (red) and field-cooled (FC) (black) regimes, with a fit to the Curie law

= C/(T-) presented in blue (dashed lines) ...................................................................................... 120

Figure 3. 13. Magnetic hysteresis at 5 and 300 K for -MnS NCs obtained from 2. The inset shows

the region around zero fields. .......................................................................................................... 121

Figure 3.S 1. Crystal structures of 1, 2, 3, 4, 5, 6 and 7 showing intermolecular C–H⋯S non-

covalent contacts and S⋯S interactions. ......................................................................................... 129

Figure 3.S 2. IR spectra of manganese alkyl xanthate precursors (1-6) ......................................... 131

7

Figure 3.S 3. The XRD patterns of manganese sulfide nanoparticles prepared by hot-injection from

[Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 200 °C for 30 min to

determine the optimum temperature for thermal decomposition. ................................................... 132

Figure 3.S 4. EDX spectra of MnS from precursors (a-f) (1-6) prepared by hot injection

thermolysis. ..................................................................................................................................... 133

Figure 3.S 5. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) synthesised by

hot injection thermolysis. ................................................................................................................ 133

Figure 3.S 6. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by hot

injection thermolysis at 250 °C, 5μm magnification. ...................................................................... 134

Figure 3.S 7. The XRD patterns of manganese sulfide nanoparticles prepared by solvent-less

thermolysis from [Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 250, 300

and 350°C for 60 min to determine the optimum temperature for thermal decomposition. ........... 134

Figure 3.S 8. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by solvent-less

thermolysis at 350 °C, 5μm magnification. ..................................................................................... 135

Figure 3.S 9. EDX spectra of MnS from precursors (a-f) (1 – 6) prepared by solvent-less

thermolysis. ..................................................................................................................................... 136


solvent-less thermolysis. ................................................................................................................. 136

Figure 3.S 11. EDX spectra of MnS thin films from precursors (a-f) (1-6) prepared by doctor blade

method. ........................................................................................................................................... 137

Figure 3.S 12. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) Deposition by

the doctor blade method. ................................................................................................................. 138

Figure 4. 1. TGA profile of (1) lead(II) ethylxanthate and (2) manganese(II) ethylxanthate.

TMEDA .......................................................................................................................................... 146

Figure 4. 2. XRD patterns and lattice parameters a, unit cell volume V and d(200) spacing of Pb1-

xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and

manganese xanthate precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x =

0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08. ............................................................................ 148

Figure 4. 3. Unit cells of (a) PbS (ICDD: 03-065-0692) and (b) MnS (ICDD: 03-065-0891) along

with their bonds. .............................................................................................................................. 149

Figure 4. 4. Approximately linear correlation between the amounts of manganese in the precursor

feedstock and the mole % Mn found in Pb1-xMnxS samples from EDX spectroscopy ................... 150

Figure 4. 5. Representative SEM secondary electron SEM images (10 kV) of Pb1-xMnxS (0≤ x ≤

0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and manganese xanthate

precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c) x = 0.04,

(d) x = 0.06 and (e) x = 0.08. .......................................................................................................... 151

8

Figure 4. 6. Raman spectra of Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared by solvent-less

thermolysis at 350 °C using lead and manganese xanthate precursors with different mole fractions

of Mn. .............................................................................................................................................. 152

Figure 4. 7. Relationship between Particle Size and band gap of undoped PbS and Pb1-xMnxS (0

≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C ................................................ 154

Figure 4. S 1. XRD for cubic PbS (ICDD: 03-065-0692) from lead(II) ethylxanthate at (a) 300 °C

and (b) 350 °C. ................................................................................................................................ 159

Figure 4.S 2. XRD for cubic MnS (ICDD: 03-065-0891) from Manganese(II)

ethylxanthate.TMEDA at (a) 300 °C and (b) 350 °C. .................................................................... 160

Figure 4. S 3. EDX spectra of of Pb1-xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less

thermolysis at 350 °C with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c)

x = 0.04, (d) x = 0.06 and (e) x = 0.08 ............................................................................................ 160

Figure 4. S 4. EDX elemental mapping (20 kV) of Pb, Mn and S for Pb1-xMnxS samples. (a) x =

0.02, (b) x = 0.04, (c) x = 0.06 and (d) x = 0.08 mole fractions of manganese .............................. 161

Figure 4. S 5. Particle size distribution histogram of the samples prepared Pb1-xMnxS by solvent-

less thermolysis at 350 °C with different mole fractions of Mn: (a) x = 0 (PbS), (b) x = 0.02, (c) x =

0.04, (d) x = 0.06 and (e) x = 0.08. ................................................................................................. 162

Figure 4. S 6. The UV-Vis-NIR absorbance spectra of undoped PbS and Pb1-xMnxS (0 ≤x≤ 0.08)

samples prepared by solvent-less thermolysis at 350 °C ................................................................ 163

Figure 4. S 7. Tauc plot (ahν)2 vs. hν showing the direct bandgaps of undoped PbS and Pb1-xMnxS

(0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C............................................ 164

Figure 5. 1. Crystal structures for stannite Cu2MnSnS4, a = 5.449 Å; c = 10.726 Å, α. β and γ= 90°,

ICDD: 0005838.30 ........................................................................................................................... 169

Figure 5. 2. Illustration of the formation of Cu2MnSnS4 nanoparticles through thermal

decomposition of copper(II) ethylxanthates (1), manganese(II) ethylxanthates (2) and tin(II)

ethylxanthates (3) and reaction using the solvent-less thermolysis................................................. 173

Figure 5. 3. Thermogravimetric analysis of (1) bis(ethylxanthate) copper(II), (2) bis(ethylxanthate)

manganese(II).TMEDA and (3) bis(ethylxanthate) tin(II) .............................................................. 175

Figure 5. 4. P-XRD patterns of the CMTS nanoparticles prepared at different temperatures ........ 176

Figure 5. 5. Room temperature Raman spectra of the CMTS nanocrystals prepared at different

temperatures. ................................................................................................................................... 179

Figure 5. 6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.

........................................................................................................................................................180

9

Figure 5. 7. Tauc plots of the of the CMTS nanoparticles prepared at different temperatures 350

°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 182

Figure 5. 8. Variation of bandgap and grain size as a function of annealing temperature. ............ 183

Figure 5. S1. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.

........................................................................................................................................................189

Figure 5. S2. EDX elemental mapping of the CMTS nanoparticles prepared at different

temperatures, (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 10 µm in all

cases. A secondary electron SEM image of the mapped area is included in each case, labelled as

SE. ................................................................................................................................................... 190

Figure 5. S3. Absorption spectra of the CMTS nanoparticles prepared at different temperatures 350

°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 190

Figure 5. S4. XRD patterns of the CMTS films prepared by spin coating from 350 C to 500 C.

........................................................................................................................................................192

Figure 5. S5. Raman spectra of the CMTS films prepared by spin coating from 350 C to 500 C.

........................................................................................................................................................192

Figure 5. S6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500

thin films prepared by spin coating ................................................................................................. 193

Figure 5. S7. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500

thin films prepared by spin coating ................................................................................................. 194

Figure 5. S8. EDX elemental mapping of the CMTS thin films prepared at different temperatures,

(a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 5 µm in all cases. A

secondary electron SEM image of the mapped area is included in each case, labelled as SE. ....... 194

Figure 5. S9. Absorption spectra of the CMTS thin films prepared at different temperatures 350

°C, 400 °C, 450 °C and 500 °C ....................................................................................................... 195

Figure 5. S10. Tauc plots of the of the CMTS thin films prepared at different temperatures 350 °C,

400 °C, 450 °C and 500 °C ............................................................................................................. 195


10.726 Å, α. β and γ= 90o, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å,

α. β and γ= 90o ICDD: 0005843.23 .................................................................................................. 202

Figure 6. 2. Thermogravimetric analysis of [Cu(S2COEt)2] (red colour), [Fe(S2COEt)3] (blue

colour), [Sn(S2COEt)2] (green colour) and [Sn(S2COEt)4] (black colour) precursors. ................... 208

Figure 6. 3. P-XRD patterns of Cu2FeSnS4 powder (1) and (2) synthesised at (a) 250°C; (b) 350°C

and (c) 450°C for 1 hour ................................................................................................................. 210

10

Figure 6. 4. Raman spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of

450°C for 1 hour ............................................................................................................................. 211

Figure 6. 5. XPS spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C

for 1 hour: (a) Fe 2p, (b) Cu 2p, (c) Sn 3d and (d) S 2p ................................................................. 213

Figure 6. 6. SEM images of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour. Scale

bar showing different magnifications. ............................................................................................. 214

Figure 6. 7. Elemental mapping of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour

showing the distribution of Cu, Fe, Sn and S. Scale bar represented 5 μm in all cases .................. 215

Figure 6. 8. Tauc plot (ahѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 Powders (1) and (2).

........................................................................................................................................................217

Figure 6. S 1. The EDX plots of Cu2FeSnS4 powder (1) and (2) synthesised at a temperature of

450°C for 1 hour. The inset of figure 6. S1 shows the compositional data of Cu2FeSnS4 powder (1)

and (2) ............................................................................................................................................. 222

Figure 6. S 2. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 powder (1) and (2) synthesised

at a temperature of 450°C for 1 hour .............................................................................................. 223

Figure 6. S 3. P-XRD patterns of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour ............................................................................................................................. 224

Figure 6. S 4. Raman spectra of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour ............................................................................................................................. 224

Figure 6. S 5. SEM images of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour ............................................................................................................................. 225

Figure 6. S 6. EDX plots of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed at a

temperature of 450°C for 1 hour. The inset image showing the atomic percent of Cu2FeSnS4 thin

films. ............................................................................................................................................... 225

Figure 6. S 7. Elemental mapping of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed

at 450°C for 1 hour, showing the distribution of Cu, Fe, Sn and S ................................................. 226

Figure 6. S 8. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 thin films deposited from (3)

and (4) and annealed at 450°C for 1 hour ....................................................................................... 226

Figure 6. S 9. Tauc plot (αhѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 thin films

deposited from (3) and (4) and annealed at 450°C for 1 hour ......................................................... 227

11

List of Tables

Table 1. 1. Band gap energies and electrical conductivity at room temperature for semiconductor

materials.4 .......................................................................................................................................... 20

Table 1. 2. Common semiconductors and their applications.43 ........................................................ 29

Table 3.S 1. X-ray crystallographic data and refinement details for (1-7) using Cu K radiation and

with H-atom parameters constrained. ............................................................................................. 127

Table 3.S 2. Selected Bond Lengths (Å) and Angles (o) for novel complexes (1-7) ...................... 128

Table 3.S 3. Details of selected intermolecular non-covalent contacts (Å) in the prepared

compounds (1-7) ............................................................................................................................. 128

Table 3.S 4. Elemental and thermal analyses of xanthates diaminemanganese(II) complexes 1 - 7.

........................................................................................................................................................130

Table 3.S 5. The unit cell parameters for the MnS synthesised by hot injection thermolysis from

precursors (1-6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite

size, EDX measurements and Raman data from these samples ...................................................... 132

Table 3.S 6. The unit cell parameters for the MnS synthesised by solvent-less thermolysis from

precursors (1 – 6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite

size and EDX measurements from these samples ........................................................................... 135

Table 3.S 7. The unit cell parameters for the MnS synthesised by doctor blade method from


size and EDX measurements from these samples ........................................................................... 137

Table 4. 1. Lattice parameters a, unit cell volume (V), band gap (Eg) and grain size of Pb1-xMnxS

(0 ≤ x ≤ 0.08) with variations in Mn/Mn+Pb molar ratios. ............................................................. 149

Table 4. S 1. Composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) ........................................................................... 159

Table 4. S 2. Summary of the required composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) calculated from

the elements in the feed and analysis of final products by EDX spectroscopy ............................... 161

Table 5. 1. Lattice constants of the CMTS nanoparticles obtained from XRD patterns. ............... 178

Table 5. 2. Electrical properties of CMTS films prepared by spin coating from 350 C to 500 C.

........................................................................................................................................................184

Table 5. S1. Chemical composition and composition ratio from EDX spectra of the CMTS

nanoparticles prepared at different temperatures ............................................................................ 189

Table 5. S2. Chemical composition and composition ratio of the CMTS thin films prepared at

different temperatures ..................................................................................................................... 193

12

Table 6. 1. Reported band gaps of CFTS nanomaterials prepared by different methods ............... 201

13

Abstract

Metal sulfide nanomaterials thin films are important in photovoltaic applications due to their

exciting properties. Xanthate complexes are well known for the deposition of metals sulfide

films and nanomaterials. This produces suitable chemical and physical properties to deposit

films with a very low level or no contamination at low temperature. Different synthetic

protocols are available for preparation of metal sulfide nanomaterials and/or thin films.

Nonetheless, the use of single source precursors is usually advantageous, as they can be used

for the synthesis of metal chalcogenide nanocrystals, and are equally suitable for the

deposition of thin films. Furthermore, a better control over stoichiometry and phase can be

achieved, due to preformed bonds between metal and chalcogen atom.

The work shows the synthesis of a series of novel manganese complexes of xanthate ligands,

their spectroscopic characterization, crystal structures and thermal decomposition have been

studied. The complexes were used as single source precursors for the production of MnS

nanocrystals and thin films. MnS nanocrystals have been synthesised by hot injection and

solvent-less thermolysis at 230 °C and 350 °C, respectively. In addition, MnS thin films have

been synthesised by doctor blade method at 350 °C. The nanocrystals and films were

characterised by powder X-ray diffraction, Raman spectra, scanning electron microscopy

and energy dispersive X-ray spectroscopy.

Additionally, xanthate complexes of lead has been used for the production of Mn-doped PbS

nanocrystals using solvent-less thermolysis, adding with a low concentration of Mn source.

The nanocrystals were characterised by several techniques to study the formation behaviour,

structure and chemical composition.

Finally, we report the use of copper, manganese, iron and tin xanthates in solvent-less

thermolysis to produce stannite Cu2MnSnS4 (CMTS) nanoparticles and Cu2FeSnS4 (CFTS)

powders at temperatures between 250 and 500 °C. Higher temperatures give the normal

tetragonal phase CMTS and CFTS, while low temperatures are contaminated with cubic

phases.

14

Declaration

I hereby declare that no portion of the work referred to in the thesis has been submitted in

support of an application for another degree or qualification of the University of Manchester

or any other university or other institute of learning.

Abdulaziz Mohammed Alanazi

15

Copyright Statement

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16

Acknowledgment

First and foremost, I would like to thank God Almighty for giving me the strength,

knowledge, ability and opportunity to undertake this research study and to persevere and

complete it satisfactorily. Without his blessings, this achievement would not have been

possible. I have also been supported by many good people to whom I would like to express

my deepest gratitude.

I would like to thank the Ministry of Education in Kingdom of Saudi Arabia and Islamic

university in Madinah for funding and support. I would like to thank and acknowledge my

supervisor, Professor Paul O’Brien who passed away for his excellent advices, and for giving

me the opportunity to research at the University of Manchester, and thanks for his keenness

and follow-up me in the first two years of my PhD stages. I would like also to thank my co-

supervisor Dr. David J. Lewis a person who has had a high impact on my improvement, he

has been instrumental in my success and has encouraged me during this long journey to

complete PhD. Dr. Lewis’s valuable advice, guidance and trust during my study made it

possible for me to complete this goal. Also many thanks for his patience, help and guidance

for writing papers and thesis. I would like to thank Prof. David Collison for his academic

and administrated roles to make sure I finish strong. I am really grateful to Post doc Dr Firoz

Alam Dr Paul McNaughter who I have worked with them on some projects and I have taken

useful comments and I appreciate their scientific advice. I’m very grateful to POB’s research

group members who have been of great help. Big thanks to technical staffs in the department

of Chemistry for their great help in the use of X-ray crystallography and analytical analysis.

Also thanks to technical staffs in the school of Materials for their help and support on the

SEM.

Finally, I would like to thank my parents for their support, prayers and encouragement

throughout my study. I would also like to thank my brothers, sisters and friends for their

assistance and supports. Last but not least, distinctive thanks to my friend Asil for sharing

this journey with kindness, cooperation, and encouragement and care. Also, Very special

thanks to my best friend Rosie, for her support and assistance in every step of my thesis.

Thank you for colouring my journey.

17

Abbreviations

1D 1- Dimensional

a, c Lattice parameter

AACVD Aerosol Assisted Chemical Vapour

Deposition

nBu n-Butyl

CCDC Cambridge Crystallographic Data Centre

CFTS Copper Iron Tin Sulfide

CMTS Copper Manganese Tin Sulfide

CVD Chemical Vapour Deposition

eV Electron Volt

EDX Energy Dispersive X-ray Spectroscopy

Eg Energy Gap

et al et alia

Et Ethyl

K Kelvin

KS kesterite

M.pt Melting Point

Me Methyl

MeOH Methanol

ml millilitres

mmol millimole

NCs Nanocrystals

nm nanometers

NPs Nanoparticles

OLA Oleylamine

p-XRD Powder X-ray Diffraction

Pr Propyl

SEM Scanning Electron Microscope

SSP Single Source precursor

ST stannite

TMEDA N,N,N′,N′-Tetramethylethylenediamine

THF Tetrahydrofuran

18

TGA Thermogravimetric Analysis

TOP Trioctylphosphine

UV/Vis ultra violet/ visible

WZ Wurtzite

ZB Zinc Blende

19

Chapter 1. Introduction

1.1. Classification of solids

Solids can be classified into two main categories: amorphous and crystalline. A material

will be referred to as an amorphous solid if the constituent atoms do not have a regular and

repeating arrangement. Examples of such structures are found in plastics, rubber and glass.

Some substances may adopt different arrangements when in a solid state; an example is

carbon that coexists as graphite crystalline or fullerenes. This phenomenon is referred to as

allotropy, the elements have allotropes.1

At the other end of the spectrum, a crystalline solid is a material containing particles that are

arranged in an orderly manner. Examples of such materials include sucrose, sodium chloride

and diamond. In general, a crystalline solid will have a sharp melting point which, when

reached, will cause the crystalline solid to become an isothermal liquid. After the cooling

process, the same arrangements can be observed. These types of element are also referred

to as true solids.1

Solids can also be classified according to the type of bond that holds them together.

According to this, crystalline materials are classified as molecular, covalent, ionic and

metallic. There are three types of solid-state materials depending on their electrical

conductivity, which are conductors, semiconductors and insulators. The distinction between

these types is made evident through band theory.2

1.2. Semiconductors

Semiconductors are solid substances that have conductivity between an insulator and most

of the metals. A material’s resistance depends on its purity and the temperature. The

resistance in semiconductors is significantly reduced owing to the addition of impurities.3

Also, when the semiconductor’s temperature is raised the resistance decreases significantly,

20

but the material does not have high conductivity. Conversely, the resistance increases by

reducing the temperature and becomes close to the resistance of the insulating materials.3

Devices made of semiconductors such as silicon, are important components of most

electronic circuits. The properties of certain common semiconductor materials are shown in

Table 1.1.4

Table 1. 1. Band gap energies and electrical conductivity at room temperature for semiconductor

materials.4

Material Band Gap (eV) Electrical Conductivity

[(Ω-m)-1]

Elemental

Si 1.11 4×10-4

Ge 0.67 2.2

III-V Compounds

GaAs 1.42 10-6

InSb 0.17 2×104

II-VI Compounds

CdS5 2.40 6.14×10-4

ZnO6 2.26 2×10-2

1.3. Intrinsic and extrinsic semiconductors

Intrinsic semiconductors are represented by elements such as silicon (Si) or germanium (Ge),

or compounds such as gallium arsenide (GaAs) or copper indium sulfide (CuInS2), which

contain no impurities in contrast to the number of thermal generated holes and electrons

present in a lattice. Although, in practical terms, these impurities do exist, their levels are

infinitely small and thus negligible.7

21

Extrinsic semiconductors represent conductors that, from the pure form, have had impurities

deliberately placed within them. Due to the intense degree of difficulty of obtaining pure

semiconductor material, these elements are not used. However, by adding small amounts of

impurities to these materials during the crystal growth and even in the later stages in selected

regions of the crystal, the process called doping takes place.8 This results in different

materials, by which the doping process becomes classified as an n-type or a p-type. In n-type

semiconductors, an extra electron in the conduction band is present while, in p-type

semiconductors, additional holes in the valence band are present.7

1.3.1. n-type doping

An n-type dopant is differentiated from the other types by the addition of negatively-charged

electrons to the semiconductor. This type represents the most common form of dopant and

is located in Group 15 of the Periodic Table. In some cases, n-type dopants are also referred

to as donors, due to the fact that they donate electrons to the semiconductor. These elements

are nitrogen, phosphorus, arsenic, antimony and bismuth. Any donor atom becomes easily

ionised, holding a free negatively-charged electron and leaving a positive ion core (Figure

1.1).9

22

Figure 1. 1. Donor levels produced by n-type doping.

1.3.2. p-type doping

p-type doping is generally produced by adding B, Ga or In to the silicon lattice of an initial

intrinsic semiconductor. In this scenario, each of the acceptor atoms will have three valence

electrons. Only three valence electrons will share the neighbouring silicon atoms in the

crystal lattice. This transformation will substantially increase the number of holes created

by the acceptor atoms, that are greater than the number of free electrons and holes that were

noted in the intrinsic semiconductor. As such, the majority of the carriers are represented

by the holes which also deliver a positive charge (Figure 1.2).10

23

Figure 1. 2. Acceptor levels produced by p-type doping.

1.3.3. p-n junction

p-n junctions are created by bringing together n and p-type semiconductor materials. In this

scenario, because of the high electron concentration in the n-type and high hole

concentration in the p-type, electrons pass from the n- to the p-type material. This is done

through the depletion region due to the non-uniform electron distribution. At an equilibrium

state, no electron continues to pass through the depletion region. The same effect is observed

regarding the holes in the valence band. In this regard, a uniform Fermi level is defined as

being flat throughout the p-n junction.11,12

1.4. Direct and indirect semiconductors

Direct and indirect semiconductors display complicated energy band diagrams, in which the

electron energy is set against the electron crystal momentum (k-vector). As shown in Figure

1.3 (from left to right), the conduction band minimum along with the valence band maximum

are placed within the same momentum value, implying that the electron can transit from the

valence to the conduction band with no change in momentum taking place. When this

24

phenomenon is possible, the material is referred to as a direct semiconductor. An example

of such material is gallium arsenide, GaAs.13

Figure 1. 3. Schematic diagram illustrating the direct and indirect band gap of a semiconductor.46

By contrast, as shown in the right hand image of Figure 1.3, the conduction band minimum

along with the valence band maximum are now moved from the momentum value, each with

its own momentum location. When this phenomenon occurs, the electron requires a change

in momentum and extra energy input. This is the case for indirect semiconductors, for which

group silicon is a representative material.13 In recent years, the potential for silicon to

become integrated and more widely applicable to the solar power industry has been

discussed, specifically due to its characteristics as an indirect semiconductor.14

In this regard, in a p-n junction photovoltaic (PV) cell, a photon of light produces an electron

hole pair if the energy of the photon is at least similar to the band gap of the material forming

the p-n junction. Consequently, materials with high carrier mobilities are preferable for high

efficiency.15 Thus, the p-type semiconductor can be utilised as light absorber in a p-n

junction solar cell duo to the electrons have higher mobility than the holes.15

In this regard, kesterite materials are seen as the main candidates for the creation of thin-film

solar cells, as they possess an increased absorption coefficient and a direct band gap; see

Section 1.9.3.2 for the structure of kesterite. An example of this is the Cu2ZnSnS4, for which

25

Chen and Chuang proposed a formation mechanism for crystals nanorods.16 Apart from the

quantum properties of such elements (e.g., Cd2+, Hg2+, S2- and Se2-, …) El-Sayed discussed

the potential of size to influence the energy distribution within a semiconductor material.17

He argued that colloidal semiconductor nanoparticles will display certain features that will

depend upon the electronic relaxation rates that occur when spherical nanoparticles are

transformed into nanorods. Size changes of small nanoparticles were also noted at a

structural level upon adsorbed strongly bound molecules.17 The implications of such

findings reveal a practical application in the photovoltaic industry.17 As this researcher

discusses, the atoms in a nanoparticle will behave quite differently to those in a particle

located in a bulk material owing to surface effects and quantum size effects that are not

present in bulk, but are notable at quantum levels.18

1.5. The semiconductor bandgap

The bandgap in solid materials is the energy difference between the unoccupied conduction

band and the fully-occupied valence band. It encompasses a range of energy values that are

forbidden to the electrons of the material.19 The high electrical conductivity of metals is due

to the presence of a partially filled (and, hence, partially unoccupied) conduction band at any

temperature. Meanwhile, semiconducting and electrically insulating materials contain a full

valence band and a more-or-less completely empty conduction band. However, in

semiconductors, an elevated temperature can supply enough energy to promote electrons

from the valence band into the conduction band.20 This gives the semiconductors electrical

conductivity properties intermediate between those of the conductors and the insulators,

since current can flow only when electrons are promoted to the conduction band. The

primary difference between semiconductors and insulators is the size of the bandgap, being

larger in insulators (e.g. 5.5 eV for diamond and 9 eV for silicon dioxide (SiO2)) than in

semiconductors (e.g. 1.1 eV for silicon (Si), 0.67 eV for germanium (Ge) and 1.43 eV for

26

gallium arsenide (GaAs)).19 The electrons cannot take on energy values intermediate

between the valence and conduction bands, but can be promoted from one to the other by

absorbing photons of sufficient energy. In general, materials with a bandgap of less than 3

eV are classified as semiconductors while those with a bandgap greater than 3 eV are

regarded as insulators because insufficient thermal energy is available at 300 K to promote

electrons to the conduction band in the latter case.19

The semiconductor bandgap is a key determinant of energy absorption in solar technology;

hence the various types of semiconductor that can be used in this field have been examined

in a wide range of studies. For instance, the bandgap in binary materials varies with

stoichiometry, e.g. Cu2S (~2.47 eV) and CuS (1.26 eV).21 As a result, the different forms

of CuxS behave distinctly, with Cu2S nanomaterials displaying p-type conduction and the

CuS nanomaterials displaying n-type conduction.21,22 See Section 1.3 for a discussion of n-

and p-type doping. Moreover, Al-Shakban et al. identified a bandgap of approximately 1.4

eV for Cu1.74S nanorods.23

Due to the combination of lower toxicity, lower cost and higher efficiency, solar cells

containing iron pyrite (FeS2) would be preferable to those containing cadmium compounds.

The bandgap of natural pyrite crystals was found to be 0.9 eV by Ennaoui et al., while single

crystals and synthetic polycrystals both displayed a bandgap of 0.95 eV.24 Moreover, FeS

thin films were produced on glass substrates by Akhtar et al. and shown to have a bandgap

of 1.87 eV.25

While the indirect bandgap of tin sulfide (SnS) is 1.1 eV, the direct bandgap of about 1.3 eV

is close to the ideal value of 1.5 eV for solar cells,26 see Section 1.4 for a discussion of direct

and indirect bandgaps. However, the literature reveals a variation in this bandgap energy

that is strongly correlated with the method of preparation. For instance, Al-Shakban et al.

used chemical vapour deposition (CVD) to produce SnS from a single-source precursor

27

under conditions of heating at various temperatures to indicate a decrease in bandgap (from

a maximum of 1.4 eV) with increasing temperature.27 Meanwhile, single-crystal p-type SnS

specimens were produced by Chamberlain and Merdan with a direct bandgap of 1.43 eV and

indirect bandgaps of 1.13 eV and 1.22 eV at 77K.28

The p-type semiconductor manganese sulfide (MnS) has a broad bandgap around 3 eV.29 A

decrease in the bandgap of thin MnS films with increased temperature of annealing was

demonstrated by Girish et al. Thus, heating at 300 ᵒC gave a bandgap of 3.95 eV, whereas

heating at 400 and 450 ᵒC gave bandgaps of 3.44 eV and 3.33 eV, respectively.30 Similarly,

Shi et al. reported a decrease in the bandgap of a CBD-produced MnS thin film from 3.18

eV to 3.15 eV after annealing.31

The quaternary semiconductors Cu2MSnS4 (M = Ni2+, Co2+, Fe2+, Mn2+) have potential

application in low-cost thin film for solar cells.32 In the experiment, Cu2CoSnS4, Cu2FeSnS4,

Cu2NiSnS4 and Cu2MnSnS4 nano-crystals have been synthesised via a solvothermal method.

Cu2MSnS4 has a band gap at 1.2–1.5 eV, which indicates its viability for application in solar

energy capture.32

Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 are suitable for low cost energy and stipulated that the

band gap of these materials is situated at 1.5 eV and 1.3 eV, respectively, which would thus

make these quaternary compounds suitable for application.33 In 2009, Chen and

collaborators analysed this potential via first principles calculations and noted that, because

of the dependence of the band structure on the Cu and Zn, the cation ordering is low and is

therefore a predictor that the band gap of Cu2ZnSnS4 is situated at 1.0 eV, which is a far

lower value than initially predicted.34

A facile, economical, environmentally sound and industry-scalable ball-milling technique

for the preparation of quaternary copper iron tin sulfide (Cu2FeSnS4) or CFTS powder with

a bandgap of 1.42 eV has been developed by Vanalakar et al. These workers confirmed the

28

identity of the pure product by means of Raman spectroscopy, X-ray diffraction (XRD) and

energy-dispersive X-ray spectroscopy (EDX) analysis.35 Meanwhile, a solution-based

approach was used by Zhang et al. to produce CFTS nanocrystals with oblate spheroidal

shapes (bandgap 1.54 eV) and triangular plate shapes (bandgap 1.46 eV).36 Moreover, CFTS

nanoparticles have been deposited onto FTO substrates by Dong et al. via spin coating to

give a bandgap of 1.53 eV.37 In addition, single phase CFTS samples with an optical bandgap

of 1.40 eV, and CZTS samples with a 1.48 eV optical bandgap, were prepared by Mokurala

et al. using thermal decomposition.38 Furthermore, the chemical spray pyrolysis technique

was used at a range of deposition temperatures by Nilang et al. to produce CFTS thin films

with an optical band gap of 1.54 eV.39

The literature also contains examples of research into the copper manganese tin sulfide

(CMTS) semiconductor. For example, Chen et al. (2015) used the sol–gel technique to

produce CMTS thin films with tuneable bandgaps ranging from 1.62 eV to 1.14 eV

depending upon the post-annealing temperature.40 Moreover, Nie et al. used chemical spray

pyrolysis to generate CMTS with a 1.19 eV bandgap,41 while other researchers used a

microwave-assisted solvothermal method to produce CMTS nanocrystals with optical

bandgaps of 1.11 eV.42

29

1.6. Classification and applications of semiconductors

There are several types of semiconductor materials that can be used within electronic

devices.

Table 1. 2. Common semiconductors and their applications.43

Material chemical

symbol/formula

Group Applications

Germanium

Silicon

Silicon carbide

Ge

Si

SiC

IV

Microchips, solar

cells

Gallium arsenide

Gallium nitride

Gallium phosphide

GaAs

GaN

GaP

III-V

Light emitting

diodes

Cadmium sulfide

CdS

II-VI

Solar cells, solid

state lasers

Tin sulfide

SnS

IV-VI

Thermal imaging,

IR detectors

30

1.7. Nanoparticle materials

Nanoparticles are defined as materials that have a diameter ranging from 1 to 100 nm.44

These materials come in a variety of shapes and sizes and are distinguished from bulk

materials through the different physical and chemical characteristics that nanoparticles

display. Such differentiations include specific optical characteristics, a higher surface area

and magnetization properties as well as lower melting points.44 In recent years, nanoparticles

have been widely investigated by researchers specifically because of these attributes, which

makes their applicability relevant to several industries, including biological labeling,

electronic devices, drug delivery systems, quantum dot catalysis as well as environmental

remediation.45,46

In addition to the relatively wide applicability of these materials across several industries,

their manipulation and adjustment for different purposes also make nanoparticles a desired

area for further investigation. In this regard, nanoparticles can be fabricated to suit specific

needs and thus vary in shape and size, thereby facilitating the creation of new materials.

Moreover, nanoparticles can achieve different morphologies and types, which place these

materials into different categories such as semiconductors, metal oxides, metals and even

biomaterials.47

To synthesise nanoparticles, two main approaches are used. The first is the so-called top-

down approach, which involves several attrition methods including laser ablation or

lithography where nanoparticles are extracted from bulk materials.48 At the other end of the

spectrum, the bottom-up approach involves attaining the molecules from their component

parts, specifically from molecules and atoms.49 This approach is favoured in the fabrication

of nanoparticles due to cost and the fact that, through this method, nanoparticles with less

defects are obtained.50,51

31

1.8. Nanocrystals of semiconductors

Semiconductor materials generate nanocrystals that have a diameter of 1 to 20 nm. These

particles exhibit very distinctive properties in contrast to the bulk materials from which they

are obtained. Several researches have focused on semiconductors specifically due to these

properties, which include special optical characteristics, electrical properties and catalytic

properties.52

The properties of nanocrystals obtained from semiconductor materials are a direct result of

their size. Firstly, with the significant reduction in size, the surface properties effects are

noted. The same effect resulting from the small size of the nanoparticle results in the electric

properties of these materials. This effect is referred to as quantum confinement and dictates

that the electrical properties of materials will change in accordance with changes in the size

of the material.53 Notably, particles will exhibit different behaviour when examined on a

small scale and when observed in bulk. Thus, quantum confinement is the spatial

confinement of electron–hole pairs in one or more dimensions within a material, and also

electronic energy levels are discrete. In this context, the smaller the particle obtained, the

wider the band gap achieved. Such manipulations in the size of the material will thus result

in distinctive optical, electrical, mechanical, magnetic and chemical reactivity properties.54

Furthermore, if one dimension of a semiconductor is smaller than the Bohr exciton radius of

the material, the band structure will be modified and blue shifted to higher energy by the

quantum confinement effect.55–59 In the limit of very small particle size, the so-called strong

confinement regime, quantized levels appear, which is distinct from the continuous band of

bulk counterparts and shows characteristics of the discrete molecular semiconductors, Figure

1.4.

32

Figure 1. 4. Band structures of bulk, nanoparticles and molecule.60

1.8.1. Compound semiconductors

As well as the Group IV elements, semiconductors often consist of compounds of Group III

and Group V elements or Group II and Group VI elements, giving an average of four valence

electrons in each type of material. The stoichiometry of semiconductors like GaAs can be

varied to give doping effects involving either a small increase in the proportion of As (n-

type doping) or a small increase in the proportion of Ga (p-type doping).

As shown in Figure 1.5, the Group II-VI the transition metal chalcogenides (TMCs) can be

used as starting materials to generate various binary, ternary and multinary semiconductors

via cation mutation. Thus, a Group III cation can substitute for a Group II cation to give a

I-III-VI semiconductor; one Group II and one Group IV cation can substitute for two Group

III atoms to give a I2–II–IV–VI4 semiconductor; one Group I and one Group III cation can

replace two Group II cations to give a I–III–II2–VI4; or half of the Group II cations can be

replaced by a different Group II cation to give I2–II–II-III-VI4. Control of the atomic ratios

via the mutation process makes it possible to engineer, tailor and optimise the material’s

33

bandgap properties for specific applications, including solar cells,34 spintronics,61

thermoelectrics62 and novel categories of topological insulators.63 However, since each of

the semiconductor categories encompasses numerous possible compositions, appropriate

research and screening is necessary to identify the correct composition.64

Figure 1. 5. Schematic representation of the structure development tree for the formation of binary,

ternary and multinary semiconductors starting from a II–VI parent compound.65

1.9. Transition metal chalcogenide semiconductors

Interest in the transition metal chalcogenide (TMC) materials has recently shown a dramatic

increase, with numerous research groups concentrating on their attractive characteristics and

wide range of applications such as in field effect transistors, sensors, solar cells and water

34

splitting photocatalysis.66 Several TMCs possess layered structures that provide distinct

electronic and chemical properties from those of bulk semi-conductors.

In addition, the potential use of these compounds as earth abundant, cheap, non-toxic and

environmentally sustainable photovoltaic (PV) materials is another source of the rise in

interest and is the main emphasis of the present review. The primary advantage of TMCs

relative to other conventional PV materials (e.g. lead perovskites, organic photovoltaics

(OPVs)) is their higher stability. The OPVs experience bleaching due to oxidation of the

photoabsorbent organic molecules in the presence of oxygen,67 while the lead perovskites

are similarly susceptible to both oxygen and water.68,69 While the conventional TMC

photovoltaics are based on the Cd(S, Se) family, the newer materials incorporate arsenic

(As), gallium (Ga) or indium (In). However, interest into other chalcogenide materials is

motivated by severe international restrictions limiting the industrial use of cadmium along

with continuing concerns surrounding the global supply and sustainable availability of In,

Ga and As.70 Although PV devices are frequently described as 'green' energy sources, they

can only be genuinely sustainable and economically practicable if the device efficiency is

high and the material cost is low. The annual energy generation potential of a range of PV

materials was modelled and compared with the production costs by Wadia et al. to

demonstrate that materials like FeS2, Cu2S and Cu2ZnSnS4 have the highest energy

production potential relative to material cost.71 The present challenge is therefore to achieve

the full potential of these materials.

The range of TMC semiconductor materials suitable for PV devices is remarkably large,

with an Inorganic Crystal Structure Database (ICSD) search indicating at least 15000 distinct

compounds. These can be divided into three primary categories, namely the binary (MxEn),

ternary (MxM′yEn) and quaternary (MxM′yM″zEn) systems, where M is a transition metal, M′

and M″ is another transition metal or other type of metal and E can be sulfur (S), selenium

35

(Se) or tellurium (Te). These categories are frequently designated by Roman numerals

indicating the oxidation state of the metal and the group of the chalcogen or pnictogen, for

example II–VI (e.g. CdS), I–III–VI2 (e.g. CuInS2) or III–V (e.g. InP).

Since the efficiency of a single p-n junction solar cell is subject to a theoretical maximum

value, a suitable photoactive semiconductor must have a 1.0 to 1.5 eV gap between the

lower-energy (valence) band and the higher-energy (conduction) band.72,73

The TMC semiconductors have found several uses in PV devices, including photo-absorbent

layers, buffer layers and anodes in dye-sensitized solar cells (DSSCs),74 where they generally

take the form of either quantum dots or nanostructured thin films.75,76 However, while the

TMCs are starting to achieve their potential in these applications, the production of a material

with a high absorption coefficient and 1.0–1.5 eV bandgap from cheap and plentiful elements

remains a significant research challenge. Films can be produced by solution processing of

TMCs to form nanocrystalline materials or inks77,78 or by other methods including chemical

bath deposition (CBD).79,80 As well as methods such as solvothermal synthesis, the widely-

available hot-injection technique has been used to produce nanocrystalline TMCs and has

proven suitable for binary, ternary and quaternary systems.

1.9.1. Binary TMCs

A wide range of binary TMCs displays properties that are suitable for PV systems. Examples

include FeS2,81,82 CdS,83 CuxS,84,85 CuSe,86 MnS87 and SnS,88 among which Cd(S, Se), FeS2

and the range of copper sulfides are possibly the most familiar and have the most attractive

properties. Following the development of simple synthetic protocols in the 1990s, the Cd(S,

Se) quantum dots became all-pervasive during the early 2000s.89–91 Although these materials

possess optimal photoelectric and electronic properties (e.g. bandgaps) that are readily

tuneable by adjusting the proportion of sulfur/selenium,92 the high toxicity of cadmium has

been well demonstrated and has resulted in stringent EU restrictions.93,94 While this would

36

seem to relegate the PV applicability of cadmium chalcogenides to the context of the

laboratory-scale test, the development of Cd-based solar cells is continuing such that, in

2016, a record efficiency of 22.1% was achieved for CdTe in a thin film device developed

by First Solar.95

Iron pyrite (FeS2, also known as fool's gold) is easily synthesised,96 has an absorption

coefficient of 105 cm-1, a 0.95 eV bandgap and exceptionally low raw material costs, all of

which should make it an ideal choice for PV devices. However, while nanostructured FeS2

has found application as a photoconductor, in a p–n heterojunction, in bulk heterojunction

inorganic–organic hybrid solar cells and in DSSCs,97–100 the electronic properties can be

adversely compromised by surface defects due to sulfur vacancies. According to Steinhagen

et al., nanocrystal devices are especially susceptible to this because of the high proportion of

grain boundaries and the high proportion of atoms likely to reside at the surface of nanoscale

particles.81 Indeed, when Shukla et al. obtained photovoltages from pyrite nanocubes by

sulfurization of a deposited colloidal ink, they determined that surface defects were the

primary source of electron–hole recombination and that the efficiency could be enhanced by

using an optimised synthetic route to decrease either the concentration of grain boundaries

or the number of defects.82

A wide range of copper sulfide phases based on the stoichiometry of CuxS all have bandgaps

of around 1.2 to 2.0 eV.84 While x values of less than 2 correlate to bandgaps near 2.0 eV

and, hence, minimal PV applicability, the indirect bandgap semiconductor Cu2S has a bulk

bandgap of 1.21 eV85 and the indirect bandgap of its selenide counterpart (Cu2Se) is 1.4

eV.86 The production of a 1.6% efficient PV device was reported by Wu et al. who spin-

coated a layer of CdS nanorods with Cu2S nanocrystals synthesised by reaction of

ammonium diethyldithiocarbamate and bis(acetylacetonato)copper(II) in a mixed dodecane

thiol/oleic acid solvent.85 Although Cu2S found frequent use in combination with CdS from

37

the 1960s through the 1980s,101 the PV cell tended to degrade over time due to Cu+ diffusion

into the CdS layer.

Quantum dots such as CdS, CdSe, PbS and ZnS have been doped with manganese(II)

sulfide;102 for instance, Punnoose et al. demonstrated a PbS quantum dot DSSC with a PV

conversion efficiency of 4.25%.103 Finally, interest in the main group binary chalcogenide

tin mono sulfide (SnS) for PV applications has arisen due to its appropriate bandgap for solar

absorption (generally between 1.1 and 1.4 eV)88 and up to 24% theoretical power conversion

efficiency. However, the maximum efficiency for the SnS PV cell, reported by

Sinsermsuksakul et al., was 4.4%;104 hence, there is significant scope for improvement. The

efforts of the present author's group have concentrated on using aerosol-assisted chemical

vapour deposition (AACVD) to produce suitable thin-film SnS semiconductors for ultimate

application in PV device designs.105–107 It is particularly interesting to note that SnS is a

van der Waals layer structure and the present group has demonstrated that the bandgap

energy can be controlled in a foreseeable, layer-dependent way by thinning the material to

the 2D limit.108

1.9.1.1. Manganese sulfide

Manganese sulfide (MnS) displays three crystalline polymorphs with distinct morphological

and physical properties. The naturally-occurring, thermodynamically stable, green-coloured

alpha (α) phase (alabandite) displays an octahedrally coordinated rock salt structure (space

group Fm3m) and forms at relatively high temperatures. Meanwhile, the metastable beta

(β) and gamma (γ) phases are both pink in colour and form at low temperatures, with β-MnS

crystallizing in the tetrahedral zinc blende structure (space group F43m) and γ-MnS

crystallizing in the tetrahedral wurtzite structure (space group P6(3)mc) at low temperatures

as shown in Figure 1.6.109 Early experimental research examining the electrical, magnetic

and optical properties of the MnS phases has been reviewed in the literature.109 Recent

38

photo-electrochemical research has focused on manganese sulfide (MnS) owing to its

hypothesised role in prebiotic synthesis on early Earth.110 To avoid the introduction of

extraneous sources of carbon, such studies make use of crystalline MnS generated in the

absence of organo-sulfur compounds. Although room-temperature formation of the

thermodynamically stable α-phase is kinetically hindered, it will readily form under

solvothermal conditions at temperatures above approximately 200 °C. Various published

synthetic routes to MnS in one or more of the three phases include the high-temperature

reaction of sulfur and manganese in elemental form111–113 and deposition as the thin-

film29,114–120 generated in at least one example by decomposition of organometallic

precursors.121 Numerous other experimental studies involved the initial aqueous synthesis

of MnS from inorganic precursors followed by drying and subsequent transformation to the

α-phase by heating at temperatures around 1000 °C.122,123 Previous studies have also

described the solvothermal synthesis of both metastable (β and γ) forms as well as the stable

α-phase.124–128 A synthesis of pure α-MnS is presented in the present work and, in agreement

with the previous studies.129

Figure 1. 6. The crystalline structures of cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD 01-

089-4952), metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288) and

hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-040-1289).

Color code: Mn, violet; S, yellow.129

39

1.9.2. Ternary TMCs

The ternary TMCs category is generated by replacing the metal of a binary metal

chalcogenide system (MxEn) with two metals giving the same total charge (MxM′yEn). The

presence of two different metals opens up bandgaps that are not available to the isoelectronic

binary metal chalcogenides.65 A frequently encountered ternary system combines two

metals in the +1 and +3 oxidation states with a chalcogen pair in the −2 oxidation state, e.g.

CuInS2, and is denoted as I–III–VI2. The I–III–VI2 system is derived from the II–VI parent

binary system, e.g. CdSe. Systems incorporating two different chalcogens (MxM′yEnE′m)

also belong to the I–III–VI2 system and are regarded as ternary in spite of containing four

distinct elements.65 As with the parent binary compounds, the ternary systems experience

quantum confinement and function as quantum dots,130 enabling these materials to interact

with the full solar spectrum via the resultant energy modulation effects. This is extremely

useful for light harvesting, making the ternary TMCs a desirable substitute for toxic binary

compounds such as the cadmium chalcogenides.

The chalcopyrite phase of the ternary compound copper indium sulfide (CuInS2) has

attracted research interest as a possible component of heterojunction PV devices due to its

absorption coefficient >105 cm-1, direct band gap of 1.5 eV, high radiation hardness and

defect tolerance.131,132 Early devices and homojunction devices in the 1970s combined

CuInS2 with CdS or InP.133–135 The high concentration of defects results in beneficial

characteristics including bandgap tuning by controlling the number of defect sites along with

a high dopant capacity.136,137 However, these properties can result in compositional

differences between identically-sized nanocrystals within a batch, thereby broadening the

ensemble properties such as the luminescence peak for colloidal nanocrystals.138

Another I–III–VI2 ternary metal chalcogenide with the chalcopyrite structure is copper

gallium selenide (CuGaSe2). Although this has a high optical absorption coefficient of 105

cm-1 and a direct bandgap of 1.66 eV,139 its use in PV devices has been obstructed by the

40

challenges involved in generating a single-phase material. New colloidal routes for the

synthesis of phase-pure CuGaSe2 have been examined in order to address this problem.140,141

1.9.3. Quaternary TMCs

The most promising TMC systems with respect to maximum efficiency are the quaternary

TMC compounds, although they are also some of the most problematic to synthesise. They

have the general formula MxM′yM″zEn, where M is a transition metal, M′ and M″ are

additional (distinct) transition or other types of metal and E can be S, Se or Te. Among the

chief quaternary systems, copper zinc tin sulfide/selenide or CZTS (Cu2ZnSn(S,Se)4) and

copper indium gallium selenide or CIGS (Cu(In,Ga)Se2) have been extensively studied.142,143

While the earliest chalcopyrite-based solar cells used CuInSe2, with its 1.04 eV bandgap, as

the absorber, it was subsequently realised that the bandgap could be tuned by substitution of

gallium for some of the indium (giving a maximum bandgap of 1.68 eV for CuGaSe2).

Further studies have shown that the power conversion efficiency can be optimised by using

CuInGaSe2, giving a bandgap between 1.10 eV and 1.25 eV.144 The CIGS PV materials

have numerous desirable characteristics, such as harmless grain boundaries and lenient phase

characteristics that facilitate compositional variety without triggering a phase-transition.145

Notably, CIGS PV materials are among the few TMCs to be commercially produced, with

several firms producing devices with greater than 15% efficiency.144

Although the above-mentioned ease of manufacture, efficiency and commercial presence

make the CIGS materials unarguably successful, they do share one significant drawback

with the ternary CIS and related compounds, namely the low availability of indium. Indium

has been classified in the 2015 Risk List issued by the British Geological Survey ranks as

having a high supply risk.70 The focus of research has consequently switched to CZTS

(Cu2ZnSnS4) as an economical, environmentally friendly and abundantly available PV

material. CZTS has a stoichiometrically tuneable direct band gap of 1.45 eV and a high

absorption coefficient.146 The significant promise of this material is demonstrated by the

41

present admirable record efficiency of 12.6%.147 Like CIGS, it is frequently generated by a

vapour deposition, sputtering or a process followed by a high-temperature annealing stage.

This high-temperature stage presents two challenges that must be addressed if CZTS is to

become commercially practicable. Firstly, volatile compounds such as SnS can be lost

during annealing, which presents problems in controlling the composition of the desired

phase and, hence, the stoichiometrically-dependent photoconversion efficiency of the

absorber layers.148–152 In practice, solar cells produced from Cu-poor films display a notably

better performance than those using stoichiometric Cu2ZnSnS4.149 Secondly, an extreme

reduction in efficiency can arise due to reaction of sulfur with the molybdenum (Mo)

electrode onto which the CZTS is frequently deposited, forming an unwanted MoS2 layer

between the electrode and the absorber.153–155

Additional challenges in the underlying materials science of CZTS arise from the existence

of three possible stable phases (namely kesterite, stannite and a primitive mixed CuAu-like

structure)1,145,156, which can negatively impact upon the electronic and optical properties of

the material.

1.9.3.1. Kesterite and Stannite

Research into economical materials for highly efficient solar cells has gained major

importance in order to address the continuing need arising from recent rapid increases in

global energy consumption. The low bandgap energies and high absorption coefficients of

the Cu-based multinary chalcogenides gives them significant potential as effective next-

generation solar cell materials. Although the most outstanding of these is CuInxGa(1-x)Se2,

with the highest known energy conversion efficiency, reproducibility and flexibility towards

a range of growth process technologies,143 the use of rare and expensive elements such as

Ga and In is regarded as an obstruction to its commercial application. Moreover, while a

power conversion efficiency (PCE) of 21.7% has been achieved using a solar cell based on

a thin CdTe film,95 this cannot be effectively used due to the presence of toxic cadmium.

42

The Cu-based chalcogenide compounds such as CZTSe (Cu2ZnSnSe4) and CZTS

(Cu2ZnSnS4), with their abundant elemental components, close-to ideal bandgaps, large

absorption coefficients and 32.2% theoretical maximum PCE, have therefore been employed

by research scientists as a means of replacing Cd, Ga and In as absorber materials.157–159 For

example, a PCE of up to 12.6% has been reported for a thin-film solar cell (TFSC) based on

the p-type CZTS,147 although synthesis of the pure kesterite phase is problematic because of

the quantity of secondary and ternary phases present.160 Several further challenges to the

development of CZTS-based thin film solar cells, including: (i) the continuous variation in

bandgap (grading) for a given film thickness; (ii) the closely comparable atomic sizes of Zn

and Cu, which facilitates intermixing and gives rise to defects;161 and (iii) deterioration in

solar cell performance due to variations in the electrostatic potential of Cu and Zn in the

CuZn + ZnCu defect.162 There is therefore pressing need to develop alternatives to kesterite

CZTS and a method for substituting other elements for Zn or Cu. For instance, attention has

focused on the p-type CFTS (Cu2FeSnS4) materials with comparable properties to the CZTS

materials, including appropriate bandgaps of 1.28 eV – 1.50 eV and optical absorption

coefficients greater than 104 cm−1. Furthermore, CFTS is entirely composed of cheap,

abundant, relatively non-toxic elements and the optical bandgap energy is decreased by the

substitution of Zn by Fe. The higher solubility of Fe in the lattice results in enhanced

conductivity and enhanced efficiency at converting solar energy to electricity.163 Notably,

in addition to functioning as an absorber layer in TFSCs, the CFTS materials can function

as counter electrodes in DSSCs. Research in this direction has suggested that CFTS may

provide a more economical substitute for platinum in DSSCs. An additional potential use

of CFTS, in the form of nanoparticles, is in the photo-catalytic degradation of dyes.

Nevertheless, in spite of their photo conversion efficiency of around 8%,164 research into the

CFTS-based solar cells remains rare.

43

1.9.3.2. Crystal structures of kesterite and stannite

The component atoms of CFTS are arranged in the unit cell as 4Cu, 2Fe, 2Sn and 8S. The

Cu-based chalcogenides CZTS (Cu2ZnSnS4) and CFTS (Cu2FeSnS4) are most frequently

found in the kesterite and stannite crystal structures, respectively. Although these structures

are closely related, they differ in the distribution of cations such as Cu+, Zn2+and Fe2+ and

are therefore assigned to different space groups. Thus, as indicated in Figure 1.7, the

kesterite structure has one Cu atom in the 2a (0, 0, 0) position, Zn in the 2c (0, ½, ¼) position

and the remaining Cu atoms occupying the 2d (0, ½, ¾) position, whereas the stannite

structure has Fe sited at the origin (2a), Cu at the 4d (0, ½, ¼) position and the Sn4+cation

remains on the 2b site in each case.165,166


10.726 Å, α. β and γ= 90, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α. ,

β and γ= 90 ICDD: 0005843.1

44

Quintero et al. studied the crystallographic properties of the I2-Fe-IV-VI4 compounds and X-

ray powder diffraction indicated a tetragonal stannite structure I42m for Cu2FeSnS4 derived

from chalcopyrite by replacing half of the Fe atoms with Sn atoms and changing the

symmetry from I42d to I42m. The stannite phase remains stable within the temperature

range of 420 to 500 °C. Structural refinements by Hall et al. and other researchers support

these deductions.167–169

While the most frequently generated tetragonal structure arises from the ordered

arrangement of metal atoms in the cubic cell, cubic polymorphous adaptations with

disordered sphalerite-like structures are also noted at high temperature. The cubic crystal

structure variation of Cu2FeSnS4 was recently identified by Evstigneeva et al.170 who

reported the existence of the CFTS compound in a cubic phase belonging to the space group

I42m. The stannite prototype with a small cubic unit cell was also confirmed in the X-ray

study.

1.10. Doping

Semiconductors containing impurities or foreign atoms within the crystal structure are

referred to as doped semiconductors. Such impurities can arise unintentionally due to

inadequate control during growth of the semiconductor crystal, but are often added

deliberately to provide available charge carriers and enhance the electrical, magnetic and

optical properties (e.g. luminescence efficiency) that are essential for practical

applications.171 Rare-earth (RE) and transition metal (TM) ion dopants influence the

nanocrystal morphology and band structure as well as generating intense emissions in a

broad range of wavelengths depending upon the concentration, crystal dimensions and

specific type of impurity. Much recent research has therefore focused on doping

semiconductor nanostructures with RE and TM materials in order to discover possible

applications in photonics and bio-photonics.172 More detailed investigation into doped

45

nanocrystals is also valuable because the bandgap of the nanocrystalline host material can

be fine-tuned, and new luminescence generated, by matching the nanocrystalline shapes and

sizes to the energy levels of luminescent centres. A range of nanocrystalline particle systems

have been examined as part of a matrix or as a free-standing powder.56 Semiconductor

doping creates allowed energy states within the bandgap that are very close to the energy

band of the dopant type, with donor impurities creating states close to the conduction band

and acceptor impurities creating states near the valence band.173 By introducing isolated

energy levels between the host's valence and conduction band, such dopants frequently form

an emissive trap. Another valuable effect is a shift in the Fermi level of the host material

towards the energy band of the highest-concentration dopant.173

The surface properties of nanocrystals are expected to have a notable influence upon the

optical and structural properties of the material because a significant proportion of the atoms

are located at or close to the surface.174 Moreover, the material's chemical and physical

properties can be optimised with specific applications and requirements in mind by selecting

appropriately from a range of metallic dopant atoms. In general, the electrical, magnetic and

optical properties of metal sulfides can be significantly modified by metal-atom doping due

to the resulting changes in electronic structure.

Numerous studies have noted the exceptional optical properties of doped semiconductor

nanoparticles, regarding them as a new class of luminescent materials.172 The majority of

studies on doped semiconductor nanostructures have involved samples in which the particles

have a range of dopant numbers per particle, with the primary aim of elucidating the physical

properties of such powder nanostructures.

One particularly intriguing example of the recent progress in the semiconductor field is the

production of a dilute magnetic semiconductor (DMS) with possible spin-based electronics

and optical applications via doping with magnetic impurities capable of generating an

46

extremely large Zeeman splitting (more than two orders of magnitude larger than that of

standard semiconductors).175,176 In addition, the quantity of charge carriers (electrons and

holes) in a semiconductor can be controlled by doping with standard impurities. Thus, as

mentioned earlier dopant with one less valence electron than the host atom provides a hole

or positive charge carrier ("p-type"), while a dopant with one more valence electron provides

an electron or negative carrier ("n-type"). These forms of doping provided the basis for p-n

semiconductor devices like computer chips.177

Due to the similarities in chemical properties such as valance band and ionic radius, Mn2+

ions are relatively easy to incorporate into zinc (Zn2+) or cadmium (Cd2+) lattices.

Consequently, the distinctive and intricate properties of ZnS and CdS nanoparticles doped

with various concentrations of Mn2+ have been noted by several researchers. With an

electronic configuration of 1s2, 2s2, 2p6, 3s2, 3p6, 3d5, the Mn2+ ions in a range of luminescent

materials displays a d5 configuration.178–181 These ions produce a broad emission peak at a

position strongly influenced by the relationship between host lattice and crystal field

strength.182–184 The bulk ZnS:Mn material has found frequent application as a phosphor,

notably in AC thin-film electron luminescent devices.185–188 It has been suggested that the

Mn2+ d-electron states function as luminescence centres under external electronic excitation

via strong interaction with the s-p electronic states of the ZnS or CdS host lattice. One

possible mechanism is the generation of an excited state in the Mn2+ ion by the

recombination of an electron with a hole trapped by the Mn2+ ion.189,190

For instance, nearly monodispersed Mn-doped ZnS and CdS nanoparticles capped with

trioctylphosphine oxide (TOPO) were synthesised by Malik et al. via a single-source method

using manganese dichloride and bis(diethyldithiocarbamato)zinc(II) or

bis(methylhexyldithiocarbamato)cadmium(II).191 The resulting particles displayed distinct

optical properties compared to those of bulk ZnS or CdS. Their nanometre sizes and

47

predominantly hexagonal crystalline phases were demonstrated by electron microscopy and

X-ray diffraction, while the presence of the expected proportion of manganese dopant in the

ZnS or CdS nanoparticles was confirmed by electron paramagnetic resonance (EPR) spectra

and inductively coupled plasma mass spectrometry (ICP-MS).191

Meanwhile, metal diethyldithiocarbamate complexes with the general formula

[M(S2CN(Et)2)n] where M = Co (III), Cu(II), Fe(III), Ni(II) or Zn(II) and n = 2 or 3 were

used by Khalid et al.192 as single-source precursors for the aerosol-assisted chemical vapour

deposition (AACVD) of thin films of iron pyrite (FeS2) or transition metal-doped iron pyrite

(MxFe1-xS2) onto glass or indium tin oxide (ITO)-coated glass. Thermogravimetric analysis

(TGA) confirmed the decomposition of each complex to the corresponding metal sulfide

within comparable temperature ranges. The iron complex, [Fe(S2CNEt2)3], was shown to

deposit as a single-phase granular cubic crystalline FeS2 film at 350 °C and as a mixed-phase

pyrite/marcasite film at 450 °C. A shift in the powder X-ray diffraction (p-XRD) peaks

confirmed the generation of an MxFe1-xS2 solid solution, while incorporation of the TMs into

the pyrite for films deposited at various mole ratios of the TM complexes was confirmed by

EDX spectroscopy.192

Similarly, Al-Dulaimi et al. used the AACVD technique at 475 °C with various mole ratios

of [Re(µ-SiPr)3(SiPr)6] and [Mo(S2CNEt2)4] complexes to deposit thin films of Mo1-xRexS2

(0 ≤ x ≤ 0.06). SEM analysis indicated a change in the thin-film morphology with increased

levels of Re dopant in the MoS2; while pure MoS2 displayed a lamellar morphology, 1.79%

Re-doping produced MoS2 clusters and 3.60% Re generated feather-like crystals, which

were also present at 6.25% Re. The p-XRD analysis showed regular variations in the peak

intensity, shape and position of the (002) planes with varying rhenium content.193

Meanwhile, the spin-coating and melt technique was used by Bakly et al. to generate thin

Cd1-xZnxS (CZS) films from bis(ethylxanthato)zinc(II), [Zn(S2COEt)2], and

48

bis(ethylxanthato)cadmium(II), [Cd(S2COEt)2], complexes. For doping ratios between 0

and 0.15, p-XRD analysis showed that the thin films were hexagonal, while a shift in the

CdS peaks to higher angles with increasing Zn content demonstrated successful doping of

CdS with Zn.194

The production of PbS semiconductor nanocrystals doped with 0.05 and 0.52% Mn2+ via

simple chemical methods has been reported by Kripal and Tripathi. Nanocrystals of PbS:Mn

with an average crystallite size of 5 to 10 nm and a cubic structure belonging to space group

Fm3m were produced. The X-ray diffraction (XRD) analysis demonstrated a mild variation

in the lattice constant (a) with the concentration of Mn2+ ions. Since the level of Mn doping

in the PbS nanocrystal was quite low (less than 1%), the variations in peak intensity for 0.05,

0.26 and 0.52% Mn2+ were minor. Successful doping of the PbS nanocrystals with Mn2+ ions

was confirmed by EPR spectroscopy.195

Finally, the chemical bath deposition (CBD) approach was successfully used by Kumar et

al. to deposit thin films of Sb-doped PbS onto a cleaned glass substrate. Doping of the pure

PbS was achieved during film growth by addition of an aqueous solution of Sb3+ ions. While

all the deposited films displayed good quality, the characterization revealed a significant

influence of Sb-doping and annealing upon their physical properties. The XRD analysis

revealed a face centred cubic crystalline structure with a preferred (200) orientation and an

increasing crystallite size with increasing concentration of Sb dopant. Due to the quantum-

size effect, the optical absorption band edge displayed a blue-shift in the doped

nanostructured films relative to that of pure PbS. Moreover, the bandgap energy values were

significantly influenced by the concentration of dopant and annealing.196

1.11. Synthesis of nanoparticle semiconductors

Significant research endeavours have focused on synthetic routes to high quality, crystalline

semiconductor thin films and nanocrystals (NCs). These synthetic approaches fall into four

49

primary categories, depending upon the state of the reaction medium, namely: vapour-phase,

solid-phase, liquid-phase, and two-phase synthesis. The chief techniques for semiconductor

NC synthesis include chemical vapour deposition (CVD),197,198 molecular beam epitaxy

(MBE),198–202 magnetron sputtering,203 laser ablation,204 ball milling205 and metal-organic

vapour chemical deposition (MOCVD).206

Nanoscale materials, especially thin films, have often been successfully synthesised using

gas- or vapour-phase methods such as chemical vapour deposition (CVD). Several reviews

are available207,208 and just the key aspects will be recapitulated herein. The molecular

precursor(s) are initially vaporised under atmospheric or lower pressures before introducing

them (e.g. via an inert carrier gas) to the heated substrate, at which point decomposition

occurs to generate the thin film or particulate product.207,209 In the absence of a capping

agent, nanoparticle growth can lead to broad particle size distributions.209

Due to limitations relating to instrumentation, precursors and control of the synthetic process

and NC quality, the vapour-phase and solid-phase approaches have been found less efficient

than the liquid-phase approach for generating well-defined semiconductor NCs. The liquid-

phase synthesised NC semiconductors are dispersed in appropriate solvents to form stable

suspensions with the aid of surfactants or capping ligands; hence, they are also known as

colloidal semiconductor nanocrystals (CS-NCs).210 The liquid-phase approach has a number

of key benefits, including tuneable bandgap energies, high dipole moments, high optical

absorption coefficients and the potential to generate multiple excitons.211 Moreover, the

low production costs and extremely high throughput capability make solution-based

methods such as inkjet printing, spin-coating and roll-to-roll casting with stable colloidal

suspensions the preferred route for the large-scale manufacturing of devices.212,213

Liquid-phase syntheses can be further subdivided into three types according to the reaction

medium, namely: aqueous syntheses, organic syntheses and aqueous/organic syntheses. The

50

advantages of aqueous syntheses are the use of environmentally sound chemical

precipitation,214,215 hydrothermal techniques,214–217 mild reaction temperatures218 and

biocompatible solvents, while the primary disadvantage is lack of control of NC

morphology. By contrast, organic-based methods such as hot-injection219 and the

solvothermal autoclave method220–222 use high-boiling organic solvents and organic ligands

to generate NCs with well-controlled morphologies.223–226 Finally, the aqueous-organic

techniques (also referred to as interface-mediated or liquid-solid-solution (LSS)

techniques)227–231 involve the generation of NCs at the interface between an aqueous phase

and an organic phase, with the reactants divided between the phases. This approach,

pioneered by Wang et al.229 and Pan et al.230 have been successfully employed to

synthesise a range of NCs including polymer nanoparticles.229 Benefits include

stoichiometric control and mild reaction conditions.231 The resulting NCs are frequently

spherical or have regular shapes facilitated by their crystal structures.

The LaMer model considers the formation of NCs in two primary stages, namely nucleation

and crystal growth.232 When the precursors are dissolved in appropriate solvents, they

chemically react to form monomers which increase in concentration until super-saturation

triggers aggregation and self-nucleation of the monomers. Monomers then continue to

aggregate onto the pre-existing nuclei and NC growth occurs when the concentration of

monomers falls below a critical level. New nuclei continue to form during NC growth,

leading to a broadening of the NC size range; hence efforts to control the size of NCs are

primarily aimed at limiting this size distribution by adjusting reaction parameters such as

reaction time, reaction temperature, reactant injection temperature (for hot-injection

methods), precursor reactivity, precursor concentration, choice of solvent, choice of

surfactant and pH.233

51

1.11.1. Hot injection method

An early strategy is to separate the nucleation and growth stages. For instance, nucleation

can be induced by rapid injection of the precursors into solvents at an elevated temperature,

with a subsequent reduction in reaction temperature to separate nucleation from growth.

This technique, termed the hot-injection method, was first employed by Murray et al. to

successfully synthesise high-quality monodisperse cadmium chalcogenide NCs using

dimethyl cadmium, Cd(CH3)2, as the source of Cd, bis(trimethylsilyl)sulfide, selenide or

telluride as chalcogenide sources, and tri-n-octylphosphine (TOP) and its oxide (TOPO) as

solvents.234 The precursors were injected at 300 °C, while CdX NCs (where X = S, Se, Te)

of average size were obtained at temperatures between 112 and 115 °C. The quality of NCs

obtained by this method can be enhanced by rapidly quenching the reaction mixture or by

introducing size-selective precipitation into the procedure. These additional strategies have

been applied to the aforementioned CdSe NC synthesis by dispersing the NCs in 1-butanol,

adding methanol until persistent opalescence was observed and then centrifuging.219

The hot injection technique has been used to synthesise ternary and quaternary chalcogenide

NCs such as CZTS and CIS. In this case, dispersal of the precursor mixture in an appropriate

solvent was followed by addition of a capping agent (e.g. oleylamine) and heating to a

selected temperature under an inert atmosphere. Control of the particle size is achieved by

varying the precursor concentrations and reaction times. For instance, CuInS2 NCs have

been synthesised by mixing CuI, In(OAc)3 and DDT in octadecene solvent, followed by

addition of oleylamine and heating to 200 °C in an Ar atmosphere with controlled particle

sizes of 3.5 to 7.3 nm being obtained at reaction times of 20 to 120 min. The apparatus used

in the hot injection approach is shown schematically in Figure 1.8.235

52

Figure 1. 8. setup of hot injection method.236

In recent years, researchers have shown an increased interest in metal sulfide production

using single source precursors (SSPs) and which are described in Section 1.13. For example,

the thermolytic synthesis of copper(I) sulfide from a copper stearate complex in alkanethiol

solution has been reported by Li et al.237

Nanoparticulate manganese(II) sulfides have also been prepared by such methods. In

particular, MnS NCs with well-defined crystal structures and shapes have also been obtained

by Peng et al. via thermolysis of the SSPs bis(diethyldithiocarbamato)manganese(II),

[Mn(DDTC)2], or [Mn(DDTC)2(1,10-phenanthroline)] in oleic acid, oleylamine and 1-

octadecene (ODE) solvent.238

Phase-pure tin(II) sulfide nanosheets have also been obtained by Khan et al. by hot injection

of the SSP dibutyl-bis(piperidinedithiocarbamato)tin(IV) in oleylamine at 230 °C, revealing

how the SnS nanosheets develop from spherical nanoparticles during the course of the

reaction.239 Meanwhile, Yousefi et al. used the simple, comparatively low-temperature,

53

hydrothermal reaction of tin(II) chloride with thioglycolic acid to synthesise SnS

nanoflowers.240 Various methods have also been applied to the synthesis of iron sulfide. For

example, Vanitha and O’Brien described the thermolysis of a single-source cubane-like Fe–

S cluster in octylamine at 180 °C to generate nanocrystalline pyrrhotite (Fe7S8), while

thermolysis in dodecylamine at 200 °C produced greigite (Fe3S4).241

1.11.2. The solvent-less thermolysis

Due to its favourable scalability, we have seen that hot-injection is the most frequently used

chemical technique for nanoparticle synthesis at the present time,234 with the heating-up

approach also being favoured for similar reasons. Nevertheless, reactions in the molten state

are not only less complicated and more easily scaled up, but are also more environmentally

and economically favourable due to the absence of any solvent.242 The practical advantages

also of solvent-free techniques also include facile control of the precursors and reaction

parameters (e.g. duration and temperature of thermolysis), along with the lack of need for

strong reagents (e.g. reducing or sulfonating agents). With respect to scaling up, the process

of heating a precursor in a furnace should be comparatively economical and

straightforward.34 The apparatus used in the melt method is shown schematically in Figure

1.9.

Figure 1. 9. setup of solvent-less thermolysis.243

54

The melting method has been successfully used to synthesise a broad range of nanomaterials,

including metals and their oxides and chalcogenides, in a wide variety of morphologies,

including nanoscale spheres, wires, disks, rods and fabrics.244–247 For example, the O’Brien

group reported a solvent-less, self-capping approach in which an asymmetric cadmium

dithiocarbamate precursor was heated to 150 – 300 °C under vacuum, with particles of ~ 5

– 7 nm being obtained at 250 °C.246 The bismuth counterpart of this precursor,

[Bi(S2CN(C18H37)(CH3))3], was subsequently used by the same group, with one hour of

heating to 150 – 300 °C under vacuum to generate spherical, crystalline Bi2S3.248 At 250 °C,

the particles obtained were ~ 8 nm in diameter with a moderately wide size distribution.

Meanwhile, a low-temperature solvent-less method was used by Khan et al. to produce a

range of ternary chalcogenides,249 while Almanqur et al. used the melt method to produce

iron sulfide nanostructures from iron(III) xanthate SSPs of the type [Fe(S2COR)3], where R

= ethyl, methyl, 1-propyl and isopropyl.250 Similarly, Alqahtani et al. produced Bi2S3 and

Sb2S3 by heating the precursors tris(O-ethylxanthate)bismuth(III) and tris(O-

ethylxanthate)antimony(III) to 200 – 300 °C under vacuum in the absence of any solvent.251

1.12. Synthesis of thin films

Since the first thin film of CZTS was deposited in 1988 via the sputtering method, various

other methods for depositing different types of thin film have been developed. Spray

pyrolysis is seen as a fast, cheap method, which is also suitable when large areas are

sprayed.252 An updated version of the sputtering method was subsequently investigated,

which showed great potential for the growth of different metal sulfide films. Additional

deposition techniques include pulsed laser deposition (PLD), photochemical deposition

(PCD), electrochemical deposition (ECD), chemical vapour deposition (CVD) and the spin

coating technique.253–255

55

The selection process of the method to be employed during the deposition stage depends on

various characteristics that the researchers attempt to analyse. Such selections will be

dictated by the type of film, the characteristics of the material and the potential applications

of the material.256–258 For example, CZTS was found to have a greater absorption coefficient

when it was deposited via the dip coating method ,259 while copper sulfide (Cu2S) was found

to produce better results via the chemical deposition technique.260 Tin sulfide and tin dioxide

nanocomposites were found to produce better results when electrochemical deposition

techniques were employed,261 while Cu2S thin films obtained via chemical bath deposition

required more parameters to be considered, such as the bath temperature and the solution

pH.262 Additional studies emphasise the importance of the deposition technique used,

particularly for the durability of the materials obtained, but also highlight the potential cost

of the materials used in deposition methods.263,264

1.12.1. Spin coating method

The spin coating technique is a simple and swift process that is used for the deposition of

material on flat surfaces. Rotatable fixtures hold the substrate in place while the coating

solution is dispersed onto the surface. The technique presents some defect prevention

properties and can be used extensively for a variety of solar cell materials.265 Chuang et al.

employed this technique for ZnO/PbS quantum dot (QD) solar cells. The authors noted the

prolonged performance and air stability of the cells obtained via this method,266 as shown in

Figure 1.10.

Figure 1. 10. Basic diagram of the spin coating technique. 267

56

1.12.2. The doctor blade method

Thin film deposition can be rapidly achieved at room temperature in the presence of non-

toxic (hence, environmentally friendly) solvents by means of the doctor blade method, which

is shown schematically in Figure 1.11.268,269 Due to its use of inexpensive high-throughput

equipment, along with low material wastage and high uniformity over large deposition areas,

this technique is highly facile, efficient, economical and scalable. Because of its

effectiveness, the doctor blade method has been used in the present work.

Figure 1. 11. Schematic picture of doctor blade coating process for thin film deposition.270

Furthermore, the process is reproducible and allows faster film growth relative to other

coating techniques such as electrochemical deposition, chemical bath deposition (CBD) or

spray coating. However, in spite of its efficiency at the laboratory and industrial scales, the

technique continues to face a range of challenges including the necessity for high-

temperature annealing in a reactive atmosphere, material loss at the annealing stage, additive

contamination, low film adhesion and undesired effects during drying, such as cracking and

increased porosity. Nevertheless, high-quality films can be generated by selecting

57

appropriate solvents and mild additives along with an optimal slurry preparation and

multiple coating protocols.

The doctor blade method has been used by Murtaza et al. to deposit thin films of antimony

sulfide (stibnite, Sb2S3) from the SSP tris(thiobenzoato)antimony(III) complex with heating

of the glass substrate for 1 h at 300, 350 and 400 °C. While the films deposited at low

temperature displayed a random distribution of thick sheet-like crystallites on the substrate

surface, those annealed at higher temperatures displayed uniform bundles of sticks.270 The

technique has also been used by Ayala et al. to deposit thin films of CZGS (Cu2ZnGeS4)

from a paste of binary metal sulfide precursors, which were generated by mixing suitable

quantities of CuS, GeS and ZnS with dilute acetic acid and triethanolamine.271

1.13. Single source precursors (SSPs)

A number of researchers have focused on the use of single source precursors, i.e. precursors

containing two or more of the constituent elements in a single molecule, in the synthesis of

nanoparticles and thin films.246,272–275 These studies have often found SSPs to be an efficient

approach to high-quality, monodispersed crystalline semiconducting nanoparticles,

affording a number of key benefits over other routes such as: it minimizes the use of toxic

gases; the decomposition temperature of the precursors can be easily controlled; ease of

purification; considerable volatility and moisture stability;276 ease of handling and

characterisation;274 the presence of pre-existing bonds that can lead to a material with

improved stoichiometry and/or fewer defects;277 and the potential contribution to a decreased

environmental effect of material processing .272 The benefits of single-source precursors

(SSPs) include the enhanced tunability and control of the resulting material structure due to

the fixed geometry and close proximity of the components within the precursor molecule.

58

Several authors have reported the synthesis of metal sulfide nanoparticles via such

techniques using metal-thiolate complexes.

1.14. SSPs for metal sulfide nanostructures

A metal sulfide SSP must meet the following requirements: (i) it must contain both sulfur

and the desired metal, preferably linked to each other chemically; (ii) it must be sufficiently

stable for ease of handling and not be susceptible to oxidation prior to decomposition; (iii)

it must decompose fairly readily to generate a pure metal sulfide product; and (iv) any side-

products must be inert and, ideally, volatile in order to avoid contaminating the target

material.278 A metal-dithiolate complex such as those presented in Figure 1.12 provides an

evident candidate with the capacity to stabilise metal centres, although the additional

heteroatoms contained by some of these ligands (e.g. phosphorus in the dithiophosphates

and oxygen in the xanthates) may generate impurities in the obtained material.278 The

prevention of oxidation is particularly important and is a frequent problem in metal sulfide

synthesis; hence, an aim of the present work is to avoid these difficulties by synthesising a

metal sulfide from xanthate in the presence of nitrogen gas.

Figure 1. 12. Some common ligands used in single source precursors to prepare metal sulfides.

59

1.14.1. Xanthates: a general introduction

As noteworthy members of the 1,1-dithiolate family, the xanthates are ligands capable of

forming metallic complexes of the form ROCS2−M+, where R is an alkyl or aryl group and

M+ = Na+, K+ etc. They were first synthesised by W. C. Zeise in 1815, who named them

after the Greek word for yellow (xanthos) due to the colour of the lead compounds.279,280

The presence of the -CS2 group enhances the reactivity of xanthates towards various metals

and informs a wide variety of applications. They have found a wide range of uses in classical

and organometallic chemistry, some of which will be cited here.

The potential use of transition metal xanthate complexes in nonlinear optical applications

has been investigated,281 with a wide range of studies over an extended period of time

examining O-alkyl dithiocarbonate (alkyl xanthate) ligands with various transition metals282–

285 and main group elements.286,287 These ligands have been shown to bond with a range of

metals in various modes including monodentate288,289 and bidentate.290 They are known to

act as chelating agents with the vast majority of transition elements, making them highly

versatile reagents in the extraction and separation of metals in mineral floatation and

analytical chemistry.291–293 The alkali metal xanthates, especially sodium and potassium, are

widely used to selectively separate sulfide minerals by the froth floatation method and as

industrial flotation agents for minerals containing thiophilic transition metals e.g. cobalt,

copper, gold, nickel and zinc.294–297 The xanthic acids have also been found to act as

reducing agents. Heavy metal ions have been extracted from aqueous solutions with the aid

of insoluble cellulose xanthate and the O-alkyl dithiocarbonate of cellulose.298 The xanthates

have also found widespread use in the chemical separation and quantitative analysis of

transition metals, alcohols and carbon disulfide.299 A technique for the direct

spectrophotometric determination of micro-quantities of Co(II), Cu(II), Mo(VI), Ni(II),

Pd(II) and Ru(III) in a range of alloys and in environmental samples (fly ash) was developed

60

by Malik et al. using sodium iso-amylxanthate in the presence of a surfactant as a solubilising

agent.300 Mercury, silver etc. are frequently extracted and separated with the aid of the alkali

metal xanthates, with the ethyl xanthates of sodium and potassium having known curative

properties in cases of acute mercury poisoning.281,301

Cellulose xanthates are widely employed for the production of rayon in the textile

industry.302 The bactericidal and bacteriostatic properties of cellulose dixanthate and

cuprous xanthate have been reported.303 Xanthate compounds are capable of inhibiting a

range of infective DNA and RNA types at concentrations that avoid harming the mitotic

activity of healthy cells.304 The potential anti-tumour effects of tin xanthate complexes have

also been demonstrated, while certain phosphine-gold(I)dithiocarbonate complexes have

displayed anti-arthritic properties.305,306

Recent studies have also described the synthesis of aryl xanthates of cobalt, [Co(S2COC6H2-

2,4,6-Me3)3], and nickel, [Ni(S2COC6H4-4-t-Bu)2].293 While earlier studies of these ligands

focused on the use of sulfur-donor ligands as analytical reagents, the recent increased interest

in their synthesis and characterisation is due to their possible biological activity307,308 and

practical uses in a diverse range of fields including electronics309 and rubber technology.310

Since the divalent transition metal bis(1,1-dithiolate) complexes are partially unsaturated,

they can form 1:1 adducts with electron donor ligands including neutral oxygen, nitrogen,

sulfur or phosphorus to give a range of coordination geometries from square pyramidal to

trigonal bipyramidal.311

61

1.14.1.1. Xanthate synthesis

As shown in Figure 1.13, the xanthate salts of alkali metals are generated by reaction of an

alcohol with sodium or potassium hydroxide and carbon disulfide:312,313

ROH + CS2 + MOH RO(C=S)SM + H2O

where R = an alkyl group; M = a monovalent metal (e.g. Na or K)

Figure 1. 13. Synthesis of alkali metal xanthates

The mechanism of this reaction is likely to be nucleophilic addition of the alkoxide ion to

carbon disulfide. A range of alcohols has been used in this reaction, with commercially

significant xanthate products including sodium ethyl xanthate (C2H5OCS2Na), sodium

isobutyl xanthate (C4H9OCS2Na), sodium isopropyl xanthate (C3H7OCS2Na), potassium

ethyl xanthate (C2H5OCS2K) and potassium amyl xanthate (CH3(CH2)4OCS2K).243 The

chemistry of such xanthate ligands has been extensively investigated and reviewed.314,315

1.14.1.2. Structure and bonding in xanthates

The xanthates are essentially salts or esters of dithiocarbonic (xanthic) acid RO(C=S)SH or

its O-ester; hence, the xanthic acids can be described as dithiocarbonates obtained by

replacing the two oxygen atoms of carbonic acid with two sulfur atoms and substituting an

alkyl group for one hydrogen atom:

62

The remaining hydrogen atom in xanthic acid is easily replaced by a metal ion or by another

alkyl group to give a dialkyl xanthate:

The rate constant indicated that the reaction followed the bimolecular reaction rate law,

giving the mechanism shown in Figure 1.14 for potassium ethyl xanthate. The second step

of this reaction proceeds slowly and is therefore the rate-limiting step.

Figure 1. 14. Bimolecular synthesis of potassium ethyl xanthate

Meanwhile, the ground-breaking work of Hoskins and Winter led to an increased interest in

the structural and synthetic chemistry of the xanthates, with more recent and wide-ranging

structural analyses by Tiekink and by Haiduc revealing the capacity of these ligands to

undergo monodentate, isobidentate or anisobidentate coordination with metal atoms.316–318

These are shown in Figure 1.15, along with other possible forms of bonding such as

bimetallic bridging via sulfur atoms or, occasionally, the oxygen atom, or (rarely) an

additional metal-oxygen atom.318

63

Figure 1. 15. Coordination behaviour of xanthate ligands. (A): monodentate; (B) isobidentate; (C)

anisobidentate; (D) and (E): bimetallic bridging through sulfur; (F) and (G): bridging to metal

through oxygen.

Heat promotes the decomposition of solid xanthates to generate products such as alcohols,

dixanthogens, dialkylxanthates, elemental sulfur, mercaptans, mercaptides and metallic

sulfides. As noted previously, the decomposition of xanthate is particularly important

primarily due to its use in mineral flotation. When dissolved in water, the xanthate salts

dissociate into xanthate anions and alkali metal cations to form strong electrolytes which

slowly hydrolyse to produce xanthic acid, which further decomposes into the alcohol and

carbon disulfide, Figure 1.16:

Figure 1. 16. The hydrolysis of dissolved the xanthate in water.

64

Numerous transition metal xanthates are known (e.g. Ag, Cr, Co, Cu, Fe, Mo, Ni, Zn) and

their importance in catalysis, metallurgy, metalloenzymes and material precursors has

stimulated significant interest in their chemistry.319 Due to their very low solubility in water,

TM xanthate complexes are produced by double decomposition of potassium or sodium

xanthate and a soluble salt of the heavy metal in solution, with precipitation of the desired

product. In the moist conditions, heavy metal xanthates are highly prone to decomposition

in the presence of carbon dioxide and oxygen, so the precipitation and filtration must be

performed under an atmosphere of nitrogen.

Figure 1. 17. The complex of metal xanthate, where M(II) = different metals and R an alkyl group.

1.14.1.3. Xanthate complexes as SSPs

As previously mentioned, the compositions and physical and optical properties of synthetic

materials can often more readily be controlled by the use of single-source precursor (SSP)

compounds, which contain the desired elements in one molecule, rather than separate

precursors for each component.320–322 Hence, while a wide range of metal (N,N-dialkyl

dithiocarbamates) [M(S2CNR2)n] have been used for a number of years as SSPs for the

synthesis of metal sulfide nanocrystals,53,88,89,105,275,323,324 a group of O-alkyl xanthate

(S2COR) complexes have been considered potentially useful as SSPs in this type of

synthesis. The metal-organic xanthates are known to decompose via the comparatively clean

and low-temperature Chugaev elimination reaction.325 Moreover, the xanthate ligand-based

SSPs have facilitated the synthesis of a wide range of metal sulfides at lower temperatures

65

than required by their corresponding N,N-dialklydithiocarbamato compounds, including

CdS,326 CZTS,327 MoS2,328 NiS and PdS329 to name but a few. The O’Brien group has

recently reported the synthesis of PbS/polymer composites via a melt process using both

lead(II) dithiocarbamate and lead(II) xanthate complexes.315 Pure cubic PbS nanocrystals

were obtained by the decomposition of [Pb(S2COnBu)2] in a polymer matrix at 150 °C, and

temperatures considerably below 275 °C were needed to decompose [Pb(S2CNnBu2)2].

Hence, the xanthate-based SSPs are useful for enhanced control of PbS nanocrystal size,

shape and preferred orientation across a wide range of temperatures. In addition, published

approaches to the synthesis of tin(II) sulfide nanomaterials have been included studies on

the use of Sn-SSPs including [SnII(S2CNR2)2] and [R'2SnIV(S2CNR2)2] for the production of

orthorhombic SnS nanoparticles and films.106,108

Alkyl xanthate complexes of copper(II), lead(II), nickel(II), tin(II) and zinc(II) have also

been synthesised. For example, although lead(II) alkylxanthates can be produced by reacting

lead(II) acetate with sodium xanthate in aqueous solution, McNaughter et al. synthesised a

range of lead(II) complexes with good solubility in organic solvents. Pure lead(II) sulfide

(PbS) was obtained from these xanthate SSPs via a melt reaction at 150, 175 or 200 °C.243

Meanwhile, bis(O-ethylxanthato)tin(II), [Sn(S2COEt)2], was synthesised and used as an SSP

in a spin coating method for the deposition of SnS thin films by Al-Shakban et al. The

[Sn(S2COEt)2] powder showed excellent solubility in a wide range of common organic

solvents, including THF, but had to be stored at −20 °C to minimise decomposition. After

spin-coating, the substrates were heated to temperatures of 150 to 400 °C to obtain pure

crystalline SnS films. While the composition and morphology were influenced by the

temperature of heating, the films were primarily orthorhombic with approximately spherical

structures along with some flakes.107

66

A one-pot synthetic protocol for the synthesis of pure, high-quality MoS2 nanosheets capped

by oleylamine was developed by Savjani et al. based upon the hot injection thermolytic

decomposition of [Mo2O2S2(S2COEt)2] as SSP. A highly-crystalline monolayer of randomly

oriented nanosheets was obtained with a combination of small flakes and high purity that

made it an optimal material for energy storage applications, e.g. supercapacitors.48

In addition, tris(xanthato)iron(III) complexes ([Fe(S2COR)3], where R = ethyl, methyl, 1-

propyl and isopropyl) SSPs have been used by Almanqur et al. to deposit iron sulfide thin

films and nanostructures via spin-coating and solid-state deposition. The potential of both

techniques for the low-temperature synthesis of iron sulfide materials with well-controlled

crystalline phase was demonstrated. When the spin coating and annealing method was used,

powder diffraction-XRD indicated the formation of troilite, while solvent-free pyrolysis was

similarly shown to generate primarily iron sulfide pyrrhotite or Fe1-xS, where x = 0 to 0.2.

The morphologies of these materials were similar, consisting of approximately spherical

crystals with a large range of particle sizes.250

Since a variety of distinct crystallographic phases and stoichiometric combinations of CuxS

has been identified, copper sulfide has gained much interest as a p-type semiconductor in a

wide range of optoelectronic devices. The phase-controlled synthesis of copper sulfide

nanoparticles from xanthato complexes by non-colloidal or colloidal methods is therefore

significant. Such an approach using bis(O-alkylxanthato)copper(II) complexes (where alkyl

= ethyl, hexyl or octyl) as SSPs was followed by Akhtar et al. via solid-state deposition,

thermolysis in oleylamine and the doctor blade method. A range of reaction times and

temperatures was used to demonstrate that the product phase depended directly upon the

method, temperature and alkyl chain length of the precursor.330

The spin-coating technique was used by Al-Shakban et al. to produce thin films of kesterite

(CZTS, Cu2ZnSnS4) from copper, zinc and tin xanthates, with the product phase depending

67

on the temperature of decomposition. Raman spectroscopy and X-ray diffraction analysis

indicated that tetragonal films were obtained by annealing between 375 and 475 °C, while

hexagonal films resulted at temperatures below 375 °C. The Cu/(Zn + Sn) ratio was

identified by EDX measurement to be between 1 and 0.64. The films heated at various

temperatures (225, 375 and 450 °C) were found to be moderately uniform according to

resistivity, carrier concentration, mobility and Hall coefficient measurements.331 In addition,

a very recent paper by the same author has described the syntheses of diphenyltin bis(2-

methoxyethylxanthate) and diphenyltin bis(iso-butylxanthate). These were characterised by

single-crystal X-ray diffraction and used as SSPs in the deposition of thin tin chalcogenide

films by AACVD. The films were characterised by scanning transmission electron

microscopy with elemental mapping and grazing incidence X-ray diffraction to indicate that

deposition from diphenyltin bis(iso-butylxanthate) produced orthorhombic SnS, while an

SnS/SnO2 nanocomposite was obtained by deposition from diphenyltin bis(2-methoxyethyl

xanthate) between 400 and 575 °C.27

Akhtar et al. used an AACVD method to deposit thin films of nickel sulfide onto silicon and

glass substrates at a range of temperatures from bis(O-alkylxanthato)nickel(II) precursor

complexes. A low deposition temperature of 250 °C was shown by p-XRD to produce a

pure nickel sulfide phase with irregular morphology, while higher deposition temperatures

resulted in mixed nickel sulfide nanophases with enhanced crystallinity.332

68

1.15. Aims and objectives

The purpose of this study is to synthesise xanthate single-source precursors for the formation

of nanomaterials and deposition of thin films. Metal xanthate complexes show interesting

thermal behaviour, and these complexes could be a good choice to produce binary, ternary

and quaternary metal sulfide at low temperatures. Furthermore, we hypothesise their

xanthate complexes may lead to low temperature synthesis of alkali metal chalcogenide

nanomaterials that have previously only been produced from a high temperature. Moreover,

the synthesis of xanthate complexes with different chain length and different annealing

temperatures could be useful to study the variations of physical properties of metal sulfide

compounds. For the growing interest in nanoparticles and thin films based solar energy

generation it is important to find cheap, nontoxic and environmentally friendly materials.

The metal sulfides that were used here are considered as the cheapest and non-toxic material

for photovoltaic cells. The other aim of this project is the development of a novel synthetic

technique for the synthesis of Pb1-xMnxS nanocrystals (x = 0 to 0.08) with detailed

compositional studies based on the p-XRD patterns and EDX. The optical properties of these

materials will be analysed by UV-Vis spectroscopy. Finally, the method which we propose

in this study is solvent-less thermolysis which has some advantages such as straight forward,

solvent free, inexpensive and single step utilizing single source precursors (SSPs).

69

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Chapter 2. Instruments section

2.1. Measurement Methodologies

A Specac single reflectance ATR instrument (4000-400 cm-1, resolution 4 cm-1) was used to

record the infrared spectra and a Barloworld SMP10 melting point apparatus was used to

record the melting points.

2.2. Elemental analysis

Using a standard reference material and a calibrated Carlo Erba EA 1108 elemental analyser,

nitrogen, hydrogen, and carbon analysis was completed. The samples were contained within

a tin, which was placed into a furnace and burnt in oxygen at 1000 degrees Celsius. At this

heat, hydrogen is removed as a water vapour and carbon, nitrogen and sulfur react to become

dioxides. Gas chromatography columns were used to analyse the gases and identify the

amount of each element within the samples. In order to analyse the metal content of samples,

a weighed volume of the specific elements, i.e. copper, iron, nickel, zinc, cadmium, were

digested in acid by heating to the optimum temperature before transferring the residue to a

volumetric flask and creating a solution with water. Using a Fison's Horizon ICP-OES, the

solution was analysed and the individual element concentrations within the solution were

measured. The School of Chemistry microanalysis team completed all the elemental analysis

throughout the duration of the project.

2.3. Thermogravimetric analysis (TGA)

Chemical and physical properties of materials can be measured using the TGA thermal

analysis method, with these properties being assessed as a function of time at constant

temperature, or temperature at constant time. Phase transitions and other physical

phenomena, like vaporisation, sublimation, and absorption and desorption, and chemical

phenomena, like solid gas reactions, oxidation or reduction, decomposition and dehydration,

87

can all be assessed using TGA. The method can also be used to identify specific material

characteristics, which are correlated with a mass gain or loss resulting from oxidation,

decomposition or volatile loss, as seen with water. A sample’s organic and inorganic content,

its degradation mechanisms and reaction kinetics, and its decomposition patterns are

analysed during TGA to determine the material characteristics. A Seiko SSC/S200 model

was used to complete these measurements using a heating rate of 10 degrees Celsius per

minute under nitrogen. The instrument is calibrated using indium metal as a reference.

2.4. X-Ray crystallography

Using a Rigaku FRX diffractometer and graphite mono-chromated Mo-Kα or Cu-Kα

radiation, single crystal X-ray diffraction data was collected. The Sheldrick (2015) SHELXL

program was used to solve the structures.1 Anisotropic atomic displacement parameters were

used to refine the non-hydrogen atoms, whilst the hydrogen atoms were carefully positioned

and allocated with isotropic thermal parameters before being permitted to connect their

parent carbon atoms.1,2

2.5. Powder X-ray Diffraction (p-XRD)

A Panalytical X’Pert PRO diffractometer was used for the p-XRD diffraction studies using

Cu-Kα radiation. The sample nature dictated the various count rate used after they had been

mounted flat and scanned between 10 to 80o with a step size of 0.05. The sample’s diffraction

patterns and the ICDD index pattern were then compared. Crystalline materials can be

identified using the powerful powder X-ray diffraction (p-XRD) technique. The different

phases within a material can also be identified using this technique along with a

measurement of structural properties, including phase composition, defect structures, strain

state, preferred orientation, epitaxy, and grain size. The p-XRD technique is ideal for studies

88

of structure due to its non-destructive technique. Bragg's law dictates the diffraction beam

position (angle θ).

𝑛= 2𝑑s𝑖𝑛𝜃

where the order of diffraction is denoted as 𝑛, incident X-ray beam wavelength is denoted as

λ, the diffraction contribution dictated by the spacing between planes is denoted as 𝑑, and

the angle between the crystallographic plane and incident beam is denoted as 𝜃.

This equation shows that the path difference between the rays reflected by consecutive

planes within the lattice can be used to calculate the angle of incidence. Figure 2.1 shows

how the synthesised material can be determined by identifying the intensities and positions

of diffraction peaks and comparing these with the crystalline materials that are known to be

present in the database. Furthermore, the equation also allows the identification and

quantification of different sample phases and the average particle size can be estimated using

the p-XRD pattern line broadening.3

Figure 2. 1. Schematic representation of Bragg’s Diffraction.

89

2.6. Raman Spectroscopy

Raman spectroscopy is a technique that uses the interaction of light with a material to gain

understanding into the material’s characteristics, the same as IR spectroscopy. The data

provided by Raman spectroscopy results depend on light scattering, while those by IR

depend on light absorption. Both Raman and IR results provide the exact vibrations of a

molecule, called a molecular fingerprint, and are used to identify a substance. However,

Raman spectroscopy can provide additional information about frequency modes and

vibrations that give insight into crystal lattices.

When light interacts with particles, the majority of the photons are scattered at the same

energy as the incident photons. This is called Rayleigh scattering. When a small number of

these photons scatter at a different frequency than the incident photon, this is called Raman

scattering. When the change in energy of the incident photon is higher than that of the

scattered photon, the scattering is called Stokes scatter. However, some molecules may

start in a vibrational excited state and, when promoted to the higher energy, may relax to a

final energy state that is lower than the initial excited state. This scattering is called anti-

Stokes (Figure 2.2).

The structural characteristics of molecular bonds can be altered due to the frequency of the

light scattered from the molecule. These characteristics can provide data on a

semiconductor’s electrical and vibrational properties that show sensitivity to free carrier

density, alloy composition, strain, microstructure and crystalline quality. Over the past

decade, the vibrational spectra of polymer films, glasses and crystals have generally been

completed using Raman spectroscopy. The method is advantageous due its spatial

resolution being as low as 1 μm, which allows it to be used with microcrystalline samples.

For this study, a Renishaw 1000 micro-Raman system with a 514 nm laser was used to

perform Raman analysis at room temperature in backscattering mode.4

90

Figure 2. 2. Top; Energy level diagram of stimulated Raman scattering, down; Raman

spectrum showing the relative intensities of the different scattering processes.

2.7. Scanning electron microscopy (SEM) and energy dispersive X-ray

spectroscopy (EDX)

A TESCAN MIRA3 FEG-SEM was used to perform the SEM analysis. Following this, the

chemical composition of the samples was completed using energy dispersive X-ray

91

spectroscopy (EDX). A sample’s images are scanned via a focused beam of electrons during

SEM electron microscopy. There are a range of cryogenic or elevated temperatures that can

be used to observe the specimens, as well as high, medium and low vacuum conditions and

wet conditions in environmental SEM. The secondary electrons that the atom’s emitted

following excitation of the electron beam, allowed the analysis of the sample’s composition

and surface morphology. A resolution of 1nm or higher can be produced by SEM, however,

the quantity of secondary electrons that are produced depends on the angle that the surface

specimen (topography) and beam meet. Therefore, the surface topography is displayed as an

image as a special detector collects the secondary electrons that have scanned the sample.

Transmitted electrons, specimen current, light cathode luminescence (CL), characteristic X-

rays, back-scattered electrons (BSE), and secondary electrons (SE) are all SEM signals.

Whilst all SEMs have secondary electron detectors as standard equipment, it is unusual for

a single machine to contain detectors for all these signals.

Where there are atom and electron beam interactions close to or on the surface of the sample,

signals are created. The fine electron beam creates a large depth of field in SEM

micrographs. Elastic scattering reflects beam electrons from the sample, which are known

as BSE. The intensity of the BSE signal is closely correlated with the specimen’s atomic

number and are therefore, frequently used in analytical SEM, alongside characteristic X-Ray

spectra. A sample’s element distribution can be determined through the use of BSE images.

When a sample’s inner shell electron is removed by an electron beam, characteristic x-rays

are emitted. The shell is then filled with a higher energy electron, which therefore releases

energy. The abundance and composition of the sample’s elements can be identified using

characteristic x-rays, whilst the identification and quantification of these elements on a

micron scale is completed using EDX analysis. This can also be utilised to determine a

sample’s chemical composition and complete elemental mapping.5,6

92

2.8. UV/Vis spectroscopy

Any absorption spectroscopy occurring in the ultraviolet to visible regions of light is referred

to as UV/Vis spectroscopy. Electronic transitions within the compound vary the rate of

absorption and these transitions are measured from the ground to the excited state during

UV-vis spectroscopy techniques and from the excited state to the ground during fluorescence

spectroscopy.

As ligands and anions strongly influence the colour of metal ion solutions, any change in

maximum absorption wavelength (λmax) in transition metals where ligands and anions are

present, can be studied using UV/Vis spectroscopy. A system’s degree of conjugation can

also be assessed using this technique as there is an absorption of light in the UV or visible

region by any molecules that have a high degree of conjugation. Toluene, hexane, ethanol

and water can be used as solvents.

Optical absorbance spectra were used in the current study to measure the sample’s band gap

with these absorbance spectra being recorded with a Shimadzu UV-1800, double beam

UV/Vis NIR spectrophotometer. A wavelength range between 200 and 1100 nanometres

was used and a Tauc plot was created by plotting the absorption coefficient as a function of

photon energy to determine the optical band gap. This Tauc plot was correlated to the

semiconductor material’s absorption edge.7

2.9. Magnetic measurements

A Quantum Design MPMS-XL SQUID magnetometer fitted with a 7T magnet was used to

complete the magnetic measurements. For this, both field-cooled and and zero-field-cooled

magnetisation curves over a temperature range of 5 - 300 K and an applied magnetic field of

100 Oe were recorded.8

93

2.10. Reference

1 W. Clegg, Crystal Structure Determination, Oxford University Press, U.S.A., Oxford ;

New York, 1996.

2 M. . Woolfson, Introduction x ray crystallography., Cambridge ; New York, 2nd edn.

3 J. H. Robertson, Acta Crystallogr. A, 1979, 35, 350–350.

4 R. Singh, Phys. Perspect., 2002, 4, 399–420.

5 C. W. Oatley, W. C. Nixon and R. F. W. Pease, Adv. Electron. Electron Phys. 1966, 21,

181–247.

6 M. A. Haque and M. T. A. Saif, Exp. Mech., 2002, 42, 123–128.

7 P. Minutolo, G. Gambi and A. D’Alessio, Symp. Int. Combust., 1996, 26, 951–957.

8 A. F. Orchard, Magnetochemistry, Oxford University Press, Oxford, New York, 2003.

94

Chapter 3. Structural investigations of α-MnS nanocrystals and

thin films synthesised from single source precursors by hot

injection, scalable solvent-less and doctor blade routes.

3.1. Introduction

Considerable attention has been given to sulfide-based nanomaterials, due to their unique

properties; they have high conductivity, are low-cost, have low toxicity, high thermal

stability and catalytic ability.1,2 Its properties give it a significant range of possible

applications across a number of fields; it has potential to be used within supercapacitors,

batteries, dye-sensitized solar cells, drug delivery and electrocatalysis.3–6 Manganese sulfide

(MnS) is a magnetic semiconductive material (Eg=3.1 eV). It has potential within a number

of short wavelength opto-electronic applications, including solar selective coatings, solar

cells, sensors, photoconductors, and optical mass memories.7–10 MnS is one of the most

promising anode materials to be found among the sulfides. It has a wide range of variance

in its possible nanostructures, which includes nanorods, nanocubes, nanowires, nanosaws,

and nanospheres. Additionally, it has a high theoretical capacity.2,11–14 Typically, MnS thin

films and powders are found in one of several polymorphic forms. The most common form

it is found within is the rock salt type structure (α−MnS). This structure crystallizes into the

zincblende (β−MnS) or a wurtzite (γ-MnS) structure when exposed to a low temperature

growing technique.15,16

Hot injection, solvent-less thermolysis, and doctor blade processes were applied in order for

the formation of manganese sulfide nanomaterials from xanthate complexes to occur. A

series of novel manganese(II) xanthate single-source precursors [(TMEDA)Mn(S2COR)2 (R

= methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl and n-octyl) were investigated. Reports

were carried out on the crystal structures of these complexes. These complexes were tested

95

as single-source precursors for the formation of manganese sulfide nanomaterials and thin

films. Oleylamine was used for the thermal decomposition of manganese(II) xanthate

complexes, allowing manganese sulfide nanomaterials to be produced at a lower temperature

of 250 oC under hot injection. This differs from both the solvent-less thermolysis and the

doctor blade technique, which require a higher decomposition temperature of 350 °C.

3.2. Author distribution

In this work, I synthesised and then characterised xanthate complexes via IR, elemental

analysis and TGA. The experimental work to produce nanocrystals was carried out by me, I

characterised the samples by XRD, Raman, SEM, EDX and UV-Visible spectroscopy.

Floriana Tuna provided the magnetic measurements and analysis of the data. The

crystallographic data of the complexes has been collected by Inigo Vitorica-Yrezabal and

George Whitehead. Firoz Alam checked the characterization of complexes and materials.

David collison provided useful editing. The original idea was provided by Paul O’Brien.

David J. Lewis supporting me in the project and he provided as well a nice and useful

discussion, and also editing the manuscript. The experimental work was done in the

laboratory of Paul O’Brien.

3.3. References:

1 S. Biswas, S. Kar and S. Chaudhuri, J. Phys. Chem. B, 2005, 109, 17526–17530.


Zou, J. Phys. Chem. C, 2012, 116, 3292–3297.

3 A. Marques, M. Marin and M.-F. Ruasse, J. Org. Chem., 2001, 66, 7588–7595.

4 M. Govindasamy, S. Manavalan, S.-M. Chen, U. Rajaji, T.-W. Chen, F. M. A. Al-

Hemaid, M. A. Ali and M. S. Elshikh, J. Electrochem. Soc., 2018, 165, B370–B377.

5 Y. Tang, T. Chen and S. Yu, Chem. Commun., 2015, 51, 9018–9021.

6 Y. Tang, T. Chen, S. Yu, Y. Qiao, S. Mu, J. Hu and F. Gao, J. Mater. Chem. A, 2015, 3,

12913–12919.

96

7 D. Fan, X. Yang, H. Wang, Y. Zhang and H. Yan, Phys. B Condens. Matter, 2003, 337,

165–169.

8 B. Piriou, J. Dexpert-Ghys and S. Mochizuki, J. Phys. Condens. Matter, 1994, 6, 7317–

7327.

9 R. Tappero, P. D’Arco and A. Lichanot, Chem. Phys. Lett., 1997, 273, 83–90.



11 Y. Gui, L. Qian and X. Qian, Mater. Chem. Phys., 2011, 125, 698–703.

12 X. Yang, Y. Wang, Y. Sui, X. Huang, T. Cui, C. Wang, B. Liu, G. Zou and B. Zou,

Langmuir, 2012, 28, 17811–17816.

13 J. Beltran-Huarac, J. Palomino, O. Resto, J. Wang, W. M. Jadwisienczak, B. R. Weiner

and G. Morell, RSC Adv., 2014, 4, 38103–38110.

14 J. Beltran-Huarac, O. Resto, J. Carpena-Nuñez, W. M. Jadwisienczak, L. F. Fonseca, B.

R. Weiner and G. Morell, ACS Appl. Mater. Interfaces, 2014, 6, 1180–1186.

15 R. L. Clendenen and H. G. Drickamer, J. Chem. Phys., 1966, 44, 4223–4228.

16 M. Kobayashi, T. Nakai, S. Mochizuki and N. Takayama, J. Phys. Chem. Solids, 1995,

56, 341–344.

97

3.4. Structural investigations of α-MnS nanocrystals and thin films

synthesised from single source precursors by hot injection, scalable

solvent-less and doctor blade routes.

Abdulaziz M. Alanazi,a,c Firoz Alam,a,b Inigo Vitorica-yrezabal,a George Whitehead,a

Floriana Tuna,a David Collison,a Paul O’Briena,b and David J. Lewisb*

a, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.

b, Department of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.

c, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,

KSA

*Email: [email protected]

3.4.1. Abstract

Manganese (II) xanthate precursors [Mn(S2COR)2(TMEDA)] (R = methyl (1), ethyl (2), n-

propyl (3), n-butyl (4), n-pentyl (5), n-hexyl (6) and n-octyl (7), TMEDA =

tetramethylethylenediamine) have been synthesised and their crystal structures have been

determined using single crystal X-ray diffraction. The complexes were used as single source

precursors to synthesise manganese sulfide (MnS) nanocrystals and thin films using hot

injection, solvent-less and doctor blade thermolysis, respectively. The nanocrystals and thin

films were characterised by powder X-ray diffraction, scanning electron microscopy (SEM),

energy dispersive X-ray (EDX) and Raman spectroscopy. Analysis of MnS obtained from

all routes indicates that the hot injection thermolysis provides superior control over

composition. Also, the oleylamine (OLA), which is used as capping agent assists the

decomposition of the complexes at lower temperatures whereas the solvent-less thermolysis

and doctor blade technique requires higher decomposition temperature of 350 °C. The

magnetic measurements recommend that α-MnS nanocrystals depict a ferromagnetic

mailto:[email protected]

98

behaviour. Magnetic hysteresis measurements reveal that α-MnS nanocrystals have large

coercive field strength (e.g., 0.723 kOe for 8.2 nm nanocrystals), which is associated with

the size and self-assembly of the materials.

3.4.2. Introduction

Semiconductor nanocrystals (NCs) or 'quantum dots' have drawn significant interest in the

research community because of their size dependant optical and electronic properties.1–11

The unique properties of these nano-materials can also be altered using composition, shape

and surface states, and have strong potential to be used in many applications such as

optoelectronics (display, lighting and photovoltaics), as well as in biological imaging and in

photodetectors.12–19

For the synthesis of metal chalcogenide nanomaterials, the use of single source molecular

precursors has been extensively reported.20–22 For example, xanthate complexes with a metal

chalcogenide bond as their single source precursor (SSP) have been demonstrated to be

effective for the synthesis of metal sulfide nanocrystals and thin films.23–27 The xanthates

were developed as broadly adaptable ligands for generating an extensive variety of complex

organic and inorganic materials with desirable chemical and physical properties.28,29 Their

advantages arise from ease of synthesis and the presence of a sulfur donor atom which can

stabilize a broad range of elements and transition metals in a variety of oxidation states.23

Metal xanthates [M(S2COR)x] have been synthesised to be an excellent precursors to a range

of metal chalcogenides previously, including complex materials such as alkaline earth metal

sulfides.30–33

Manganese sulfide (MnS) is a p-type magnetic semiconductor with potential application in

short wavelength or high temperature optoelectronics, having a wide band gap of 2.7-3.7

eV.34–36 It also finds application as a magnetic semiconductor and in luminescent materials,

optical mass memory, photo-conductors, sensors and solar selective coatings.37–40

99

Manganese sulfide (MnS) is generally found in three crystal forms, which are shown in

Figure 3.1, namely the stable cubic rock-salt RS α-MnS, metastable cubic zincblende ZB β-

MnS, and hexagonal wurtzite WZ γ-MnS structures. The ZB and WZ phases are unstable or

metastable, being easily transformed into the stable octahedrally coordinated RS phase under

high temperature or pressure.41 A range of synthetic methods has been applied to the

generation of MnS nanostructures including chemical bath deposition (CBD), hydrothermal,

microwave, solvothermal and sonochemical methods.42–47 Synthetic routes for the controlled

synthesis of single-phase MnS NCs have also been reported.48–50 For example, Hyeon et al,

synthesised hexagonal MnS with the wurtzite structure by heating a mixture of sulfur and

MnCl2 in oleyamine at 280 °C.48 Moreover, the use of a Mn(II) dithiocarbamate complex in

the production of manganese sulfide has been reported in a study that examined the effect

of the counter anion upon the morphology and phase of the synthesized product.27 The

properties of the synthesised NCs are strongly dependent upon the specific method of

preparation. Solvent-less thermolysis has a facile and comparatively economical single-step

approach that uses xanthates as single source precursors and does not necessitate the use of

intricate apparatus. The technique is simple, cost-effective, solventless, atom efficient,

environmental friendly and has a significant potential for scaling up. This work examines

the synthesis of novel bis(O-alkylxanthato) manganese(II) (alkyl = Me, Et, n-Pr, n-But, n-Pen,

n-Hex and n-Oct) complexes stabilised by the bidentate N-donor ligand

tetramethylethylenediamine (TMEDA) and their use as precursors for the synthesis of MnS

NCs as well thin films by (i) a hot injection thermolysis, using oleylamine (OLA) as a

capping agent and trioctylphosphine (TOP) as the dispersion medium, (ii) Solvent-less

thermolysis and (iii) doctor blade technique for the deposition of thin films. To the best of

our knowledge, these approaches have not previously been applied to the synthesis of MnS

NCs and thin films using manganese(II) xanthate complexes.

100

Figure 3. 1. The crystalline structures of (a) cubic rock-salt (RS) α-MnS, a, b, c = 5.224 Å (ICDD

01-089-4952), (b) metastable cubic zincblende (ZB) β-MnS, a, b, c = 5.615 Å (ICDD 00-040-1288)

and (c) hexagonal wurtzite (WZ) γ-MnS structures, a and b = 3.979 Å and c = 6.446 Å (ICDD 00-

040-1289). Colour code: Mn, violet; S, yellow.41

3.4.3. Experimental

3.4.3.1. Materials and instrumentation

All chemicals were purchased from Sigma Aldrich and used as received. Melting points were

determined with a Stuart melting point apparatus (Cole-Palmer, UK). Infrared spectra (IR)

were recorded using a Nicolet iS5 Thermo Scientific ATR instrument (4000–400 cm−1,

resolution 4 cm−1). Elemental analyses (EA) and Thermogravimetric analyses (TGA) were

carried out by the Micro-elemental Analysis Service in the School of Chemistry at the

University of Manchester. EA was performed for all complexes using a Flash 2000 Thermo

Scientific elemental analyser and TGA data were obtained with Mettler Toledo TGA/DSC

stare system in the range 30–600 °C at a heating rate of 10 °C min−1 under nitrogen flow.

Powder X-ray diffraction (p-XRD) analyses were carried out using an X-Pert diffractometer

with a Cu-Kα1 source (λ = 1.54059 Å), the samples were scanned between 10° to 80°, the

applied voltage and current was 40 kV and 30 mA, respectively. Scanning electronic

microscopy (SEM) and energy dispersive X-ray spectroscopy analysis is carried out using

TESCAN MIRA3 FEG-SEM. The EDX was used to know the chemical composition of the

samples. Raman spectra were measured using a Renishaw 1000 Micro- Raman System

101

equipped with a 514 nm laser. Single crystal X-ray diffraction data for all the complexes

were obtained using Mo-Kα or Cu-Kα radiation on a Rigaku FR-X diffractometer. The

structures were solved by SHELXL (Sheldrick, 2015) program.51 Crystals were grown using

vapour diffusion of hexane in to a solution of precursor in acetone. Non-hydrogen atoms

were refined with anisotropic atomic displacement parameters. Hydrogen atoms were placed

in calculated positions, assigned isotropic thermal parameters and allowed to ride on their

parent carbon atoms.

3.4.3.2. Synthesis of [Mn(S2COMe)2.(TMEDA)], (1)

All the potassium xanthate reported herein are prepared according to the previously

published papers.31,52 Briefly, potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in

excess methanol and stirred for 2 h at room temperature. The mixture was then cooled to 0

°C. Carbon disulfide (1.04 g, 0.83 mL, 13.63 mmol) was added drop-wise and the mixture

stirred for 1 h. 50 ml of an aqueous solution of Mn(CH3COO)2.4H2O (1.6 g, 6.8 mmol) was

added drop-wise to the reaction mixture, which was stirred for 30 min to form a

brown/yellow solution. TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring

for 1 h to form a brown precipitates. The solid residue was filtered off and washed with

deionised water. The final product was dried in a vacuum oven for overnight. Then the

product was recrystallized from acetone. Yield: 83.5% (3.5g). Melting point: 138 °C.

Elemental analysis: Calc (%): C, 31.17; H, 5.76; S, 33.22; N, 7.27; Mn, 14.27. Found (%):

C, 30.98; H, 5.56; S, 33.22; N, 7.02; Mn, 13.94. IR (νmax/cm-1): 2995 (w), 1140-1193 (s),

1037(s).

3.4.3.3. Synthesis of [Mn(S2COEt)2.(TMEDA)], (2)

The complex 2 was prepared in the same way as mentioned in the 1, using excess ethanol

instead of methanol. Yield: 88 % (3.7g). Melting point: 137 °C. Elemental analysis: Calc

102

(%): C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S, 31.26;

N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).

3.4.3.4. Synthesis of [Mn(S2COn-Pr)2.(TMEDA)], (3)

The complex 3 was prepared in the same way as 1, using n-propanol. Yield: 77.1% (3.8g).

Melting point: 134 °C. Elemental analysis: Calc (%): C, 38.09; H, 6.86; S, 29.00; N, 6.35;

Mn, 12.46. Found (%): C, 37.88; H, 6.67; S, 29.37; N, 6.12; Mn, 12.18. IR (νmax/cm-1): 2968

(w), 1145-1179 (s), 1043(s).

3.4.3.5. Synthesis of [Mn(S2COn-But)2.(TMEDA)], (4)

The complex 4 was prepared in the same way as 1, using n-butanol. Yield: 76.3% (4.1g).



(w), 1043 (s), 1142-1180 (s).

3.4.3.6. Synthesis of [Mn(S2CO n-Pent)2.(TMEDA)], (5)

The complex 5 was prepared in the same way as 1, using 1-pentanol. Yield: 78.3% (4.5g).



(w), 1040 (s), 1145-1180(s).

3.4.3.7. Synthesis of [Mn(S2CO n-Hex)2.(TMEDA)], (6)

The complex 6 was prepared in the same way as 1, using 1-hexanol. Yield: 82.9% (5.1g).



(w), 1038 (s), 1142-1182 (s).

103

3.4.3.8. Synthesis of [Mn(S2CO n-Oct)2.(TMEDA)], (7)

The complex 7 was prepared in the same way as 1, using 1-octanol. Yield: 80.9% (5.5g).


Mn, 9.45. Found (%): C, 49.05; H, 8.42; S, 21.91; N, 4.65; Mn, 9.28.

3.4.3.9. Synthesis of MnS nanocrystals using hot injection thermolysis

The MnS nanocrystals were synthesized by dispersing (0.2 g) of manganese alkylxanthate

in 2.0 mL of trioctylphosphine (TOP) and then injected into 8.0 ml of pre-heated oleylamine

(OLA) at 230 °C with continuous stirring under nitrogen atmosphere. The temperature was

maintained at 230 oC for 30 min, after which the reaction mixture was removed from the

heating source for cooling. Then, precipitation was done by adding methanol (12.0 ml) into

the reaction mixture and then solid material was washed using methanol and separated by

centrifugation.

3.4.3.10. Synthesis of MnS nanocrystals using solvent-less thermolysis

Solvent-less thermolysis were performed by placing 0.4 g of the manganese alkylxanthate

in a ceramic boat under a stream of argon (300 cm3 min-1) in a furnace tube which was then

heated to 350 °C and the heating is continued at this temperature for 1 h. After cool down to

room temperature the MnS nanocrystals were collected for characterizations.

3.4.3.11. Deposition of MnS thin films using doctor blade technique

Thin films MnS were deposited on pre cleaned glass substrates using doctor blade technique.

In a typical deposition process, 0.02g of manganese alkylxanthate was be slurry in 0.2 ml of

Tetrahydrofuran (THF). As prepared complex slurry was pasted on the cleaned glass

substrate and distributed uniformly on the glass substrates using a sharp blade made up of

stainless steel to form wet thin films of MnS. The films were then placed in to furnace tube

which was then heated to 350 °C for 1h.

104

3.4.4. Results and discussion

3.4.4.1. Precursor crystal structures

We report here the synthesis and single-crystal structures of six novel manganese xanthates:

[Mn(S2COMe)2.TMEDA] (1), [Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3),

[Mn(S2COnBut)2.TMEDA] (4), [Mn(S2COnPen)2.TMEDA] (5), and

[Mn(S2COnHex)2.TMEDA] (6). We also prepared [Mn(S2COnOct)2.TMEDA] (7), which

was characterised only by X-ray diffraction analysis, elemental analyses and melting point.

These complexes were prepared from the reaction of a previously prepared potassium

alkylxanthate (from the insertion of CS2 into the relevant potassium alkoxide) and

manganese(II) acetate tetrahydrate with the subsequent addition of TMEDA. All the

complexes were soluble in common organic solvents such as chloroform, THF and toluene.

These complexes were stored at − 20°C to avoid premature decomposition. Crystals suitable

for X-ray analysis were grown from the slow evaporation of chloroform solution at room

temperature. The structures of the complexes are shown in Figure 3. 2. All 2, 3, 4, 6 and 7

adopt monoclinic crystal systems with space groups C2/c, P21/c, P21/c, I2/a and I2/a,

respectively, while 1 orthorhombic Pbca and 5 is triclinic P1.

In all cases, the central Mn ions were coordinated by 6 atoms, bound by two xanthate ligands

and single TMEDA ligand, with N and S donors arranged in a distorted octahedron.

Furthermore, no differences in Mn–S or Mn–N bond distances were observed in the

structures of 2, 6 and 7; therefore, the ligands were considered to be in a symmetric

(isobidentate) mode. However, the Mn–N bond distances in 5 were significantly different,

and a relatively small difference was observed in the cases of 1, 3 and 4, therefore, the ligands

were considered to be in an asymmetric mode.

The molecular structure of (2) [Mn(S2COCH2CH3)2 (TMEDA)] is shown in Figure 3. 2, and

the selected geometric parameters are presented in Table 3.S1. The shorter Mn–S and the

longest Mn–S bond lengths involving the xanthate ligands were 2.5645 and 2.6750 Å,

105

respectively, as listed in Table 3.S2, and were in good agreement with those reported for

other analogous 1:1 adducts of Mn–dithiocarbonato (xanthate) complexes.23

The symmetric mode of coordination of the xanthate ligands was reflected in the near

equivalence of the associated C–S bond distances. Within each of the xanthate ligands, the

shorter Mn–S bond had the S atom approximately trans to a N atom, and the two S atoms

forming the longer Mn–S bonds were approximately trans to each other. Another difference

between the structures was that the shortest Mn–N bond distance was observed in complex

6, and thus, the N–Mn–N angle was the shortest angle compared with other complexes.

As the alkyl chain length increased in the symmetrical structures, the difference in the

bonding modes of the two ligands became more obvious. The difference in the Mn–S bond

distances in the symmetrical binding decreased with an increase in the length of the alkyl

chain. The ∆(Mn–S) = (longer Mn–S bond distance − shorter Mn–S distance) values for the

remaining ligand were 0.11, 0.08 and 0.03 Å for 2, 6 and 7, respectively. In contrast, the

difference in the Mn–S bond distances in the asymmetric binding of 1, 3, 4 and 5 varied with

an increase in the length of the alkyl chain.

The relatively short C–O bond distances of 1.333 (2) Å for one ligand in complex 1, 2, 3, 4

and 5 were almost the same. However, in 6 and 7, the C–O bond distances were 1.361 (5)

and 1.341 (9) Å, respectively, which were longer than those of the other complexes. The

data shown in Table 3.S2 are consistent with a significant contribution of the resonance form

of the xanthate anion that features a formal C=O bond and the negative charges on each of

the S atoms. In the case of compound 2 the bidentate N-donor ligands had the same Mn–N1

and Mn–N2 distances (2.293 (15) Å). Because of the restricted ligand bite, the angles N–

Mn–N and S–Mn–S were lower than 90° in a regular octahedron. The N–Mn–N angles

averaged at approximately 79.22° (8) and S–Mn–S angles at 69.10° (15), as shown in Table

3.S2. The molecular structures of other novel complexes are shown in Figure 3. 2, and

selected bond distances and angles are given in Table 3.S2.

106

The most distinct difference between these compounds was how the ligand frameworks and

the presence of hydrogen bonds affected the crystal packing in the extended solid state for

all of these complexes. As shown in Figure 3.S1, all the complexes displayed intermolecular

hydrogen bonds through the sulfur atoms of the neighbouring molecules (C–HS), except

complex 5, wherein the (C–HS) interaction was not observed. The distances of these

interactions were slightly shorter than the sum of the contact radii (van der Waals radii),53

as shown in Table 3.S3.

Furthermore, 2, 3 and 6 had two main modes of association between molecules, one of them

was the H from the adduct contact with the S from the other molecule (N–C–HS) and the

H from the alkyl group contact with S from the other molecule (C–C–HS), as shown in

Figure 3.S1. In contrast, 1 and 7 had one mode of association between molecules, which was

the (N–C–HS) interaction in 1 and 4 and the (C–C–HS) interaction in complex 7. The

complex 5 also exhibited interchelate distances between S from the molecule and S from

another molecule (3.491 Å).

107

Figure 3. 2. The molecular structures of the manganese xanthates. [Mn(S2COMe)2.TMEDA] (1),

[Mn(S2COEt)2.TMEDA] (2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),

[Mn(S2COnPen)2.TMEDA] (5), [Mn(S2COnHex)2.TMEDA] (6) and [Mn(S2COnOct)2.TMEDA] (7).

H atoms are omitted for clarity. Violet = Mn, yellow = S, red = O, blue= N, grey = C.

108

3.4.4.2. Thermogravimetric analysis of molecular precursors 1–7

Thermal analyses of all the complexes were conducted up to 600°C under a nitrogen

atmosphere. Complexes 2, 3 and 4 decomposed cleanly in one step to form MnS at 150–

350°C, while 1 decomposed around 200 °C. With increasing alkyl chain length, the TGA

profiles for 5 and 6 changed from a single step to a two-step breakdown. The inset picture

in figure 3.3 shows the first thermal decomposition step for 5 and 6 at approximately 150

°C, which was similar to the single decomposition of 2. In the case of 5 and 6 precursors,

the mass residue obtained from the TGA profiles for the first decomposition stage (57.5%)

agrees with the theoretical value calculated for the removal of one molecule of xanthate and

half from another one (58%). While in the secondary step, there is a mass loss in the

temperature range of 250°C to 350 °C that is consistent with the theoretical value for

production of MnS. All the six complexes gave the final solid residue amounts that matched

with the calculated value for MnS. The comparison for the final experimental residues and

the calculated values for MnS are given in Table 3.S4.

Figure 3. 3. Thermogravimetric analysis (TGA) profiles of complexes (1-6) and inset picture for

complexes (5 and 6).

109

3.4.4.3. MnS nanocrystals using hot-injection thermolysis

MnS nanocrystals were synthesized using hot-injection of dispersing manganese

alkylxanthate in trioctylphosphine (TOP) and then injected into pre-heated oleylamine

(OLA) at 230 °C. The OLA is used as a coordinating agent and often catalyses the

degradation of complexes at lower temperatures than the other methods.54,55 During the

optimization and the case of complex 2 we have observed that the complex showed no sign

of decomposition below 190°C, whereas at 200°C, complex 2 decomposed partially, leading

to poor crystallinity in the resulting materials as shown in Figure 3.S3. For this reason a

higher temperature has been applied for the decomposition of all complexes.

At 250°C, the complete decomposition of the complexes occurred yielding products with

good crystallinity, as shown by the p-XRD measurements. The p-XRD pattern of all the

samples prepared using hot-injection are shown in Figure 3.4. Diffraction peaks at 2θ values

of 29.62, 34.33, 49.35, 58.62, 61.45, and 72.37 correspond to the (111), (200), (220),

(311), (222) and (400) planes, respectively, of the cubic α–MnS (JCPDS 03−065−0891).56

The intensity profile of the p-XRD pattern also matched well with the standard pattern, with

the highest intensity peak being the (200) plane with the 2θ value of 34.33°. The pattern

showed significant changes in the intensity of peaks depending on the chain length. In the

case of complexes from 2 to 4 the change in intensity was more obvious along the (220)

plane, which showed a decrease in the peak intensity with an increase in the chain length.

The Scherrer equation was used to estimate the crystallite size of the MnS nanocrystals as

shown in Table 3.S5. Kan et al. have successfully synthesized a cubic α-MnS nanocrystals

of different sizes ranging from 20 to 80 nm by a colloidal synthesis route through the reaction

of MnCl2 and S[Si(CH3)3]2 in trioctylphosphineoxide.57 The lattice parameters were

calculated using the p-XRD data, unit cell, volume (V) of the cells for all the samples are

given in Table 3.S5.

110

Figure 3. 4. P-XRD patterns of MnS prepared at 250 °C via hot injection from precursors 1-

6.The standard pattern ( black sticks) is cubic α–MnS (ICDD No. 03-065-0891).56

The atomic percentage of all the elements present in the sample synthesized using the hot-

injection was determined by EDX spectroscopy (Figure 3.S4) and the atom percentages

observed from the EDX spectra suggest manganese sulfide was formed in agreement with

the XRD measurements (ESI Figure 3.S4,Table 3.S5).

The morphology of the synthesized MnS nanocrystals was observed by the SEM analysis

and images are shown in Figure 3.5. All the images are obtained at the same magnification

for comparison. The MnS nanocrystals obtained from 1-5 are irregularly shapes (Figure

3.5.(a to e)), while those obtained from complex 6 have spherical morphology (Figure 3.5.f).

111

Figure 3. 5. Representative secondary electron SEM images (10 kV) of MnS samples prepared using

precursor (a-f) (1-6) prepared by hot injection thermolysis at 250 °C, taken at magnification of 1µm.

Figure 3.S5 shows micro-Raman spectra of the MnS nanocrystals prepared by the hot-

injection. The Raman spectra revealed that the MnS synthesised from precursor 1 exhibited

a single band at 635.89 cm−1 which is corresponding to the strong photoluminescence band

as earlier reported. This peak is approximately at the same energy when the chain length was

increased in the cases of 2, 3, 4, 5 and 6, as shown in Figure 3.S5 (Table 3.S5). Similar

results have been previous reported in the literature.58,59

3.4.4.4. MnS nanocrystals using solvent-less thermolysis

MnS nanocrystals NCs were also synthesized using solvent-less thermolysis. This route is

an unexpansive and simple toward the production of nanocrystals semiconductor materials.

Generally, this method produces high yields in comparison with other method for instance

hot injection. The procedure involved the placement some grams of the complexes in a

112

ceramic boat in a furnace tube which was then heated for 1 h. The α-MnS NCs were obtained

after cool down to room temperature.

The NCs obtained from complex (2) [Mn(S2COEt)2.(TMEDA)] at different temperatures

were analysed using XRD to identify the best temperature for obtaining the good

crystallinity of MnS. At low temperatures of 250 and 300 °C, we found that the complete

conversion of the precursor had not occurred as evidenced by XRD (Figure 3.S7), while at

350 °C, complete conversion of the complex 2 occurred giving products with good

crystallinity.

Therefore, all the precursors were decomposed at 350°C, which resulted in highly

crystalline products. With longer heating times (1 h), all six precursors (1−6) generated MnS,

with a pattern that matched that of the cubic α–MnS (JCPDS 03−065−0891, Figure 3.6).56

The diffraction peaks observed at 2 values of 29.6°, 34.4°, 49.3°, 56.8°, 61.5° and 72.6°

correspond to the (111), (200), (220), (311), (222) and (400) planes of cubic α–MnS,

respectively. The unit cell parameters for the NCs are in good agreement with the previously

reported values for the bulk phase.49 The size of the crystalline was estimated using the

domains Debye–Scherrer equation as shown in Table 3.S6. These sizes are smaller than the

sizes of the nanocrystals obtained by using the hot-injection thermolysis.

113

Figure 3. 6. P-XRD patterns of MnS prepared at 350 °C via solvent-less thermolysis of precursors

(1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-065-0891).56

The SEM images of the MnS nanocrystals grown from the precursors using solvent-less

thermolysis at a scale bar of 1-μm are shown in Figure 3.7. For precursors 1 and 2, the

nanocrystals were found to be well-defined and quasi-spherical (Figures 7(a, b)). For

precursors 3 to 6 the products are irregular in appearance with some agglomeration (Figure

3.7.(c–f)). Magnified images (scale bar of 5 μm) are shown in Figure 3.S8. The EDX

spectroscopy of the nanocrystals has been done to quantity atomic percentage. The atomic

percentage of the NCs obtained from EDX from all the precursors (1-6) are shown in Figure

3.S9, Table 3.S6.

114

Figure 3. 7. Representative secondary electron SEM images (10 kV) of MnS samples using precursor

(a-f) (1-6) prepared by a solvent-less thermolysis at 350 °C.

Figure 3.S10 shows the micro-Raman spectra of the MnS nanocrystals prepared using the

solvent-less thermolysis. The Raman spectra revealed that the MnS synthesised from

precursor 1 exhibited a band at 635.18 cm−1 which is corresponding to the strong

photoluminescence band as earlier reported. This peak was almost the same as that obtained

by increasing the chain length of 2, 3, 4, 5 and 6 (Table 3.S6). The similar observation for

raman data has been previously reported in the literature.58,59

115

3.4.4.5. MnS thin films by Doctor Blading

MnS thin films were deposited using doctor’s blade using manganese alkylxanthate

complexes. For the preparation of MnS thin films, MnS thin films was prepared by the

incremental addition of THF to the precursor to make a uniform and lump-free paste. Thus

prepared uniform slurry was coated onto a glass substrate by a doctor blade technique. After

natural drying at room temperature, the thin films were annealed at 350°C for 1 h.

The p-XRD pattern of MnS thin films obtained from all the precursors are shown in Figure.

8. The diffraction peaks of the thin films prepared from complexes 1–6 were indexed to the

cubic manganese sulfide α–MnS (ICDD # 03-065-0891)56. The pattern showed significant

changes in the intensity of the peaks depending on the chain length. This change in the

intensity was indicated that the preferred orientation along the (200) and (220) plane at 2θ =

34.4° and 49.4°, respectively, which showed an increase in the peak intensity with an

increase in the chain length. Crystallites that have preferred orientation have been observed

in other thin films grown by chemical bath deposition technique.59 This suggests that the

substrate or molecular precursor structure may apply control over the nucleation and growth

kinetics of manganese sulfide thin films under these conditions. The grain size of the

crystallites was estimated as shown in Table 3.S7. These sizes of the crystallites are almost

the same as the sizes of the nanocrystals obtained by using the hot injection thermolysis but

are larger than those of the NCs obtained using the solvent-less thermolysis.

116

Figure 3. 8. P-XRD patterns of MnS thin films prepared at 350 °C Deposition by the doctor blade

method from precursor (1-6). The standard pattern is cubic manganese sulfide, MnS (ICDD No. 03-

065-0891).56

The SEM images of the MnS thin films obtained from all the precursors are shown in Figure

3.9. (a-d). For all precursors, the deposited thin films consisted of cube-like MnS crystals

deposited randomly on the surface of the glass substrate at the 1-µm magnification. The

morphology shown by the films deposited by using the doctor blade method was clearly

different than that of the samples obtained by the hot-injection or the solvent-less

thermolysis. The compositional analyses performed using EDX spectroscopy (Figure 3.S11)

revealed the presence of manganese and sulfur in all thin film samples. Table 3.S7 lists the

atomic percentage of each element, the cubic lattice parameter and the crystallite size.

117

Figure 3. 9. Representative secondary electron SEM images (10 kV) of MnS thin films using

precursor (a-f) (1-6) deposited by the Doctor Blade method at 350 °C.

The Figure 3.S12 shows the micro-Raman spectra of MnS thin films prepared using the

doctor blade technique. The Raman spectra revealed that the MnS thin films obtained from

the precursors 1, 2, 3, 4, 5 and 6 exhibited bands at 635.89 cm−1, 636.60 cm−1, 636.21 cm−1,

635.18 cm−1, 635.89 cm−1 and 636.92 cm−1, respectively (Table 3.S7). These peak positions

are corresponding to the strong photoluminescence band as earlier reported.58,59

3.4.4.6. Magnetic properties of MnS nanocrystal

The successful synthesis of α-MnS nanocrystals lets us to study their magnetic properties

and the α-MnS which was obtained from complex 2 was the only one that used to study the

magnetic properties because of the similarity of the other samples. The room temperature X-

band EPR spectrum of -MnS NCs obtained from complex 2 displays a strong signal with

g = 2.003, characteristic of magnetic nanocrystals (Figure 3.10). The magnetisation of the

nanocrystals was measured as a function of temperature, in field cooled (FC) and zero-field-

cooled (ZFC) regimes, under the applied field of 100 Oe (Figure 3.11). To compare with

118

different reports, the magnetic properties of α-MnS NCs have been investigated by Kan et

al. where different sizes between of 20 and 80 nm, α-MnS NCs were antiferromagnetic AFM

with reduced interaction strength in smaller NCs. However, those NCs actually were

aggregates with smaller of particles, which led to that their hysteresis loop is closed.57 Puglisi

et al. reported the magnetic properties of single-crystal octahedral α-MnS NCs of different

size (14, 20, and 29 nm). Below 50 K the NCs showed increased in (FC) magnetization and

a maximum of the (ZFC) magnetization at 25 K, which are both confirmed of a transition

between a superparamagnetic (SPM) and ferromagnetic (FM) type.49

Figure 3. 10. X-band EPR spectrum of -MnS NCs obtained from complex 2.

119

Figure 3. 11. Thermal dependence of the magnetisation for -MnS NCs obtained from complex 2,

measured in zero-field cooled (ZFC) (red circles) and field-cooled (FC) (black squares) regimes, with

the difference MFC-MZFC plotted in blue. Insert: Plot of –d(MFC-MZFC)/dT for the same nanocrystals.

Irreversible magnetic behaviour is observed below 40 K, which marks a transition from the

SPM to FM, the latter characterised by blocking of the magnetisation. The presence of FM-

like regions in the material is also evident in the T-dependence of the magnetisation

difference in Figure 3.11 (blue triangles). Above 70 K, the ZFC and FC magnetisation curves

fully superpose and data could be fitted to a Curie-Weiss law, = C/(T-), providing a Curie-

Weiss constant = -254 K (Figure 3.12).

The negative sign of indicates that the α-MnS nanocrystals obtained from 2 is

antiferromagnetic. The value is less negative than the bulk value of -465 K,60 and close to

the value that reported by Puglisi et al where = -272 K for 29 nm.49 Then, the AFM

interactions become less effective for smaller NCs, in approximately agreement with

previous results. The existence of the FM structure at the surface of the α-MnS NCs is

additionally supported by the hysteresis measured at 5 and 300 K, as shown in Figure 3.13.

At 300 K the saturated magnetization was smaller than 5 K, and there was no hysteresis

loops. The hysteresis curve recorded at 5 K shows that the magnetisation does not saturate

120

up to the magnetic field of 70 kOe, indicative of large anisotropy. Cycling of the

magnetisation between 70 kOe and -70 kOe reveals a hysteresis loop with a coercive Hc field

of 0.723 KOe. This field is larger than observed for similar nanocrystals. Puglisi et al.

confirmed that α-MnS NCs samples were obtained by the isothermal magnetization at 5 K

showed an open loop with size-dependent Hc = 0.009 kOe (14 nm), 0.081 kOe (20 nm),

0.180 kOe (29 nm).49 Yang et al. reported that at low-temperature hysteresis loops was

presented in the FM region since they display open loops withsize-dependent Hc ranging

from 0.01 kOe (14 nm) to 1.265 kOe (40 nm).61 So, this result is to the best of our knowledge,

the first demonstration of a large coercive field (0.723 kOe at 5 K with small size of 8.2 nm)

in α-MnS nanocrystals. It is noted that the magnetization of FM materials is depends on the

size, shape, and structure of these materials.62

Figure 3. 12. Plot of 1/ versus temperature for -MnS NCs obtained from complex 2, measured in

zero-field cooled (ZFC) (red) and field-cooled (FC) (black) regimes, with a fit to the Curie law =

C/(T-) presented in blue (dashed lines).

121

Figure 3. 13. Magnetic hysteresis at 5 and 300 K for -MnS NCs obtained from 2. The inset shows

the region around zero fields.

3.4.5. Conclusion

The synthesis and the single-crystal X-ray structure of seven novel

tetramethylethylenediamine manganese(II) bis(alkylxanthate) complexes [methylxanthate

(1), ethylxanthate (2), n-propylxanthate (3), n-butylxanthate (4), n-pentylxanthate (5), n-

hexylxanthate (6) and n-octylxanthate (7)] were reported. Complexes 2, 3, 4, 6 and 7 adopted

a monoclinic crystal system, while 1 was orthorhombic and 5 was triclinic. All the

compounds displayed intermolecular hydrogen bonds through the sulfur atoms of the

neighbouring molecules (C–HS), except complex 5, wherein the (C–HS) interaction was

not observed. The distances of these interactions were slightly shorter than the sum of the

contact radii (van der Waals radii). Furthermore, 4 and 5 exhibited intramolecular S-S

distances of 3.491 Å and 3.565 Å, respectively. The decomposition of the complexes was

studied using TGA measurements. The series of alkyl–xanthato manganese (II) complexes

were found to change from a single-step decomposition pathway to a two-step pathway with

an increase in the alkyl chain length. The two-step pathway implied that the decomposition

122

of one ligand occurred before that of the other. These six complexes were tested as single-

source precursors for the formation of MnS nanocrystals. The OLA used as the capping

agent in the hot injection thermolysis helped in the decomposition of the complexes at lower

temperatures, whereas the solvent-less thermolysis and the doctor blade technique required

a high decomposition temperature of 350°C. The XRD studies showed that all the precursors

broke down cleanly at 250 and 350°C by the hot-injection, solvent-less and doctor blade

methods, respectively, to form cubic rock-salt (RS) α-MnS. In comparison, the pattern

obtained by using the hot-injection and the solvent-less thermolysis were showed significant

changes in the intensity of the peaks depending on the chain length. This change in intensity

was more obvious along the (220) plane, which indicated a decrease in the peak intensity

with an increase in the chain length. In contrast, the peak intensity obtained by using the

doctor blade increased with an increase in the chain length. Moreover, the size of the α-MnS

nanocrystals synthesised using the hot-injection and the doctor blade methods was higher

than that of the nanocrystals synthesised by using the solvent-less thermolysis. The Raman

peaks were almost the same when the chain length was increased and was in good agreement

with that reported by previous studies. The magnetic measurements display the nanocrystals

have the ferromagnetic behaviour and large coercive field (0.723 kOe for 8.2 nm

nanocrystals). This report provides easy approaches to combine α-MnS nanocrystal with

suitable magnetic properties, which might have potential applications for the short

wavelength magneto-optical nanocrystal in the future.

3.4.6. Acknowledgements

A. Alanazi is thankful to the Ministry of Higher Education in Saudi Arabia for funding and

the University of Islamic, Saudi Arabia for permission to study in the United Kingdom. We

thank Dr P.D. McNaughter and Salman Alanazi for useful comments. We acknowledge the

123

EPSRC National EPR Facility at the University of Manchester for support with magnetic

and EPR measurements.

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3.4.8. Supporting Information

Table 3.S 1. X-ray crystallographic data and refinement details for (1-7) using Cu K radiation and

with H-atom parameters constrained.

Complex (1) (2) (3) (4) (5) (6) (7)

Chemical

formula

C10H22MnN2O2S4 C12H26MnN2O2S4 C14H30MnN2O2S4 C16H34MnN2O2S4 C18H38MnN2O2S4 C20H42MnN2O2S4 C24H50MnN2O2S4

Mr 385.47 413.53 441.58 469.63 497.68 525.73 581.84

Crystal

system,

space group

Orthorhombic,

Pbca

Monoclinic,

C2/c

Monoclinic,

P21/c

Monoclinic,

P21/c

Triclinic,

P¯1

Monoclinic,

I2/a

Monoclinic,

I2/a

Temperature

(K)

293 100 100 150 100 240 150

a, (Å)

b, (Å)

c, (Å)

15.2336 (8)

16.3399 (7)

13.8528 (7)

20.8959 (13)

8.0893 (4)

15.3732 (9)

11.8338 (5)

11.9042 (4)

15.5003 (6)

12.5433 (3)

21.2751 (5)

9.3308 (2)

7.5898 (4)

11.8599 (5)

16.0804 (7)

15.6601 (3)

8.15454 (13)

23.5223 (5)

13.8999 (11)

8.5486 (6)

27.052 (3)

(°)

(°)

(°)

90

90

90

132.491 (7)

90

90

106.751 (2)

90

102.885 (2)

70.704 (4),

78.932 (4),

72.591 (4)

108.507 (2)

97.371 (10)

V (Å3) 3448.2 (3) 1916.1 (2) 2090.90 (14) 2427.32 (10) 1296.60 (11) 2848.48 (10) 3187.9 (5)

Z 8 4 4 4 2 4 4

(mm-1) 10.75 9.71 8.94 7.73 7.26 6.64 5.98

Crystal size

(mm)

0.3 × 0.1 × 0.04 0.22 × 0.14 ×

0.06

0.24 × 0.13 ×

0.06

0.16 × 0.11 ×

0.01

0.20 × 0.07 ×

0.01

0.46 × 0.31 ×

0.02

0.4 × 0.35 × 0.1

Tmin, Tmax 0.282, 1.000 0.504, 0.593 0.469, 0.616 0.353, 1.000 0.743, 1.000 0.445, 1.000 0.664, 1.000

No. of

measured,

independent

and

observed [I

> 2(I)]

reflections

15125, 3288,

2271

6616, 1868, 1670

16722, 4099,

3538

11697, 4724,

3860

14258, 4999,

4661

29808, 2598,

2422

10324, 3236,

2083

Rint 0.096 0.045 0.053 0.039 0.034 0.042 0.065

(sin /)max

(Å-1)

0.623 0.617 0.617 0.617 0.617 0.602 0.628

R[F2 >

2(F2)],

wR(F2), S

0.070, 0.223,

1.05

0.026, 0.063,

1.05

0.028, 0.066,

1.02

0.037, 0.101,

1.06

0.031, 0.079,

1.06

0.068, 0.183,

1.10

0.099, 0.334,

1.13

No. of

reflections

3288 1868 4099 4724 4999 2598 3236

No. of

parameters

178 99 214 291 250 191 180

max, min

(e Å-3)

1.78, -0.73 0.34, -0.22 0.34, -0.24 0.54, -0.45 0.30, -0.47 1.34, -0.45 0.66, -0.80

128

Table 3.S 2. Selected Bond Lengths (Å) and Angles (o) for novel complexes (1-7).

Table 3.S 3. Details of selected intermolecular non-covalent contacts (Å) in the prepared compounds

(1-7).

Complexes N–C–HS interactions

distance

C–C–HS interactions

distance

1 2.886 a) -

2 2.840 a) 2.842 a)

3 2.894 a) 2.970 a)

4 2.997 a) -

5 - -

6 2.927 a) 2.885 a)

7 - 2.756 a)

Sum of the contact radii = 3.00 53

Reference:

53. A. Bondi, J. Phys. Chem., 1964, 68, 441–451.

129

Figure 3.S 1. Crystal structures of 1, 2, 3, 4, 5, 6 and 7 showing intermolecular C–H⋯S non-covalent

contacts and S⋯S interactions.

130

Table 3.S 4. Elemental and thermal analyses of xanthates diaminemanganese(II) complexes 1 - 7.

Complexes Elements analysis : Calc

(found) %

M. Pt

(oC)

Temperature

of TGA (oC)

Mass loss

(%)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

3.4.8.1. Infra-red spectroscopy

The Figure 3.S2 shows the FTIR spectra of all the complexes. The vC=S and vC–O–C are

the two important bands of the xanthate moiety because the additional π–electron flows from

the oxygen atom to the sulfur atoms via a planar delocalized π–orbital system. The IR spectra

of the as-synthesized complexes [Mn(S2COMe)2.TMEDA] (1), [Mn(S2COEt)2.TMEDA]

(2), [Mn(S2COnPr)2.TMEDA] (3), [Mn(S2COnBut)2.TMEDA] (4),

[Mn(S2COnPFen)2.TMEDA] (5) and [Mn(S2COnHex)2.TMEDA] (6) revealed that the vC=S

vibration was at approximately 1034‒1046 cm−1, while the band around 1140‒1190 cm−1

was attributable to the stretching vibrations of the v(C–O–C) asymmetric group, as shown

in Figure 3. S2. Moreover, as reported by Bonati and Ugo et al. for analogous

dithiocarbamate complexes, the vC–S stretching frequencies may be used to distinguish

between the monodentate and the bidentate behaviours of the 1,1-dithiolate ligands. In the

C H S N Mn (Calc.)

Found

31.17 5.76 33.22 7.27 14.27 138 200 - 350 (22.6)

(30.98) (5.56) (33.22) (7.02) (13.94) 24.9

34.86 6.34 30.96 6.78 13.30 137 200 - 350 (21.1)

(34.94) (6.28) (31.26) (6.70) (13.01) 24.4

38.09 6.86 29.00 6.35 12.46 134 200 - 350 (19.7)

(37.88) (6.67) (29.37) (6.12) (12.18) 20.5

40.93 7.30 27.26 5.97 11.71 85 200 - 350 (18.5)

(40.78) (7.15) (27.58) (5.61) (11.55) 19.9

43.45 7.70 25.73 5.63 11.05 65 200 - 350 (17.5)

(43.41) (7.69) (25.98) (5.42) (10.86) 17.77

45.70 8.06 24.35 5.33 10.46 63 200 - 350 (16.5)

(45.30) (7.99) (24.32) (5.01) (10.20) 17.36

49.55 8.67 22.00 4.82 9.45 60 - - (49.05) (8.42) (21.91) (4.65) (9.28)

131

case of monodentate dithiolate ligands, a doublet peak appeared around 1000 cm−1 separated

by ≥20 cm−1, which could be attributed to the non-equivalence of two C=S stretching

vibrations.54 In contrast, in the case of bidentate dithiolate ligands, a strong singlet was

observed in the ~1000 cm−1 region, which was indicative of a symmetrically bound

dithiolate moiety. In the present series of manganese complexes, we observed only one

strong band at approximately 1030 cm−1, which indicated that all the xanthate ligands were

bidentate and symmetrically bonded.

Figure 3.S 2. IR spectra of manganese alkyl xanthate precursors (1-6).

132

3.4.8.2. Manganese sulfide nanoparticles by the hot injection thermolysis:

Figure 3.S 3. The XRD patterns of manganese sulfide nanoparticles prepared by hot-injection from

[Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 200 °C for 30 min to

determine the optimum temperature for thermal decomposition.

Table 3.S 5. The unit cell parameters for the MnS synthesised by hot injection thermolysis from


size, EDX measurements and Raman data from these samples.

MnS from Complexes

Lattice constant a

Volume (Å3)

Crystallite size

EDX measurements Raman shift (cm-1)

(Å) (nm) Mn (at%) S (at%)

(1) 5.214 141.75 19.5 ± 2.01 48.82 51.18 635.89

(2) 5.216 141.91 17.8 ± 2.12 48.65 51.35 636.21

(3) 5.227 141.99 17.0 ± 1.84 48.73 51.27 635.89

(4) 5.220 142.24 14.9 ± 1.75 49.01 50.99 635.90

(5) 5.221 142.32 10.0 ± 1.62 48.93 51.07 635.87

(6) 5.224 142.56 9.18 ± 1.52 48.32 51.68 634.88

133

Figure 3.S 4. EDX spectra of MnS from precursors (a-f) (1-6) prepared by hot injection thermolysis.


hot injection thermolysis.

134

Figure 3.S 6. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by hot injection

thermolysis at 250 °C, 5μm magnification.

3.4.8.3. Manganese sulfide nanoparticles by the solvent-less thermolysis:

Figure 3.S 7. The XRD patterns of manganese sulfide nanoparticles prepared by solvent-less

thermolysis from [Mn(S2COEt)2(TMEDA)] (2) complex heated at different temperature 250, 300

and 350°C for 60 min to determine the optimum temperature for thermal decomposition.

135

Table 3.S 6. The unit cell parameters for the MnS synthesised by solvent-less thermolysis from

precursors (1 – 6), with (ICDD No. 03-065-0891) as the MnS reference pattern, volume, crystallite

size and EDX measurements from these samples.

MnS from Complexes

Lattice constant a

Volume (Å3)

Crystallite size

EDX measurements Raman shift (cm-1)


(1) 5.225 142.65 8.2 ± 1.35 48.44 51.56 635.18

(2) 5.219 142.15 6.8 ± 1.21 48.72 51.28 635.89

(3) 5.223 142.48 6.3 ± 1.14 48.59 51.41 637.02

(4) 5.211 142.50 8.9 ± 1.08 49.27 50.73 636.21

(5) 5.223 142.48 7.6 ± 1.23 50.08 49.92 633.98

(6) 5.210 142.42 8.7 ± 1.85 48.68 51.32 634.52

Figure 3.S 8. SEM images of MnS nanoparticles from complex (a-f) (1-6) prepared by solvent-less

thermolysis at 350 °C, 5μm magnification.

136

Figure 3.S 9. EDX spectra of MnS from precursors (a-f) (1 – 6) prepared by solvent-less thermolysis.


solvent-less thermolysis.

137

3.4.8.4. Deposition of manganese sulfide thin films by the doctor blade method:

Table 3.S 7. The unit cell parameters for the MnS synthesised by doctor blade method from


size and EDX measurements from these samples.

MnS from

Complexes

Lattice

constant a

Volume

(Å3)

Crystallite

size

EDX measurements Raman shift

(cm-1)


(1) 5.20 140.61 20.8 ± 2.38 51.89 48.11 635.89

(2) 5.22 142.24 14.2 ± 2.18 51.38 48.62 636.60

(3) 5.20 140.61 13.4 ± 1.98 50.23 49.77 636.21

(4) 5.22 142.24 17.6 ± 2.03 50.36 49.64 635.18

(5) 5.21 142.42 16.5 ± 2.08 50.60 49.40 635.89

(6) 5.22 142.24 16.5 ± 1.86 50.34 49.66 636.92

Figure 3.S 11. EDX spectra of MnS thin films from precursors (a-f) (1-6) prepared by doctor blade

method.

138

Figure 3.S 12. Raman spectra of cubic rock-salt (RS) α-MnS from complexes (1-6) Deposition by

the doctor blade method.

139

Chapter 4. The influence of single precursor on manganese

incorporation into Mn-doped PbS (Pb1-xMnxS) nanoparticles by

solvent-less thermolysis.

4.1. Introduction

The rare properties of nanocrystalline materials, when compared with their large-grained

analogs, have led to the rapid development of nanotechnology. Significant evidence suggests

that reduction of chalcogenides’ (such as lead sulfide) particle sizes to several tens of

nanometers or below, will change their properties notably.1–7 Doped nanocrystals with

magnetic metal ions, such as transition ions or Mn2+, can result in an array of spectroscopic

and magnetic properties which might have utility in practical application. If a structure is

impure, containing magnetic metal ions, its optical and magnetic properties will be

dependent upon the concentration of these magnetic metal ions.8 Lead sulfide (PbS) is an

important IV–VI semiconductor. Its relatively small bulk band gap (Eg=0.41 eV)9 and a

large exciton Bohr radius of 18 nm gives it a range of applications: it may be used as an

infrared detector, photometer, nonlinear element or sensor.10–12 Contrastingly, PbS

nanocrystals doped with Mn2+ ions produce a material which displays abnormal

magnetooptical and switching properties. Additionally, the doping of Mn2+ ions may be

efficiently used in nano-spintronics, spin-photonics and magneto-electronics.13–16

Within this chapter, the uses of [Pb(S2COEt)2] (1) and [Mn(S2COEt)2.TMEDA] (2)

(TMEDA= Tetramethylethylenediamine) as single source precursors for the synthesis of

undoped PbS and doped Pb1−xMnxS (0 ≤ x ≤ 0.08) with the use of a facile solvent-less

thermolysis has been investigated. The thermogravimetric analysis (TGA) reveals that both

precursors are capable of successful decomposition within a similar temperature range. The

materials produced by this method were further investigated with the use of powder X-ray

140

diffraction (p-XRD), scanning electron microscopy (SEM), energy dispersive X-ray

spectroscopy (EDX), Raman spectroscopy and UV-Vis absorption spectroscopy.



analysis and TGA. The experimental work to produce nanocrystals was carried out by me, I

characterised the samples by XRD, Raman, SEM, EDX and UV-Visible spectroscopy. The

original idea was provided by Paul O’Brien. David J. Lewis supporting me in the project and

he provided as well a nice and useful discussion, and also editing the manuscript. The

experimental work was done in the laboratory of Paul O’Brien.

4.3. References

1 S. I. Sadovnikov, N. S. Kozhevnikova, V. G. Pushin and A. A. Rempel, Inorg. Mater., 2012, 48,

21–27.

2 H. Cao, G. Wang, S. Zhang and X. Zhang, Nanotechnology, 2006, 17, 3280–3287.

3 D. D. W. Grinolds, P. R. Brown, D. K. Harris, V. Bulovic and M. G. Bawendi, Nano Lett., 2015,

15, 21–26.

4 A. Pimachev and Y. Dahnovsky, J. Phys. Chem. C, 2015, 119, 16941–16946.

5 J. K. Wu, L. M. Lyu, C. W. Liao, Y. N. Wang and M. H. Huang, Chem. Eur. J., 2012, 18,

14473–14478.

6 H. Zhao, H. Liang, F. Vidal, F. Rosei, A. Vomiero and D. Ma, J. Phys. Chem. C, 2014, 118,

20585–20593.

7 J. J. Peterson and T. D. Krauss, Nano Lett., 2006, 6, 510–514.

8 Y. T. Nien, K. H. Hwang, I. G. Chen and K. Yu, J. Alloys Compd., 2008, 455, 519–523.

9 D. J. Asunskis, I. L. Bolotin and L. Hanley, J. Phys. Chem. C, 2008, 112, 9555–9558.

10 I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J. C. Martins, F. Vanhaecke, A.

Vantomme, C. Delerue, G. Allan and Z. Hens, ACS Nano, 2009, 3, 3023–3030.

11 R. Kripal and U. M. Tripathi, J. Mater. Sci. Mater. Electron., 2018, 29, 12195–12205.

12 R. Kripal, C. Rudowicz and U. M. Tripathi, Appl. Magn. Reson., 2019, 50, 785–795.

13 R. Beaulac, P. I. Archer, S. T. Ochsenbein and D. R. Gamelin, Adv. Funct. Mater., 2008, 18,

3873–3891.

14 V. Proshchenko and Y. Dahnovsky, J. Phys. Chem. C, 2014, 118, 28314–28321.

15 D. A. Bussian, S. A. Crooker, M. Yin, M. Brynda, A. L. Efros and V. I. Klimov, Nat. Mater.,

2009, 8, 35–40.

16 P. I. Archer, S. A. Santangelo and D. R. Gamelin, Nano Lett., 2007, 7, 1037–1043.

141

4.4. The influence of single precursor on manganese incorporation into

Mn-doped PbS (Pb1-xMnxS) nanoparticles by solvent-less thermolysis.

Abdulaziz. M. Alanazi,a,c David J. Lewis*b and Paul O’Briena,b



c, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,

KSA

*Email: [email protected]

4.4.1. Abstract

[Bis(O-ethylxanthate) lead(II)] (1) and [bis(O-ethylxanthate) manganese(II).(TMEDA)] (2)

were synthesized and used as single source precursors for the preparation of Pb1-xMnxS (x =

0, 0.02, 0.04, 0.06 and 0.08) nanoparticles using solvent-less thermolysis at 350 °C. P-XRD

revealed a cubic crystal structure, with lattice parameter a decreasing linearly as a function

of Mn content. For all samples the elemental compositions and stoichiometries were

determined by EDX spectroscopy. Raman spectroscopy indicates that the intensity of the

weak band observed at 270 cm-1 and 430 cm-1 increased with increasing amounts of

manganese. Incorporation of Mn2+ into PbS led to an increase in the band gap from 0.87 eV

to 0.89 eV, while the particle sizes decreases in the range of 24.80 to 22.07 nm.

4.4.2. Introduction

Recently, much effort has been made to the research of doped metal chalcogenide

nanoparticle materials. These kind of nanoparticles show different physical and chemical

properties in comparison with their bulk materials, such as size-dependent difference of the

band gap energy.1,2 Additionally, impurity ions doped into these nanoparticles can affect the


142

electronic structure and transition probabilities.3 In particular, when doped with magnetic

ions (e.g. Mn2+), these materials can produce unique magnetic and magneto-optical

properties and provide opportunities for the new field of photonic and spintronics.4–6

Lead sulfde (PbS) is an important II–VI semiconductor material, with a rather small bulk

band gap energy (Eg=0.41 eV at 300 K)7,8 and a large exciton Bohr radius of 18 nm,9 which

has applications such as optical switches, sensors, infrared detectors, photovoltaic solar cells,

and storage batteries.10–13 Several approaches have been reported for the preparation of PbS

and MnS including solvothermal,14,15 hydrothermal,16,17 hot injection18,19 and melt

techniques.20

Solvent-less thermolysis has benefits over other routes, such as it is a simple technique in

which a solid state precursor is decomposed and is carried out by thermal treatment under

inert conditions. This technique has confirmed to be an effective way of producing metal

chalcogenide nanomaterials with a varied range of morphologies for example nanorods,21

nanowires,22 nanospheres,23 and nanodisks.24 In comparison with the other chemical

techniques Solvent-less thermolysis offers a simple, an economic, environmental-friendly

and unexpansive way to scale up production.25

The use of single-source precursors provides significant benefits which are useful precursors

for preparation of a range of metal sulfide nanomaterials or thin films.26–31 These precursors

can be prepared simply in large quantities, are generally air-stable, easy to handle, purify

and characterise.32,33 Indeed, great success has been achieved using the thermal

decomposition of lead/manganese complexes of thiobenzoate,34,35

diethyldithiocarbamate36,37 as single-source precursors. The use of metal xanthate precursors

for the preparation of PbS38 and MnS is promising owing to the low decomposition

temperature of metal xanthate complexes (100–350 °C).

143

To our current knowledge, the synthesis of Mn2+ incorporated PbS nanoparticles using

solvent-less thermolysis from the single source precursors has not been reported. In this

study, we synthesised and characterized two precursors [Pb(S2COEt)2] (1) and

[Mn(S2COEt)2.TMEDA] (2) (TMEDA= tetramethylethylenediamine) in solvent-less

thermolysis for the preparation of undoped PbS and doped Pb1−xMnxS (0 ≤ x ≤ 0.08)

nanoparticles at 350 °C. The thermogravimetric analysis (TGA) reveals that both precursors

exhibit successful decomposition in a similar temperature range. The materials produced are

investigated by using powder X-ray diffraction (p-XRD), scanning electron microscopy

(SEM), energy dispersive X-ray (EDX) spectroscopy, Raman spectroscopy and UV-Vis

absorption spectroscopy.

4.4.3. Experimental

4.4.3.1. Chemicals

Acetone (≥ 99.8%, Sigma-Aldrich), potassium hydroxide (≥ 85%), carbon disulfide (≥ 99%),

manganese(II) acetate tetrahydrate (≥ 99%, Sigma-Aldrich), lead(II) acetate trihydrate (≥

99.9%, Sigma-Aldrich) and N,N,N′,N′-Tetramethylethylenediamine (≥ 99%, Sigma-

Aldrich) were used as received with no further purification.

4.4.3.2. Instrumentation

Melting points were determined with a Stuart melting point apparatus (Cole-Palmer, UK);

infrared spectra (IR) were recorded using a Nicolet iS5 Thermo Scientific ATR instrument

(4000–400 cm−1, resolution 4 cm−1). Elemental analyses (EA) and Thermogravimetric

analyses (TGA) were carried out by the Micro-elemental Analysis Service in the School of

Chemistry at the University of Manchester. EA was performed for all complexes using a

Flash 2000 Thermo Scientific elemental analyser and TGA data were obtained with Mettler

Toledo TGA/DSC stare system in the range 30–600 °C at a heating rate of 10 °C min−1 under

144

nitrogen flow. Powder X-ray diffraction (p-XRD) analyses were carried out using an X-Pert

diffractometer with a Cu-Kα1 source (λ = 1.54059 Å), the samples were scanned between

10° and 80°, with an applied voltage of 40 kV and a current of 30 mA. Scanning electron

microscopy (SEM) was carried out using a Philips XL 30 FEG. The voltage used was 40

kV. EDX spectroscopy (Philips EDAX DX4 X-ray micro-analyser SEM) was used to

determine elemental composition as well used for elemental mapping in order to know the

spatial distribution of elements in the sample. Raman spectra were measured using a

Renishaw 1000 Micro Raman System equipped with a 514 nm laser and UV-Vis spectra

were collected on a Lambda 1050, using 3.09 mM solution of Pb1−xMnxS nanoparticles in

ethanol.

4.4.3.3. Synthesis of [bis(O-ethylxanthato) Lead(II)] (1)

Synthesis of [Pb(S2COEt)2] was carried out by following literature.38 Briefly, [Pb(S2COEt)2]

(1) was prepared by a chemical reaction between an aqueous solution of potassium

ethylxanthate ligand (1.6 g, 9.9 mmol) and an aqueous solution of lead(II) acetate

(Pb(CH3COO)2. 4H2O, (0.5 g, 3.3 mmol)) at room temperature while stirring for 60 min to

form a white precipitate. The powder was filtered off and washed with water, and the crude

product was dried in a vacuum oven. The product was recrystallizesed from acetone. Yield:

84% (1.8 g). Melting point: 135 °C. Elemental analysis: Calc (%): C, 16.03; H, 2.24; S,

28.47; Pb, 46.13. Found (%): C, 16.36; H, 2.19; S, 28.62; Pb, 46.53%. IR (νmax/cm-1): 2969

(w), 1121-1138 (s), 1056(s).

4.4.3.4. Synthesis of [ bis(O-ethylxanthato) Manganese(II). (TMEDA)](2)

Potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in 20 ml methanol and

stirred for 2 h at room temperature. Carbon disulfide (1.04 g, 0.83 ml, 13.63 mmol)

was added drop-wise at 0 °C and the mixture stirred for 1 h. 50 ml of an aqueous

145

solution of Mn(CH3COO)2.4H2O (1.60 g, 6.80 mmol) was added drop-wise to the

reaction mixture, which was stirred for 0.5 h to form a brown/yellow solution.

TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring for 60 min to

form a brown precipitate. The solid residue was isolated by filtration and washed with

water, and the product was dried in a vacuum oven. The product was crystallized from

acetone. Yield: 88% (11.2 g). Melting point: 137 °C. Elemental analysis: Calc (%):

C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S,

31.26; N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).

4.4.3.5. Synthesis of Pb1−xMnxS nanoparticles by the solvent-less thermolysis

Metal sulfides were prepared by the thermal decomposition of the complexes (1) and (2),

mixed in different mole fractions of Mn (x = 0, 0.02, 0.04, 0.06 and 0.08) (Table 4. S1).

Both complexes were mixed in the required molar ratios, and the mixture was ground in air

using a pestle and mortar. The powder placed in a ceramic boat under a stream of argon

(300 cm3 min-1), and at 350 °C for 30 min. After this time the heating was turned off and

the combustion boat was allowed to cool naturally to room temperature, and the results

powder were collected for analysis.

4.4.4. Results and discussion

The thermal stability of Pb and Mn complexes was analysed using thermogravimetric

analysis (TGA) (Figure. 4.1). The TGA profile for both complexes displayed decomposition

in a single step, and complete decomposition occurred at around 200 °C. In the case of Pb

complex, one-step decomposition was observed with rapid mass loss of 53%, which is

consistent with the calculated value to produce PbS (53%). Similarly, the TGA for Mn

complex indicated a residual mass of 24%, which was close to calculated value to produce

MnS (21%).

146

Figure 4. 1. TGA profile of (1) lead(II) ethylxanthate and (2) manganese(II) ethylxanthate. TMEDA.

The thermal decomposition of (1) or (2) was performed at 300 °C and 350 °C, under Ar.

Decomposition of the complexes produced black residues which were structurally

characterised using p-XRD. The p-XRD pattern of the powder (Figure. 4.S1) prepared from

(1) alone could be indexed to cubic PbS (ICDD: 03-065-0692).39 The relatively intense (200)

plane indicates the preferred orientation in the pattern. Peaks observed at 2θ value of 25.86°,

29.96°, 42.97°, 50.84°, 53.31°, 62.43°, 68.77°, 70.83°, and 78.81°, correspond to the (111),

(200), (220), (311), (222), (400), (331), (420), and (422) Bragg planes of PbS, with no

indication of secondary phases. P-XRD patterns obtained from the decomposition of (2) at

300 °C and 350 °C are shown in Figure. 4.S2, with diffraction peaks indexing to cubic MnS

(ICDD: 03-065-0891).40 The diffraction peaks observed at 2θ values of 29.6°, 34.4°, 49.3°,

56.8°, 61.5° and 72.6° correspond to the (111), (200), (220), (311), (222) and (400) planes

of cubic α–MnS.

Furthermore, TGA profile indicated that both complexes decomposed completely in the

temperature range of 320–350 °C, and therefore, the temperature of 350 °C was used to

147

ensure the congruent decomposition of the complexes. Figure 4.2 shows p-XRD patterns of

undoped and Mn-doped PbS nanoparticles with various Mn concentrations (0, 2, 4, 6, and

8%), which were synthesised using solvent-less thermolysis. The introduction of manganese,

where x = 0.02, 0.04, 0.06 and 0.08, shifts the reflections associated with cubic PbS to larger

2θ implying a contraction of the lattice. The peaks were sharp for every sample, and the shift

in the peaks was also observed, thereby verifying their crystalline nature.

The diffraction peaks for the Pb1-xMnxS nanoparticles were observed at 2θ values between

those found for PbS and MnS. The peaks displayed a gradual shift with the change in the

ratio of the mole fraction of Mn. The shift in the peaks is consistent with Vegardian

behaviour where compositional result in a changes linear change in unit cell parameters. The

peaks shifted towards a higher theta value with an increase in manganese content, which can

be attributed to a reduction of the lattice parameters with the substitution of the larger Pb2+

(ionic radius 1.33 Å) for smaller Mn2+ (ionic radius 0.80 Å).41,42 The unit cells of both the

cubic PbS and MnS, along with their bond distances are shown in Figure 4.3.

The influence of the Mn2+ content on the lattice constants of the Pb1-xMnxS nanoparticles

indicated a linear decrease in the lattice parameter with an increase in the mole fraction of

Mn. The observed linearity provided further evidence in support of the successful

incorporation of Mn into the nanoparticles. As shown in Figure 4.2, lattice parameters were

plotted against the variations in Mn/Mn + Pb molar ratios, and it can be clearly seen that unit

cell volume is a linear function with respect to Mn2+ content, in the Pb1-xMnxS nanoparticles.

Table 4. 1 illustrates the unit cell lattice parameters ɑ and the unit cell volume V for PbS and

Pb1-xMnxS (0 ≤ x ≤ 0.08). X-ray-diffraction data was used to calculate these parameters,

which involved using the lattice relation for cubic structures: namely, 1/d2 = (h2 + k2 + l2)/ɑ2

and V = ɑ 3, where d represents the space between adjacent lattice planes and (hkl) are the

Miller indices of the plane.43

148

Figure 4. 2. XRD patterns and lattice parameters a, unit cell volume V and d(200) spacing of Pb1-

xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and

manganese xanthate precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x =

0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08.

149

Figure 4. 3. Unit cells of (a) PbS (ICDD: 03-065-0692) and (b) MnS (ICDD: 03-065-0891) along

with their bonds.

Table 4. 1. Lattice parameters a, unit cell volume (V), band gap (Eg) and grain size of Pb1-xMnxS

(0 ≤ x ≤ 0.08) with variations in Mn/Mn+Pb molar ratios.

X Composition Lattice

parameter

a (Å)

V (Å)3 d spacing

(Å) (200)

Band

gap (eV)

Grain size (nm) estimated

from

XRD data SEM micrograph

0 PbS 5.947 ± 0.003 210.33 ±0.09 2.979 0.87 24.62 ± 3.45 24.80 ±1.29

2 Pb0.98Mn0.02S 5.946 ±0.003 210.22 ±0.09 2.977 0.87 23.94 ± 2.84 24.05 ± 1.83

4 Pb0.96Mn0.04S 5.945 ±0.002 210.11 ±0.08 2.975 0.88 23.89 ± 2.85 23.51 ± 1.39

6 Pb0.94Mn0.06S 5.944 ±0.002 210.01 ±0.08 2.973 0.88 23.67 ± 2.93 22.82 ± 0.86

8 Pb0.92Mn0.08S 5.943 ±0.002 209.90 ±0.08 2.970 0.89 22.29 ± 2.17 22.07 ± 0.73

EDX spectroscopy was used to determine the percentage of each element contained within

the synthesised nanoparticles (Figure. 4. S3). Qualitative analysis indicated the existence

only of requirement elements (i.e., Pb, S, and Mn), and so the quantitative analysis showed

that an increase in manganese concentration was accompanied by a decrease in lead content

when Mn/(Mn + Pb) composition ranged between x = 0 to x = 0.08. A linear trend between

manganese and lead was identified, whereas in the case of sulfur, the percentage was almost

the same. The stoichiometry of the nanomaterials was found to be close to the expected

value. There is an approximately quantitative linear relationship between the amount of

manganese in the precursor powders and the amount of manganese determined by the EDX

(a)

(b)

150

in the Pb1-xMnxS products (Figure. 4. 5). The spatial distribution of all elements in the

materials was analysed using EDX elemental mapping. The results indicated a uniform

distribution of Pb, S, and Mn throughout the samples (Figure.4. S4) at the microscale, and

gives credence to the p-XRD data which suggests that this is a true solid solution.

Figure. 4.6 Shows the morphologies of the synthesised Pb1-xMnxS nanoparticles, as observed

using SEM. The SEM image of pure PbS (x = 0) revealed cube-shaped particles (Figure.

4.6a), which transformed into cube-shaped particles with slightly sharper edges as the mole

fraction of Mn content increased. In accordance with the XRD result, close monitoring of

the SEM images of nanoparticles with different Mn concentration. SEM images have also

been utilized to estimate the particle size of the samples. (Table 4.1 and Figure 4.S5) show

the particle size distribution histogram of samples prepared by solvent-less thermolysis at

350 °C with different mole fractions of Mn. It can be seen that the particles size ranges from

24.8 to 22.1 nm depending on the Mn content. It was observed that the particles size of the

nanoparticles are decreased with increase in Mn doping.

Figure 4. 4. Approximately linear correlation between the amounts of manganese in the precursor

feedstock and the mole % Mn found in Pb1-xMnxS samples from EDX spectroscopy.

151

Figure 4. 5. Representative SEM secondary electron SEM images (10 kV) of Pb1-xMnxS (0≤ x ≤

0.08) samples prepared by solvent-less thermolysis at 350 °C using lead and manganese xanthate

precursors with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c) x = 0.04, (d)

x = 0.06 and (e) x = 0.08.

152

Raman spectroscopy was used to investigate the Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared

from mixtures of the lead and manganese ethyl xanthates at 350 °C (Figure. 4.7). The pure

PbS sample displayed a strong peak at 136 cm-1, and it was assigned to a combination of

longitudinal and transverse acoustic modes [LO(L) + TO(L)].44 The other two small broad

bands were located at 270 cm-1 and 430 cm-1, which corresponded to a two-phonon process

and first overtones, respectively.45–47 The strong peak at 967 cm−1 can be attributed to the

photodegradation of PbS.48 For Pb1-xMnxS (x = 0.02, 0.04, 0.06, and 0.08), a significant

change was observed in the Raman spectra with an increase of manganese in the

nanoparticles. The intensity of the weak band observed at 270 cm-1 and 430 cm-1 increased

with the introduction of greater amounts of manganese, but no other significant effects were

observed.

Figure 4. 6. Raman spectra of Pb1-xMnxS (0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis

at 350 °C using lead and manganese xanthate precursors with different mole fractions of Mn.

153

4.4.4.1. Optical properties

The changes in the optical properties of the undoped and doped lead chalcogenides were

observed by UV–Vis–NIR spectroscopy (Figure. 4. S6). The incorporation of manganese

into PbS resulted in the red shift of the absorption spectrum. The estimated energy band gaps

were calculated by Tauc plots to be found 0.87, 0.87, 0.88, 0.89 and 0.90 eV for undoped

PbS and doped Pb1-xMnxS (x = 0.02, 0.04, 0.06, and 0.08) respectively, which correspond to

wavelengths from 1100–1500 nm in the near infrared region in the spectrum (Figure. 4. S7).

Hence, in conclusion we can finally say that the band gap of synthesized PbS NPs is about

0.87 eV as determined from UV-Vis-NIR data which is about 2 times higher than its bulk

value of PbS (0.41eV).19,49–52 From Table 4.1 it is clearly seen that within the quantum

confinement regime band gap becomes a function of the grain size and increases with

decreasing particle size.53–56 Thus, with the subsequent doping of Mn2+ the particle size

decreases but remains comparable to the Bohr radius (18 nm). Thus absorption edge shifts

to shorter wavelength (blue shift) again as a result of decrease in particle size due to which

the optical band gap increases.56–58 Similar changes in the band gap energy for PbS

nanoparticles with smaller crystallite sizes have been reported for PbS nanoparticles by Saah

and Akhtar et al.19,50 The value of the band gap was found to vary from 0.88 to 1.71 eV,

depending on the particles size.

154

Figure 4. 7. Relationship between Particle Size and band gap of undoped PbS and Pb1-xMnxS (0 ≤x≤

0.08) samples prepared by solvent-less thermolysis at 350 °C.

4.4.5. Conclusion

[Bis(O-ethylxanthate) lead(II)] (1) and [bis(O-ethylxanthate) manganese(II).(TMEDA)] (2)

complexes have been used as single source precursors (SSPs) for the synthesis of single

phase Pb1-xMnxS (0 ≤x≤ 0.08) nanoparticles over the entire range of composition by the

solvent-less thermolysis. The TGA curve for both complexes shows decomposition in a

single step, and the successfully decomposition occurs around 200 °C. The p-XRD shows

the shift in peaks, the change in the lattice parameter and the change in the composition

indicate the successful integration of Mn in the crystal lattice of PbS. SEM images exhibited

slightly changes in the morphology as the amount of Mn2+ was increased in the samples. The

elemental compositions of all the samples were examined via EDX spectroscopic mapping,

with the latter technique revealing uniform spatial distribution of elements in all samples.

The Raman spectroscopy indicates that the intensity of the weak band observed at 270 cm-1

and 430 cm-1 increased with the increase amounts of manganese. The band gaps of Pb1-

xMnxS nanoparticles were found to vary from 0.87 eV to 0.89 eV with increasing Mn2+ mole

155

fraction (x). The particle sizes reduces due to the incorporation of Mn2+, while the band gap

increases. The estimated sizes of the nanoparticles found from the SEM are consistent with

the XRD results. This study shows that the use of SSPs is possibly advantageous for the

synthesis of nanoparticles and can be used to other systems for the tuning of their properties.




acknowledge the EPSRC National Facility at the University of Manchester.

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159


Table 4. S 1. Composition of Pb1-xMnxS (0 ≤ x ≤ 0.08).

Composition (x)

[Mn]/[Mn]+[Pb]

[Pb(S2COEt)2] (1) [Mn(S2COEt)2.(TMEDA)] (2)

0 0.22 mmol 0

0.02 5.93 mmol 0.24 mmol

0.04 2.91mmol 0.24 mmol

0.06 1.89 mmol 0.24 mmol

0.08 1.39 mmol 0.24 mmol

Figure 4. S 1. XRD for cubic PbS (ICDD: 03-065-0692) from lead(II) ethylxanthate at (a) 300 °C

and (b) 350 °C.

160

Figure 4.S 2. XRD for cubic MnS (ICDD: 03-065-0891) from Manganese(II) ethylxanthate.TMEDA

at (a) 300 °C and (b) 350 °C.

Figure 4. S 3. EDX spectra of of Pb1-xMnxS (0≤ x ≤ 0.08) samples prepared by solvent-less

thermolysis at 350 °C with different mole fractions of manganese: (a) x = 0 (PbS), (b) x = 0.02, (c)

x = 0.04, (d) x = 0.06 and (e) x = 0.08.

161

Table 4. S 2. Summary of the required composition of Pb1-xMnxS (0 ≤ x ≤ 0.08) calculated from

the elements in the feed and analysis of final products by EDX spectroscopy.

Composition

[Mn]/[Mn]+[Pb]

Target

composition

Required

Stoichiometry

EDX

Pb:Mn:S

Material

stoichiometry

0 PbS 50:50 51:0:49 PbS

0.02 Pb0.98Mn0.02S 49:1:50 46.83:1.03:52.14 Pb0.978Mn0.022S

0.04 Pb0.96Mn0.04S 48:2:50 46.51:2.07:51.42 Pb0.957Mn0.042S

0.06 Pb0.94Mn0.06S 47:3:50 45.30:3.14:51.56 Pb0.936Mn0.064S

0.08 Pb0.92Mn0.08S 46:4:50 45.10:3.91:50.99 Pb0.921Mn0.079S

Figure 4. S 4. EDX elemental mapping (20 kV) of Pb, Mn and S for Pb1-xMnxS samples. (a) x =

0.02, (b) x = 0.04, (c) x = 0.06 and (d) x = 0.08 mole fractions of manganese.

162

Figure 4. S 5. Particle size distribution histogram of the samples prepared Pb1-xMnxS by solvent-

less thermolysis at 350 °C with different mole fractions of Mn: (a) x = 0 (PbS), (b) x = 0.02, (c) x =

0.04, (d) x = 0.06 and (e) x = 0.08.

163

Figure 4. S 6. The UV-Vis-NIR absorbance spectra of undoped PbS and Pb1-xMnxS (0 ≤x≤ 0.08)

samples prepared by solvent-less thermolysis at 350 °C.

164

Figure 4. S 7. Tauc plot (ahν)2 vs. hν showing the direct bandgaps of undoped PbS and Pb1-xMnxS

(0 ≤x≤ 0.08) samples prepared by solvent-less thermolysis at 350 °C.

165

Chapter 5. Effects of annealing temperature on the structural

and optical properties of CMTS (Cu2MnSnS4) nanoparticle

prepared by solvent-less thermolysis

5.1. Introduction

Solar cells which have their basis in quaternary materials have a large photoelectric

conversion efficiency, of over 20%.1,2 Materials such as the quaternary chalcogenide

Cu2ZnSnS4 (CZTS) and the naturally occurring mineral Cu2MnSnS4 (CMTS) have a direct

band gap energy of 1.0–1.4 eV and a substantial absorption coefficient of over 104 cm-1.

These properties one of the absorbent materials which has the greatest potential for use in

sustainable and efficient solar cells.3–5 In comparison with CZTS, CMTS may have greater

potential to produce low-cost solar cells, as it is composed of earth-abundant and low-cost

Mn and Sn. Therefore, the development of an inexpensive, straightforward, solvent-free and

non-toxic fabrication method for high quality and single phase CMTS materials is desirable.

Development of a such a method is critical in order for CMTS to meet photovoltaic

technology requirements.

Firstly, the synthesis and characterisation of M(S2COR)2, M(S2COR)3 and (R´3P)2CuS2COR

R´ = Ph, R =Et M = Cu, Mn , Sn complexes were described. As [Cu(S2COEt)2],

[Mn(S2COEt)2.TMEDA] and [Sn(S2COEt)2] precursors annealed, the temperature-

dependent phase of Cu2MnSnS4 nanoparticles and thin films was successfully completed.

Through the X-ray diffraction patterns and Raman spectra conducted on the samples

annealed between 350 and 500 °C, the samples were revealed as having a tetragonal structure

with a space group of I42m.

166



analysis and TGA. The experimental work to produce nanomaterials and thin films was

carried out by me, I characterised the samples by XRD, Raman, SEM, EDX and UV-Visible

spectroscopy. Mohamed Missous and Abdelmajid Salhi provided conductivity

measurements and analysis of the data. The original idea was provided by Paul O’Brien.

David J. Lewis supporting me in the project and he provided as well a nice and useful

discussion, and also editing the manuscript. The experimental work was done in the

laboratory of Paul O’Brien.

5.3. References:

1 P. Jackson, D. Hariskos, R. Wuerz, O. Kiowski, A. Bauer, T. M. Friedlmeier and M.

Powalla, Phys. Status Solidi RRL – Rapid Res. Lett., 2015, 9, 28–31.

2 K. Ito and T. Nakazawa, Jpn. J. Appl. Phys., 1988, 27, 2094.

3 M. Quintero, A. Barreto, P. Grima, R. Tovar, E. Quintero, G. S. Porras, J. Ruiz, J. C.

Woolley, G. Lamarche and A.-M. Lamarche, Mater. Res. Bull., 1999, 34, 2263–2270.


Compd., 2015, 640, 23–28.

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5046.

167

5.4. Effects of annealing temperature on the structural and optical

properties of CMTS (Cu2MnSnS4) nanoparticle prepared by solvent-less

thermolysis

Abdulaziz M. Alanazi,a,d Abdelmajid Salhi,c Mohamed Missousc and David J. Lewisb*



c, Department of Electrical and Electronic Engineering, The University of Manchester, Sackville

Street, Manchester, M13 9PL, UK.

d, Department of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,

KSA.

E-mail: [email protected]

5.4.1. Abstract

Earth-abundant Cu2MnSnS4 (CMTS) nanoparticles were prepared through a cheap, simple

and non-toxic solvent-less technique by using a mixture of copper(II), manganese(II) and

tin(II) ethylxanthate molecular precursors. The effect of annealing temperatures on the

structure, composition, morphology, and optical properties of the nanoparticles has been

studied. Characterization by X-ray diffraction, Raman spectroscopy, scanning electron

microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and UV-Vis absorption

spectroscopy confirm that the nanoparticles are stoichiometric stannite CMTS. XRD reveals

that at high annealing temperatures (500°C) CMTS is produced as a single phase, whereas

samples annealed at lower temperatures (especially at 350°C) are contaminated with MnS.

Scherrer’s formula shows that there is a correlation between annealing temperature and grain

size. Elemental mapping of the CMTS nanoparticles reveals uniform elemental distributions

of Cu, Mn, Sn and S for every sample tested. The estimated band gap energies of CMTS are


168

in the range 1.75 to 1.40 eV and decrease with increase the annealing temperature. Champion

high-purity CMTS thin films exhibit a band gap energy of 1.16 eV, carrier concentrations of

4.89×1018, carrier mobility of 16.85 cm2 v-1 s-1, and electrical resistivity of 7.57×10-2 Ω cm.

5.4.2. Introduction

The very low price of energy generation with Si modules (<$1/W)has had an adverse

effect upon thin-film solar cell manufacturers.1 In efforts to counteract this,

researchers have concentrated upon very low cost or very high efficiency materials.

Whilst there is little difference in efficiency between Cu(In, Ga)Se2 (CIGS) thin-film

solar cells and established silicon-based PV technology, the quantities of gallium and

indium in the Earth’s crust are limited, and as they become scarcer, they will

inevitably become more expensive.2–5 Because these devices contain the rare and

expensive metals, In and Ga, the application of these devices commercially, is

constrained. However, due to the low cost, nontoxicity and abundance of elements,

thin film and nanoparticles solar cells have started being constructed using sulfide

minerals such as kesterite copper zinc tin sulfide (CZTS),6 copper zinc tin selenide

(CZTSe)7 and the sulfur-selenium alloy (CZTSSe)8–10 as substitutes to chalcogenide

materials, like CdTe and Cu(In, Ga)Se2.11,12

In addition to thin CZTS films, researchers have also concentrated their efforts on the

Cu2FeSnS4 (CFTS), which has a high optical absorption coefficient and a wide

bandgap energy. However, these features are preparation-method dependent and are

influenced by the resulting film’s crystalline structures.13–17

Copper manganese tin sulfide (Cu2MSnS4, CMTS where M= Mn) is another

compound in this group of materials, offering potential for use as a p-type

semiconductor. The absorption coefficient of CMTS, which crystallizes in the stannite

structure (space group: I42m), is high (≈104 cm-1) and it has a direct band gap energy

169

in the range of 1.0 eV to 1.4 eV; these are favourable qualities suited to PV

applications, and the crystal structure of CMTS is shown in Figure 5.1.13,18,19 CMTS’s

ferroelectric, magnetoelectric and optical properties have become the subject of

investigation in recent years.20–22 In addition, studies have explored various methods

of synthesising CMTS compounds including dip coating, electrospinning, hot

injection, microwave irradiation and spray pyrolysis techniques.18,23–26 However, each

of these synthesis methods demands special reaction conditions and involves a

complex solution phase using toxic organic solvents. The majority of research is

dedicated to synthesising CMTS thin films24,27–29

Figure 5. 1. Crystal structures for stannite Cu2MnSnS4, a = 5.449 Å; c = 10.726 Å, α. β and γ= 90°,

ICDD: 0005838.30

Xanthates have recently been used as single source precursors for metal sulfide

nanoparticles.31–35 The general chemical formula for xanthates is [M(S2COR)n],

where R is an alkyl group. Because of the pre-formed M–S bonds, xanthates are good

precursors to deposit metal sulfide thin films.36,37 Also, in comparison to other

precursors, their decomposition occurs at lower temperatures.38 A report has

described the synthesis of a number of xanthates and dithiocarbamates for uses as

170

metal sulfides; using thermogravimetric analysis (TGA) for investigated thermal

decomposition profiles of the compounds. The results of their studies indicate that in

both thin-film and nanoparticle forms, metal xanthates to be practical precursors for

Cu2ZnSnS4.39

Recently, we described the synthesis of CFTS from single source precursors using

solvent-less thermolysis which gave materials with optical band gap energy of 1.5

eV.40 Solventless thermolysis has advantages over other methods, as it is a simple

method in which solid state decomposition of a precursor is accomplished by thermal

treatment under inert conditions.41,42 This approach has confirmed to be an active way

of producing metal chalcogenide nanomaterials with a extensive range of

morphologies for example nanodisks,43 nanospheres,44 nanowires,45 and nanorods.46

Compared to the other solution based chemical approaches solvent-less thermolysis

offers a simple and economical way to scale up production. Furthermore, it offers

environmental benefits, removes the need for harsh reactants and the yields are

usually high.32

This paper considers the synthesis of several copper, manganese and tin O-

ethylxanthates and assessed their suitability as precursors in mixed complexes

reactions to form the quaternary sulfide CMTS.

5.4.3. Experimental

5.4.3.1. Chemicals

Potassium ethyl xanthogenate (96%, Sigma-Aldrich), Tin(II) chloride (99.9%),

copper(II) sulphate (98%) and manganese(II) acetate tetrahydrate (≥99%, Sigma-

Aldrich) N,N,N′,N′-Tetramethylethylenediamine (≥99%, Sigma-Aldrich) were used

with no further purification.

171

5.4.3.2. Instrumentation

Elemental analysis of the precursors was carried out by the chemistry microanalysis

laboratory at the University of Manchester. TGA was conducted from 25 °C to 600

°C under nitrogen using Mettler Toledo TGA/DCS system. Fourier transform infrared

(FTIR) spectra were obtained using a Specac single reflectance ATR and melting

points were obtained using a Barloworld SMP10 apparatus. Powder-X-ray diffraction

(p-XRD) of all the samples was carried out using a Bruker X-pert diffractometer. The

samples were scanned between 20° and 80° using CuKa radiation, the applied voltage

was 40 kV and the current 30 mA. Scanning electronic microscopy (SEM) and energy

dispersive X-ray spectroscopy analysis is carried out using TESCAN MIRA3 FEG-

SEM. The EDS was used to know the chemical composition of the samples. Raman

spectra were measured using a Renishaw 1000 Micro-Raman System equipped with

a 514 nm laser. UV-Vis spectra were collected on a Shimadzu UV-1800, using 3.09

mM solution of CMTS nanoparticles in ethanol.

5.4.3.3. Synthesis of bis (O-ethylxanthato) copper(II) (1)

The synthesis of [Cu(S2COEt)2] was carried out according to the literature.47 Briefly,

a 20 ml aqueous solution of potassium ethylxanthate (1.6 g, 9.9 mmol) and

CuSO4.5H2O (1.2 g, 4.9 mmol) were mixed at room temperature and stirred for 60

minutes. An orange precipitate was isolated by filtration and washed with deionised

water. The product was dried in a vacuum oven overnight at 30 °C. Yield: 87% (1.8

g). Melting point: 187 °C. Elemental analysis: Calc (%): C, 23.58; H, 3.30; S, 41.85;

Cu, 20.80. Found (%): C, 23.67; H, 3.21; S, 41.39; Cu, 21.09. IR (νmax/cm-1): 2979.63-

2932.92 (w), 1462.34-1367.20 (s), 1239.20(s), 1119.11(s), 846 (w).

5.4.3.4. Synthesis of bis (O- ethylxanthato) manganese(II).(TMEDA) (2)

Potassium hydroxide (0.76 g, 13.63 mmol) was dissolved in 20 ml methanol and

stirred for 2 h at room temperature. Carbon disulfide (1.04 g, 0.83 ml, 13.63 mmol)

172

was added drop-wise at 0 °C and the mixture stirred for 1 h. 50 ml of an aqueous

solution of Mn(CH3COO)2.4H2O (1.60 g, 6.80 mmol) was added drop-wise to the

reaction mixture, which was stirred for 0.5 h to form a brown/yellow solution.

TMEDA (0.79 g, 6.76 mmol) was added to the solution while stirring for 60 min to

form a brown precipitate. The solid residue was isolated by filtration and washed with

water, and the product was dried in a vacuum oven. The product was crystallized from

acetone. Yield: 88% (11.2 g). Melting point: 137 °C. Elemental analysis: Calc (%):

C, 34.86; H, 6.34; S, 30.96; N, 6.78; Mn, 13.30. Found (%): C, 34.94; H, 6.28; S,

31.26; N, 6.70; Mn, 13.01. IR (νmax/cm-1): 2980 (w), 1142-1185(s), 1032(s).

5.4.3.5. Synthesis of bis (O- ethylxanthato) tin(II) (3)

The synthesis of [Sn(S2COEt)2] was carried out by following the literature.36 Briefly,

tin(II) ethylxanthate was produced by adding an aqueous solution of K(S2COEt) (5.0

g, 31.1 mmol) into an aqueous solution of tin(II) chloride (2.95 g, 15.5 mmol) in 50

ml deionised water while stirring which continued for 60 min and resulted in a black

precipitate. The precipitate was filtered, and the product was dried at room

temperature. Yield: 88% (6.9 g). Melting point: 47 °C. Elemental analysis: Calc (%):

C, 19.98; H, 2.79; S, 35.45; Sn, 32.91. Found (%): C, 20.04; H, 2.72; S, 35.19; Sn,

32.87. IR (νmax/cm-1): 2977.2-2935.8 (w), 1462.8-1364.9 (s), 1272.8 (s), 1113.8 (s),

860 (w).

5.4.3.6. Synthesis of (O-ethylxanthato) copper(I) triphenylphosphine (4)

The synthesis of [(Ph3P)2CuS2COEt] was carried out by following the literature.48 A

mixture of triphenylphosphine (2.1 g, 8.0 mmol) and CuCl (0.4 g, 4.0 mmol) was

dissolved in 40 ml of chloroform wich was subsequently added to potassium

ethylxanthate (0.6 g, 4.0 mmol) in 40 ml of chloroform. The precipitate was filtered

to obtain a clear yellow solution after 1 h stirring. At 20 C yellow crystals of O-

ethylxanthato copper(I) triphenylphosphine were obtained. Yield: 85% (2.6 g).

173

Melting point: 185–191 C. Elemental analysis: calc. (%): C, 66.1; H, 4.97; S, 9.02;

P, 8.74; Cu, 8.96. Found (%): C, 65.7; H, 5.08; S, 8.77; P, 8.44; Cu, 8.74. IR (νmax/cm-

1): 3048 (w), 2992 (w), 1478 (m) 1433 (m), 1290 (s), 1142 (m), 1041 (m), 1009 (s),

849.5 (s), 740.8 (m), 617.7 (s), 559.2 (s).

5.4.3.7. Synthesis of Copper manganese tin sulfide quaternary system (Cu2MnSnS4)

using solvent-less thermolysis

For the synthesis of Cu2MnSnS4 nanoparticles, 2 mmol copper(II) ethylxanthates (1),

1 mmol manganese(II) ethylxanthates (2) and 1 mmol tin(II) ethylxanthates (3) were

mixed together. Then the mixture placed into a ceramic boat in a tube furnace and

annealed at 350, 400, 450 and 500 °C under nitrogen for 30 min, and the digram of

this process is shown in Figure 5.2. The obtained black residue was then characterised

by p-XRD, EDX, Raman spectroscopy, SEM and UV spectroscopy.

Figure 5. 2. Illustration of the formation of Cu2MnSnS4 nanoparticles through thermal

decomposition of copper(II) ethylxanthates (1), manganese(II) ethylxanthates (2) and tin(II)

ethylxanthates (3) and reaction using the solvent-less thermolysis.

174

5.4.4. Results and dissections

Ethylxanthate complexes of copper [Cu(S2COEt)2] (1), manganese

[Mn(S2COEt)2.TMEDA] (2) ; (TMEDA= N,N,N′,N′-tetramethylethylenediamine)

and tin [Sn(S2COEt)2] (3) were used in combination to produce CMTS at different

temperatures. The thermal stability of the complexes in the solid state was studied by

thermogravimetric analysis (TGA) in the range of 30 to 600 °C (10 °C min−1 in N2)

and resulting profiles are shown in Figure 5.3. The copper xanthate complex (1) is

stable up to 150 °C, after which there are two decomposition steps that involve mass

loss.

The first decomposition starts in the range of 150 to 180 °C and 58% of the original

mass is lost which corresponds to the calculated value of 58% for one ethyl xanthate

and half of another one. The solid residue left after decomposition was ca. 31% (calc.

31% for CuS) in the temperature range of 200 to 450 °C. The manganese xanthate

complex (2) is stable up to 150 °C, after which there is sharp single step

decomposition with major mass loss in the range of 150 to 180 °C. The TGA profile

shows that the solid residue left after decomposition was ca. 22% (calc. 21% for MnS)

in the temperature range of 200 to 350 °C. Similarly, the decomposition of the tin

xanthate complex (3) initiates at an even lower temperature of 120 °C and

decomposition is completed around 150–155 °C in a single step. The TGA profile

shows that the residual mass was ca. 42% (calc. for SnS 42%).

We therefore conclude that the decomposition of the complexes leads to the formation

of metal sulfides at temperatures, higher than 200 °C under the TGA conditions

employed here.

175

Figure 5. 3. Thermogravimetric analysis of (1) bis(ethylxanthate) copper(II), (2) bis(ethylxanthate)

manganese(II).TMEDA and (3) bis(ethylxanthate) tin(II).

5.4.4.1. XRD Characterisation

Figure 5.4 displays the XRD patterns exhibited in CMTS produced at between 350-

500°C. Prominent diffraction peaks observed at 2θ = 18.10°, 27.75°, 28.30°, 32.15°,

47.03°, 55.70° and 75.89° correspond to the (101), (110), (112), (200), (204), (312)

and (316) planes of stannite CMTS. This data corresponds with standard PDF data

(JCPDS no. 51-0757).49,50

When the temperature for annealing is increased, there is a resultant significant

increase in the (112) peak intensity, so this peak is considered as preferred orientation

as well. On the other hand, the remaining diffraction peaks remain distinct and do not

exhibit evidence of any impurities. Lattice parameters of a=b=5.50 Ǻ and c=10.85 Ǻ

were calculated. These are in accordance with the results obtained in previous

studies.49,50

176

When the temperature was lowered, diffraction peaks from a different crystalline

phase were observed at 2θ= 26.83° (marked with #), 2θ= 34.33°and 49.35° (marked

with *). We attribute these to cubic CuS2 (JCPDS no 00-019-0381) and MnS (JCPDS

no 03-065-0891). There were no observable peaks from other crystalline entities from

materials produced at higher temperatures (450-500°C).

Table 5.1 reports the full-width at half-maximum (FWHM) of the (112) peak. This

Table demonstrates that as the annealing temperature is increased there is a reciprocal

effect on the FWHM: an increase in temperature from 350°C to 500°C was shown to

cause a FWHM decrease from 0.46 to 0.36. This data can be used to estimate

relative average grain size using Scherrer’s formula51 as diffraction peak width is

directly related to internal strain, grain size and structural defects.

Figure 5. 4. P-XRD patterns of the CMTS nanoparticles prepared at different temperatures.

* Corresponds to peaks attributed to cubic MnS and # Corresponds to peaks attributed to cubic CuS2

177

Using Scherrer’s formula, the average size of CMTS grains were estimated to be 9.96

± 2.66, 10.47 ± 3.88, 14.26 ± 3.48 and 16.26 ± 2.12 nm at annealing temperatures

350, 400, 450 and 500 °C respectively. Therefore, it is evident that there is a positive

correlation between temperature and CMTS nanoparticle grain size. On the other

hand, the increase in annealing temperature did not affect the interplanar spacing of

CMTS samples.

Due to internal stresses, dislocations can sometimes occur. Dislocations are a

significant type of crystal defect and affect the material’s properties including

strength, rigidity and malleability. Therefore, it is important to mitigate the effects of

dislocations in crystal formation. Dislocation rate can be measured as the length of

dislocation lines occurring in each unit volume of crystal.52 The expression of

dislocation can vary contingent on structural properties, grain size and crystallite

formation. To estimate any resulting dislocation, the XRD line profile analysis

method can be used. This has been expressed in the Williamson-Smallman equation

as:52–54

δ = 1/D2

where D is the estimated crystal size. The dislocation density (δ) estimations

calculated using this method is displayed in Table 5.1. As evident from the values in

Table 5.1, CMTS samples annealed at lower temperatures typically exhibit more

defects (represented by decreased δ values). Chen et al. has reported that the smallest

value of δ obtained in 580°C confirms the good crystallinity of the film.28

Furthermore, the cell structure tetragonal distortion (c/2a) of the stannite

nanoparticles samples were calculated by reference to the lattice parameter ratio.

Calculations showed that at temperatures of 350, 400, 450 and 500 °C, the

corresponding c/2a were

178

0.9864, 0.9862, 0.9868 and 0.9887. This demonstrates that all stannite samples

displayed value around 1, or just less than 1and probably therefore stannite. This is

supported by existing literature.55

Table 5. 1. Lattice constants of the CMTS nanoparticles obtained from XRD patterns.

Sample

ID

d-

spacing

of

(112) (A)

Lattice

constant

a (A)

Lattice

constant

c (A)

Volume

of

crystal

(A3)

Tetragonal

distortion

(c/2a)

FWHM

of the

(112)

peak

(degree)

Crystallite

size D

(nm)

Lattice

Strain

10-3

Dislocation

density δ

(line per

m2)

CMTS-

350 °C

3.1508 5.5001 ± 0.05

10.8508 ± 0.06

328.246 ± 3.81

0.9864 0.4653 9.96 ± 2.66

8.1 10.08×1013

CMTS-

400 °C

3.1498 5.5073 ± 0.01

10.8626 ± 0.07

329.466 ± 2.30

0.9862 0.4344 10.47 ± 3.88

7.5 9.12×1013

CMTS-

450 °C

3.1508 5.4999 ± 0.02

10.8554 ± 0.04

328.363 ± 1.89

0.9868 0.3952 14.26 ± 3.48

6.8 4.92×1013

CMTS-

500 °C

3.1533 5.5063 ± 0.04

10.8889 ± 0.06

330.144 ± 1.63

0.9887 0.3638 16.26 ± 2.12

6.3 3.78×1013

5.4.4.2. Raman spectroscopy

The phase purity of the CMTS nanoparticles formed can be further examined using

Raman spectroscopy (λexc=514 nm). All the samples across all the temperatures

exhibited similar Raman spectra with a prominent peak at 327 cm-1. This peak

represents the A phonon mode which is typically the strongest mode in closely-

connected chalcopyrite crystals.24 Aside from this A mode peak, the spectra of CMTS-

350 °C and CMTS-400 °C also show a weak peak at 471 cm-1. This is likely to arise

from Cu2-xS.56

However, materials that were synthesised at lower temperatures exhibit no peaks at

292 cm-1 or 635 cm-1 as would be expected to indicate the MnS phase.57,58 This may

be due to the low concentration of MnS in both samples.

179

At the higher temperatures of 450 and 500°C, the Raman spectra display one major

peak at 328 cm-1, with no peaks representing MnS and Cu2-xS. This is further evidence

that there are fewer impurities when the crystals are annealed at a higher temperature.

Figure 5. 5. Room temperature Raman spectra of the CMTS nanocrystals prepared at different

temperatures.

5.4.4.3. Nanoparticles composition and morphology

The CMTS nanoparticles were analysed using energy dispersive x-ray (EDX)

spectrometry to determine elemental composition and structural configurations.

These results are displayed in Table 5.S1 and Figure 5.S1. As annealing temperature

was increased, a general decrease in Cu and Mn concentration was evident, which

may be attributed to their evaporation as chalcogenides. Nonetheless, all samples

examined display Mn-rich and Cu-rich composition as Mn/Sn and Cu/(Mn+Sn) > 1.

180

SEM images were also produced to explore the surface morphology of the materials.

These are shown in Figure 5.6 and display a positive correlation between temperature

and average particle size and particle accumulation.

For instance, the CMTS-350 and CMTS-400°C samples, the crystals were spherical

with small crystals, in accordance with the XRD peaks observed. Figure 5.6 (C)

shows the nanoparticles at 450°C, these crystals have a noticeably larger grain size.

At 500°C, the crystals showed agglomerates of spheres.

EDX elemental mapping of CMTS was employed to investigate the elemental

homogeneity in a spatial sense. As is clear from Figure 5.S2, there appears to be a

uniform distribution of Cu, Mn, Sn and S using a scale bar of 10 μm as the image

displays an even dispersion of colour of all four elements. This imaging analysis was

Figure 5. 6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.

181

conducted on more than five different areas of each sample, and there was no

significant variation between the sites, indicating that the imaging is reliable. This

therefore further suggests that the CMTS produced are high quality with low levels

of impurity.


Figure 5.S3 shows the optical absorption spectra of these samples in the wavelength

range of 200–800 nm. The band gap (Eg) can be calculated by reference to the UV-

Visible spectra using the Tauc relation (αhѵ)2= A(hѵ-Eg).59 Where A is the energy

independent constant, α is the absorpation coefficient, h is Planck's constant, ѵ is the

photon frequency and Eg is the band gap energy.

The optical band gap energy can therefore be obtained from extrapolation of the

straight line portion of the (αhѵ)2 versus hѵ and reading the point of interception on the

horizontal photon energy axis, as shown in Figure 5.7. The band gaps were calculated

to be 1.75, 1.64, 1.60 and 1.40 eV for CMTS produced at temperatures 350, 400, 450

and 500 °C, respectively. These values are in the same range as those obtained in

previous studies.50,55 Chen et al. reported the changes of the optical bandgap in CMTS

thin films by spin-coating which found to be between 1.69 to 1.18 eV.50 Moreover,

Cui et al. has been successfully synthesized CMTS nanocrystal by a

182

Figure 5. 7. Tauc plots of the of the CMTS nanoparticles prepared at different temperatures 350 °C,

400 °C, 450 °C and 500 °C.

solvothermal method, and note that the band gap of this material was 1.28 eV which

indicating a potential applications in solar cells.13

Thus, increases in annealing temperature have the effect of gradually decreasing band

gap values. This may be attributed to the increased crystalline purity and

consequential compositional changes as the annealing temperature increases.60,61

As discussed above, there is a positive correlation between annealing temperature and

grain size. Therefore, as annealing temperature is increased, grain boundary density

will decrease, resulting in less electron scattering at grain boundaries.62 Thus, band

gap values will decrease as it becomes less difficult for electronic transitions to occur

between the valence band and the conduction band. This is evident as the sample with

the smallest grain size as 9.96 nm and the largest dislocation density at (10.08×1013

lines per m2), CMTS-350, also exhibits the largest band gap value at 1.75 eV. This is

shown in figure 5.8.

183

Alternatively, the changes in Eg may be attributable to the presence of secondary

phases in CMTS such as MnS. Thus, in the samples annealed at 350 and 400 °C, MnS

(Eg = 3.1 eV) was present. This may have resulted in greater Eg values.63 Furthermore,

the CMTS-500 optical gap is significantly smaller than the other samples investigated.

This may be attributed to sample purity and the lack of secondary phases.

Figure 5. 8. Variation of bandgap and grain size as a function of annealing temperature.

5.4.4.5. Electrical properties

The optical properties that we measure for these materials recommend that they may

well be valuable for applications in the absorber layers in solar cells because of the

strong optical transitions between the energy bands and high absorption coefficient (a

> 104 cm-1). We therefore studied the electronic properties of these materials as thin

films deposited using spin coating. Electrical properties of CMTS thin films deposited

at various temperatures are investigated on Hall measurements. All the CMTS films

184

deposited at various substrate temperatures exhibit the p-type conductivity, a

desirable requirement for the fabrication of heterojunction solar cells. The resistivity

(ρ), carrier mobility (μ) and carrier concentration (p) are shown in Table 5.2.

Table 5. 2. Electrical properties of CMTS films prepared by spin coating from 350 C to 500 C.

T 350 C 400 C 450 C 500 C References

ρ ( Ω cm ) 3.62×10-2 2.59×10-1 1.69×10-1 7.57×10-2 64

μ (cm2 v-1 s-1) 38.39 4.17 849.42 16.85 64,65

p (cm-3) 4.02×1018 4.40×1018 3.99×1014 4.89×1018 48

Conductivity p-type p-type p-type p-type 40,48,64

The resistivity (ρ) increased from 0.0362 Ω cm to around 0.0757 Ω cm for the

temperatures from 350 C to 500 C, respectively. In addition, we found that the carrier

concentration (p) in these films is increased from 4.02×10+18 cm-3 to 4.89×10+18 cm-3

for the from 350 C to 500 C, respectively. However, the carrier mobility (μ)

decreased from 38.39 cm2 v-1 s-1 for 350 C to 16.85 cm2 v-1 s-1 for 500 C. The same

finding for CMTS samples have been reported by Nie et al.19 The obtained values of

hole mobility and carrier density suggest that CMTS could be a potential material for

photovoltaic applications.66 Full details of the CMTS thin films preparation,

characterisation and electronic measurements are given in the ESI.†

5.4.5. Conclusions

A simple, inexpensive and non-toxic solvent-less thermolysis is introduced to prepare

Cu2MnSnS4 (CMTS) nanoparticles using single source precursors. The influence of

annealing temperatures on the grown nanoparticles was optimized. Specifically, the

secondary phase was existed with CMTS nanoparticles at low temperature. However,

185

at high annealing temperatures produced nanoparticles of CMTS with an increase

degree of crystallinity, large particles size and low dislocation density. Pure CMTS

was obtained with increasing the annealing temperatures to 450 and 500 °C. The SEM

morphology displays that increasing the annealing temperatures usually leads to

improved nanoparticle morphology of CMTS. The EDX data showed that the Mn

ratio decreased with the increase the annealing temperatures. Analysis of UV spectra

for the annealed CMTS nanoparticles displays that the band gap energy shifts toward

lower energies gradually with increasing the annealing temperatures from 1.75 to 1.40

eV; in specific, the CMTS-350 sample displays the largest band gap of 1.75 eV. The

decrease of band gap values with increasing annealing temperatures which could be

attributed to the joint effects of the upgrading of the crystalline value and the reducing

of the Mn ratio in the CMTS nanoparticles.




acknowledge the EPSRC National Facility at the University of Manchester.

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52 A. A. Akl and A. S. Hassanien, Superlattices Microstruct., 2015, 85, 67–81.

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64 S. K. Swami, A. Kumar and V. Dutta, Energy Procedia, 2013, 33, 198–202.

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29, 7613–7620.


Cells, 2011, 95, 1421–1436.

189


5.4.8.1. Solvent-less thermolysis

Table 5. S1. Chemical composition and composition ratio from EDX spectra of the CMTS

nanoparticles prepared at different temperatures.

Figure 5. S1. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500.

190

Figure 5. S2. EDX elemental mapping of the CMTS nanoparticles prepared at different

temperatures, (a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 10 µm in all

cases. A secondary electron SEM image of the mapped area is included in each case, labelled as SE.

Figure 5. S3. Absorption spectra of the CMTS nanoparticles prepared at different temperatures 350

°C, 400 °C, 450 °C and 500 °C.

191

5.4.8.2. Spin coating technique

The deposition of CMTS thin films using spin coating technique Glass substrates were cut

to 20 mm × 15 mm, and cleaned by acetone and water and allowed to dry. The solutions

were prepared by dissolving the mixture of 2 mmol triphenylphosphine copper(I)

ethylxanthates, 1 mmol TMEDA.manganese(II) ethylxanthates and 1 mmol tin (II)

ethylxanthates in tetrahydrofuran (Chloroform, 6 ml). A clear black solution was obtained.

The solution was used to deposit CMTS thin films on cleaned glass substrates using spin

coating techniques (Ossila, 24 V DC, 2.01 A) at 700 rpm for 30 s and allowed to dry. The

resulting films were placed into a tube furnace and heated at 450 oC for 60 min, under an

inert atmosphere. After that the furnace was turned off and the tube was allowed to cool

down to room temperature.

5.4.8.3. Electrical properties of CMTS thin films

The electrical properties of the CMTS thin films were characterized using Hall measurement

in a four-probe configuration. Conductive silver paste was used to form the four contact

electrodes. The Hall measurements performed on all CMTS samples of dimension

7mm×7mm shows that the majority carriers are holes, indicating the p-type conductivity in

the CMTS films deposited at different temperatures.

192

Figure 5. S4. XRD patterns of the CMTS films prepared by spin coating from 350 C to 500 C.

Figure 5. S5. Raman spectra of the CMTS films prepared by spin coating from 350 C to 500 C.

193

Figure 5. S6. SEM images of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500

thin films prepared by spin coating.

Table 5. S2. Chemical composition and composition ratio of the CMTS thin films prepared at

different temperatures.

194

Figure 5. S7. EDX spectra of (a) CMTS-350, (b) CMTS-400, (c) CMTS-450 and (d) CMTS-500

thin films prepared by spin coating.

Figure 5. S8. EDX elemental mapping of the CMTS thin films prepared at different temperatures,

(a) 350 °C, (b) 400 °C, (c) 450 °C and (d) 500 °C. Scale bars represent 5 µm in all cases. A secondary

electron SEM image of the mapped area is included in each case, labelled as SE.

195

Figure 5. S9. Absorption spectra of the CMTS thin films prepared at different temperatures 350 °C,

400 °C, 450 °C and 500 °C.

Figure 5. S10. Tauc plots of the of the CMTS thin films prepared at different temperatures 350 °C,

400 °C, 450 °C and 500 °C.

196

Chapter 6. A molecular precursor route to quaternary

chalcogenide CFTS (Cu2FeSnS4) powders as potential solar

absorber materials

6.1. Introduction

This chapter was published as the article “a molecular precursor route to quaternary chalcogenide

CFTS (Cu2FeSnS4) powders as potential solar absorber materials” in Journal of Royal Society

Chemistry Advances (2019).1 In recent years, both the demand for and the rates of energy

consumption have steeply and rapidly increased on a global scale. Due to this increase, the research

of low-cost and high-efficiency solar cell material has become a matter of crucial importance. Cu

based multinary chalcogenides have great potential to be the new generation of solar cell materials;

their high absorption coefficient and low band gap energy increase their potential.2 There is a

newfound interest in photovoltaics at present, stimulating research into new, alternative materials

and the approaches that might be for fabricating low-cost thin-film solar cells. There are various

semiconductive materials that have been researched in this line, including CdTe/CdSe, Cu(InxGa1-

x)Se2 (CIGS), dyesensitized TiO2, organic materials, and so on.3–6 As indium and gallium are in

limited availability and as cadium is high in toxicity, interest in identifying and resarching low-cost,

non-toxic, earth-abundant photovoltaic materials has risen.6–9 One potential alternative is the

quaternary semiconductor Cu2FeSnS4 (CFTS), a strong candidate due to its optimal band gap p

(1.28–1.50 eV) and its optical absorption coefficients (> 104 cm−1). Additionally, CFTS is formed of

inexpensive, relatively non-toxic and earth-abundant elements.5,10,11 For the purposes of this

research, CFTS has been produced using a metal xanthate precursor and synthesised using a

straightforward and cost-efficient solvent-free thermolysis.

In this work, the synthesis and characterisation of M(S2COR)2, M(S2COR)3, M(S2COR)4 and

(R´3P)2CuS2COR [R´ = Ph, R =Et; M = Cu, Fe , Sn complexes are described. As [Cu(S2COEt)2],

[Fe(S2COEt)3], [Sn(S2COEt)2] or [Sn(S2COEt)4] precursors were annealed, the temperature-

dependent phase of a Cu2FeSnS4 powder was successfully prepared. As the X-ray diffraction patterns

197

and Raman spectra of powder annealed between 250 and 450 °C, it was shown that the powders were

organised in a tetragonal structure with a space group of I42m. The four probes method was used to

test for the carrier mobility and carrier density of the films when heated at 450 °C. The film’s

conduction type was found to be p-type.



analysis and TGA. The experimental work to produce nanomaterials and thin films was

carried out by me, I characterised the samples by XRD, Raman, SEM, EDX and UV-Visible

spectroscopy. Firoz Alam checked the characterization of complexes and materials.

Mohamed Missous and Abdelmajid Salhi provided conductivity measurements and analysis

of the data. Andrew Thomas provided XPS measurements and analysis of the data. The

original idea was provided by Paul O’Brien. David J. Lewis supporting me in the project and

he provided as well a nice and useful discussion, and also editing the manuscript. The

experimental work was done in the laboratory of Paul O’Brien.

6.3. Citation

A. M. Alanazi, F. Alam, A. Salhi, M. Missous, A. G. Thomas, P. O’Brien and D. J. Lewis, RSC

Adv., 2019, 9, 24146–24153.

6.4. References:


J. Lewis, RSC Adv., 2019, 9, 24146–24153.

2 Y. Zhang, X. Sun, P. Zhang, X. Yuan, F. Huang and W. Zhang, J. Appl. Phys., 2012,

111, 063709.

3 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462–465.




Commun., 2012, 48, 4956–4958.

198

6 C. Wadia, A. P. Alivisatos and D. M. Kammen, Environ. Sci. Technol., 2009, 43, 2072–

2077.

7 H. Katagiri, K. Jimbo, W. S. Maw, K. Oishi, M. Yamazaki, H. Araki and A. Takeuchi,

Thin Solid Films, 2009, 517, 2455–2460.


Cells, 2011, 95, 1421–1436.

9 D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov and D. B. Mitzi, Prog.

Photovolt. Res. Appl., 2012, 20, 6–11.

10 F. Ozel, M. Kus, A. Yar, E. Arkan, M. Can, A. Aljabour, N. M. Varal and M. Ersoz, J.

Mater. Sci., 2015, 50, 777–783.

11 S. A. Vanalakar, P. S. Patil and J. H. Kim, Sol. Energy Mater. Sol. Cells, 2018, 182,

204–219.

199

6.5. Manuscript 1: A molecular precursor route to quaternary

chalcogenide CFTS (Cu2FeSnS4) powders as potential solar absorber

materials

Abdulaziz M. Alanazi,a,d Firoz Alam,a,b Abdelmajid Salhi,c Mohamed Missous,c Andrew G.

Thomas,b Paul O’Briena,b and David J. Lewis.b*

a School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.

b School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.

cSchool of Electrical and Electronic Engineering, The University of Manchester, Sackville

Street, Manchester, M13 9PL, UK.

d School of Chemistry, Islamic university, Prince Naif Ibn Abdulaziz Rd, Madinah, 42351,

KSA.

E-mail: [email protected]

6.5.1. Abstract

In the present work we report on the synthesis of tetragonal phase of stannite Cu2FeSnS4

(CFTS) powder from Sn(II) and Sn(IV) using a solvent free melt method using a mixture of

Cu, Fe, Sn(II) / Sn(IV) O-ethylxanthates and annealed at different temperatures. The as-

synthesized powders were characterized by powder X-ray diffraction (p-XRD), Raman

spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy,

scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy,

which confirm the successful synthesis of stannite CFTS. Optical measurements shows that

Cu2FeSnS4 powders have visible light absorption onsets in the far red with direct band gap

energies in the range 1.32 eV - 1.39 eV which are suitable for acting as efficient absorber

layers in solar cells. Electronic characterisation of these materials deposited as thin films by


200

spin coating show that they are p type semiconductors with respectable carrier mobilities of

ca. 60 cm2 V-1 s-1 with carrier densities in the order of 1014 cm-1.

6.5.2. Introduction

Among inorganic semiconductors, quaternary metal chalcogenides materials have attracted

interest as light absorbers in photovoltaic applications.1–8 Copper iron tin sulfide

(Cu2FeSnS4) has drawn considerable attention in photovoltaics because of its p-type

conductivity, suitable band-gap 1.2-1.5 eV (Table 6.1) and high absorption coefficient

(> 104 cm−1).9–12 The structure of Cu2FeSnS4 is similar to the zinc blende structure. The

structures are adopted which depend on the configuration of the tetrahedral holes which are

called stannite (CFTS) and kesterite (CZTS), respectively.13 The stannite is tetragonal with

unit cell parameters a = 5.449 Å, c = 10.726 Å with a space group I42m as shown in Figure

6. 1(a), and the kesterite is tetragonal with unit cell parameters a = 5.434 Å, c = 10.856 Å

Figure 6. 1(b) with a space group I42m.13 Therefore, it also consists of inexpensive, non-

toxic and earth-abundant materials. However, the commercialized solar cells technologies

such as CdTe and Cu2InGaS4 (CIGS) have commonly used p-type semiconductors, which are

expensive, rare and toxic such as In, Ga and Cd.14 Therefore, the development of low-cost,

nontoxic and environmental friendly alternatives are needed to make high-efficiency solar

cells. Copper based chalcogenides such as Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe)

have been used as solar absorber materials in thin film solar cells.15,16

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Table 6. 1. Reported band gaps of CFTS nanomaterials prepared by different methods

Method Band gap (eV) Refs.

Hot-injection 1.28 23

Solvothermal 1.33 9

Microwave irradiation 1.71 24

Electrospinning 1.24 25

a liquid reflux 1.32 26

solution-based 1.46 27

One of the challenges in the synthesis of CZTSSe materials is to obtain pure and

stoichiometric kesteritic materials materials as the optoelectronic properties seem to be

sensitive to in particular the Cu and Zn ratios.10,17–22

One of the alternative to CZTS is Cu2FeSnS4 (CFTS) , which has been used as an Pt-free counter

electrode in Dye-sensitized solar cells (DSSCs) as well as an absorber material in thin film solar

cells.23

CZTS and CFTS have suitable optical band gaps of around 1.4 eV and good absorption

coefficients (typically α > 104 cm-1) in the visible spectral range which is comparable to

CIGS materials, making them favourable candidates for photovoltaic applications.24,25

Hence, CZTS and CFTS thin film solar cells have reached power conversion efficiencies of

12.6% and 8.03%, respectively, where these materials are used as part of the absorber layer.26

In addition CZTS and CFTS show p-type conductivities which can be useful for pairing to

n-type materials in cell architectures.27

Up to now a range of methods have been reported for the synthesis of CFTS materials of

different shapes and sizes.28–30 Some reports focus on developing solution based processes

as an alternate to vacuum deposition. This offers the advantage of high productivity and low

processing temperatures.31 Other methods such as solvothermal process,29 hot injection28 and

202

microwave irradiation32 have been used for solution based synthesis of CFTS. However, the

solvothermal and hot injection processes have certain conditions, which give low yield, use

toxic chemicals (Ethylenediamine and Oleylamine) and require more steps like heat

treatment for 18-24 h, centrifugation and vacuum drying. Thus, it is necessary to design

inexpensive approaches for synthesis of CFTS materials. In this paper, we produce CFTS

from direct thermal decomposition of metal xanthate precursors. To the best of our

knowledge, the synthesis of CFTS powders using solvent free thermolysis has not been

reported so far. The method which we propose has many advantages and has been used to

prepare CFTS powders in large quantities in the laboratory. The technique is straight

forward, solvent free, inexpensive and single step utilizing single source precursors (SSPs)

such as xanthates and dithiocarbamate.33–35 Here, we use metal xanthate precursors because

their decomposition happens at a lower temperature and the by-products are gaseous.34–39


10.726 Å, α. β and γ= 90o, ICDD: 0005838 (b) kesterite type structure a = 5.434 Å; c = 10.856 Å, α.

β and γ= 90o ICDD: 0005843.23

203

6.5.3. Materials and experimental

6.5.3.1. Materials

Tin(II) chloride (99.9%), tin(IV) chloride (98%), carbon disulfide (99.9%), Iron(III) chloride

(97%)], copper(II) sulphate (98%), chloroform (99.8%), hexane (97%), toluene (99.7%) and

ethanol (99.8%) were purchased from Sigma-Aldrich or Alfa Aesar and used as received.

A Phillips X-PERT PRO with CuKa incident beam (λ=1.54059 Å) was used to record X-ray

diffraction patterns. The samples were scanned in the 2θ range of 10° to 80° for a period of

1 h. Scanning electron microscopy (SEM) was carried out using a Philips XL 30 FEG. The

voltage used was 40 kV. Carbon coating was carried out using an Edwards E306A coating

unit. EDX spectroscopy (Philips EDAX DX4 X-ray micro-analyser SEM) was used to

determine elemental composition as well used for elemental mapping in order to know the

spatial distribution of elements in the sample. The optical properties of the CFTS powders

were characterized by UV–Vis-NIR absorption spectroscopy recorded on a Shimadzu UV-

1800. Raman spectra were collected using a Renishaw 1000 Micro-Raman system equipped

with a 50× objective and a 514 nm laser. X-ray photoelectron spectroscopy (XPS)

measurements were performed using either a Kratos Axis Ultra or SPECS XPS instrument.

Both facilities are equipped with monochromated Al Kα X-ray sources with a photon energy

of 1486.6 eV. Emitted photoelectrons were collected using either a 165 mm hemispherical

energy analyser (Kratos) or a 150 mm hemispherical energy analyser (Phoibos 150 SPECS),

respectively. The peaks were calibrated through referencing C 1s to 284.8 eV. Infrared

spectra were recorded on a Specac single reflectance ATR instrument (4000-400 cm-1,

resolution 4 cm-1). Melting points were determined using a Barloworld SMP10 device and

the Elemental analyses of complexes were done using a Flash 2000 Thermo Scientific

elemental analyser. Thermogravimetric analysis (TGA) was performed using a Mettler

Toledo TGA/DSC 1 system under an atmosphere of dry nitrogen.

204

6.5.3.2. Synthesis of metal xanthate complexes

6.5.3.3. Synthesis of potassium ethylxanthate

The synthesis of the potassium ethylxanthate was conducted in the following way. Potassium

hydroxide (11.29 g, 0.2 mmol) was dissolved in ethanol (75 ml) and cooled in an ice bath.

Carbon disulfide (15.32 g, 12.16 ml, 0.2 mmol) was added dropwise while stirring. The

ethanol was evaporated at room temperature to obtain the product, 71.8% yield.

6.5.3.4. Synthesis of bis (O- ethylxanthato) copper(II)

The synthesis of [Cu(S2COEt)2] was carried out according to the literature.40 Briefly, an

aqueous solution of potassium ethylxanthate (1.596 g, 9.9 mmol) and CuSO4.5H2O (1.242

g, 4.9 mmol) mixed at room temperature while stirring and the stirring was continue for 60

minutes. Orange precipitate was obtained and washed with deionised water. The precipitate

was filtered, and then product was finally dried in a vacuum oven overnight at room

temperature. Yield: 87%. Melting point: 187 oC. Elemental analysis: Calc (%): C, 23.58; H,

3.30; S, 41.85; Cu, 20.80%. Found (%): C, 23.67; H, 3.21; S, 41.39; Cu, 21.09%. IR

(νmax/cm-1): 2979.63-2932.92 (w), 1462.34-1367.20 (s), 1239.20(s), 1119.11(s), 846 (w).

6.5.3.5. Synthesis of (O-ethylxanthato)copper(I) triphenylphosphine

The synthesis of [(Ph3P)2CuS2COEt] was carried out by following the literature.16 A mixture

of triphenylphosphine (2.09 g, 0.008 mol) and CuCl (0.40 g, 0.0040 mol) was dissolved in

40 ml of chloroform and later it was added to the potassium ethylxanthate (0.641 g, 0.0040

mol) that was dissolved in 40 ml of chloroform. After stirring for 1 h at room temperature a

white precipitate was obtained. The precipitate was filtered to obtain a clear yellow solution.

At -20 oC the yellow crystals of O-ethylxanthato copper(I) triphenylphosphine was obtained.

Yield: 85%. Melting point: 185-191 oC. Elemental analysis: Calc (%): C, 66.1; H, 4.97; S,

9.02; P, 8.74; Cu, 8.96. Found (%): C, 65.7; H, 5.08; S, 8.77; P, 8.44; Cu, 8.74. IR (νmax/cm-

205

1): 3048 (w), 2992 (w), 1478 (m) 1433 (m), 1290 (s), 1142 (m), 1041 (m), 1009 (s), 849.5

(s), 740.8 (m), 617.7 (s), 559.2 (s).

6.5.3.6. Synthesis of tris (O- ethylxanthato) Iron(III)

The synthesis of [Fe(S2COEt)3] was carried out by following the literature.41 Briefly, an

aqueous solution of potassium ethylxanthate (1.596 g, 9.9 mmol) and an aqueous solution

of FeCl3 (0.538 g, 3.3 mmol) mixed at room temperature while stirring and the stirring was

continue for 60 minutes. The black precipitates was obtained and washed with deionised

water. The precipitate was filtered using whatman paper, and the product was finally dried

in a vacuum oven overnight at room temperature. Yield: 85%. Melting point: 118 oC.

Elemental analysis: Calc (%): C, 25.79; H, 3.61; S, 45.81; Fe, 13.34%. Found (%): C, 25.59;

H, 3.35; S, 45.36; Fe, 12.70%. IR (νmax/cm-1): 2987.63-2979.92 (w), 1458.55-1425.36 (s),

1233.18(s), 1059.25 (s), 856 (w).

6.5.3.7. Synthesis of bis (O- ethylxanthato) tin(II)

The synthesis of [Sn(S2COEt)2] was carried out by following the literature.42 Briefly, tin(II)

ethylxanthate was produced by adding an aqueous solution of K(S2COEt) (5 g, 31.1 mmol)

into an aqueous solution of tin(II) chloride (2.95 g, 15.5 mmol) in 50 ml deionised water

while stirring and the stirring was continue for 60 minutes that results in a black precipitates.

The precipitate was filtered using whatman paper, and the product was finally dried at room

temperature. Yield: 87.5%. Melting point: 47 oC. Elemental analysis: Calc (%): C, 19.98; H,

2.79; S, 35.45; Sn, 32.91. Found (%): C, 20.04; H, 2.72; S, 35.19; Sn, 32.87 IR (νmax/cm-1):

2977.24-2935.84 (w), 1462.82-1364.91 (s), 1272.76(s), 1113.75 (s), 860 (w).

6.5.3.8. Synthesis of tetrakis (O- ethylxanthato) tin(IV)

The synthesis of [Sn(S2COEt)4] was carried out using a technique that was modified from

literature.43 Briefly, SnCl4 (1 g, 3.8 mmol) was dissolved in 50 ml of toluene and added drop

206

by drop to the potassium ethylxanthate (2.5g, 15.3 mmol) in toluene at room temperature.

The reaction mixture was stirred for 60 minutes then precursor solution was evaporated

under reduced pressure and then oily residue was shaken by adding 50 ml of hexane. The

yellow crystals of [Sn(S2COEt)4] were extracted from the solution. Yield: 91.3%. Melting

point = 73 °C. Elemental analysis: Calc (%):C, 23.91; H, 3.34; S, 42.43; Sn, 19.69. Found:

C, 23.98; H, 3.29; S, 42.01; Sn, 20.11. IR (νmax/cm-1): 2987.29 (w), 1462.48-1365.84 (s),

1247.83 (s), 1025.47 (s), 860 (w).

6.5.3.9. Synthesis of Cu2FeSnS4 powders

For the synthesis of CFTS powders, 2 mmol copper(II) ethylxanthates, 1 mmol iron(III)

ethylxanthates and 1 mmol tin (II) or (IV) ethylxanthates were mixed together. Then the

mixture was heated in a furnace at 250 oC, 350 oC and 450 oC, for 1 hour under a nitrogen

atmosphere. The CFTS powders were allowed to cool-down to room temperature in the inert

atmosphere. The CFTS powder synthesised using Sn(II) and Sn(IV) are named as (1) and

(2), respectively. In addition to the synthesis of CFTS powders, we also deposited the CFTS

thin films using spin coating technique from Sn(II) and Sn(IV), which are named as (3) and

(4), respectively. Full details on the synthesis and characterisation of these thin film samples

can be found in the Supporting Information.

6.5.4. Result and discussion

6.5.4.1. Thermogravimetric analysis (TGA) of precursors

The synthesis of [Cu(S2COEt)2], [Fe(S2COEt)3], [Sn(S2COEt)2] and [Sn(S2COEt)4]

complexes were performed and their suitability for melt reactions was measured through

thermal stability measurement in a nitrogen atmosphere. Figure 6. 2 shows the TGA profiles

of [Sn(S2COEt)2], [Cu(S2COEt)2], [Sn(S2COEt)4] and [Fe(S2COEt)3], respectively. The

[Cu(S2COEt)2] and [Sn(S2COEt)4] complexes display a two- step decomposition pattern. In

case of [Cu(S2COEt)2] precursor, the mass residue obtained from the TGA profiles for the

207

first decomposition stage (58%) agreed with the theoretical value calculated for the removal

of one molecule of xanthate and half from another one (58%). While in the second step, there

is a mass loss of 31.15% in the temperature range of 200 to 450 °C that is agreed with

theoretical value (31.3%) of CuS. In the case of [Sn(S2COEt)4] the first step involves a

degradation of the mass loss 58.6% in the temperatures range of 45.61 to 120 oC obtained

from the TGA profile, which corresponds and agreed with the theoretical value calculated

for the removal of three molecules of xanthate (60.1%), and the final decomposition residue

obtained after 150°C was found to be SnS2 which is almost in conformity with the mass loss

data obtained from the TGA profile (32%) and the theoretical value (33.4). On contrast, the

[Sn(S2COEt)2] and [Fe(S2COEt)3] precursor complexes have a single step decomposition.

The single step decomposition of [Sn(S2COEt)2] occurred in the temperature range of 304-

396 °C with a mass loss of 41.6% and for [Fe(S2COEt)3] is 27% in the temperature range

of 73.19-400.70°C which are in good agreement with the theoretical values of SnS (41.7%)

and FeS2 (28.6%), respectively. Other researchers have been observed with often metal

xanthates.35,36 For instance, Almanqur et al, have successfully synthesised a series of iron

alkyl xanthate complexes to deposit iron sulfide thin films and nanostructures using the spin

coating and the solventless pyrolysis methods. The TGA profiles of these complexes showed

approximately the same with a rapid residue loss within the temperature range of 120 to 300

°C, and final step occurred between 320 to 500 °C. All complexes showed the final solid

residue amounts that matched with the calculated values for FeS2 or FeS.34

Al-Shakban et al, have synthesised the SnS thin films from diphenyltin bis(iso-

butylxanthate) complexes using aerosol-assisted chemical vapor deposition (AACVD). The

TGA profile of this complex showed two-step decomposition, the first step of which

involves elimination of the alkyl groups, followed by carbonyl sulfide (SCO). Then, the

final step may involve the loss of another carbonyl sulfide.44

208

Figure 6. 2. Thermogravimetric analysis of [Cu(S2COEt)2] (red colour), [Fe(S2COEt)3] (blue colour),

[Sn(S2COEt)2] (green colour) and [Sn(S2COEt)4] (black colour) precursors.

6.5.4.2. Bulk Structural characterisation of CFTS powders

The powder XRD patterns of CFTS synthesized at different temperatures using Sn(II) and

Sn(IV) precursors are shown in Figure 6. 3. The diffraction peaks observed at 2θ values of

28.50, 32.85, 33.36, 36.97, 47.15, 47.50, 50.93, 56.66, 70.04 and 76.69 correspond

to the (112), (200), (004), (202), (220), (204), (301), (116), (008) and (316) planes of the

tetragonal phase of stannite, respectively. The calculated lattice parameters for powder (1)

and (2) are a = 5.4501 Å, c = 10.7468 and a = 5.4467 Å, c = 10.7510 Å, respectively which

are in good agreement with the reported literature values for CFTS.29

The XRD peaks intensities increased with increasing the temperature without affecting the

phase of the powder. The average domain size of both powder (1) and (2) are approximately

13 ± 1.15 nm calculated using Scherrer’s formula. The comparison between kesterite and

stannite with experimentally determined and calculated value is represented by the tetragonal

209

distortion (deviation of the c/2a ratio from 1, where, c and a are the lattice parameters). The

tetragonal distortion parameter is important for the resulting electronic structure of the

material,45 for example, strong deviations away from the ideal structure caused by a changes

in crystal field can lead to non-degenerate valence band maxima.46–48 Therefore, it is

important to look into the tetragonal distortion values found experimentally and

theoretically. In kesterite CZTS, the c/2a ratio has been reported to be greater than 1 in a

neutron diffraction study done on powder samples.45 However, in stannite CFTS this ratio

has been reported to be less than 1, as estimated using XRD studies. In our study, the ratio

of stannite CFTS was determined to be 0.99, which is slightly less than 1 and thus this value

is in good agreement with the values in the literature.13,45

In order to prove the pristine nature of the synthesized powders and to rule out the existence

of secondary phases that were not distinguished by the XRD, Raman spectroscopy was

performed. Figure 6. 4 shows the Raman spectra of CFTS, which exhibits a large peak at

312.22 cm−1 and 317.25 cm−1 corresponding to tetragonal CFTS in both (1) and (2),

respectively. It is reported in the literature that this is the A1 symmetric vibrational motion

of sulfur atoms in CFTS.49–51

We also note the absence of Raman peaks corresponding to FeS (214 and 282 cm−1) and

Cu2SnS3 (267, 303 and 356 cm−1) which are common contaminants of CFTS,52 and is

consistent with the XRD patterns of the powders being a single crystalline phase (Figure 6.

3).

210

Figure 6. 3. P-XRD patterns of Cu2FeSnS4 powder (1) and (2) synthesised at (a) 250°C; (b) 350°C

and (c) 450°C for 1 hour.

211

Figure 6. 4. Raman spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C

for 1 hour.

Figure 6. 5(a-d) show the Fe 2p, Cu 2p, Sn 3d and S 2p X-ray photoelectron spectra recorded

from powders prepared using a Sn(II) and Sn(IV) precursor. The Sn 3d spectra show no

difference between the two samples, with peaks at 486.9 eV and 495.3 eV arising from the

spin orbit split 3d5/2 and 3d3/2, respectively. It is difficult to determine the Sn oxidation state

from the Sn 3d XPS spectrum since the literature reports both Sn(II) and Sn(IV) compounds

with binding energies in the region. It is possible there is some surface oxidation for both

synthesis methods. The S 2p spectra in figure 6. 5d are fitted with three spin orbit split

doublets from S2p3/2 and S2p1/2. Both samples show significant surface oxidation with a

substantial sulphate derived peak with the S 2p3/2 at a binding energy of 168.8 eV. There is

a clear sulfide derived doublet with the 2p3/2 at a binding energy of 161.6 eV and some

residual contamination at the surface attributed to S-O, S-H or S-C at the surface.53 We found

that the Fe 2p and Cu 2p spectra in Figure 6. 5(a, b) are difficult to fit. It is well established

212

that the delectrons in these transition metals lead to a range of multiplet split features, and

complex shake up structures.54,55 Simple analysis of the binding energies of the features in

the Cu 2p3/2 region are consistent with the presence of CuO at the surface.

The strong, narrow peak at a binding energy of 932.2 eV, however, is similar to that of the

mineral chalcopyrite (CuFeS2) and more intense than would be expected for CuO.55 The Fe

2p spectra also suggest some oxidation at the surface. The binding energy of the lowest

energy peak of 711.7 eV is often attributed to the presence of FeSO4 and Fe2O3. The former

is consistent with the binding energy of sulphate derived S.54 The satellite feature at a binding

energy of 725.22 eV is 2p peak in figure 6.5a and the lower energy satellite at 716.5

indicative of the presence of Fe(III) seemingly confirming the oxidation of Fe to Fe2O3. It is

clear that the high binding energy satellite and the 711.7 eV peaks are much more

pronounced for the material synthesized from the Sn(IV) precursor, but the reasons for this

are unclear. Unfortunately, a lack of high quality XPS spectra from FeCuSnS2 standards

means it is difficult to determine contributions from this material.

213

Figure 6. 5. XPS spectra of Cu2FeSnS4 powder (1) and (2) synthesized at a temperature of 450°C

for 1 hour: (a) Fe 2p, (b) Cu 2p, (c) Sn 3d and (d) S 2p.

214

6.5.4.3. Microscopic Characterisation of CFTS powders

The scanning electron microscopy (SEM) images of the CFTS powders at different

magnifications are shown in Figure 6.6. The CFTS (2) show that the quaternary

chalcogenides particles were largely agglomerated with variation in their size, while the

agglomerated (same size) particles are obtained in CFTS (1). In both cases agglomeration of

crystals is seen without any definite shape. The compositional data and EDS spectra of CFTS

powder synthesized at 450 oC are shown in Figure 6. S1. The atomic % of Cu, Fe, Sn2+ and

S were 27.76, 13.19, 13.73 and 45.32, respectively in (1), while in (2) the atomic % of Cu,

Fe, Sn4+ and S were 25.41, 14.09, 15.40 and 45.10, respectively which indicates that both

Cu2FeSnS4 powders have the required stoichiometry. Elemental mapping of CFTS is used

to investigate the homogeneity in terms of material composition at the microscale. Figure

6.7 shows the elemental mapping CFTS powders. It is clear from the Figure 6.7 that the

distributions of Cu, Fe, Sn and S elements in the sample are uniform in a scale bar of 5µm.

Figure 6. 6. SEM images of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour. Scale

bar showing different magnifications.

215

Figure 6. 7. Elemental mapping of Cu2FeSnS4 powder (1) and (2) synthesised at 450 °C for 1 hour

showing the distribution of Cu, Fe, Sn and S. Scale bar represented 5 μm in all cases.

216


The UV-Vis-NIR absorbance spectra of the CFTS powders dissolved in ethanol are shown

in Figure 6. S2. In order to quantify the band gap energy of stannite Cu2FeSnS4 powders

synthesized at 450 oC for 1 hour, the optical absorption measurements were done in the

wavelength range of 400-1100 nm. Figure 6.8 shows (ahѵ)2 versus hѵ with a straight line

fitting, indicating the direct bands gaps of 1.32eV and 1.39eV for (1) and (2), respectively,

which are in good agreement with the literature values.29,56 Ideally, the absorber material of

an efficient solar cell should be a direct bandgap semiconductor because of strong optical

transitions between the energy bands and high absorption coefficient (α >104 cm-1). The

calculated limiting efficiency for a single band gap solar cell of Eg = 1.3 - 1.4 eV in a

simulated solar spectrum (AMG 1.5, i.e. fixed incident light) is around 30%. Hence, the

optical properties that we measure for these materials suggest that they may well be useful

for applications in the absorber layers in solar cells. The optimum thickness of absorber

layers is inversely proportional to the absorption coefficient. Hence these materials would

potentially be suitable as absorber layers for thin film cells in particular. We therefore studied

the electronic properties of these materials as thin films deposited using spin coating. Four-

probe Hall measurements performed on CFTS thin films revealed that the majority carriers

are holes (p-type), whilst the carrier mobility ranged between 58 – 60 cm2 V-1 s-1. The

estimated carrier densities in these films are of the order of 1014 cm-3. Full details of the

CFTS thin film preparation, characterisation and electronic measurements are given in the

Supporting Information.

217

Figure 6. 8. Tauc plot (ahѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 Powders (1) and (2).

218

6.5.5. Conclusions

Copper, iron and tin O-ethylxanthate complexes have been successfully synthesized. The

complexes were found to decompose in the temperature range of 150 - 450°C to give the

metal sulfide as the final product in conformity with the mass loss data and were used for

the synthesis of CFTS powders. The CFTS powder (1) and (2) have been successfully

synthesised from both Sn(II) and Sn(IV) precursors respectively using pyrolysis in the

temperature range of 250 to 450 °C. The stannite phase is obtained for both CFTS powders,

which was ascertained from a tetragonal distortion parameter c/2a of less than 1 in all cases.

Absorption measurements confirm that Cu2FeSnS4 powder (1) and (2) are direct band gap

semiconductors having bandgap energies of 1.32 eV and 1.39 eV, respectively and thus are

suitable for photovoltaic absorber layer applications.


A. Al-A. thankful to Ministry of Higher Education in Saudi Arabia for funding and the

University of Islamic, Saudi Arabia for permission to study in the United Kingdom. DJL and

FA are funded by EPSRC grant EP/R020590/1.

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6.5.8.1. Powder from solvent-less thermolysis

Figure 6. S 1. The EDX plots of Cu2FeSnS4 powder (1) and (2) synthesised at a temperature of

450°C for 1 hour. The inset of figure 6. S1 shows the compositional data of Cu2FeSnS4 powder (1)

and (2).

223

Figure 6. S 2. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 powder (1) and (2) synthesised at

a temperature of 450°C for 1 hour.

6.5.8.2. The deposition of CFTS thin films using spin coating technique

Glass substrates were cut to 20 mm × 15 mm, and cleaned by acetone and water and allowed

to dry. The solutions were prepared by dissolving the mixture of 2 mmol triphenylphosphine

copper(I) ethylxanthates, 1 mmol iron(III) ethylxanthates and 1 mmol tin (II) ethylxanthates

or tin (IV) ethylxanthates in tetrahydrofuran (THF, 6 ml). A clear black solution was

obtained. The solution was used to deposit CFTS thin films on cleaned glass substrates using

spin coating techniques (Ossila, 24 V DC, 2.01 A) at 700 rpm for 30 s and allowed to dry.

The resulting films were placed into a tube furnace and heated at 450 oC for 60 min, under

an inert atmosphere. After that the furnace was turned off and the tube was allowed to cool

down to room temperature. The CFTS thin films deposited from Sn(II) and Sn(IV) are named

as (3) and (4), respectively.

224

Figure 6. S 3. P-XRD patterns of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour.

Figure 6. S 4. Raman spectra of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour.

225

Figure 6. S 5. SEM images of Cu2FeSnS4 thin films deposited using (3) and (4) and annealed at

450°C for 1 hour.

Figure 6. S 6. EDX plots of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed at a

temperature of 450°C for 1 hour. The inset image showing the atomic percent of Cu2FeSnS4 thin

films.

226

Figure 6. S 7. Elemental mapping of Cu2FeSnS4 thin films deposited from (3) and (4) and annealed

at 450°C for 1 hour, showing the distribution of Cu, Fe, Sn and S.

Figure 6. S 8. The UV-Vis-NIR absorbance spectra of Cu2FeSnS4 thin films deposited from (3) and

(4) and annealed at 450°C for 1 hour.

227

Figure 6. S 9. Tauc plot (αhѵ)2 vs. hѵ showing the direct bandgap of Cu2FeSnS4 thin films deposited

from (3) and (4) and annealed at 450°C for 1 hour.

228

6.5.8.3. Electrical properties of CFTS thin films

The electrical properties of the CFTS thin films were characterized using Hall measurement

in a four-probe configuration. Conductive silver paste was used to form the four contact

electrodes. The Hall measurements performed on both CFTS samples of dimension

7mm×7mm shows that the majority carriers are holes, indicating the p-type

conductivity in the CFTS films deposited using (3) and (4). The carrier mobility in

CFTS thin films obtained from (3) and (4) are 58 cm2/V.s and 60 cm2/V.s,

respectively. The estimated carrier densities are 8.2×1014 cm-3 and 4.6×1014 cm-3 for

CFTS thin films obtained from (3) and (4), respectively. The same finding for CFTS

samples have been reported by Prabhakar et al.1 The obtained values of hole mobility

and carrier density suggest that CFTS could be a potential material for photovoltaic

applications.2

6.5.8.4. References

1. R.R. Prabhakar, N.H. Loc, M.H. Kumar, P.P. Boix, S. Juan, R.A. John, S.K.

Batabyal,L.H. Wong, ACS Appl. Mater. Interfaces, 2014, 6, 17661–17667.

2. D. B. Mitzi, O. Gunawan, T. K.Todorov, K. Wang, S. Guha, Sol. Energy Mater.

Sol. Cells, 2011, 95, 1421-1436.

https://www.sciencedirect.com/science/article/pii/S0927024810006719#!





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Chapter 7. Conclusion and Future Work

7.1. Conclusion

An increasingly popular choice for semiconducting materials is the nanoscale rather than

bulk. In this area layered transition chalcogenides (TMCs) are drawing increased attention

due to optical, electric, magnetic properties of the materials that change at a large rate in

going to the nanoscale from that of bulk scale. The modification of both transition metals

and chalcogenide components through intercalation and replacement may result in the

production of novel properties. TMCs are non-poisonous, of negligible cost and abundant

semiconductor materials that can be used for several applications.

TMCs attract interest as a result of their impressive array of characteristics, in addition to

the broad array of uses, including solar cells, sensors, field effect transistors, and water

splitting photocatalysts. A key property of TMCs, and the main topic of this study, is their

photovoltaic (PV) potential. Since some of these compounds are relatively affordable,

plentiful and non-toxic, their capacity to be used in sustainable energy production is

unrivalled.

These valuable properties have resulted in detailed investigation and application of these

compounds. TMCs can be described in accordance with the following formula MXn, (M =

transition metal, X = chalcogenide S, Se or Te). While there are multiple methods for the

synthesis of metal chalcogenides at the nanoscale, it is clear, that to facilitate industrial

processes, an uncomplicated procedure is the favoured choice, such as the solvent-less

thermolysis, which is relatively inexpensive and nontoxic. Furthermore, this process is

economic, environmental-friendly and typical yields are usually high.

Single source precursors for the synthesis of metal sulfide nanomaterials provide many

additional benefits, which include consistency in thermal responses, and improved stability

230

both in exposure to air and in response to moisture. Furthermore, using a single source

precursor enables precise design to manage the decomposition temperature, enabling purer

nanoparticle production, with lower rates of imperfection. Downstream processing is

enhanced by using a single source precursor, as impurity levels are reduced and purification

is easier whilst the metal stoichiometry is preserved. In addition, single source precursors

are appropriate for production of powdered and thin-film nanomaterials. Lastly, but of vital

importance, using single source precursors, as compared to dual or multiple sources,

facilitates stringent stoichiometry, phase and morphology management, with greater ease.

Researchers including our team, have elaborated the application of metal xanthates

[M(S2COR)x] (M = transition metal, R = alkyl chain), using AA-CVD to produce thin films

metal sulfides and solvent-less and hot-injection thermolysis to produce nanostructured

metal sulfides.1–5 The selection of metal xanthates as a single source precursor for metal

sulfides is favourable due to their clean decomposition, observed at low temperatures.

In this study, a series of metal (Fe, Mn, Cu, Pb and Sn) complexes of organoxanthates were

synthesised. These complexes were characterized by IR spectroscopy, elemental analysis

and single crystal X-ray diffraction. Thermal stability of the complexes was analysed by

TGA. The complexes were employed as potential single source precursors for annealing and

depositing of metal sulfide nanomaterials and thin films by doctor blade deposition, hot

injection and solvent-less thermolysis, respectively. The syntheses of the complexes are

simple and use low cost chemicals.

Chapter 3 presented the synthesis of several novel bis(O-alkylxanthato)manganese(II) (alkyl

= Me (1), Et (2), nPr (3), nBut (4), nPen (5), nHex (6) and nOct (7)) complexes stabilised by

the bidentate N-donor ligand tetramethylethylenediamine (TMEDA), and their use as single

source precursors to produce Mn-S materials. They are characterised by elemental analysis,

infrared spectroscopy and thermogravimetric analysis. The X-ray single crystal structures of

231

2, 3, 4, 6 and 7 are based on a monoclinic crystal system with space groups C2/c, P21/c,

P21/c, I2/a and I2/a, respectively, while 1 is orthorhombic Pbca and 5 is triclinic P1. In all

the cases, the central Mn ions were coordinated by six atoms, bound by two chelating

xanthate ligands and a chelating TMEDA ligand in a distorted octahedral manner. Moreover,

the coordination of each of the bidentate ligands was symmetric for 2, 6 and 7 and

asymmetric for 1, 3, 4 and 5. Most of the compounds displayed intermolecular hydrogen

bonds through the sulfur atoms of the neighbouring molecules (C–HS). The distances of

these interactions were slightly shorter than the sum of the contact radii (van der Waals radii).

Hot injection was the first method used to produce manganese sulfide nanocrystals from

these complexes. Thermolysis of these complexes in oleylamine at 250 °C was also chosen,

and the temperature and capping agent were important factors in controlling the phase and

shape of the deposited material. Moreover, α-MnS nanomaterials were also annealed from

these complexes at temperature of 350 °C by using solvent-less thermolysis, which is

straightforward, solvent-free and inexpensive. Finally, α-MnS thin films were also deposited

from these complexes on glass substrates at temperature of 350 °C by using the doctor blade

method. The Mn-S materials were characterised using powder X-ray diffraction (p-XRD),

Raman spectroscopy, Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray

Spectroscopy analysis (EDS). The XRD studies showed that all the precursors broke down

cleanly by these methods to form cubic rock-salt (RS) α-MnS. The Raman peaks were almost

the same when the chain length of the precursor was increased in all methods. The difference

was in the morphologies, which were spherical, irregular and cube-shaped from hot

injection, solvent-less thermolysis and doctor blade route, respectively.

Chapter 4 describes nanomaterials of Mn doped in PbS, which were synthesised from the

precursors [Pb(S2COEt)2] (1) and [Mn(S2COEt)2(TMEDA)] (2) , using solvent-less

thermolysis. Different molecular ratios of Pb:Mn precursors were used to obtain

232

nanomaterials of stoichiometry Pb1-xMnxS (0 ≤x≤ 0.08) and showed the enhancement of the

morphology of the materials. The samples were characterised by powder XRD, Raman

spectroscopy, SEM and EDX. The p-XRD shows the shift in peaks, the change in the lattice

parameter and the change in the composition, which indicate that the successful integration

of Mn in the crystal lattice of PbS. The characterisation by powder XRD, SEM and EDX

indicated that increase in mole fraction of Mn2+ results in decrease of cell constant a and

volume of unit cell. Incorporation of Mn2+ into PbS led to an increase in the band gap from

0.87 eV to 0.89 eV, while the particle sizes decrease in the range of 24.80 to 22.07 nm.

Copper and tin based chalcogenides are widely employed for their potential use in solar

energy applications. The band gap of these materials can be tuned between 1.0 to 2.0 eV by

addition of elements such as Zn, Fe, Mn, In and Ga. Chapters 5and 6 of the thesis mainly

targeted the development of nanomaterials of Cu2MnSnS4 (CMTS) and Cu2FeSnS4 (CFTS)

by solvent-less thermolysis using a mixed precursor approach.

In Chapter 5 the synthesis of Cu2MnSnS4 nanocrystals from simple ethylxanthate complexes

of copper, manganese and tin, which were used in combination to produce a pure phase at

different temperatures, was reported. The effect of annealing temperatures on the structure,

composition, morphology, and optical properties of the processed precursor nanocrystals has

been studied. Characterization by X-ray diffraction, Raman spectroscopy, SEM, EDX

spectroscopy and UV-Vis absorption spectroscopy confirm that the nanocrystals are

nominally stoichiometric stannite, CMTS. The estimated band gap energies of CMTS

gradually decreased from 1.67 to 1.38 eV with increase in the annealing temperatures, and

this range of band gaps is suitable for photovoltaic applications.

Chapter 6 presented the synthesis of Cu2FeSnS4 powders from mixtures of ethylxanthate

complexes of copper, iron and tin(II) or tin(IV). The CFTS powders synthesised using Sn(II)

and Sn(IV) are named as (1) and (2), respectively. The CFTS powders (1) and (2) have been

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successfully synthesised from Sn(II) and Sn(IV), respectively using the solvent-less

thermolysis in the temperature range of 250 to 450 °C. Pure stannite phase is obtained for

both CFTS powders. The strong Raman peaks clearly indicate the purity of CFTS powders.

The average domain size of both powders (1) and (2) are approximately 13 ± 1.15 nm

calculated using Scherrer’s formula. Moreover, absorption measurements confirm that

CFTS powders (1) and (2) are direct band gap semiconductors, having bandgaps of 1.32 eV

and 1.39 eV, respectively, suitable for photovoltaic applications.

To summarise, xanthate precursors have been proved to be good candidates for the synthesis

of crystalline and pure phase metal sulfide nanoparticles in high yields, short duration of

time and in low temperatures. The reagents used in synthesis were also comparatively non-

toxic, high natural abundance and very low cost. The unexpansive, non-toxic,

straightforward and solvent-less thermolysis of the xanthate precursors could produce

nanoparticles with different morphologies, particle size and band gap by controlling the

reaction conditions, viz. precursor’s concentration, growth temperature and different doping

ratios.

7.2. Future work

In order to further the role of xanthate complexes as single source precursors, future work

could focus on exploring more of the different complexes and using different methods. Four

areas suggested by the work in this thesis for early study are listed below.

Synthesis of single source precursors for manganese telluride and selenide to

compare the chemical, physical, optical and magnetic properties with those of

manganese sulfide.

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The synthesis of Mn doped in different transition metal sulfides using single source

precursor such as Cd bis(ethylxanthato)cadmium(II) and bis(ethylxanthato)zinc(II)

to enhance the chemical, physical and optical properties of the host materials.

Deposition of ternary or quaternary materials using different combination of these

metal complexes and different methods, which may lead to novel phases and hence

novel properties.

Preparation of alloys of the composition Cu2Zn1−xFexSnS4 and Cu2Mn1−xFexSnS4

from ethylxanthate complexes as single source precursors using solvent-less

thermolysis to study the chemical, physical, optical and electrical properties for

photovoltaic applications.

7.3. References

1 M. Al-Shakban, P. D. Matthews, E. A. Lewis, J. Raftery, I. Vitorica-Yrezabal, S. J. Haigh,

D. J. Lewis and P. O’Brien, J. Mater. Sci., 2019, 54, 2315–2323.

2 S. C. Masikane, P. D. McNaughter, D. J. Lewis, I. Vitorica‐Yrezabal, B. P. Doyle, E.

Carleschi, P. O’Brien and N. Revaprasadu, Eur. J. Inorg. Chem., 2019, 2019, 1421–1432.

3 T. Alqahtani, R. J. Cernik, P. O’Brien and D. J. Lewis, J. Mater. Chem. C, 2019, 7, 5112–

5121.

4 L. Almanqur, F. Alam, G. Whitehead, I. Vitorica-Yrezabal, P. O’Brien and D. J. Lewis,

J. Cryst. Growth, 2019, 522, 175–182.

5 E. A. Lewis, P. D. McNaughter, Z. Yin, Y. Chen, J. R. Brent, S. A. Saah, J. Raftery, J. A.

M. Awudza, M. A. Malik, P. O’Brien and S. J. Haigh, Chem. Mater., 2015, 27, 2127–

2136.

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Exploring the Potential of Metal Xanthate Precursors for