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i
Two-dimensional semiconductors for future
optical applications
A thesis submitted in fulfilment of the requirements for the degree of Doctor of
Philosophy
Benjamin John Carey
BEng (Electrical and Electronic Engineering)
School of Engineering
College of Science, Engineering and Health
RMIT University
August 2017
ii
Abstract of Thesis
Field of two dimensional (2D) materials has experienced rapid development and
received considerable attention in recent years. Although various methods of
producing 2D materials have been presented with ample success across numerous
families of materials and for many applications, there are still several factors
limiting their performance in various fields. The field of 2D materials is mostly
focused on stratified crystals, out of which ultra-thin (a single or few monolayers)
planes can be readily extracted. There are however still many unexplored
materials which would be suitable candidates in their 2D morphologies for future
applications. However, due to their naturally occurring non-stratified crystals,
their synthesis and implementation in 2D form have posed significant challenges
to date.
Access to colloidal suspensions of 2D sheets provides a pragmatic avenue for
numerous applications. The processes for obtaining colloidal suspensions can be
scaled up to the industrial scales with relative ease. These processes can be as
simple as applying ultrasound energy into a suitable solvent containing bulk
layered materials. However, there are still several factors which affect the quality
iii
of the sheets within these suspensions and hence their suitability for use in various
applications. This issue is highlighted by the fact that several solvents which assist
high yield production can also inhibit potential applications due to surface bound
solvent residues and breakdown products which can be difficult to remove.
Thus, the first objective of this thesis was to investigate the solvent effects within
the ultrasound exfoliation process for obtaining colloidal suspensions of 2D
materials. For this tungsten disulphide (WS2) was used as it is a well-known
material with a high potential for future implementation across numerous fields.
This objective is to attempt alternative strategies for WS2 exfoliation which
provide high yield, while also providing less detrimental effects on the desired
properties. In this PhD research, comparative evaluation of organic solvents used
in grinding assisted ultrasonic exfoliation revealed the importance of the pre-
exfoliation grinding step and that careful consideration of the solvent(s) used in
both grinding and ultrasonication is necessary. It was shown that such alterations
vastly affect the quality of the resultant suspension. Through these considerations
it was found possible to provide comparatively high yields of unobstructed 2D WS2
Although the outcomes of the previous objective provide insight into the solvent
effects on production and application of colloidal suspensions of 2D
semiconductors, the exfoliation method is limited to materials which exist
naturally with a layered (stratified) crystal system. It is not possible to use this
exfoliation process non-stratified materials to produce 2D sheets. Thus, the
following objective was to develop the processes which may be utilized in the
synthesis of 2D semiconductors from non-stratified materials.
iv
In this PhD work, in order to achieve the synthesis of 2D semiconductors for non-
stratified materials, the self-limiting oxide layer which exists at the interface of
many liquid-metals was utilized. These surface skins are in essence a naturally
occurring 2D material and therefore can offer utilization potential for creating
large area atomically thin coatings. By manipulating the interfacial layer of a
gallium based liquid metal alloy, this PhD research investigates the possibility of
forming suspensions of ultrathin non-stratified semiconductors. The
investigations shown, demonstrated that with relevant chemistry the interfacial
layer could be utilized. A eutectic alloy of gallium, indium, tin and zinc was
repeatedly reacted with a solution of sodium polysulphide to form 2D
semiconducting zinc sulphide (ZnS).
This concept was extended further to the direct deposition of ultrathin metal
oxides. It was shown within this thesis that liquid gallium could be efficiently used
to deposit a 2D layer of gallium oxide. The oxide was then chemically treated in
vapour phase reactions to form 2D semiconducting gallium sulphide (GaS). Using
this concept GaS based optical and electronic devices were developed on the
wafer-scale demonstrating the potential for this concept.
In conclusion, this Ph.D. research resulted in several new advances and novel
synthesis paths in the field of 2D materials and the methods and outcomes
presented herein will serve as launching points for many future investigations into
the synthesis and application of 2D materials.
v
Declaration
I certify that except where due acknowledgement has been made, the work is that
of the author alone; the work has not been submitted previously, in whole or in
part, to qualify for any other academic award; the content of the thesis is the result
of work which has been carried out since the official commencement date of the
approved research program; any editorial work, paid or unpaid, carried out by a
third party is acknowledged; and, ethics procedures and guidelines have been
followed.
I acknowledge the support I have received for my research through the provision
of an Australian Government Research Training Program Scholarship.
Benjamin John Carey
25th of August 2017
vi
Acknowledgements
I would like to express my gratitude toward all the people whose help, guidance
and support have made this Ph.D. thesis Possible
Firstly, I would like to thank Professor Kourosh Kalantar-zadeh providing me with
the opportunity to undertake this work and for offering the guidance and
motivation to follow it to completion.
Secondly, I would like to thank my secondary supervisors Dr. Torben Daeneke and
Dr. Jianzhen Ou. Thank you for all the guidance, support, and education you have
given me. Without you, this thesis would not have been possible.
Thirdly, I would also like to extend my appreciation to the CSIRO specifically my
past CSIRO supervisor Professor Serge Zhuiykov and my new CSIRO supervisor Dr.
Anthony Chesman who have always offered support and assistance in any way
possible.
Many thanks to the numerous research facilities and technical staff at RMIT
University including the Micro-Nano Research Facility (MNRF), the former
vii
Microelectronics and Materials Technology Centre (MMTC), the RMIT microscopy
and Microanalysis Facilities (RMMF) and the Centre of Excellence for Nanoscale
BioPhotonics (CNBP).
I would like to thank the technical staff, researchers and students of which there
are too many to name, both from within RMIT and from other institutions. Thank
you for all the help and support that you have offered. A special mention is due
to professor Salvy Russo for assistance with density function theory computation.
I would like to thank my peers and friends, who have helped keep morale thought
the years, Mr. Christopher Harrison, Ms Emily Nguyen, Ms. Rhiannon Clarke, Mr
Naresh (Shaun) Pillai, Mr. Paul Atkin, Mr. Nam Ha, Mr Steffen Schoenhardt and Mr
Peter Thurgood for all the support, heated discussions and distractions throughout
the last years. You have made this Ph.D. tolerable.
Lastly, I would like to thank my family for all the love and support, especially my
parents who have always offered all the support and assistance they can. And to
my fiancé who has never given up on me.
viii
Table of Contents
Abstract of Thesis .................................................................................................... ii
Declaration ............................................................................................................... v
Acknowledgements ................................................................................................ vi
Table of Contents .................................................................................................. viii
List of Figures .........................................................................................................xiii
List of Tables ........................................................................................................ xxiv
Abbreviations ........................................................................................................ xxv
Chapter 1 ................................................................................................................. 1
Motivations and Objectives ..................................................................................... 1
1.1 Motivations ....................................................................................................... 1
1.2 Organisation of thesis ...................................................................................... 4
1.3 References ......................................................................................................... 5
Chapter 2 ................................................................................................................. 7
Literature Review ..................................................................................................... 7
2.1 Two dimensional (2D) materials .................................................................... 7
2.1.1 Tungsten disulphide ......................................................................... 10
ix
2.1.2 Gallium sulphide ............................................................................... 11
2.2 Liquid exfoliation of stratified crystal .......................................................... 12
2.3 Potential of liquid metal as a reaction medium for synthesising non-
stratified 2D materials ......................................................................................... 15
2.3.1 High yield synthesis of colloidal materials .................................... 17
2.3.2 Large scale printing .......................................................................... 18
2.4 This Ph.D. work ............................................................................................... 19
2.4.2 Chapter 4 ........................................................................................... 20
2.4.3 Chapter 5 ........................................................................................... 20
2.4.4 Chapter 6 ........................................................................................... 20
2.5 References ....................................................................................................... 21
Chapter 3 ............................................................................................................... 30
Two Solvent Grinding Sonication Method for the Synthesis of Two-dimensional
Tungsten Disulphide Flakes ................................................................................... 30
3.1 Introduction .................................................................................................... 30
3.2 Experimental Section ..................................................................................... 32
3.2.1 Materials ............................................................................................ 32
3.2.2 Synthesis of Two-dimensional WS2 flakes ..................................... 32
x
3.3.3 Measurement and Characterisation .............................................. 32
3.3 Results and Discussion .................................................................................. 34
3.4 Conclusions ..................................................................................................... 52
3.5 References ....................................................................................................... 54
Chapter 4 ............................................................................................................... 56
A Liquid Metal Flow-Reaction for the Synthesis of Ultra-Thin Zinc Sulphide ........ 56
4.1 Introduction .................................................................................................... 56
4.2 Experimental Section ..................................................................................... 58
4.2.1 Materials ............................................................................................ 58
4.2. Preparation of Galinstan .................................................................... 58
4.3.3 Preparation of Zn-Galinstan Alloy: ................................................. 59
4.3.4 Preparation of Polysulphide Solution ............................................ 59
4.3.5 Synthesis of ZnS ................................................................................ 59
4.3.6 Characterisation ................................................................................ 60
4.3 Results and Discussion .................................................................................. 61
4.4 Conclusions ..................................................................................................... 78
4.5 References ....................................................................................................... 79
xi
Chapter 5 ............................................................................................................... 81
Wafer Scale Two Dimensional Semiconductors from Printed Oxide Skin of Liquid
Metals .................................................................................................................... 81
5.1 Introduction .................................................................................................... 81
5.2 Experimental section ..................................................................................... 84
5.2.1Materials ............................................................................................. 84
5.2.2 Preparation of materials .................................................................. 84
5.2.3 Synthesis of 2D GaS .......................................................................... 84
5.2.4 Fabrication of structures ................................................................. 85
5.2.5 Fabrication of transistors ................................................................. 86
5.2.6 Characterisation: .............................................................................. 86
5.2.7 Computational analysis of band structure .................................... 88
5.3 Results and Discussion .................................................................................. 89
5.4 Conclusions ................................................................................................... 111
5.5 References ..................................................................................................... 113
Chapter 6 ............................................................................................................. 116
Conclusions and Future Work.............................................................................. 116
xii
6.1 Conclusions ................................................................................................... 116
6.1.1 Stage 1 .............................................................................................. 117
6.1.2 Stage 2 .............................................................................................. 117
6.1.3 Stage 3 .............................................................................................. 118
6.2 Future Work .................................................................................................. 119
6.3 Journal Publications ..................................................................................... 120
6.3.1 First Author Publications ............................................................... 120
6.3.2 Co-author Publications .................................................................. 120
xiii
List of Figures
Figure 2.1 Comparison of the band gap values for different 2D semiconductor
material families studied so far. The crystal structure is also
displayed to highlight the similarities and differences between the
different families. The horizontal degraded bars indicate the range
of band-gap values that can be spanned by changing the number
of layers, straining or alloying.
8
Figure 2.2 Stick and ball representation of GaS crystal structure. Displaying
the top view (top) and side view (bottom). The blue balls represent
Gallium atoms and yellow Sulphur
11
Figure 2.3 Sonication-assisted exfoliation. The layered crystal is
sonicated in a solvent, resulting in exfoliation and nanosheet
formation. In “good” solvents—those with appropriate
surface energy—the exfoliated nanosheets are stabilized
against reaggregation. Otherwise, for “bad” solvents
reaggregation and sedimentation will occur. This mechanism
also describes the dispersion of graphene oxide in polar
solvents, such as water. NB, solvent molecules are not shown
in this figure.
13
Figure 3.1 (a) Image of the 2D WS2 flakes suspensions prepared with NMP (i),
ACN (ii), ethanol/water (iii) and the two-solvent method (iv). (b)
35
xiv
UV-Vis absorption spectra of the 2D WS2 suspensions with the A,
B and C excitons marked. Note both the NMP and the two solvent
(ACN-ETH/Water) solutions were diluted 1 part in 4 from the
optical image for the UV-Vis measurement. (b) & (c) Raman
spectra of 2D WS2 flakes. Spectra have been normalized to the
𝐴1𝑔(𝛤) phonon peak. (c) The position of the 2𝐿𝐴(𝑀) and 𝐴1𝑔(𝛤)
phonons (from bulk WS2) have been marked and spectra have
been vertically offset for clarity.
Figure 3.2 (a) Photograph and (b) UV-Vis absorption spectra of WS2
suspension after the process involved grinding in ethanol/water
(50/50) and sonication in ACN.
39
Figure 3.3 Sample AFM images of the exfoliated WS2 flakes deposited on Si
substrates.
40
Figure 3.4 (a-d) Statistical distribution of WS2 flake thickness analysed
from AFM imagining. (e-h) statistical distribution of lateral
dimensions analysed from TEM imagining. The blue overlay is
an approximation of flake size based on DLS Approximation
of 2D flake lateral dimensions as demonstrated by Lotya et al.
13 (i-l) TEM images of the WS2 flakes synthesized with
different solvent methods.
41
Figure 3.5 Conductive Atomic Force Micrograph of WS2 flakes exfoliated via
the two-solvent method on Au substrate representing both (a)
43
xv
height and (c) tip current of the same area. B and D are the profiles
the white dashed line in A and B respectively.
Figure 3.6 (a) XRD of bulk WS2 powder and WS2 powder after grinding
(pre-exfoliation). The results have been normalized to the
002 peak, offset vertically and the 002 peak has been clipped
for clarity. (b) Paracrystallinity factor (g) of the vertical crystal
planes for bulk WS2 powder (black) and WS2 powder after
grinding in NMP (green), ACN (purple) and ethanol/water
(50/50, red)
44
Figure 3.7 High resolution transmission electron micrograph (HRTEM) of the
exfoliated WS2. Insert is a selected area electron diffraction (SAED)
image of the same section.
49
Figure 3.8 Thermal gravimetric analysis (TGA) of the dried 2D WS2 flakes
prepared with both only NMP and ACN-ethanol/water two
solvent methods tested in nitrogen gas. The boiling points of
both NMP and water are marked (202°C and 100°C at
standard pressure)
50
Figure 4.1 Images are taken from a video recording of the reaction with
graphical representation of the surface of the liquid metal.
62
Figure 4.2 Depiction of the synthetic method. (a) Alloying of liquid
Galinstan and Zn powder. (b) Flow-reaction between Zn-
64
xvi
Galinstan alloy and polysulphide (i) Dispersion of ZnS and by-
products. (ii) ZnS forming on the interface between Na2Sx
solution and the alloy. (c) Washing process displaying (i)
solution before washing and iodine treatment, (ii) solution
after centrifugation (iii) resuspended ZnS in DMF.
Figure 4.3 Morphology of the 2D ZnS. (a) TEM image of the ZnS flakes.
(b) HRTEM image of the ZnS which lattice dimensions marked,
with fast Fourier transform (FFT) pattern (inset). (c) Statistical
distribution of the lateral dimension of the ZnS flakes. (d)
AFM micrograph of a flake with height profile (insert) along
the blue line.
66
Figure 4.4 TEM images of 2D ZnS sheets produced via the reaction of
Na2Sx with the Zn alloy.
68
Figure 4.5 TEM images (a-c) and HRTEM (d, FFT insert) of ZnS produced
with a modified reaction chamber where the Na2Sx is not
pumped directly into the Galinstan from underneath, but
instead from the side.
69
Figure 4.6 TEM image (a) of ZnS produced via a modified reaction
chamber where a 0.1 mol L-1M Na2S solution is also
substituting the polysulphide solution and is not pumped
directly into Galinstan from underneath, but instead from the
side. (b & c) An AFM image and profile, which demonstrate
70
xvii
that the thickness of the flakes is not altered by a change in
the reaction protocol.
Figure 4.7 XPS spectra of the (a) S 2p and (b) Zn 2p regions of the ZnS
flakes. Peak fitting is demonstrated for all the relevant shells.
71
Figure 4.8 Figure 4.8. Schematic representation of the simplified flow
dynamics within the liquid metal reactor. F, V, A, T, D, t, and
N represent the volume of the flow rate, the volume of
bubbles, the surface area of bubbles (i.e. interfacial area), the
thickness of ZnS form, density of ZnS, time of reaction, and
the total number of bubbles during the reaction, respectively.
72
Figure 4.9 TGA of the centrifuged ZnS solution demonstrating weight
reduction relative to (a) temperature and (b) time. The red
dashed line indicates the solid weight of the dried ZnS which
was utilized for the yield calculations. (a, inset) The change in
weight as the N2 flow is replaced with an air flow at 600°C.
74
Figure 4.10. Spectroscopic characterisation of the ZnS suspensions. (a) PL
emission spectrum excited at 275 nm with simplified
electronic band structure (inset). The red lines indicate the
conduction and valence band and the black line is the fermi
level. (b) UV-Vis absorption spectrum with Tauc plot (inset).
75
xviii
Figure 4.11 Valence band analysis from XPS with line fitted on the valance
band edge for clear determination of energy difference
between the valence band maxima and the Fermi level.
77
Figure 5.1. GaS representation and printing process of the 2D layers: stick and
ball representation of GaS crystal: (a) side view of bilayer GaS,
showing a unit cell c = 15.492Å made of two GaS layers. (b) Top
view of the GaS crystal. The GaS crystal lattice is composed of Ga-
Ga and Ga-S covalent bonds, which extend in two dimensions,
forming a stratified crystal made of planes that are held together
by van der Waals attractions. The Ga atoms are green and the S
atoms are blue. (c and d) Functionalisation of the substrate with
FDTES changes the contact angle between a Ga drop and the
substrate by 12.9°. The SiO2 substrate becomes more hydrophobic
and thus resists wetting by liquid Ga. (e to h) Schematics of the
synthesis process for patterning 2D GaS via printing the skin oxide
of liquid Ga. (e) Lithography process for establishing the negative
pattern of the photoresist. (f) Covering the exposed area of the
substrate with vaporised FDTES. (g) Placing Ga liquid metal and
removing it with soft PDMS that leaves a cracked layer of Ga oxide.
(h) Two concurrent steps of chemical vapour treatment: firstly,
GaCl3 layer is formed via exposure to HCl vapour and secondly,
sulphurisation by exposure to S vapour forming GaS.
89
xix
Figure 5.2 (a) AFM image of cracked layer of gallium oxide and corresponding
profile (b).
91
Figure 5.3 Analysis of 2D In2S3 synthesised via printing of liquid indium. (a)
Raman spectrum, (b) Indium 3d region of X-ray photoelectron
spectrum (XPS) and (c) AFM image including profile (d) along the
blue line shown in (c).
93
Figure 5.4 Material characterisations of printed GaS: (a) X-ray Diffraction
(XRD) of printed GaS film (after multiple printing steps) with XRD
of the printed oxide skin and SiO2/Si substrate for comparison and
GaS planes indicated (traces have been normalised and offset for
clarity). (b) Raman spectrum of the 2D GaS with GaS vibrational
peak shifts indicated. (c & d) XPS of the 2D GaS for the regions of
interest (c) Ga 3d and (d) S 2p.
94
Figure 5.5 (a-c) XPS study of GaS film displaying (a) survey spectrum, (b) Cl 2p
and (c) O 1s regions of the spectrum. (d-f) EDX of printed 2D GaOx
film (d), the film after treatment with HCl (e) and after
sulphurisation (f).
96
Figure 5.6 XPS patterns of the printed metal oxide film before (a-e) and after
(f-j) treatment with HCl vapour. (a & f) Survey, (b & g) Ga 3d, (c &
h) Ga 3s, (d & i) Cl 2p and (e & j) O 1s regions.
97
xx
Figure 5.7 Morphology and characteristics of the printed 2D films: (a) AFM
image with (b) height profile) of the printed 2D GaS layer
confirming bilayer deposition. The profile is offset to the
substrate’s surface. (c&d) Confocal PL map shows a patterned area
made of 2D GaS (green) on a SiO2/Si substrate (black)
demonstrating a near-uniform 2D layer is obtained in a large area
along with the PL spectra from areas noted on map (c) patterns 1
and 2). Due to the excitation wavelength limitation of confocal PL,
the deep trap emissions are used for assessing the uniformity of
the PL pattern. (e) PL emission spectra demonstrating
contributions of the interband transitions (red) and deep trap
recombinations (black). The deep trap emission spectrum is scaled
by a factor of 10 for clarity. (f) HRTEM of a flake mechanically
scratched from the printed GaS film.
98
Figure 5.8. AFM image (a) of 2D GaS film and statistical distribution of the
measured pixel heights (b) of the substrate (blue) and film (red)
within the areas in the blue and red squares, respectively. Scale
bar in (a) is 5 µm.
100
Figure 5.9 Statistical Raman analysis of the 2D GaS films. Raman line scan
analysis. (a) Optical image, the line scan was performed on the
black line indicated. (b) Contour plot with phonon modes
101
xxi
indicated. (c) Relative intensity of the E12gphonon peak with
standard error indicated.
Figure 5.10 Raman spectra (a) of 2D GaS spaced at approximately 1 mm
intervals. (b & c) Analysis of crystal grains of the printed 2D GaS
film featuring (b) a HRTEM image of a grain boundary and (c) a
map of grain boundaries determined via polarised confocal PL as
has been previously demonstrated by Pan et al. Different shades
of grey indicate areas that luminesce at different polarisation
angles which are associated with crystal domains and their lattice
orientation within the 2D GaS layer.
102
Figure 5.11 (a) HRTEM of a GaS flake scratched from the printed film.
Structural defects are visible in the crystal, which are mainly
ascribed to lattice dislocations (example in the red square). Inset:
magnified image highlighting the dislocation of the crystal lattice.
(b) SCLC measurement of 2D GaS. Ohmic and trap filled limited TFL
regions are marked with their respective colours.
104
Figure 5.12 Optical and electronic properties of 2D GaS. Hybrid density
functional theory (DFT) computation of the band structures in
bilayer (a) and bulk (b) GaS. (c) Tauc plot used for determining the
electronic band transition with simplified electronic band diagram
(inset). Red lines are the valence and conduction band edges, black
is the Fermi level and blue is the location of the deep trap band.
105
xxii
(d) PESA demonstrating the valence band maximum (VBM) energy
is −5.15 eV. (e) XPS valance analysis revealing an energy difference
of 1.8 eV between the VBM and Fermi level. (f) PL decay with fitted
exponential (red) and mean excitonic decay (blue) time indicated.
Figure 5.13 Hybrid DFT computation of the band structures in (a) tri-layer,
and (b) four-layer GaS.
106
Figure 5.14 Schematic representation for device fabrication. (a) Clean
SiO2/Si substrate. (b) Tungsten (W) film e-beam deposited onto
the substrate. (c) Photoresist is deposited and then exposed
through a photolithography mask. (d) Remaining photoresist
after image reversal and developing. (e) Immersion of the of the
unprotected W in the etchant. (f) Sample after etching and lift-
off. (g) Deposition of liquid Ga. (h) Formation of 2D GaOx
between electrodes after Ga is removed. (i) Exposure to HCl
vapour. (j) GaCl3 film between electrodes. (k) Exposure to S
vapour. (l) Final device. Extra photolithography steps can also
be included as presented in Figure 5.1.
107
Figure 5.15 Characterisation of 2D GaS FETs. (a) Schematic representation
of back gated GaS FET with tungsten/WS2 electrodes featuring
band energy diagram. (b) Raman spectrum of the W electrodes
after sulphurisation. Phonon modes of WS2 are indicated
demonstrating the growth of WS2 on the skin of the electrodes.
108
xxiii
(c) optical images of the fabricated devices and electrode gap
(inset). The scale bars are 500 µm and 20 µm respectively. (d)
I/V characteristics of the FET at different gate voltages. (e) I/V
phototransistor characteristics with no gate bias.
Figure 5.16. Statistical analysis of photodetector devices. (a) Responsivity of
each individual photodetector device and (b) distribution of
responsivities. (c) On/Off ratio of each device and (d)
distribution of ratios.
110
xxiv
List of Tables
Table 3.1. Summary of Raman shift peak intensities and locations for
the 2𝐿𝐴(𝑀), 𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) phonons (intensities are
normalized to the A1g(𝛤) peak).
38
Table 3.2 Summary of vertical plane XRD results for the comparison of the
crystallographic effects of grinding WS2 with different solvents.
46
Table 4.1 Thermodynamics of the compounds relevant to the
interfacial reaction.
63
Table 4.2 Quantum yield estimations based on comparative analysis
with quinine sulphate.
76
Table 5.1 Comparison of established methods for deposition of 2D materials. 112
xxv
Abbreviations
2D Two-Dimensional
ACN Acetonitrile
AFM Atomic Force Microscopy
Au Gold
Bi Bismuth
CBM Conduction Band Minimum
Cl Chlorine
CVD Chemical Vapour Deposition
DFT Density Functional Theory
DI De-Ionized
DLS Dynamic Light Scattering
DMF N,N-dimethylformamide
EDX Energy Dispersive X-ray Spectroscopy
FDTES 1h,1h,2h,2h-perfluorodecyltriethoxysilane
FET Field Effect Transistor
FFT Fast Fourier Transform
Ga Gallium
GaCl3 Gallium Chloride
GaOx Gallium Oxide
GaS Gallium Sulphide
HBr Hydrobromic acid
HCl Hydrochloric acid
HRTEM High Resolution Transmission Electron Microscopy
I Iodine
In Indium
xxvi
In2S3 Indium Sulphide
IPA Isopropyl Alcohol
MoS2 Molybdenum Disulphide
N2 Nitrogen
Na Sodium
NMP N-methyl-2- Pyrrolidine
O Oxygen
PESA Photoelectron Spectroscopy in Air
PF Paracrystallinity Factor
PL Photoluminescence
QD Quantum Dots
S Sulphur
SCLC Space Charge Limited Current
Se Selenium
Si Silicon
SiO2 Silicon Dioxide
Sn Tin
STEM Scanning Transmission Electron Microscopy
Te Telurium
TEM Transmission Electron Microscopy
TFL Trap Filled Limited
TGA Thermo Gravimetric Analysis
UV Ultra Violet
W Tungsten
WS2 Tungsten Disulphide
XPS X-ray Photoelectron Spectroscopy
xxvii
XRD X-ray Diffraction
Zn Zinc
ZnO Zinc Oxide
ZnS Zinc Sulphide
1
Chapter 1
Motivations and Objectives
1.1 Motivations
Ever since the isolation of the atomically thin form of carbon known as
graphene in 20041 the properties and applications of two-dimensional (2D)
materials have been the focus of consistent and ongoing studies. 2D
materials which consist of atomically thin sheets have received much
attention as they present many promising avenues for future technological
development due to their diverse and remarkable characteristics.2-5 These,
materials can be useful for many future applications in industries such as
electronics, optics, and chemical engineering.
However, there are still many barriers to the adoption of using 2D
semiconductors in the electronics and optics industries stemming from the
challenges involved in large scale production of these materials. In
particular 2D semiconductors could potentially revolutionise these
2
industries due to their inherent material properties4. Many methods to
produce 2D materials have been demonstrated to date, both top-down
(exfoliation of sheets from a layered bulk crystal) and bottom up
(preferential growth of atomically thin layers) however, there are many
more challenges which must be overcome before large scale
industrialisation can be realised.
This thesis will discuss various synthesis approaches, initially investigating
the process and problems of the exfoliation of metal-chalcogenides from
layered crystals to produce 2D semiconducting materials. There are still
many knowledge gaps in the literature which hamper the potential for the
development of commercial electronic and optical devices based up liquid
exfoliated 2D semiconductors.
❖ The first inquisition of this Ph.D. work is to improve on such liquid
exfoliation. In hope of finding methodology capable of efficient and
effective production as well as an investigation into the fundamental
factors affecting this process.
The question of ‘what if the desired 2D material does not exist in a layered
crystal system?’ unavoidably arises. If there is no stratified form of the
desired material then exfoliation techniques are consequently not possible.
Therefore, a new medium is required to create these ultrathin layers of the
material desired. This medium could either exist as atomically thin
precursors which can be chemically altered or as a reaction medium which
facilitates the 2D formation.
3
The requirement of a new medium leads to the consideration of utilising naturally
occurring interfacial compounds such as the oxide layer formed upon the surface of
certain metals. Many metals such as aluminium, gallium, indium and tin will form an
atomically thin and self-limiting layer of oxide when exposed to air.6,7 These
atomically thin layers are in essence naturally occurring 2D materials. Furthermore,
unlike chemical vapour deposition (CVD) the compounds grown on the surface of
these metals hypothetically do not require nucleation points.8,9 As a result, if the
surface skin of these materials can be utilised it could produce large surfaces that are
atomically thin and with the least number of boundaries that can be hypothetically
obtained
Extending upon this concept, If the metal precursor is liquid then this ultrathin layer
forms at the interface between the metal and the environment and in cases such as
gallium and gallium based alloys also have very low binding energy to the bulk metal7
resulting in an atomically thin layer which can be separated from the bulk. This leads
to the consideration of low melting point metal such as gallium, indium and gallium
based alloys as prime candidates for investigation as a new medium allowing access
to various 2D metal compounds.
❖ The later sections of this Ph.D. work will be a quest to explore this idea for
potentially using liquid metal as a reaction medium and precursor for the
synthesis of 2D semiconductors for future optical devices.
4
1.2 Organisation of thesis
The primary objectives of this Ph.D. research are to investigate and
demonstrate new understandings of the synthesis of 2D semiconductors
and to demonstrate potential for future optical technological progress.
Chapter 2 of this thesis presents a review of current relevant literature and
research on the field of 2D materials synthesis and metal-chalcogenides.
This chapter will also serve to provide background knowledge and provide
further rationale to the reader on the relevant 2D materials discussed in
later chapters and on the metal/metal alloys used therein.
Chapter 3 of this thesis investigates grinding assisted liquid phase
exfoliation of tungsten disulphide (WS2) and provides an understanding of
this process specifically the role the solvent plays during exfoliation. This
chapter will also discuss a potential solution to many of the issues involved
in liquid phase exfoliation by presenting a ‘two-solvent grinding sonication
method’.
Chapter 4 of this thesis presents the concept and investigation of utilising a
liquid metal alloy as a reaction medium for the production of 2D zinc
sulphide (ZnS). This chapter will demonstrate the viability of using a liquid
metal alloy (in this case an alloy of zinc, gallium, indium and tin) to create
2D sheets of a material which does not have a layered structure and may
offer access to a simple, scalable solution for synthesis with the devised
‘flow reactor’ system.
5
Chapter 5 of this thesis investigates another potential step toward
producing high quality, large area 2D sheets in a scalable method via the
printed oxide skin of liquid gallium. This chapter presents an investigation
of producing true wafer scale 2D semiconductors and investigates their
potential as basic optical and electronic devices, including transistors and
photodetectors.
Chapter 6 of this thesis will contain a discussion of the conclusions of the research
performed in chapters 3, 4, and 5. This chapter will also discuss the future
endeavours to be undertaken based on the work present in this thesis.
1.3 References
1 Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, 666-669 (2004).
2 Xu, M., Liang, T., Shi, M. & Chen, H. Graphene-like two-dimensional materials. Chem. Rev. 113, 3766-3798 (2013).
3 Novoselov, K. S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197-200 (2005).
4 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 7, 699-712 (2012).
5 Miro, P., Audiffred, M. & Heine, T. An atlas of two-dimensional materials. Chem. Soc. Rev. 43, 6537-6554 (2014).
6 Downs, A. J. Chemistry of aluminium, gallium, indium and thallium. (Springer Science & Business Media, 1993).
6
7 Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS App. Mater. Interfaces 6, 18369-18379 (2014).
8 Tang, S. et al. Nucleation and growth of single crystal graphene on hexagonal boron nitride. Carbon 50, 329-331 (2012).
9 Ling, X. et al. Role of the seeding promoter in MoS2 growth by chemical
vapor deposition. Nano letters 14, 464-472 (2014).
7
Chapter 2
Literature Review
2.1 Two dimensional (2D) materials
2D materials which comprise of atomically thin sheets of material present
many promising avenues for future technologies due to their diverse and
remarkable characteristics1-5. These materials offer a wide range of
exploitable bandgaps for optical applications6 ranging from 2D conductors
(e.g. graphene2,7), semiconductors3,6 to wide bandgap insulators.8
8
Figure 2.1. Comparison of the band gap values for different 2D
semiconductor material families studied so far. The crystal structure is also
displayed to highlight the similarities and differences between the different
families. The horizontal degraded bars indicate the range of band-gap
values that can be spanned by changing the number of layers, straining or
alloying. [Reproduced with permission: Castellanos-Gomez, Nature
Photonics 10, 202–204 (2016)6]
2D semiconductors exist across many different families of crystal systems
including monoelemental pnictogens such as 2D black phosphorous known
as phosphorene.9 and recently 2D forms of arsenic, antimony, and
bismuth10 however, these forms still require much more fundamental
studies. There are also many forms of bi-elemental metal compounds of
9
various bandgaps (displayed in Figure 2.1.) which have received significant
attention. Of these metallic compounds chalcogenides present many great
candidates. 2D metal chalcogenides include transition metal
dichalcogenides (TMDs) of the form MX2 (where M = Mo, W, Re etc, and X
= S, Se, Te)5 and post-transition metal chalcogenides (PTMCs) which come
in many forms as follows:
❖ MX (where M = Ga, In, Sn and X = S, Se)11-13
❖ MX2 (where M = Sn and X = S, Se) 14
❖ M2X3 (where M = Ga, In, Sn, Bi and X = S, Se)15-17
TMDs and PTMC have recently attracted significant attention for many
applications including electronic and optical devices such as transistors11,18
and photodetectors,19-21 sensors for gas14,22 and biosensors,23 catalysis,24,25
and energy storage.26,27
2D materials can be produced using several different methodologies such
as exfoliation techniques of bulk stratified crystals (compromising of
covalently bonded sheets held together via van der Walls forces) either
through mechanical plying of the layers7,28 or via agitation in liquid phase
through ion intercalation,29,30 laser ablation,31,32 or ultrasound.33,34 As well
as grown directly onto substrates via methods such as chemical vapour
deposition,35-37 atomic layer deposition,38 or thermolysis.39
❖ Exfoliation from bulk crystals
➢ Mechanical exfoliation
➢ Liquid-phase exfoliation (LPE)
10
▪ Ionic intercalation
▪ Laser ablation
▪ Ultrasonic exfoliation
❖ Growth of ultrathin layers
➢ Chemical vapour deposition (CVD)
▪ Metal-organic CVD
➢ Atomic layer deposition
➢ Thermolysis
2.1.1 Tungsten disulphide
One particularly promising 2D TMD is tungsten disulphide (WS2).5,40 In
comparison to many the other 2D TMDs, 2D WS2 offers better chemical
stabilities, while featuring certain favourable electronic characteristics.41,42
There are now well-established methods for obtaining durable p and n type
semiconducting 2D WS2 that makes it amongst the most suitable 2D TMDs
for developing future integrated circuits, optical components, catalysts as
well as analytical and biological systems.3,43 Producing high quality, large
surface area and relatively pure 2D WS2 flakes utilising a scalable method is
still a challenge and further research is necessary to investigate the
suitability of liquid phase exfoliation techniques for this purpose.
11
2.1.2 Gallium sulphide
A representative example of the MX form of PTMCs is gallium (II) sulphide (GaS),
which has a hexagonal crystal structure with a unit cell of a = b = 3.587 Å, c = 15.492
Å (c is equal to two fundamental layers of GaS as shown in Figure 2.2). 2D GaS has
been recently explored for applications in transistors11, energy storage44,
optoelectronics20, gas sensing45, and nonlinear optics46.
Figure 2.2. Stick and ball representation of GaS crystal structure. Displaying the top
view (top) and side view (bottom). The blue balls represent Gallium atoms and yellow
Sulphur
The electronic band structure of 2D GaS is of particular interest.4,47,48 Especially,
bilayer GaS, which has a thickness equivalent to c of the unit cell, is an indirect
bandgap semiconductor with a conduction band minimum (CBM) at the M point in
bouillon space and an associated bandgap of ~3.1 eV and a direct transition at the
point with only a ~210 meV wider bandgap.48 Due to this modest energy difference
between CBMs at the M and points, free carriers can be exchanged between
12
valleys via room temperature thermal excitation48. Therefore, significant radiative
exciton decay occurs, which produces photoluminescence (PL)45.
Significant changes to the band structure of GaS occur when the number of layers is
increased. Both direct and indirect bandgaps narrow simultaneously, due to the
strong interlayer interactions along the c axis. For bulk GaS, the bandgap is decreased
by 0.4-0.6 eV in comparison to that of bilayer11,47,49.
2.2 Liquid exfoliation of stratified crystal
Liquid phase exfoliation can be used as a high yield, scalable method for
producing 2D TMDs, however, the relevant outcomes from this process are
highly sensitive to the procedures’ parameters.31-35,50,51 A vast host of liquid
phase exfoliation techniques have been introduced. Such as Intercalation,
29,50 laser ablation,31,32 and ultrasound exfoliation.33,34 With grinding
assisted ultrasonic exfoliation being one of the most attractive methods due
to its high yield while avoiding the use of hazardous reagents such as
butyllithium or hydrazine.29,30
13
Figure 2.3. Sonication-assisted exfoliation. The layered crystal is sonicated
in a solvent, resulting in exfoliation and nanosheet formation. In “good”
solvents—those with appropriate surface energy—the exfoliated
nanosheets are stabilized against reaggregation. Otherwise, for “bad”
solvents reaggregation and sedimentation will occur. This mechanism also
describes the dispersion of graphene oxide in polar solvents, such as water.
NB, solvent molecules are not shown in this figure. [Reproduced with
permission: Nicolosi, et al. Science 340, 1226419 (2013). 50]
Many studies have investigated the importance of solvents50,52-54 and
surfactants50,55,56 during exfoliation via sonication. During ultrasonic
exfoliation, the solvent selection plays an important part since physical
properties such as boiling point,53 surface tension and energy,34,42 along
with solubility parameters54 affect the process and hence the resulting
morphology and stability of the 2D flakes as well as the electronic and
optical properties of the suspended flakes.52,57 Of these many studies have
recommended N-methyl-2-pyrrolidone (NMP), for its high production yield.
34,51,54,57,58 However, residual NMP solvent residues can often be found on
the resulting 2D materials due to its relatively high boiling point (202°C) and
surface tension (40.79 mJ m2 at 20°C),59 even when post processing
14
methods aiming at the solvent removal are used, this surface bound solvent
residue often retards the desired qualities of the material due to the
additional expression of NMP’s.54 Furthermore, in the presence of oxygen
NMP forms many breakdown products even at moderate temperatures (as
low as 320k60).
The most common of these products60,61
• N-methyl succinimide
• formyl-pyrrolidone
• N-hydroxymethyl pyrrolidone
• 5-hydroxy-N-methyl pyrrolidone
• 2-pyrrolidone
• Methylamine
• Formamide
• Acetamide
• N-methyl formamide
• N-ethyl acetamide
• Dimethyl acetamide
Residues and degradation products from NMP and other
solvents/surfactants are detrimental to many applications of 2D
semiconductors such as catalysis and sensing which require a pristine
surface.24,25,58 Furthermore the electronic properties of the resulting flakes
are altered by organic residues.62,63
15
There are still many challenges and unknowns when utilising sonication
exfoliation methodologies and further work is necessary in order to
understand the process and improve the quality of the resulting 2D
semiconducting flakes.
2.3 Potential of liquid metal as a reaction medium for
synthesising non-stratified 2D materials
The element gallium (Ga) was first predicted by Mendeleev and isolated by
de Boisbaudran in 1875.64 Ga is a metal with a melting temperature just
below 30°C. When exposed to an ambient environment containing oxygen,
it establishes an atomically thin, self-limiting oxide on its surface65,66 which
has been shown to comprise of only one-unit cell.67
Reports from as early as the 1960’s show that transition and post-transition
metals such as zinc,68 antimony,69 bismuth,69 and palladium70 have been
alloyed with gallium and used in many applications such as:
• medicine71-73
• dentistry70,74
• soldering75
• soft electronics76-78
• antennas79,80
• sensors81
• coolants82
16
• microfluidics83
Many alloys of gallium have even lower melting temperatures than that of
gallium itself. The most recognized of these room temperature liquid metal
alloys include:
❖ Eutectic gallium-indium (Egain; 85:15 wt% Ga:In) melting point
~15°C.84,85
❖ Eutectic gallium-indium-tin (Galinstan®; 68:22:10 wt% Ga:In:Sn) melting
point ~−19°C.86,87
Gallium and its alloys are gaining popularity for many applications
previously held by mercury due to their low toxicity and their low vapor
pressure. 65,88 Ga also has a low bulk viscosity (approximately twice that of
water).89 Furthermore, with such low melting temperatures and boiling
points of >1300°C these alloys have tremendous dynamic range.79
One of the main considerations regarding Ga based alloys has been the
heterogeneity, which depends on the thermodynamics of the elements
within the alloys.90 Other important factors are the properties that emerge
as a result of the interfacial characteristics of the self-limiting surface
compounds. Such properties depend on whether the alloyed metal takes
part in the formation of the surface layer or whether gallium remains as the
dominant compound forming metal at the interface.79,90,91
17
2.3.1 High yield synthesis of colloidal materials
Hypothetically if the self-limiting interfacial layer may also be able to host
compounds other than oxides, for instance, chalcogenide compounds,
provided that the environment consists of chalcogen rich reagents, and if
the atomically thin interfacial layer could be controlled to be predominantly
composed of the alloyed additive rather than gallium, liquid metal based
reactions could provide access to 2D crystals of a hypothetically immense
number of metal compounds.
If gallium alloys can also be utilized as a liquid reaction medium by delivering
co-alloyed metals as the predominant interfacial component, this synthetic
approach could lead to an entirely new avenue for production of 2D metal
compounds. It has been previously shown that the surface formation is
dependent upon the enthalpy and Gibbs free energy of the potential
products.66,92
Also based on the previous hypothesises, by replacing the ambient oxygen
with chalcogen rich reagents, potentially 2D metal chalcogen can be
obtained. To expand this concept further, the approach would be especially
attractive for the synthesis of 2D compounds that do not occur as a
stratified crystal naturally and are thus inaccessible via traditional
exfoliation techniques.34,50,53,93
Hypothetically if a metal such as Zn was alloyed into Galinstan, the surface
formation should comprise of zinc compounds based on the
18
thermodynamic considerations.94 A chemical flow-reaction causing
repeated exposure could utilise this for a high yield synthesis of Zn based
ultrathin materials.
Sodium sulphide (Na2S) and sodium polysulphide (Na2Sx) are soluble and
reactive compounds, which have been previously used in the synthesis of
colloidal zinc sulphide (ZnS) from zinc metal and zinc oxide (ZnO).95,96 ZnS is
a well-known luminescent material that has been used for many optical
applications.97-99 ZnS is frequently found to be synthesized as quantum dots
or nanoparticles,95,100 however, multiple attempts at making ultrathin ZnS
sheets such as through vapor phase101 and thermo-chemical102 methods
have been met with limited success or been synthetically challenging.93
2.3.2 Large scale printing
The initial step in fabricating devices based on 2D materials is the formation of the
2D sheet on a chosen substrate. Many methods have been proposed for the
synthesis of 2D materials including (as discussed in section 2.1) techniques for
directly growing 2D layers on substrates. However the production of large-scale, high
quality and homogeneous 2D sheets has proven to be a major challenge with many
reports of direct growth limited to expensive and impractical substrates such as
sapphire.37,103 So far only one report has addressed the wafer-scale homogeneity for
the deposition of MoS2 using a metal–organic chemical vapour based technique.104
However, the temperatures used are above 550C, which is incompatible with many
electronic industry processes, and more negatively, the deposition process takes up
to 24 hours, which significantly adds to the cost and practicality.
19
The surface oxide of Ga and Ga alloys has been shown to adhere strongly to (wet)
many materials particularly oxides (e.g. SiO2)66. Ga and Ga alloys have previously
been used for printing of conducting electrical tracks.105-107 However, after the
placement of the liquid metal with its self-oxide on the substrate, the metal-oxide
experiences well dispersed van der Waals forces66 while the bulk liquid metal
contains relatively low binding energy to its surface oxide skin and thus can be
removed readily. Furthermore, via functionalisation with fluorocarbons it has been
shown possible to control the deposition of suspended 2D flakes108 and using the
similar functionalisation techniques it is possible to inhibit the wetting effect of Ga.109
If it is possible to deposit only the oxide skin adhered to the substrate than an
ultrathin layer of oxide could be produced which may provide a suitable precursor
for the development of Ga based 2D semiconductors.
2.4 This Ph.D. work
Considering the discussions presented in chapter 1 and the aforementioned
studies in this chapter, the author has identified several major research gaps
in the literature which could offer vast potential for the future in the fields
of 2D materials. In light of this, the succeeding chapter will deal with the
following prospects.
20
2.4.1 Chapter 3
This chapter will deal with the issues regarding solvent effects on grinding
assisted LPE of stratified crystals. In particular, it will explore the effects of
solvent on the exfoliation of WS2 and attempt to overcome the drawbacks
of high yield solvent NMP by exploring combinations of alternate solvents.
2.4.2 Chapter 4
This chapter will deal with the potential to manipulate the interfacial
compounds in Ga based liquid metal alloys. Such that it will explore the
possibility for the use of selectively alloying precursor metals into a Ga alloy
as a reactor media in order to produce 2D sheets of non-stratified materials.
The primary goal will be the production of ultrathin ZnS sheets by
continuously reacting Na2Sx with an alloy of Galinstan and zinc.
2.4.3 Chapter 5
This chapter will deal primarily with wafer scale production of 2D
semiconducting films and devices based upon such semiconductors via the
deposition of the oxide skin of liquid gallium. This chapter will aim to use
deposited oxide layer as a precursor for sulphurisation to produce devices
based upon 2D GaS.
2.4.4 Chapter 6
This chapter will present an overview of the conclusions and research outcomes
present in chapters 3, 4, and 5.
21
Furthermore, this work will form a starting platform for future works based upon
many more candidate compounds synthesised in similar manners to this. For
instance, many of the metals which could be considered potential candidates for
these methods have previously had their oxide forms shown to produce other
chalcogenides (e.g. GaSe, In2Se3, SnS2, SnSe2, etc.), oxides (Ga2O3, In2O3, SnO2 etc.)
and pnictogens (e.g. GaN, GaP, InP, etc.).
2.5 References
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3 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 7, 699-712 (2012).
4 Miro, P., Audiffred, M. & Heine, T. An atlas of two-dimensional materials. Chem. Soc. Rev. 43, 6537-6554 (2014).
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22
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23
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33 Stengl, V., Henych, J., Slusna, M. & Ecorchard, P. Ultrasound exfoliation of inorganic analogues of graphene. Nanoscale Res. Lett. 9, 167, doi:10.1186/1556-276x-9-167 (2014).
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35 Das, S., Kim, M., Lee, J.-w. & Choi, W. Synthesis, Properties, and Applications of 2-D Materials: A Comprehensive Review. Crit. Rev. Solid State Mater. Sci. 39, 231-252, doi:10.1080/10408436.2013.836075 (2014).
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24
37 Xu, Z.-Q. et al. Synthesis and Transfer of Large-Area Monolayer WS2 Crystals: Moving Toward the Recyclable Use of Sapphire Substrates. ACS nano (2015).
38 Tan, L. K. et al. Atomic layer deposition of a MoS2 film. Nanoscale 6, 10584-10588 (2014).
39 Liu, K.-K. et al. Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Nano Lett. 12, 1538-1544 (2012).
40 Chen, Y. et al. Tunable Band Gap Photoluminescence from Atomically Thin Transition-Metal Dichalcogenide Alloys. Acs Nano 7, 4610-4616, doi:10.1021/nn401420h (2013).
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42 Tang, Q. & Zhou, Z. Graphene-analogous low-dimensional materials. Prog. Mater. Sci. 58, 1244-1315 (2013).
43 Yuan, L. & Huang, L. Exciton dynamics and annihilation in WS2 2D semiconductors. Nanoscale 7, 7402-7408 (2015).
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45 Yang, S. et al. High performance few-layer GaS photodetector and its unique photo-response in different gas environments. Nanoscale 6, 2582-2587 (2014).
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47 Jung, C. S. et al. Red-to-ultraviolet emission tuning of two-dimensional gallium sulfide/selenide. ACS Nano 9, 9585–9593 (2015).
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49 Polian, A., Chervin, J. & Besson, J. Phonon modes and stability of GaS up to 200 kilobars. Phys. Rev. B 22, 3049 (1980).
50 Nicolosi, V., Chhowalla, M., Kanatzidis, M. G., Strano, M. S. & Coleman, J. N. Liquid exfoliation of layered materials. Science 340, 1226419 (2013).
25
51 Coleman, J. N. Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 46, 14-22 (2013).
52 Nguyen, E. P. et al. Investigation of two-solvent grinding assisted liquid phase exfoliation of layered MoS2. Chem. Mater. 27, 53-59 (2015).
53 Zhou, K.-G., Mao, N.-N., Wang, H.-X., Peng, Y. & Zhang, H.-L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Ange. Chem. Int. Ed. 50, 10839-10842, doi:10.1002/anie.201105364 (2011).
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30
Chapter 3
Two Solvent Grinding Sonication Method for the
Synthesis of Two-dimensional Tungsten
Disulphide Flakes
3.1 Introduction
As discussed in chapter 2 and inspired the unique of two-dimensional (2D)
transition metal dichalcogenides (TMDs) this chapter focusses on some of
the boundaries which need to be crossed for large scale production of 2D
TMDCs.
In this chapter, the effects of solvent selection were experimentally
assessed for grinding assisted ultrasound liquid phase exfoliation by
evaluating a methodology that comprises of using different solvents for the
grinding and sonication processes. A popular high yield solvent, N-methyl-
2-pyrrolidone (NMP), which has been recommended in several studies was
31
employed as a high yield reference. 1-3 Even though high yields are obtained
when using NMP, solvent residues were found to be difficult to remove due
to its relatively high boiling point (202°C) and surface tension (40.79 mJ m2
at 20°C). 4 Residual NMP can often be found on the resulting 2D materials,
even when post processing methods aiming at the solvent removal are
used.5 The inherent instability of NMP is further complicating its removal
since NMP is known to polymerize at elevated temperatures which occur
during sonication and solvent evaporation.6 Solvent residues and
degradation products are detrimental to many applications of 2D TMDs
such as catalysis and sensing which require a pristine surface. Furthermore,
the electronic properties of the resulting flakes are likely to be altered by
organic residues. In order to find alternative solutions, able to overcome the
limitations of exfoliation in NMP, acetonitrile (ACN) and a 50/50
ethanol/water mixture were initially investigated as solvents. These good
process solvents are chosen as they offer moderate boiling points and their
residues can be readily removed.
The outcomes of this investigation have been peer reviewed and published
in Chemical Communications.7
32
3.2 Experimental Section
3.2.1 Materials
Tungsten disulphide (WS2) 2 µm powder (CAS Number 12138-09-9) was
purchased from Sigma-Aldrich®. N-methyl-1-pyrrolidine (NMP), acetonitrile (ACN)
and absolute ethanol were purchased from commercial chemical suppliers and
were not purified or treated in any way and the water used was Milli-Q filtered
water (>18 MΩ).
3.2.2 Synthesis of Two-dimensional WS2 flakes
For each of the four batches 1 g of WS2 was ground with a mortar and pestle for
30 minutes with 1 ml of solvent added periodically for a total of 5 ml/g. in the case
of the single solvent method additional solvent was added to form 15 ml of slurry.
For the two-solvent method, the grinding solvent was left to dry for 1 hour at
room temperature before the second solvent was added to form 15 ml of slurry.
The solutions were then probe sonicated at 100 W for 90 minutes in an ice bath
to prevent excessive heating. The solutions were then centrifuged at 2500 RPM
for 45 minutes and the supernatant was collected.
3.3.3 Measurement and Characterisation
Transmission electron microscope (TEM) imaging was performed with a JEOL
1010 TEM operating at 100 kV, and JEOL 2100F scanning transmission electron
microscope (STEM) operating at 80 kV both fitted with SC600 CCD Cameras (Gatan
Orius), 10-100 µl dripped onto a 300-mesh copper holey carbon grid (ProSciTech,
33
Australia).Atomic force microscopy (AFM) imaging was performed using a Phillips
D3100 scanning probe microscope (AFM/SPM) in tapping mode, 5 µL of solution
was drop-casted onto a standard silicon substrate. Conductive AFM images were
obtained using a Bruker MultiMode with an installed Peak Force TUNA module
(MM8-PFTUNA for MultiMode8 AFM system).
UV-Vis absorption was measured using a Cary 60 spectrometer (Algilent
Technologies) with baseline correction in a standard UV glass cuvette. Samples
were diluted 1:4 in the case of NMP and the two-solvent method exfoliated flakes.
Raman spectroscopy was carried out using a Renishaw inVia confocal microscope
system. Specimens were illuminated with an argon laser (514 nm wavelength)
through a 50× objective, laser was approximately 7 mW and the spot size in the
range of 1 μm. 20 µL of solution was drop-casted onto a gold coated (200 nm)
silicon substrate.
X-ray diffraction (XRD) patterns were obtained using RMIT Bruker D4 Endeavour
wide-angle diffractometric with a (Cu K-alpha) 0.15418 nm X-ray source. Dynamic-
Light scattering (DLS) particle sizing was completed using an ALV Fast DLS particle
sizing spectrometer. Thermo-gravimetric analysis (TGA) was carried out using a
Perkin Elmer, Pyris1 TGA from 50°C-800°C under nitrogen gas flow and then from
800-850°C in air. 5 mg of dried WS2 flakes were used, the NMP exfoliated flakes
were dried in a vacuum oven (~500 mmHg) at 70°C for 100 hours. In the case of
the two solvent exfoliated flakes the sample was dried at 70°C for 5 hours under
a stream of nitrogen. Prior to the actual TGA measurement the sample was kept
34
at 70°C for 30 minutes while recording the weight loss. No weight loss was
observed indicating that a stable state had been reached and the sample was dry.
3.3 Results and Discussion
The liquid exfoliation process using NMP, ACN and ethanol/water solvents,
involved the WS2 bulk powder separately ground in each solvent and then
sonicated in the same solvent as grinding. Figure 3.1a shows an image of
the resulting 2D WS2 suspensions. While the NMP solution resulted in high
yield (Figure 3.1a i), as expected, the observed yields for the ACN and
ethanol/water solutions were low, evident through the low coloration of
the suspensions (Figure 3.1a ii and iii).
A fourth suspension was investigated with a modified process step to
possibly increase the yield. In this case, the grinding and sonication solvents
were chosen to be different. This process is referred to as the ‘two solvent
method’ which has previously been shown effective for obtaining 2D
molybdenum disulphide.8 ACN was used during the grinding step and the
50/50 ethanol/water mixture during the sonication step (after the grinding
solvent was evaporated). In this case, a dramatic increase of the yield was
evident due to the more intense coloration of the fourth suspension (Figure
3.1a iv). The following characterisations will demonstrate why the choice of
ACN as the grinding solvent and the ethanol/water mix as the sonication
solvent provided this improvement.
35
Figure 3.1. (a) Image of the 2D WS2 flakes suspensions prepared with NMP
(i), ACN (ii), ethanol/water (iii) and the two-solvent method (iv). (b) UV-Vis
absorption spectra of the 2D WS2 suspensions with the A, B and C excitons
marked. Note both the NMP and the two solvent (ACN-ETH/Water)
solutions were diluted 1 part in 4 from the optical image for the UV-Vis
measurement. (b) & (c) Raman spectra of 2D WS2 flakes. Spectra have been
normalized to the 𝐴1𝑔(𝛤) phonon peak. (c) The position of the 2𝐿𝐴(𝑀) and
𝐴1𝑔(𝛤) phonons (from bulk WS2) have been marked and spectra have been
vertically offset for clarity.
The UV-vis spectra of the suspensions can be observed in Figure 3.1b. From this
the concentrations of the solutions were calculated using the UV-Vis absorption
36
data and the methodology outlined by O’Neill et al.3, whereas the resonant
absorbance per unit length [(𝐴/𝑙)𝑟] is directly related to the concentration
independent of scattering effects:
(𝐴/𝑙)𝑟 = 𝛼𝑟𝐶
where 𝛼𝑟 is the resonant absorption coefficient. 3 The value of 𝛼𝑟 for WS2 was
obtained from Coleman et al.’s work.2 This method yields concentrations of
0.14 mg/ml, 1.46 µg/ml, 0.82 µg/ml and 0.1 mg/ml for NMP, ACN, ethanol/water
and the two-solvent method respectively. Indicating the superiority of the two-
solvent method over both ACN and ethanol/water.
The peaks of 635, 530 and 475 nm (A, B and C respectively) are clearly seen for
the two-solvent processed suspension which are known to be due to the excitonic
absorption of 2D WS2 suspensions, emphasising the existence of a direct bandgap
associated with a small number of layers. 9,10 Excitonic peak A is also observed for
flakes exfoliated in NMP. The excitonic peaks B and C are however not observed
in this case. The disappearance and reduction of the intensity of the excitonic
peaks have been suggested to be associated with the decrease in the lateral
dimensions of the flakes when NMP is used.11 In this case, however, it seems that
the interaction between NMP and/or the NMP decomposition products and the
WS2 surface are likely to alter the electronic structure of the semiconducting
flakes leading to more intense absorption of high energy photons obscuring
excitonic effects.
37
The phonon peaks 2𝐿𝐴(𝑀), 𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) in the Raman spectrum of
the non-exfoliated (bulk) powder are observed at 350.6, 356 and 420.4 cm1
shift to higher wave numbers (~353, ~358 and ~425 cm1, respectively) after
exfoliation (Figure 3.1b). The shift of the Raman peaks and the increase of
the 𝐸2𝑔1 (𝛤) phonon (Table 3.1) is characteristic for the formation of thin WS2
flakes.12
Raman spectra for NMP exfoliated flakes (Figure 3.1d) showed strong
fluorescence induced by the excitation laser evident in the intense
background signal and the deviation from a flat baseline. This behaviour is
not observed in the two-solvent method exfoliated flakes, highlighting
differences in the electronic structure of the resulting material even after
the solvent drying stage. The phonon peaks are quenched for NMP
exfoliated flakes and a characteristic NMP Raman signal is observed
featuring peaks at 1450 and 1600 cm1 highlighting that a significant
amount of the NMP solvent residues and decomposition products remained
on the 2D WS2 surface, despite of drying the flakes at 70°C for 4 hours. These
residues are likely to interfere with 2D WS2 applications that require pristine
surfaces such as catalysis, gas/bio sensing and certain electronic devices.
The NMP effect also reduces the accuracy of the number of layer
assessment using Raman spectroscopy. However, samples prepared
following the two-solvent exfoliation method showed no signs of residues
in the Raman spectrum confirming the two-solvent approach as a superior
exfoliation technique.
38
Table 3.1. Summary of Raman shift peak intensities and locations for the 2𝐿𝐴(𝑀),
𝐸2𝑔1 (𝛤) and 𝐴1𝑔(𝛤) phonons (intensities are normalized to the A1g(𝛤) peak).
2𝐿𝐴(𝑀) 𝐸2𝑔1 (𝛤) 𝐴1𝑔(𝛤)
Intensity Peak shift
(cm−1) Intensity Peak shift
(cm−1) Intensity Peak shift
(cm−1)
Bulk 0.52 350.6 0.22 356.0 1 420.4
NMP 0.42 352.5 0.28 357.7 1 422.5
ACN 0.41 354.2 0.36 358.2 1 425.5
Ethanol/water 0.41 352.8 0.48 359.8 1 428.0
ACN-ethanol/water 0.48 353.0 0.51 358.9 1 425.2
The information of phonon peak intensities and positions are recorded in Table
3.1. Here it is clear that the 2D and quasi-2D WS2 flakes produce a comparative
increase in the 𝐸2𝑔1 (𝛤) phonon peak which is a phonon in the horizontal plane as
opposed to the 2𝐿𝐴(𝑀) and 𝐴1𝑔(𝛤) phonon peaks that are vertical plane
phonons and thus relay upon interlayer interactions.12
The reverse methodology where the implementation of the two-solvent solution
was reversed (grinding in ethanol/water, sonication in ACN) was attempted as a
control test. The procedure was accomplished using an almost identical method
to the ACN grinding, ethanol/water sonication solution with the exception of an
increase in post-grinding drying time (which was necessary due to the higher
boiling point of water). In the case of the reverse method the WS2 ethanol/water
slurry was left to dry overnight (at room temperature).
39
Figure 3.2. (a) Photograph and (b) UV-Vis absorption spectra of WS2 suspension
after the process involved grinding in ethanol/water (50/50) and sonication in
ACN.
The reverse two solvent method was however unable to effectively increase the
yield. As can be seen in Figure 3.2, the reverse two-solvent method produced low
optical contrast and very little UV-Vis absorbance characteristics analogous to the
results produced by both ACN and ethanol/water single solvent methods. In fact,
the concentration of the reverse method was calculated (see above for method)
to be reduced to 0.77 µg/ml, less than both of the former methods.
40
Figure 3.3. Sample tapping mode AFM images of the exfoliated WS2 flakes
deposited on Si substrates.
Typical AFM images of the WS2 solutions are displayed in Figure 3.3. The statistical
analysis presented in Figure 3.3(a-d) were assessed from AFM images such as (and
including) these by assessing the height of approximately 100 flakes. Similarly,
both the AFM and TEM images such as (and including) those presented in Figure
3.3(i-l) were used for statistical analysis of the lateral dimensions.
41
Figure 3.4. (a-d) Statistical distribution of WS2 flake thickness analysed from
AFM imagining. (e-h) statistical distribution of lateral dimensions analysed
from TEM imaging. The blue overlay is an approximation of flake size based
on DLS Approximation of 2D flake lateral dimensions as demonstrated by
Lotya et al. 13 (i-l) TEM images of the WS2 flakes synthesized with different
solvent methods.
The thickness distribution and polydispersity of the exfoliated WS2 flakes
was assessed by AFM, TEM and dynamic light scattering (DLS) techniques.
42
The physical dimensions are summarized in Figure 3.4. Here the effect that
solvent selection has upon the physical characteristics of the produced
flakes becomes fairly evident. NMP produces many thin flakes (≤4 layers)
with relatively large surface areas (median = 98 nm) while ethanol/water
produced flakes of similar lateral polydispersion (median = 87 nm), however
with a tendency towards thicker, less two-dimensional flakes (≥5 layers).
ACN produced a large fraction of single and two-layer flakes with a large
horizontal polydispersity and overall reduced the lateral sizing (median = 51
nm). The two-solvent method produced flakes with size characteristics of
90 nm median lateral dimension with many ≤4-layer flakes, suggesting a
combination of the flake properties between the ethanol/water and ACN
systems which was overall effectively closer to that of the archetypal NMP
based exfoliation.
43
Figure 3.5. Conductive Atomic Force Micrograph of WS2 flakes exfoliated via the
two-solvent method on Au substrate representing both (a) height and (c) tip
current of the same area. B and D are the profiles the white dashed line in A and
B respectively.
It has been shown that liquid exfoliated transition metal dichalcogonides (TMDs)
can contain defects in the crystal structure, resulting in metallic edges.11
Conductive AFM (Figure 3.5) has been used for elucidating this effect. With the
profiles in Figure 3.5 it is apparent that the edges of most of the WS2 flakes are
conductive.
44
Figure 3.6. (a) XRD of bulk WS2 powder and WS2 powder after grinding (pre-
exfoliation). The results have been normalized to the 002 peak, offset
vertically and the 002 peak has been clipped for clarity. (b) Paracrystallinity
factor (g) of the vertical crystal planes for bulk WS2 powder (black) and WS2
powder after grinding in NMP (green), ACN (purple) and ethanol/water
(50/50, red)
To elucidate the origin of the yield variation and the yield increase following
the implementation of the two-solvent method, X-ray diffraction (XRD)
patterns of the ground (non-exfoliated) and dried powders were taken prior
to sonication. Figure 3.6a) shows several solvent dependent variations in
the XRD pattern indicating changes in crystallinity after grinding the bulk
45
WS2. The 002, 004, 006 and 008 peaks (at 14.5°, 29°, 44°, and 60°,
respectively) are of particular interest since these peaks are arising from the
symmetry of the vertical crystal planes (layer to layer crystallinity). The
paracrystallinity factor (PF) allows to assess the crystallinity of a sample
(Figure 3.6b) via the determination of the distortions in short term order of
the crystal lattice. The PF was calculated using the cold-work distortion
method, 14 which has been previously used for similar materials.15 The
paracrystallinity of the WS2 was calculated as
𝑔 = √⟨𝑑2⟩ − ⟨𝑑⟩2
⟨𝑑⟩2
where g is the paracrystallinity factor, and d is the planar spacing.15
46
Table 3.2. Summary of vertical plane XRD results for the comparison of the
crystallographic effects of grinding WS2 with different solvents.
Paracrystallinity (%)
Crystal plane Bulk NMP ACN
Ethanol/
water
002 0.793 0.730 0.979 0.847
004 1.244 1.128 1.347 1.075
006 1.266 1.259 1.264 1.265
008 0.754 0.545 0.766 0.642
Normalized Peak Intensity
Crystal plane Bulk NMP ACN
Ethanol/
water
002 1 1 1 1
004 0.082 0.102 0.072 0.083
006 0.083 0.102 0.065 0.075
008 0.023 0.004 0.022 0.009
Normalized Integral Intensity
Crystal Plane Bulk NMP ACN
Ethanol/
water
002 1 1 1 1
004 0.117 0.139 0.102 0.104
006 0.143 0.150 0.110 0.112
008 0.115 0.022 0.080 0.041
Full-width half-maximum (FWHM) (2θ °)
Crystal plane Bulk NMP ACN
Ethanol/
water
002 0.085 0.105 0.130 0.105
004 0.125 0.140 0.165 0.140
006 0.095 0.180 0.219 0.185
008 0.569 1.517 0.888 0.818
47
Increased PF values indicate lower short-term order. A breakdown of
vertical order as indicated by the PF is just seen for the ACN ground samples.
This reduction in the short-term order of the 002 plane can also be indicated
by the broadening of the 002 peak (the calculated FWHM is presented in
Table 3.2). The FWHM of the 002 peak is most effectively increased when
grinding with ACN. NMP and ethanol/water did however also broaden the
002 peak indicating some reduction in the vertical order. The XRD patterns
are providing an explanation for the increased exfoliation yield when using
the two-solvent method. The vertical order is most effectively broken down
by grinding in ACN. Zhou et al. investigated the suitability of different
ethanol/water mixtures for the sonication step during the WS2 exfoliation
and found a correlation between the exfoliation yield and the Hansen
solubility parameters (HSP) of the solvent.16 The HSP distances 𝑅𝑎 were
calculated using:
𝑅𝑎 = [4(𝛿𝐷,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝐷,𝑠𝑜𝑙𝑢𝑡𝑒)2
+ (𝛿𝑃,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝑃,𝑠𝑜𝑙𝑢𝑡𝑒)2
+ (𝛿𝐻,𝑠𝑜𝑙𝑣𝑒𝑛𝑡 − 𝛿𝐻,𝑠𝑜𝑙𝑢𝑡𝑒)2
]0.5
where 𝛿𝐷, 𝛿𝑃, and 𝛿𝐻 are the dispersive, polar, and hydrogen-bonding solubility
parameters of the solvent and solute, respectively. In the case of mixed solvent,
the HSP values can be calculated as a linear function of composition thus for the
50/50 ethanol/water mixtures the each effective HSP (D, P and H) value is simply
the mean of ethanol and water.16The HSP values for WS2 and the solvents were
48
obtained from Coleman et al. and Hansen Solubility Parameters, A User's
Handbook, 2nd Ed respectively.2,17
A HSP-distance of 13.1 is calculated for the ethanol/water mixture while
ACN leads to a calculated HSP-distance of 19.2. A lower HSP-distance leads
to a higher expected solubility. The observed difference in HSP-distance
between the solvent systems was comparatively large. These results
demonstrate that the required solvent parameters during grinding and
sonication may be very different and potentially unrelated.
49
Figure 3.7. High resolution transmission electron micrograph (HRTEM) of the
exfoliated WS2. Insert is a selected area electron diffraction (SAED) image of the
same section.
Figure 3.7 displays a HRTEM image of the 2D WS2 here both the 1.0.0 and 1.0.1
crystal planes are visible in two distinct crystal domains. These lattice spacing
correlate with the XRD pattern in Figure 3.6 and Table 3.2 at 32.7° and 33.5°.
50
Figure 3.8. Thermal gravimetric analysis (TGA) of the dried 2D WS2 flakes
prepared with both only NMP and ACN-ethanol/water two solvent methods
tested in nitrogen gas. The boiling points of both NMP and water are
marked (202°C and 100°C at standard pressure)
TGA (Figure 3.8) was employed to assess the extent of NMP (for the single
solvent process using NMP) as well as ACN and ethanol (for the two-solvent
method) residues on the dried 2D WS2 flakes. It can be seen that due to the
large surface area of the 2D WS2 flakes over 60% w/w of the product
constitutes surface bound solvent molecules and solvent decomposition
products, even after drying in vacuum at elevated temperatures (100 hr at
70°C). High temperatures of over 200°C were necessary to start the NMP
desorption process. Flakes prepared following the two-solvent method
show weight loss at significantly lower temperatures indicating that
51
predominantly water and potentially ethanol residue were present on the
WS2 surface which were removed with comparative ease. This exceedingly
high absorbed solvent to solid ratio could potentially open up many
doorways to applications such as dehumidification, solvent absorption and
gas/bio sensing. This property of 2D WS2 can be rationalized considering the
large aspect ratio of the flakes and their surface polarity.
52
3.4 Conclusions
A two-solvent exfoliation method was developed and investigated for the
synthesis of 2D WS2 flakes at high yields. For comparison, the resulted 2D
flake suspension was assessed in reference to those obtained using the
single solvent method. It was demonstrated that the choice of a separate
grinding solvent prior to the sonication in liquid phase exfoliation played a
major role in the determination of the process that has been largely
overlooked to date. The two-solvent method based on grinding with ACN
and sonication in ethanol/water mix resulted in much higher yield in
comparison to single solution grinding sonication method using ACN and
ethanol/water solutions alone.
AFM, TEM and DLS characterisations revealed that the lateral size and
thickness of the exfoliated flakes depend highly on the chosen solvent
during both the grinding and sonication steps. They showed that the two-
solvent method generated flakes comparable to those of NMP solution in
terms of lateral size and thickness. Raman and UV-Vis spectroscopy
provided strong evidence that NMP residues remained on the surface of the
exfoliated flakes, interfering with many of the desired properties of 2D WS2.
However, such adverse effects of the residues seem to be reduced using the
two-solvent method.
XRD was performed on the WS2 powder after the grinding step in order to
understand the origin of the yield variation. The XRD results confirmed that
53
ACN does reduce interlayer crystallinity more efficiently than the other
solvents.
A >60% w/w solvent content was later confirmed with TGA for both NMP
and two solvent exfoliated flakes. The NMP contamination required
however a higher temperature for its removal.
This work helps to increase the understanding of the importance of the
grinding step during the grinding assisted ultrasound exfoliation of 2D
materials which has been poorly understood to date. It also suggests that
choice of two separate solvents for grinding and sonication processes are
highly beneficial. Future work should carefully evaluate, if surface bound
molecules on the 2D WS2 surface can interfere with or even be utilized for
the intended applications. Additionally, further research of different two
solvent method combinations may lead to increased exfoliation yields and
larger lateral dimensions of the flakes with clean surfaces.
54
3.5 References
1 Coleman, J. N. Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 46, 14-22 (2013).
2 Coleman, J. N. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 331, 568-571 (2011).
3 O'Neill, A., Khan, U. & Coleman, J. N. Preparation of High Concentration Dispersions of Exfoliated MoS2 with Increased Flake Size. Chem. Mater. 24, 2414-2421 (2012).
4 Lide, D. R. Physical Constants of Organic Compounds. Vol. 69 (CRC Press, 2005).
5 Halim, U. et al. A Rational Design of Cosolvent Exfoliation of Layered Materials by Directly Probing Liquid–Solid Interaction. Nat. Commun. 4, 2213 (2013).
6 Berrueco, C. et al. Sample Contamination with NMP-oxidation Products and Byproduct-free NMP Removal from Sample Solutions. Energy Fuels 23, 3008-3015 (2009).
7 Carey, B. J. et al. Two solvent grinding sonication method for the synthesis of two-dimensional tungsten disulphide flakes. Chemical Communications 51, 3770-3773 (2015).
8 Nguyen, E. P. et al. Investigation of two-solvent grinding assisted liquid phase exfoliation of layered MoS2. Chem. Mater. 27, 53-59 (2015).
9 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nano 7, 699-712 (2012).
10 Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N. & Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat. Nanotechnol. 7, 699-712 (2012).
11 Backes, C. et al. Edge and confinement effects allow in situ measurement of size and thickness of liquid-exfoliated nanosheets. Nat. Commun. 5, 4576 (2014).
12 Berkdemir, A. et al. Identification of individual and few layers of WS2 using Raman Spectroscopy. Sci. Rep. 3, 1755 (2013).
13 Lotya, M., Rakovich, A., Donegan, J. F. & Coleman, J. N. Measuring the lateral size of liquid-exfoliated nanosheets with dynamic light scattering. Nanotechnology 24, 265703 (2013).
55
14 Warren, B. E. & Averbach, B. L. The effect of cold-work distortion on X-ray patterns. J. App. Phys. 21, 595-599 (1950).
15 Palit, D., Srivastava, S. K., Chakravorti, M. C. & Samantaray, B. K. Study of layer disorder and microstructural parameters of molybdenum-tungsten mixed sulpho-selenide Mo0.5W0.5SxSe2-x (0 ≤ x ≤ 2) by X-ray line profile analysis. J. Mater. Sci. Lett. 15, 1636-1637 (1996).
16 Zhou, K.-G., Mao, N.-N., Wang, H.-X., Peng, Y. & Zhang, H.-L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Ange. Chem. Int. Ed. 50, 10839-10842 (2011).
17 Hansen, C. M. Hansen solubility parameters: a user's handbook. (CRC press, 2012).
56
Chapter 4
A Liquid Metal Flow-Reaction for the Synthesis of
Ultra-Thin Zinc Sulphide
4.1 Introduction
As discussed in chapter 2 the exfoliation method as presented in chapter 3
is limited to materials with stratified crystal structures. This chapter
focusses on the potential to utilize the naturally occurring surface layer of
gallium (Ga) alloys as a medium for synthesising non-stratified two-
dimensional (2D) zinc sulphide (ZnS).
This chapter introduces a galinstan based flow-reactor for the production of
(2D) ZnS, a well-known luminescent material that has been traditionally
used for many optical applications.1,2 ZnS is frequently found to be
synthesized as quantum dots or nanoparticles,3,4 however, multiple
attempts at making ultrathin ZnS sheets such as through vapor phase5 and
57
thermo-chemical6 methods have been met with limited success or been
synthetically challenging.7 In this work, thin ZnS sheets were produced by
continuously reacting sodium polysulphide (Na2Sx, where 2 ≤ x ≤ 5) with an
alloy of galinstan containing 11.7 wt% zinc, which is calculated to be a
eutectic mixture.8 Sodium sulphide (Na2S) and Na2Sx are reactive
compounds, which have been previously used in the synthesis of ZnS from
zinc metal and zinc oxide (ZnO).3,9
The Outcomes of this investigation are undergoing peer review at this time
by Chemistry, A European Journal
58
4.2 Experimental Section
4.2.1 Materials
Gallium (Ga, 99.99%), indium (In, 99.99%) and tin (Sn, 99.9%) were purchased
from Roto Metals. Sulphur (S, 99.98%), zinc (Zn, 99.9%), sodium sulphide (Na2S,
>97%), iodine (I, 99.99%) and N,N-dimethylformamide (DMF, 99.8%) were
purchased from Sigma-Aldrich. Methanol (99.9%) was purchased from VWR
chemicals. All chemicals were used without further processing. Macroscopic
metal precursors were chosen to avoid the formation of large amounts of surface
oxide, which can occur with micron-sized powders during storage.
4.2. Preparation of Galinstan
Ga, In and Sn were placed in a single glass beaker at a Ga:In:Sn ratio of
68.5:22.5:10 wt%. Tin and indium were purchased in the form of chips and ingots
which could be cut to size in order to achieve an accurate mass fraction. A large
ingot of gallium was melted in a hot water bath (~80°C) and the liquid metal was
transferred and weighed using plastic pipettes. Care was taken to achieve
accurate mass fractions. The system containing all three metals was then heated
to 300°C, which is well above the melting point of tin. Heat was applied until all
metals were molten, with the mixture then stirred thoroughly with a glass rod
before the alloy was cooled to room temperature. The liquid metal was separated
from oxides that formed during the melting process by decanting the liquid metal
using plastic pipettes, and only collecting liquid metal from the centre of the melt.
59
The alloy, which remained liquid at room temperature, is called Galinstan
throughout the manuscript
4.3.3 Preparation of Zn-Galinstan Alloy:
In order to alloy Zn into the mixture, 30 g of Galinstan and 4 g (11.7 wt%) of Zn
powder were mixed together inside a N2 glovebox (O2 content ~ 10-100 ppm) via
light grinding with a mortar and pestle for 10 minutes. After grinding, the alloy
was transferred into a plastic vial using plastic pipettes. The Zn-Galinstan alloy was
then stored in the same glovebox.
4.3.4 Preparation of Polysulphide Solution
20 ml of methanol was degassed with N2 and then transferred into a glovebox.
Na2S (~155 mg, 2 mMol) and sulphur (~65 mg, 2 mMol) were measured by weight
and then added to the solvent in a single glass vial. The solution was stirred at
room temperature for 1 hour before filtration with a 0.22 µm syringe driven filter.
4.3.5 Synthesis of ZnS
The 2D ZnS was synthesized in a flow reaction involving pumping the polysulphide
solution (0.1 mol L-1 Na2Sx in CH3OH) through the liquid metal as shown in Figure
4.2b. A centrifuge tube was modified in order to function as a reaction chamber.
A small hole was drilled into the bottom of the tube and a rubber tube was pushed
into this hole, which was then sealed with parafilm. 5 ml of polysulphide solution
and approximately 3.5 ml of the alloy were added into the centrifuge tube. The
60
polysulphide solution was then cyclically and continuously pumped through the
liquid metal at a rate of 5 ml/min for 2 hours.
After the reaction was completed the methanol phase was removed carefully with
a pipette (the liquid metal remained at the bottom of the tube) and centrifuged
at 15 kRPM for 30 min, the supernatant was then removed and the sediment re-
dispersed in DI water that was centrifuged again. This washing process was
repeated twice. The sediment was then resuspended in a 0.1 mol L-1 iodine
solution in DMF. The sample was then washed thrice more with pure DMF and
finally resuspended in DMF prior to use.
4.3.6 Characterisation
X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific
K-alpha system. The X-ray source was a monochromated Al Kα source and the
measurement was performed on 50 µl of the suspension drop cast onto a Si wafer.
Photoluminescence (PL) spectra were collected using a PerkinElmer LS50 on a
diluted sample (100:1). UV-Vis absorption measurements were performed on a
PerkinElmer Lambda 1050 fitted with an integrating sphere. TEM images were
taken using both a JEOL 1010 TEM and a JEOL 2100F STEM with 100 kV and 200 kV
accelerating voltages, respectively. AFM images were collected using a Bruker
Dimension Icon AFM.
TGA was performed on a PerkinElmer TGA Pyris 7 in a N2 environment (flow rate
20 ml/min). Initially the temperature was increased to 80°C at a rate of 10 °C/min
and held for 30 minutes, before increasing the temperature again to 200°C and
61
holding for a further 10 minutes, with the sample then heated to 750°C at 20
°C/min and the N2 flow replaced with air at 600 °C. To isolate a sample for TGA a
suspension of the material was centrifuged for 1 hour at 15 kRPM. Care was taken
to ensure that the entire sample was transferred into the TGA pan.
4.3 Results and Discussion
The flow-reaction was performed as outlined in Figure 4.2 and in the
Experimental section. The methanolic Na2Sx solution was then continuously
circulated through the Zn-Galinstan alloy from the base of the reaction
vessel with a peristaltic pump, with discrete droplets of the Na2Sx solution
rising through the denser liquid metal (Figure 4.1). The polysulphide reacts
with the metal species at the liquid/liquid interface, leading to sulphide
formation.
62
Figure 4.1. Images are taken from a video recording of the reaction with
graphical representation of the surface of the liquid metal.
Thermodynamic considerations lead to the conclusion that the reaction
between the polysulphide ions and zinc results in the greatest reduction in
Gibbs free energy (see Table 4.1), with the reaction proceeding according
to the following equation:
yZn + Na2Sx → yZnS + Na2Sx−y
63
Table 4.1. Thermodynamics of the compounds relevant to the interfacial
reaction.
Material Enthalpy of formation
(kJ mol−1)
ΔfH°
Gibbs energy
(kJ mol−1)
ΔfG°
Standard entropy
(J mol−1 K−1)
S°
Reference
GaS −144.4 −159.6 21 10
InS −138 −131.8 67 11
SnS −100 −98.3 77 11
ZnS −192.6 −201.3 57.7 11
S2− 33.1 85.8 −14.6 11
Sx2− a 13 – 6.6 77.6 – 66 −22 – 100 12
a x≥2 and x≤5
The possible formation of other products, such as GaS, InS and SnS feature
significantly smaller reductions of Gibbs free energy and are thus not
expected to preferentially form. Degassed methanol was used to avoid the
possibility of forming zinc oxide/hydroxide.
64
Figure 4.2. Depiction of the synthetic method. (a) Alloying of liquid
Galinstan and Zn powder. (b) Flow-reaction between Zn-Galinstan alloy and
polysulphide (i) Dispersion of ZnS and by-products. (ii) ZnS forming on the
interface between Na2Sx solution and the alloy. (c) Washing process
displaying (i) solution before washing and iodine treatment, (ii) solution
after centrifugation (iii) resuspended ZnS in DMF.
Once the reaction was completed, the suspension was decanted from the
liquid metal. A reaction workup procedure, which consists of a multi-stage
65
centrifuge based washing procedure outlined in Figure 4.2c and the
Methods section, was devised to remove remaining polysulphide (Na2Sx), as
well as small metallic droplets of the alloy that were suspended due to the
physical agitation during the synthesis.13 De-ionized (DI) water assisted in
the removal of the polysulphide by transforming Na2Sx into sodium sulphate
(NaSO4), which is highly soluble in H2O.14 Thus, after repeated washings with
water, residual Na2Sx and NaSO4 were completely removed.
After the first purification stage, the suspension was predominantly
composed of ZnS, with some residual metallic precursor particles
remaining. These metallic contaminants were removed using a solution of
N,N-dimethylformamide (DMF) containing iodine, which effectively
transforms any residual unreacted metals such as Ga, In, Sn or Zn into easily
dissolved GaI3/InI3/SnI4/ZnI2. This reaction was followed by repeated
washing in clean DMF which was sufficient to remove any iodine and metal
iodides (Figure 4.2c, ii). The final product of ZnS flakes was resuspended in
DMF and stored prior to further characterisation (Figure 4.2c, iii).
66
Figure 4.3. Morphology of the 2D ZnS. (a) TEM image of the ZnS flakes. (b)
HRTEM image of the ZnS which lattice dimensions marked, with fast Fourier
transform (FFT) pattern (inset). (c) Statistical distribution of the lateral
dimension of the ZnS flakes. (d) AFM micrograph of a flake with height
profile (insert) along the blue line.
Transmission electron microscopy (TEM) revealed that large, thin sheets
with lateral dimensions measuring several microns (predominantly 1–3 µm)
67
were obtained (Figure 4.3a,c). These results, as well as those shown in
Figure 4.4, are a good representation of the distribution of flake sizes.
High-resolution TEM imaging (HRTEM) revealed that the nanosheets are
composed of many small nanocrystals, with the FFT of the area indicating
the sheets had a polycrystalline structure (Figure 4.3b). The structure
possibly formed due to the large number of nucleation sites that developed
at the interface between the polysulphide solution and the liquid metal.
During the synthesis, the interfacial layer is expected to protrude as the
bubble expands, allowing additional nucleation sites to appear, ultimately
leading to the observed polycrystallinity in a single nanosheet. The lattice
spacings of 3.3 Å and 3.1 Å correspond to the 100 and 002 planes,
respectively, of hexagonal ZnS, which is also known as wurtzite ZnS. The
synthesis of wurtzite ZnS typically requires elevated temperatures,15
however, the process described in this work takes place at room
temperature, which can be seen as a considerable advantage over other
synthetic strategies.
Atomic force microscopy (AFM, Figure 4.3d) revealed that the synthesized
2D ZnS nanosheets have a thickness in the order of a few nm, which
corresponds to several unit cells of ZnS (measured in the c direction). The
observation of wrinkles in the TEM also suggests atomically-thin nature of
the sheets. Furthermore, it can be seen in these images that some residual
metal appears on the sheets, sometimes in the form of small balls on the
flakes’ surface
68
Figure 4.4. TEM images of 2D ZnS sheets produced via the reaction of Na2Sx
with the Zn alloy.
Interestingly, the configuration of the flow reactor has a significant impact
on the efficacy and outcome of the reaction and the morphology of the
product. When the polysulphide solution was introduced to the liquid alloy
from the side, bubbling was not observed. This is indicative of the formation
of semi-permanent microchannels within the liquid metal, through which
the methanolic solution may flow, reducing the reaction interface. This
could be attributed to different fluid mechanics that applied to the process,
such as the change in pressure when injecting the solution from the side,
due to a lower height of the liquid metal column. It was also observed that
this alternative approach significantly reduced the lateral size and alters the
shape of the ZnS forming nanoparticles, nanoplatelets and nanoflakes as
opposed to sheets (see Figure 4.5). This indicates that a prolonged
residence time of polysulphide in the alloy may lead to laterally larger sheet
sizes. Furthermore, the crystallinity of the sample also appears to be
improved, which were attribute to the longer reaction times that may apply
within the microchannels.
69
Figure 4.5. TEM images (a-c) and HRTEM (d, FFT insert) of ZnS produced
with a modified reaction chamber where the Na2Sx is not pumped directly
into the Galinstan from underneath, but instead from the side.
Substituting Na2Sx for Na2S gave smaller lateral dimensions, however the
thickness of the flakes remained unaffected by this change (Figure 4.6).
These changes, indicating that the observed thickness corresponds to the
sulphide that grew in a self-limiting interfacial surface reaction.
70
Figure 4.6. TEM image (a) of ZnS produced via a modified reaction chamber
where a 0.1 mol L-1M Na2S solution is also substituting the polysulphide
solution and is not pumped directly into Galinstan from underneath, but
instead from the side. (b & c) An AFM image and profile, which demonstrate
that the thickness of the flakes is not altered by a change in the reaction
protocol.
71
Figure 4.7. XPS spectra of the (a) S 2p and (b) Zn 2p regions of the ZnS flakes.
Peak fitting is demonstrated for all the relevant shells.
Elemental analysis of the product utilising X-ray photoelectron
spectroscopy (XPS) revealed that the chemical composition of the sample is
stoichiometric ZnS. The S 2p region of the XPS spectra features a doublet
which in agreement with S2− (Figure 4.7a). The Zn 2p region displays a
doublet consistent with Zn2+ (Figure 4.7b).15,16 A small peak at 160.7 eV was
observed, which corresponds to the Ga 3s binding energy, indicating the
presence of small amounts of residual gallium.
72
Figure 4.8. Schematic representation of the simplified flow dynamics within
the liquid metal reactor. F, V, A, T, D, t, and N represent the volume of the
flow rate, the volume of bubbles, the surface area of bubbles (i.e. interfacial
area), the thickness of ZnS form, density of ZnS, time of reaction, and the
total number of bubbles during the reaction, respectively.
The flow dynamics of the reactions are depicted in Figure 4.8. Knowing that
the flow of the solution is 5 ml/min (F) and counting the average bubble
rate, the average bubble size was determined to be 125 μl (V). Assuming
the bubbles maintain a constant volume and a spherical shape, the
interfacial area was calculated to be 1.21×10−4 m2 (A) based on simple
geometry. Furthermore, assuming the ZnS layer formed at the interface has
a constant 6 nm thickness (T) The potential volume of ZnS formed at the
interface of the bubble can be calculated. Knowing the density of ZnS (D)
73
each bubble could potentially produce 2.97 μg of ZnS or 17.8 mg (M) over
the course of the 2-hour reaction (t).
Thermogravimetric analysis (TGA) was utilized to determine the solid
content of the final suspension, which permitted an estimation of a reaction
yield of 2.14 mg (Figure 4.9). The theoretical maximum reaction yield of
17.5 mg was obtained using the measured thickness of the ZnS nanosheets
and the total surface area of the Na2Sx droplets passing through the metal
alloy. For this purpose, the average bubble volume was estimate by
correlating the number of observed bubbles with the known flow rate and
assuming the bubbles are approximately spherical within the liquid metal,
and that the entire interface is covered with ZnS (Figure 4.8). As a result, an
overall reaction yield of 12% is reported for the flow reaction. Increasing the
reaction time, by prolonging the time the injected polysulphide solution
resides within the liquid metal, should result in even higher overall reaction
yields. This may be achieved by simply increasing the height of the liquid
metal column.
74
Figure 4.9. TGA of the centrifuged ZnS solution demonstrating weight
reduction relative to (a) temperature and (b) time. The red dashed line
indicates the solid weight of the dried ZnS which was utilized for the yield
calculations. (a, inset) The change in weight as the N2 flow is replaced with
an air flow at 600°C.
Furthermore, an improved workup procedure may limit the amount of
product lost during repeated centrifugation steps. TGA allowed the
estimation of the total amount of residual metal to be less than 5 wt% from
the increase in weight at this temperature range attributed to the oxidation
of the sample (see Figure 4.9). Based on the expected formation of
Zn3O(SO4) due to the oxidation of ZnS and the expected formation of Ga2O3,
In2O3, SnO2 and ZnO the mass fractions of the present metal were estimated
based upon literature values.11,17
75
Figure 4.10. Spectroscopic characterisation of the ZnS suspensions. (a) PL
emission spectrum excited at 275 nm with simplified electronic band
structure (inset). The red lines indicate the conduction and valence band
and the black line is the fermi level. (b) UV-Vis absorption spectrum with
Tauc plot (inset).
The optical properties of a ZnS nanosheet suspension are shown in
Figure 4.10. Photoluminescence (PL) spectroscopy revealed a dominant
emission at approximately 3.15 eV with a quantum yield of 1.41% (see Table
4.2 & Figure 4.10a).
76
Table 4.2. Quantum yield estimations based on comparative analysis with
quinine sulphate.
Sample Solvent Refractive index
Absorbance at 325 nm
Integrated emission
Quantum yield
Quinine sulphate
0.1 mol L-1 H2SO4
1.33 0.0469 3.822 × 105 54% (reference 18)
ZnS DMF 1.43 0.0267 5.511 × ×103 1.41% (this work)
These findings are in excellent agreement with previous reports on ZnS.1,19
The observed emission peak also correlates very well with the Tauc plot
derived from UV-Vis absorption spectroscopy (Figure 4.10b), allowing us to
infer that the 3.15 eV emission is due to the band edge of the material. A
simplified band structure of the synthesized ZnS nanosheets based on the
values obtained from optical characterisations and XPS valence band
analysis (Figure 4.11) is displayed as an inset in Figure 4.10a.
77
Figure 4.11. Valence band analysis from XPS with line fitted on the valance
band edge for clear determination of energy difference between the
valence band maxima and the Fermi level.
These observations indicate that the 2D ZnS nanosheets feature p-type
semiconductivity. Interestingly, the determined band gap is slightly lower
than what has been previously reported for h-ZnS (3.4–3.7 eV).2,3,5,15,16 This
may be rationalized with the presence of intrinsic defects within the
wurtzite crystal structure, which are known to be introduced when the
synthesis is conducted at low temperatures.20 These defects have been
reported to reduce the electronic bandgap of ZnS to approximately 3.25
eV.20
78
4.4 Conclusions
In conclusion, this chapter demonstrated a gallium-based room
temperature liquid alloy acting as a reaction medium, which introduces a
new pathway for synthesising 2D ZnS. The reaction was implemented in a
flow reactor and relied on the reaction occurring at the liquid/liquid
interface, resulting in 2D nanosheet formation. The findings are particularly
important since ZnS is a naturally non-stratified crystal system, for which
conventional 2D synthesis techniques, such as mechanical exfoliation,
cannot be applied. The report introduces ambient-temperature liquid
metals as suitable reaction media that can fulfil the function that is
traditionally reserved for covalent systems (solvents) and ionic liquids.
Overall, the liquid metal reaction route is a scalable process, providing
access to 2D compounds within a relatively short reaction time. The process
is likely applicable to other metals, which can be alloyed into the liquid
metal, likely giving access to many other 2D compounds that might
otherwise be inaccessible due to their lack of a stratified crystal structure.
Similarly, precursor solutions, apart from polysulphides, may be
investigated, providing access to 2D morphologies of non-sulphide metal
compounds.
79
4.5 References
1 Katayama, H., Oda, S. & Kukimoto, H. ZnS blue‐light‐emitting diodes with an external quantum efficiency of 5× 10− 4. App. Phys. Lett. 27, 697-699 (1975).
2 Denzler, D., Olschewski, M. & Sattler, K. Luminescence studies of localized gap states in colloidal ZnS nanocrystals. J. App. Phys. 84, 2841-2845 (1998).
3 She, Y.-Y., Juan, Y. & Qiu, K.-Q. Synthesis of ZnS nanoparticles by solid-liquid chemical reaction with ZnO and Na2S under ultrasonic. Trans. Nonferrous Met. Soc. China 20, s211-s215 (2010).
4 Xu, J., Ji, W., Lin, J., Tang, S. & Du, Y. Preparation of ZnS nanoparticles by ultrasonic radiation method. App. Phys. A: Mater. Sci. Process. 66, 639-641 (1998).
5 Yu, S.-H., Yang, J., Qian, Y.-T. & Yoshimura, M. Optical properties of ZnS nanosheets, ZnO dendrites, and their lamellar precursor ZnS·(NH2CH2CH2NH2) 0.5. Chem. Phys. Lett. 361, 362-366 (2002).
6 Zhao, Z., Geng, F., Cong, H., Bai, J. & Cheng, H.-M. A simple solution route to controlled synthesis of ZnS submicrospheres, nanosheets and nanorods. Nanotechnology 17, 4731 (2006).
7 Sun, Y. et al. Fabrication of flexible and freestanding zinc chalcogenide single layers. Nat. Commun. 3, 1057 (2012).
8 Brunet, L., Caillard, J. & André, P. Thermodynamic Calculation of n-Component Eutectic Mixtures. Int. J. Mod. Phys. C 15, 675-687 (2004).
9 Scheel, H. Crystallization of sulfides from alkali polysulfide fluxes. J. Cryst. Growth 24, 669-673 (1974).
10 Nishinaga, T. & Lieth, R. Thermodynamical calculations on the chemical transport of GaS in a closed tube system. J. Cryst. Growth 20, 109-115 (1973).
11 Lide, D. R. CRC handbook of chemistry and physics. (CRC press, 2004).
12 Kamyshny, A., Gun, J., Rizkov, D., Voitsekovski, T. & Lev, O. Equilibrium distribution of polysulfide ions in aqueous solutions at different temperatures by rapid single phase derivatization. Enviro. Sci. Tech. 41, 2395-2400 (2007).
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13 Yamaguchi, A., Mashima, Y. & Iyoda, T. Reversible Size Control of Liquid-Metal Nanoparticles under Ultrasonication. Ange. Chem. Int. Ed. 54, 12809-12813, doi:10.1002/anie.201506469 (2015).
14 Okorafor, O. C. Solubility and Density Isotherms for the Sodium Sulfate−Water−Methanol System. J. Chem. Eng. Data 44, 488-490, doi:10.1021/je980243v (1999).
15 Chen, X. et al. Kinetically controlled synthesis of wurtzite ZnS nanorods through mild thermolysis of a covalent organic− inorganic network. Inorg. Chem. 42, 3100-3106 (2003).
16 Clark, R. M. et al. Two-step synthesis of luminescent MoS2–ZnS hybrid quantum dots. Nanoscale 7, 16763-16772 (2015).
17 Behl, M. Desulfurization by reactive adsorption on oxide/metal composites, University of Illinois at Urbana-Champaign, (2014).
18 Melhuish, W. Quantum efficiencies of fluorescence of organic substances: effect of solvent and concentration of the fluorescent solute1. J. Phys. Chem. 65, 229-235 (1961).
19 Kitai, A. Luminescent materials and applications. Vol. 25 (John Wiley & Sons, 2008).
20 Patel, S. P., Pivin, J., Chandra, R., Kanjilal, D. & Kumar, L. Intrinsic defects and structural phase of ZnS nanocrystalline thin films: effects of substrate temperature. J. Mater. Sci. Mater. Electron. 27, 5640-5645 (2016).
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Chapter 5
Wafer Scale Two Dimensional Semiconductors from
Printed Oxide Skin of Liquid Metals
5.1 Introduction
As discussed in chapter 2 and expanding upon the concepts investigated in
chapter 4 this chapter discusses the potential to use the naturally occurring
oxide surface of gallium (Ga) as a precursor directly deposited onto an oxide
substrate as a precursor for wafer scale production of gallium sulphide
(GaS).
In this chapter, a new method is developed for the deposition and patterning of
wafer scale 2D post-transition metal chalcogenide (PTMC) compounds. The 2D
gallium sulphide deposition process described here utilises a novel approach of
placing a bulk of gallium (Ga) liquid metal directly onto a silicon dioxide (SiO2)
coated substrate that leaves a layer of 2D oxide of gallium on the wettable areas
(Figure 5.1c,d). There are many properties of Ga that make it a compelling
82
material for this process: liquid Ga has a low bulk viscosity (approximately twice
that of water)1 that flows with ease, and unlike mercury, Ga has low toxicity and
essentially no vapour pressure at room temperature2. Ga and Ga alloys have
previously been used for printing of conducting electrical tracks3-5 however, here
instead of using the bulk metal the oxide skin which is left on the substrate is
utilized. Like many post-transition metals, Ga rapidly (within milliseconds) forms a
thin oxide layer on its surface when exposed to oxygen6. This gallium oxide layer
is initially one-unit cell thick7, which under ambient atmospheric conditions grows
very slowly with time8. The atomically thin film is very robust and can
mechanically stabilise the liquid metal against deformations. The surface oxide
has been shown to adhere strongly to (wet) many materials and particularly to
oxides (e.g. SiO2)9. Certain surface modifications of oxide substrates, such as the
functionalisation with fluorocarbon chains, can inhibit this wetting effect10.
The technique presented here is based on depositing and transforming the native
interfacial metal oxide layer of metal precursors in liquid form. Group III and IV
metals such as Ga, In and Sn have low melting points. It is known that in an oxygen
containing atmosphere, these metals quickly form an atomically-thin protective
and highly wettable oxide layer in a self-limiting reaction6. The presence of the
oxide layer increases the adhesion of the post-transition liquid metal on oxygen
terminated substrates via weak but surface dispersed Van der Waals forces9,11.
After the placement of the liquid metal with its self-limiting skin onto the
substrate, the metal can be removed readily, due to the relatively low binding
energy to its surface oxide skin, which leaves the atomically thin oxide layer
83
behind on the substrate. Harnessing these phenomena, this work devised a model
process that uses low melting point Ga (29.7C) to deposit wafer scale, printable
2D GaS (and also extend it to 2D indium sulphide). Here, the oxide skin of Ga is
placed onto a patterned substrate that are then sulphurised via a relatively low
temperature two-step process, producing large area patterned 2D GaS.
Controlling the surface chemistry of the substrate allows for selective patterning
of the deposited material.
The outcomes of this investigation have been peer reviewed and published in
Nature Communications.
84
5.2 Experimental section
5.2.1Materials
Gallium (Ga) (99.99%), Sulphur (S) (99.98%), Hydrochloric acid (HCl) (36.5-38%),
Hydrobromic acid (HBr) (48%) and 1h-1h-2h-2h-perflourodecyltriethoxysilane
(FDTES) (97%) were purchased from Sigma-Aldrich®. AZ-1512 photoresist, AZ-
400k developer, acetone and isopropyl-alcohol (IPA) were purchased from VWR®
chemicals. Indium (In) (99.99%) was purchased from RotoMetals®. PDMS and
cross-linking agent were purchased from the Dow Corning Corporation. All
chemicals were used without further processing unless otherwise stated.
5.2.2 Preparation of materials
Ga was placed in a 1% v/v solution of HCl to dissolve the surface oxide. PDMS was
cured with a 10:1 ratio (elastomer:crosslinker) and allowed set for a minimum of
48 hours at room temperature under vacuum and then plasma treated under O2
flow. Wafers of 200 nm silicon dioxide (SiO2) on Si (SiO2/Si) were washed with
acetone, IPA and distilled water in sequence, baked at 125°C and cleaned with an
O2 RF plasma cleaner prior to use.
5.2.3 Synthesis of 2D GaS
The Ga in HCl was heated to 60°C and placed onto a SiO2/Si substrate also heated
to 60°C as described in Figure 5.1. The liquid gallium was then spread across the
substrate followed by its removal via wiping using soft PDMS. The gallium oxide
remains on the silicon oxide surface during the wiping process due to the very
85
strong van der Waals adhesion between gallium oxides and the substrate.
However, the liquid gallium has a much weaker adherence to the deposited
gallium oxide and thus can be easily removed by wiping the excess liquid metal,
leaving a thin layer of cracked gallium oxide behind. The printed wafer was then
cooled to room temperature. The substrate was then held directly over warm
(45°C), fuming HCl (37%) for 5 min to achieve chlorination. The chlorinated sample
was then sulphurised via a chemical vapour method in a tube furnace under N2
flow (~100 ccm). For sulphurisation, S powder was placed in a tungsten boat half
way down the furnace and heated to 400°C for 90 min, while the GaCl3 samples
were placed face down on a tungsten boat downstream heated to approximately
300°C. 2D In2S3 was synthesised in the same manner; however, a few minor
changes were applied. Indium metal was heated to 170°C for melting the metal
and during the deposition of the oxide. The oxide was then treated with
hydrobromic acid (HBr) fumes as opposed to HCl. This acid was chosen due to the
more favourable melting temperature of indium bromide which is lower than that
of indium chloride. The HBr treated sample was then sulphurised at 400°C.
5.2.4 Fabrication of structures
AZ 1512 was spin-coated onto a cleaned SiO2/Si wafer at a speed of 6000 rpm to
achieve a thickness of approximately 1 µm. The wafer was then soft baked at 95°C
for 60 s before UV exposure (350-450 nm) through a patterned photomask. The
exposed wafer was then developed in a 1:4 solution of AZ 400K and H2O for 45 s
then rinsed thoroughly with H2O. The wafer with patterned photoresist was then
suspended face down on the roof of a Pyrex® Petri dish containing 200 µl of
86
FDTES. The Petri dish was placed on a 120°C hotplate for 2 hours to functionalise
the exposed SiO2 with a monolayer of fluorocarbon. The roof of the Petri dish was
then flipped and the hotplate heated to 180°C for 30 min to evaporate any excess
FDTES. The wafer was then washed with acetone, IPA and water to remove the
photoresist. The patterned substrate was then printed with Ga as outlined in
section 5.3.3.
5.2.5 Fabrication of transistors
20 nm of tungsten (W) was electron beam (e-beam) deposited onto a SiO2/Si
wafer. AZ 5214E was then spin-coated onto the W coated wafer at a speed of
4000 rpm. The wafer was then soft baked at 110°C for 60 s before UV exposure
through a patterned photomask. This step was followed by reverse baking of the
wafer at 120°C for 120 s followed by flood exposure to UV. The exposed wafer
was then developed in a 1:4 solution of AZ 400K and H2O for 30 s then rinsed
thoroughly with H2O. The substrate was then placed in 50 ml of a 30% solution of
H2O2 and ~100mg of NaOH for 10 s to remove all unprotected W. 2D GaS was then
printed as outlined in section 5.3.3.
5.2.6 Characterisation:
X-ray photoelectron spectroscopy (XPS) was performed using a Themo Scientific
K-alpha system. The X-ray source was a monochromated Al Kα source. EDX
spectra were collected using a FEI Nova nano-SEM fitted with AZTEC X-ray
detector. Spectra were collected with a 3 KeV accelerating voltage chosen in order
to provide sufficient range of measurement whilst attempting to limit substrate
87
contributions. XRD was performed using a Bruker D4 Endeavour featuring a CuKα
X-ray source ( = 1.54 Å). Raman measurements were conducted utilising two
systems a WITec alpha 300R and a Horiba Scientific LabRAM HR evolution Raman
spectrometer both using a 100× objective, a 532 nm laser with a spot diameter of
∼500 nm was used.
AFM images were collected using a Bruker Dimension Icon AFM with scanasyst®
software in peak-force tapping mode. Confocal PL maps were gathered with a
Nikon N-STORM in confocal mode with excitations at 488 and 640 nm.
PL and UV-Vis spectroscopy were conducted on GaS printed on quartz substrates
as opposed to Si/SiO2, the method was otherwise unchanged. PL spectra were
collected using a PerkinElmer LS50 and were averaged over 10 scans. UV-Vis
measurements were performed on a PerkinElmer Lambda 1050 fitted with an
integrating sphere. Additional PL measurements including decay-time
measurements were conducted using on a custom build instrument using a
Fianium, WhiteLase SC400-8 supercontinuum optical source delivering
approximately 200 W average power to the sample with a 10 nm optical
bandwidth and a 100× air objective. A waveplate polariser was incorporated into
the excitation path to obtain polarised PL maps. Polarised maps were collected
successively in increments of 5°.
TEM images were taken using a JEOL 2100F STEM with a 200 kV accelerating
voltage. Preparation of TEM samples involved exfoliating flakes from the printed
GaS film by gently scraping with a razor blade. Photoelectron spectroscopy in-air
(PESA) was conducted using a Riken Keiki AC-2, photoelectron spectrometer. FET
88
and photodetector characterisations were performed using a Keithely SCS 4200
semiconductor Characterisation system, Keithely 2001 multimeter, and a ABET
technologies 150 W Xenon light source. Space charge limited currents (SCLC) were
measured using a Keithly 2600 source meter.
5.2.7 Computational analysis of band structure
Spin-dependent hybrid density functional theory (Hybrid DFT) calculations were
performed using Gaussian basis set ab initio package CRYSTAL14.12 The B3LYP
hybrid exchange-correlation functional was used13, augmented with an empirical
London-type correction to the energy to include dispersion contributions to the
total energy. The correction term was the one proposed by Grimme14 and
successfully used with B3LYP to calculate cohesive energies in dispersion bonded
molecular crystals.15 For all atoms a triple zeta valance (TZV) basis set, with
polarisation functions, was used to model the electrons.16,17 A 9×9×9 Monkhorst-
Pack k-point mesh18 was used to calculate band structure and total electronic
density of states for GaS bulk, while a 9×9×1 k-point mesh was used for the mono-
layer, bi-layer, tri-layer and four-layer slabs. Each structure was each
independently geometry optimized prior to calculation of the electronic
properties.
89
5.3 Results and Discussion
90
Figure 5.1. GaS representation and printing process of the 2D layers: stick and ball
representation of GaS crystal: (a) side view of bilayer GaS, showing a unit cell c =
15.492Å made of two GaS layers. (b) Top view of the GaS crystal. The GaS crystal
lattice is composed of Ga-Ga and Ga-S covalent bonds, which extend in two
dimensions, forming a stratified crystal made of planes that are held together by
van der Waals attractions. The Ga atoms are green and the S atoms are blue. (c
and d) Functionalisation of the substrate with FDTES changes the contact angle
between a Ga drop and the substrate by 12.9°. The SiO2 substrate becomes more
hydrophobic and thus resists wetting by liquid Ga. (e to h) Schematics of the
synthesis process for patterning 2D GaS via printing the skin oxide of liquid Ga. (e)
Lithography process for establishing the negative pattern of the photoresist. (f)
Covering the exposed area of the substrate with vaporised FDTES. (g) Placing Ga
liquid metal and removing it with soft PDMS that leaves a cracked layer of Ga
oxide. (h) Two concurrent steps of chemical vapour treatment: firstly, GaCl3 layer
is formed via exposure to HCl vapour and secondly, sulphurisation by exposure to
S vapour forming GaS.
In this work, liquid Ga is placed on a SiO2/Si substrate (as shown in Figure 5.1e-h
and described in the Experimental section), leaving an atomically thin-cracked
layer of gallium oxide behind, upon removal of the liquid metal (Figure 5.2). As
the liquid gallium/gallium oxide is printed, a scattered oxide layer appears with a
strong adherence on the substrate. This cracked thin film of flat gallium oxide
nano-flakes allows for the easy removal of the liquid metal by scraping with a slab
91
of PDMS. This process is implementable even as the liquid metal is printed fresh
because of the rapid formation of the oxide layer.
Selective area deposition of gallium oxide is achieved by selective treatment of
the substrate using 1h-1h-2h-2h-perflourodecyltriethoxysilane (FDTES), as shown
in Figure 5.1 e&f, resulting in non-wettable regions on the surface. The treated
areas of the substrate prohibit the adhesion of the 2D gallium oxide layer (Figure
5.1g), effectively facilitating selected area deposition. The printed pattern is found
to be visible by eye, due to interference effects (Figure 5.1g inset).
Figure 5.2. (a) AFM image of cracked layer of gallium oxide and corresponding
profile (b).
A hydrochloric acid (HCl) vapour treatment (Figure 5.1h) is used as an
intermediary step between patterned oxide deposition and sulphurisation of the
92
oxide. The HCl vapour transforms the thin gallium oxide layer into primarily
GaCl39,19, which aids subsequent sulphurisation via two means. Firstly, gallium
oxide is chemically inert and its direct sulphurisation requires high temperatures
(>900C) together with exposure to toxic H2S gas6. The reduced process
temperature is highly advantageous since it allows the inclusion of a broad range
of substrates, including certain polymers and glasses. Secondly, the temperature
used herein during sulphurisation is still higher than the melting point of the
halide, thus encouraging efficient recrystallisation and continuity in the resultant
2D layer. Eventually, sulphurisation of GaCl3 (Figure 5.1h) is conducted at low
temperature (~300°C) that is compatible with existing electronic device
processing standards. This process can be extended to forming other 2D PTMCs.
An example for depositing atomically thin In2S3 is also presented in Figure 5.3,
here it can be seen that the In2S3 film is smooth and continuous in a relatively
large area range. The thickness of the 2D In2S3 is on average 2.6 nm, which is about
three times the fundamental plane thickness (0.9 nm) of this material.
93
Figure 5.3: Analysis of 2D In2S3 synthesised via printing of liquid indium. (a) Raman
spectrum, (b) Indium 3d region of X-ray photoelectron spectrum (XPS) and (c)
AFM image including profile (d) along the blue line shown in (c).
94
Figure 5.4. Material characterisations of printed GaS: (a) X-ray Diffraction (XRD)
of printed GaS film (after multiple printing steps) with XRD of the printed oxide
skin and SiO2/Si substrate for comparison and GaS planes indicated (traces have
been normalised and offset for clarity). (b) Raman spectrum of the 2D GaS with
GaS vibrational peak shifts indicated. (c & d) XPS of the 2D GaS for the regions of
interest (c) Ga 3d and (d) S 2p.
X-ray diffraction (XRD) is utilised to assess the obtained phase and phase purity of
the sulphurised few-layer films (several layers of GaS are sequentially printed to
increase the XRD signal intensity). The XRD patterns shown in Figure 5.4 present
GaS peaks and the lack of peaks from other stoichiometries (e.g. Ga2S3)
demonstrate that the synthesised film has a stratified structure and the desired
95
monosulphide phase20-23. Furthermore, The GaS peak corresponding to the 002
plane (11.6°) reveals a layer spacing of 7.62 Å, which is in good agreement with
previous studies20,23 Raman spectroscopy (Figure 5.4b) conducted on a single
printed film reveals the characteristic features of a few layer thick GaS, including
the A11g (187.5 cm-1), A1
2g (374.8 cm-1) and E12g (303.75 cm-1) vibrational peaks,
matching those reported previously20,24. The observed relative intensity of the
phonon peaks between the out-of-plane (A11g, A1
2g) and in-plane (E12g) vibrations
of 0.25 and 0.56 for A11g/E1
2g and A12g/E1
2g respectively, are uniquely associated
with bilayer GaS23,24. X-ray photoelectron spectroscopy (XPS) further confirms the
stoichiometry of the printed layer. The main Ga 3d peaks (Figure 5.4c) are
associated with stoichiometric GaS (Ga2+), with minor contributions from Ga at
other oxidation states such as Ga0 and Ga3+. A small peak from elemental Ga is
also observed in the Ga 3d region, which has been associated with the slow
decomposition of GaS in moist air.20 The S 2p and Ga 3s peaks (Figure 5.4d) as well
as analysis of the Cl 2p and O 1s ranges (Figure 5.5) further confirm the
stoichiometry of the 2D material.
The O 1s range (Figure 5.5c) of the XPS spectrum demonstrates a significant
contribution from SiO2 (532.65 eV) while there are other minor contributions from
other oxides including gallium-oxides20. The Cl 2p range (Figure 5.5b) of the XPS
spectra also depicts some residual Cl. The intensity of this peak however (~1000
CPS above baseline ~5800 CPS) is almost negligible when compared to that of Ga
or S peaks which are significantly more intense (~16000 and ~8000 CPS above
96
baseline respectively). XPS analysis of the printed 2D GaOx and GaCl3 are shown in
Figure 5.6.
Figure 5.5. (a-c) XPS study of GaS film displaying (a) survey spectrum, (b) Cl 2p and
(c) O 1s regions of the spectrum. (d-f) EDX of printed 2D GaOx film (d), the film
after treatment with HCl (e) and after sulphurisation (f).
97
Figure 5.6. XPS patterns of the printed metal oxide film before (a-e) and after (f-j)
treatment with HCl vapour. (a & f) Survey, (b & g) Ga 3d, (c & h) Ga 3s, (d & i) Cl
2p and (e & j) O 1s regions.
98
Figure 5.7. Morphology and characteristics of the printed 2D films: (a) AFM
image with (b) height profile) of the printed 2D GaS layer confirming bilayer
deposition. The profile is offset to the substrate’s surface. (c&d) Confocal PL map
shows a patterned area made of 2D GaS (green) on a SiO2/Si substrate (black)
demonstrating a near-uniform 2D layer is obtained in a large area along with the
PL spectra from areas noted on map (c) patterns 1 and 2). Due to the excitation
wavelength limitation of confocal PL, the deep trap emissions are used for
assessing the uniformity of the PL pattern. (e) PL emission spectra demonstrating
contributions of the interband transitions (red) and deep trap recombinations
(black). The deep trap emission spectrum is scaled by a factor of 10 for clarity. (f)
HRTEM of a flake mechanically scratched from the printed GaS film.
99
Atomic force microscopy (AFM, Figure 5.7a-b) reveals that the films have a
thickness of approximately 1.5 nm, corresponding to two layers of GaS, which is
in good agreement with the Raman spectrum. High resolution transmission
electron microscopy (HRTEM) (Figure 3f) shows a lattice spacing of 3.1 Å which
correlates to the 101 plane of GaS21.
Confocal PL microscopy (Figure 5.7d) verifies that the GaS has been successfully
patterned. This PL is only seen after sulphurisation (Figure 5.7c-e). The PL has two
major components centred at 393 and 641 nm. The peak at 393 nm is due to the
direct bandgap recombination at the Γ point. The energy difference associated
with this peak is in excellent agreement with our density functional theory (see
section 5.3.7 for computation information) band structure calculations of bilayer
GaS (Figure 5.12a). Additionally, defects in GaS crystals are known to produce
deep trap recombination states25, which are associated with the 641 nm peak.
One of the main outcomes of this work is the deposition of patterned 2D GaS
displaying near-uniform PL on a large scale with consistent PL intensity (Figure
5.7c). The dimensions of the patterned area shown in Figure 5.7d exceed 1 mm,
proving that 2D GaS covering areas of mm2 can be readily achieved.
100
Figure 5.8. AFM image (a) of 2D GaS film and statistical distribution of the
measured pixel heights (b) of the substrate (blue) and film (red) within the areas
in the blue and red squares, respectively. Scale bar in (a) is 5 µm.
The surface of the 2D film is also shown to be atomically smooth (Figure 5.8). The
statistical distribution the 2D GaS shows that the film is highly continuous and the
height distribution of the substrate and the film are similar for values below than
the mean height. However, above the mean, the distribution is extended further
to the higher values for the substrate measurement leading to the skewed
appearance. This is due to the hydrophobic nature of the substrate which leads to
the deposition of some particles on its surface. In comparison, the surface of 2D
GaS is much cleaner than the substrate, indicating controlled film growth. The
standard deviation for the 2D GaS film is 0.137 nm which is associated with a
101
continuous deposited film and the absence of nano or sub-micron size cracks as
well as the absence of regions of overlapping growth that could occur at domain
boundaries. Furthermore, there is no contribution from the substrate level (0 nm)
in the red pattern attesting the absence of cracks and holes even further.
Figure 5.9. Statistical Raman analysis of the 2D GaS films. Raman line scan analysis.
(a) Optical image, the line scan was performed on the black line indicated. (b)
Contour plot with phonon modes indicated. (c) Relative intensity of the
E12gphonon peak with standard error indicated.
A statistical analysis using Raman spectroscopy investigating the uniformity of the
bilayer GaS is also in Figure 5.9. The measurements and analysis show a good
102
uniformity for the deposited bilayer GaS. The relative intensity of the E12g peak is
shown to have a standard error of only 2.5% (Figure 5.9c) across an area spanning
100 µm.
Raman spectra of 2D GaS (Figure 5.10a) spaced at much wider area than
presented in Figure 5.9. The relative intensity of the E12g peak only slightly varies
slightly when macroscopic distances of millimetres are investigated,
demonstrating the strong consistency of the bilayer GaS across a large surface
area. It can be seen from (Figure 5.10b) that the boundaries are laterally
connected with no overlap, however there are significant lattice defects around
the boundaries. Furthermore, from (Figure 5.10c) the domain sizes are seen to
range from ~1 µm2 to 10’s of µm2.
Figure 5.10. (a) Raman spectra of 2D GaS spaced at approximately 1 mm intervals.
(b & c) Analysis of crystal grains of the printed 2D GaS film featuring (b) a HRTEM
image of a grain boundary and (c) a map of grain boundaries determined via
polarised confocal PL as has been previously demonstrated by Pan et al26.
Different shades of grey indicate areas that luminesce at different polarisation
angles which are associated with crystal domains and their lattice orientation
within the 2D GaS layer.
103
Additional HRTEM image are presented in Figure 5.11a as part of an investigation
of the concentration and nature of defects. The dislocation points shown in
Figure 5.11a introduce strain which cause bent lattice planes around the
dislocation points.27 From analysing the dislocations seen in many HRTEM images
the defect density was calculated to be ~6.25×1011 cm-2. Using the threshold
voltage of the trap filled limited (TFL) region of the space charge limited current
(SCLC, Figure 5.11b) and the method as outlined by Shi et al28, a trap density of
~2.85×1011 cm-2 was determined which is in agreement with the determined
defect density from HRTEM.
104
Figure 5.11. (a) HRTEM of a GaS flake scratched from the printed film. Structural
defects are visible in the crystal, which are mainly ascribed to lattice dislocations
(example in the red square). Inset: magnified image highlighting the dislocation of
the crystal lattice. (b) SCLC measurement of 2D GaS. Ohmic and trap filled limited
TFL regions are marked with their respective colours.
105
Figure 5.12. Optical and electronic properties of 2D GaS. Hybrid DFT computation
of the band structures in bilayer (a) and bulk (b) GaS. (c) Tauc plot used for
determining the electronic band transition with simplified electronic band
diagram (inset). Red lines are the valence and conduction band edges, black is the
Fermi level and blue is the location of the deep trap band. (d) PESA demonstrating
the valence band maximum (VBM) energy is −5.15 eV. (e) XPS valence analysis
revealing an energy difference of 1.8 eV between the VBM and Fermi level. (f) PL
decay with fitted exponential (red) and mean excitonic decay (blue) time
indicated.
DFT calculations show indirect and direct recombination paths of 3.08 and
3.29 eV, respectively (Figure 5.12a). The bandgap of the 2D GaS film is determined
as 3.02 eV, using UV-Vis absorption spectroscopy (Figure 5.12c). The measured
bandgap is in good agreement with the indirect transition of bilayer GaS further
106
evidencing the near-uniform nature of the films. Significant PL is also experienced
(Figure 5.7e). The measured PL values (from deep trap emissions and direct
recombinations) are within suitable wavelength for establishing optical devices in
the middle and high end of the visible range. To obtain a more comprehensive
picture of the semiconducting properties for the printed 2D GaS, the band
structure of the material is analysed utilising the PL measurements (Figure 5.7e)
in combination with photo-electron spectroscopy in-air (PESA) and XPS valence
spectra analysis (Figures 5.12d-e). PESA determines a valence band maximum
energy of 5.15 eV and the XPS valance spectrum determined the Fermi level to
reside at 3.35 eV, leading to the energy diagram shown in Figure 5.12c inset. This
shows that the material is n-doped. It is also possible to obtain the p-doped areas
of this 2D GaS by selectively exposing it to gases such as NO229, in order to obtain
the complimentary semiconductor for creating electronics. For comparison, the
DFT calculations for three and four-layer GaS are presented in Figure 5.13.
Figure 5.13. Hybrid DFT computation of the band structures in (a) tri-layer, and
(b) four-layer GaS.
107
The PL decay of deep trap emissions (Figure 5.12f) within 2D GaS show a mean
excitonic recombination time of 315 ps, which is faster than many other 2D
semiconductors including 2D MoS2 (~800 ps)30,31.
Figure 5.14. Schematic representation for device fabrication. (a) Clean SiO2/Si
substrate. (b) Tungsten (W) film e-beam deposited onto the substrate. (c)
Photoresist is deposited and then exposed through a photolithography mask. (d)
Remaining photoresist after image reversal and developing. (e) Immersion of the
of the unprotected W in the etchant. (f) Sample after etching and lift-off. (g)
Deposition of liquid Ga. (h) Formation of 2D GaOx between electrodes after Ga is
removed. (i) Exposure to HCl vapour. (j) GaCl3 film between electrodes. (k)
Exposure to S vapour. (l) Final device. Extra photolithography steps can also be
included as presented in Figure 5.1.
108
Figure 5.15. Characterisation of 2D GaS FETs. (a) Schematic representation of back
gated GaS FET with tungsten/WS2 electrodes featuring band energy diagram. (b)
Raman spectrum of the W electrodes after sulphurisation. Phonon modes of WS2
are indicated demonstrating the growth of WS2 on the skin of the electrodes. (c)
optical images of the fabricated devices and electrode gap (inset). The scale bars
are 500 µm and 20 µm respectively. (d) I/V characteristics of the FET at different
gate voltages. (e) I/V phototransistor characteristics with no gate bias.
To investigate the potential for electronic and optoelectronic devices, field effect
transistors (FETs) are developed via the deposition of tungsten (W) electrodes
109
prior to deposition of Ga (Figure 5.15a,c). This fabrication process of the devices
is outlined in the experimental section and shown in Figure 5.14. W is chosen as
an electrode material for several reasons. Firstly, Ga does not amalgamate with
W as it does with many other metals32. Secondly, W readily forms WS2 under the
same conditions which is use for sulphurising the gallium oxide layer
(Figure 5.15b). WS2 is known to act as a hole injector which can be used to
overcome the Schottky barrier33. A maximum electron mobility of ~0.2 cm2 V-1 s-1
is obtained, which is better than previously reported mobilities for GaS22 and
higher than that of many other printable semiconductors34. Five FETs at different
locations were tested and the mobility measurements have been consistent. The
on/off ratios of the FETs are above ~150 (Figure 5.15d). Furthermore, a median
optical on/off ratios of ~170 in phototransistors is achieved (Figure 5.15e) and a
median responsivity of ~6.4 A W-1 under a solar simulator, which is comparable
with previous reports23. A statistical analysis of the performance of many
phototransistors on fabricated using the here reported wafer-scale process was
conducted. 66 devices were fabricated across two ~1×4 cm substrates (30 and 36
devices, respectively). Of these devices 59 were functional (27 and 32 on each
substrate, respectively) resulting in an average success rate of 89.4% for the
device fabrication. This distribution of responsivity and optical on/off ratio are
shown in Figure 5.16.
110
Figure 5.16. Statistical analysis of photodetector devices. (a) Responsivity of each
individual photodetector device and (b) distribution of responsivities. (c) On/Off
ratio of each device and (d) distribution of ratios.
111
5.4 Conclusions
This chapter describes a printing method that allows the selective patterning of
atomically thin semiconducting layers of 2D PTMCs. The process relies on the
efficient transformation of an ultra-thin oxide layer on the surface of liquid
elemental gallium onto an oxide-coated substrate. Low temperature
sulphurisation leads to the formation of highly uniformly distributed
semiconducting GaS bilayers of ~1.5 nm thickness. There are several advantages
of this method including the relative simplicity and low temperature, cost
effectiveness and scalability of the process (a comparative assessment is
presented in Table 5.1). The method is offers etchless patterning and deposition
of 2D semiconductors on a wafer scale. Furthermore, the development of
electronic and optical devices based on 2D GaS were demonstrated, with the
performance better than or comparable to previous reports on exfoliated GaS
layers. This process for printing 2D semiconductors, which can be extended to the
deposition of other 2D PTMCs and will provide a new paradigm for future
electronics, optics, optoelectronics, sensors and other devices.
112
Table 5.1. Comparison of established methods for deposition of 2D materials.
Temperature (°C)
Time (hours)
Materials Lateral dimensions
Comments References
Atomic layer deposition (ALD)
30035 ~235 MoS235 µm’s35 Requires
annealing at 800°C35
35 Tan et al 2014
Chemical vapour deposition (CVD)
80036, 85037
~236, ~137
WS236,
MoS237
µm’s36,37 36 Elias et al 2013 37Jeon et al 2015
Metalorganic CVD
55038 ~2638 MoS238 µm’s-
mm’s38 38 Kang et al
2015
Thermolysis 100039 1.539 MoS239 mm’s39 39 Liu et al
2012
Mechanical Exfoliation
GaS22, MoS2
40,
WSe240
µm’s-10s µm22,40
Difficult to create reproducibility
22 Late et al 2012 40 Li et al 2015
Liquid phase epitaxy (LPE)
~920 GaS20, MoS2
41, WS2
41, h-BN41
nm’s- 100s nm20, 10s nm-µm’s41
20 Harvey et al 2015. 41 Coleman et al 2011
This work 300 ~2 GaS, In2S3 mm’s
This work can be possibly expanded to create other types of 2D metal compounds.
Gallium can alloy a large number of different metals. As such, the self-limiting
oxide layer can be tuned depending on the type and concentration of the alloyed
metal, allowing the possibility of printing a large variety of pure and mixed oxides.
The 2D oxides can be deposited and patterned on wafer scale and
chalcogenetated. This means that gallium (or many other low melting liquid
metals) can be potentially used as a reaction media to establish desired 2D metal
based compounds which should be further investigated.
113
5.5 References
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4 Boley, J. W., White, E. L. & Kramer, R. K. Mechanically sintered gallium–indium nanoparticles. Adv. Mater. 27, 2355-2360 (2015).
5 Wang, Q., Yu, Y., Yang, J. & Liu, J. Fast Fabrication of Flexible Functional Circuits Based on Liquid Metal Dual‐Trans Printing. Adv. Mater. 27, 7109-7116 (2015).
6 Downs, A. J. Chemistry of aluminium, gallium, indium and thallium. (Springer Science & Business Media, 1993).
7 Regan, M. et al. X-ray study of the oxidation of liquid-gallium surfaces. Phys. Rev. B 55, 10786 (1997).
8 Plech, A., Klemradt, U., Metzger, H. & Peisl, J. In situ x-ray reflectivity study of the oxidation kinetics of liquid gallium and the liquid alloy. J. Phys. Condens. Matter. 10, 971 (1998).
9 Doudrick, K. et al. Different shades of oxide: From nanoscale wetting mechanisms to contact printing of gallium-based liquid metals. Langmuir 30, 6867-6877 (2014).
10 Yoon, Y., Kim, D. & Lee, J.-B. Hierarchical micro/nano structures for super-hydrophobic surfaces and super-lyophobic surface against liquid metal. Micro Nano Syst. Lett. 2, 1-18 (2014).
11 Dickey, M. D. Emerging Applications of Liquid Metals Featuring Surface Oxides. ACS App. Mater. Interfaces 6, 18369-18379 (2014).
12 Dovesi, R. et al. CRYSTAL14: A program for the ab initio investigation of crystalline solids. Int. J. Quantum Chem. 114, 1287-1317 (2014).
13 Becke, A. D. Density‐functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648-5652 (1993).
14 Grimme, S. Semiempirical GGA‐type density functional constructed with a long‐range dispersion correction. J. Comput. Chem. 27, 1787-1799 (2006).
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15 Civalleri, B., Zicovich-Wilson, C. M., Valenzano, L. & Ugliengo, P. B3LYP augmented with an empirical dispersion term (B3LYP-D*) as applied to molecular crystals. Cryst. Eng. Comm. 10, 405-410 (2008).
16 Peintinger, M. F., Oliveira, D. V. & Bredow, T. Consistent Gaussian basis sets of triple‐zeta valence with polarization quality for solid‐state calculations. J. Comput. Chem. 34, 451-459 (2013).
17 Novoselov, K. S. et al. Two-dimensional gas of massless Dirac fermions in graphene. Nature 438, 197-200 (2005).
18 Monkhorst, H. J. & Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 13, 5188 (1976).
19 Kim, D. et al. Recovery of nonwetting characteristics by surface modification of gallium-based liquid metal droplets using hydrochloric acid vapor. ACS Appl. Mater. Interfaces 5, 179-185 (2012).
20 Harvey, A. et al. Preparation of Gallium Sulfide Nanosheets by Liquid Exfoliation and Their Application As Hydrogen Evolution Catalysts. Chem. Mater. 27, 3483-3493 (2015).
21 Shen, G., Chen, D., Chen, P.-C. & Zhou, C. Vapor-Solid Growth of One-Dimensional Layer-Structured Gallium Sulfide Nanostructures. Acs Nano 3, 1115-1120 (2009).
22 Late, D. J. et al. GaS and GaSe ultrathin layer transistors. Adv. Mater. 24, 3549-3554 (2012).
23 Hu, P. et al. Highly Responsive Ultrathin GaS Nanosheet Photodetectors on Rigid and Flexible Substrates. Nano Lett. 13, 1649-1654 (2013).
24 Jung, C. S. et al. Red-to-ultraviolet emission tuning of two-dimensional gallium sulfide/selenide. ACS Nano 9, 9585–9593 (2015).
25 Aydinli, A., Gasanly, N. & Gökşen, K. Donor–acceptor pair recombination in gallium sulfide. J. Appl. Phys. 88, 7144-7149 (2000).
26 Yang, S. et al. High performance few-layer GaS photodetector and its unique photo-response in different gas environments. Nanoscale 6, 2582-2587 (2014).
27 Shi, H. et al. Exciton dynamics in monolayer and few-layer MoS2 2D crystals. ACS Nano 7, 1072–1080 (2014).
28 Yuan, L. & Huang, L. Exciton dynamics and annihilation in WS2 2D semiconductors. Nanoscale 7, 7402-7408 (2015).
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29 Liu, T., Sen, P. & Kim, C.-J. Characterization of nontoxic liquid-metal alloy galinstan for applications in microdevices. J. Microelectromech. Syst. 21, 443-450 (2012).
30 Jin, Z., Li, X., Mullen, J. T. & Kim, K. W. Intrinsic transport properties of electrons and holes in monolayer transition-metal dichalcogenides. Phys. Rev. B 90, 045422 (2014).
31 Kwak, D., Lim, J. A., Kang, B., Lee, W. H. & Cho, K. Self‐Organization of Inkjet‐Printed Organic Semiconductor Films Prepared in Inkjet‐Etched Microwells. Adv. Funct. Mater. 23, 5224-5231 (2013).
32 Tan, L. K. et al. Atomic layer deposition of a MoS2 film. Nanoscale 6, 10584-10588 (2014).
33 Elias, A. L. et al. Controlled synthesis and transfer of large-area WS2 sheets: from single layer to few layers. Acs Nano 7, 5235-5242 (2013).
34 Jeon, J. et al. Layer-controlled CVD growth of large-area two-dimensional MoS2 films. Nanoscale 7, 1688-1695 (2015).
35 Kang, K. et al. High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity. Nature 520, 656-660 (2015).
36 Liu, K.-K. et al. Growth of large-area and highly crystalline MoS2 thin layers on insulating substrates. Nano letters 12, 1538-1544 (2012).
37 Li, H., Wu, J., Yin, Z. & Zhang, H. Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and WSe2 nanosheets. Acc. Chem. Res. 47, 1067-1075 (2014).
38 Coleman, J. N. et al. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 331, 568-571 (2011).
39 Mayrhofer, P. H., Mitterer, C., Hultman, L. & Clemens, H. Microstructural design of hard coatings. Prog. Mater. Sci. 51, 1032-1114 (2006).
40 Shi, D. et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 347, 519-522 (2015).
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116
Chapter 6
Conclusions and Future Work
6.1 Conclusions
The work presented in this PhD thesis was intended to investigate the
research gaps presented in chapter 2 and overcome the present challenges
outlined therein. As outlined in chapter 1 this research was undertaken to
explore potential pathways for the future development and
industrialisation of two-dimensional (2D) semiconductors.
The research outlined within this thesis focused on two main objectives; the
first was to investigate solvents’ effects on the exfoliation of layered
materials by ultrasonication and the changes the solvent imbue upon
colloidal suspensions of 2D semiconductors, and the second was to explore
the concept of novel, pragmatic synthesis of 2D semiconductors via the
utilisation of the surface chemistry of certain metals (e.g. gallium). This
117
research was organised and pursued in three stages presented in chapters
3, 4 and 5, respectively.
6.1.1 Stage 1
The work presented in this thesis (chapter 3) investigates colloidal suspensions
of 2D semiconductors, the effects that solvents have during synthesis and the
impacts solvents can have upon the material properties. Therein the major
conclusions arrive from the comparison of a new method combining two well
known yet low yield solvents with the high yield, well known solvent n-methyl-
2-pyridine. Using Raman and UV-Vis absorption analysis it becomes apparent
that residues from solvents can adversely affect the desired characteristics.
This work aims to highlight the importance solvent particularly during
ultrasound exfoliation of 2D materials which has been poorly understood
prior. This stage demonstrates the importance of solvent in colloidal systems
of 2D semiconductors and needs that should be considered
6.1.2 Stage 2
While the previous stage deals with exfoliation of stratified crystals for
producing high yield suspensions of 2D semiconductors, this stage focuses
on producing non-stratified systems.
This stage demonstrates how a Ga based alloy can be used as a reaction
medium for synthesising 2D ZnS. The reaction relies on the chemistry
occurring at the liquid/liquid interface between the reagent and alloy which
118
results in the formation of 2D nanosheets. ZnS is not a stratified material
and hence, conventional 2D synthesis techniques, such as exfoliation
cannot be applied. Thus, this stage introduces room-temperature liquid
metals as suitable reaction media for synthesising ultrathin sheets. The
process can be explored for other metals, and likely granting access to many
other 2D compounds that are currently inaccessible due to a lack of
stratified crystal systems.
6.1.3 Stage 3
This final stage investigates the transformation of the ultra-thin oxide layer
on the surface of liquid elemental gallium onto an oxide-coated substrate
and subsequent sulphurisation for the wafer scale formation of
semiconducting 2D gallium sulphide (GaS). This method is found to be highly
versatile allowing for patterning via pre-functionalisation of the substrate
and device fabrication via pre-deposition of electrodes. Several advantages
of this method such as the relative simplicity and low temperature, cost
effectiveness and scalability of the process provide much potential for
future development and industrialisation. The method is compatible with
many current industrial processes. This method can potentially be extended
to the deposition of other 2D post-transition metal chalcogenides for future
development.
119
6.2 Future Work
The research presented in this Ph.D. thesis open opportunities for future
investigation further extending the scope of this work.
The work presented in chapter 4 in which a Ga based alloy was used as a medium
for synthesising zinc sulphide, has a lot of potential for expansion into many
further materials by manipulating the interfacial chemistry. Hypothetically the
methodology outlined in this chapter could be expanded to produce 2D sheets of
many other compounds either by changing the composition of the metal alloy or
by selecting different reagents. It may be possible to produce 2D sheets based on
alternative metals alloyed into Ga alloys or potentially even tin (Sn) and bismuth
(Bi) based alloys and utilising the correct reagents may be possible to create
suspensions with many different combinations.
The work presented in chapter 5 where Ga and indium (In) were used to produce
large areas of atomically thin gallium and indium sulphides (GaS and In2S3
respectively), presents many potential avenues for further work. The process
outlined in chapter 5 could potentially be expanded to many other low melting-
point metals such as Sn and Bi. Furthermore, investigation into low melting-point
metal alloys such as Ga and Bi based alloys which could hypothetically enable a
vast number of atomically thin metal compounds, although this would require
extensive investigation into the chemical makeup of the atomically thin oxide
deposited. There are also possibilities to expand upon the chemical reactants that
120
the oxide layer is exposed to resulting in different atomically thin metal
compounds
6.3 Journal Publications
This Ph.D. research resulting in the follow articles published in peer
reviewed journals.
6.3.1 First Author Publications (presented in this thesis).
Wafer-scale two-dimensional semiconductors from printed oxide skin of
liquid metals.
Carey, B. J.; Ou, J. Z.; Clark, R. M.; Berean, K. J.; Zavabeti, A.; Chesman, A. S.;
Russo, S. P.; Lau, D. W.; Xu, Z.-Q.; Bao, Q.; Kahevei, O.; Gibson B. C.; Dickey,
M. D.; Kaner, R. B. Daeneke, T.; Kalantar-zadeh, K., Nat. Commun. 2017, 8,
14482.
Two solvent grinding sonication method for the synthesis of two-
dimensional tungsten disulphide flakes.
Carey, B. J.; Daeneke, T.; Nguyen, E. P.; Wang, Y.; Ou, J. Z.; Zhuiykov, S.;
Kalantar-zadeh, K., Chem. Commun. 2015, 51 (18), 3770-3773.
A Liquid Metal Flow-Reaction for the Synthesis of Ultra-Thin Zinc Sulfide.
Carey B. J.; Zavabeti, A.; Mohiuddin, M.; Chesman, A. S. R.; Kalantar-zadeh K.;
Daeneke, T., Chem. Eur. J. 2017, (Under Review)
6.3.2 Co-author Publications (not presented in this thesis).
Exfoliation solvent dependent plasmon resonances in two-dimensional sub-
stoichiometric molybdenum oxide nanoflakes.
Alsaif, M. M.; Field, M. R.; Daeneke, T.; Chrimes, A. F.; Zhang, W.; Carey, B. J.;
Berean, K. J.; Walia, S.; van Embden, J.; Zhang, B., ACS App Mater Interfaces
2016, 8 (5), 3482-3493.
121
2D WS 2/carbon dot hybrids with enhanced photocatalytic activity.
Atkin, P.; Daeneke, T.; Wang, Y.; Carey, B. J.; Berean, K.; Clark, R.; Ou, J.;
Trinchi, A.; Cole, I.; Kalantar-Zadeh, K., Journal of Materials Chemistry A 2016,
4 (35), 13563-13571.
2D MoS2 PDMS Nanocomposites for NO2 Separation.
Berean, K. J.; Ou, J. Z.; Daeneke, T.; Carey, B. J.; Nguyen, E. P.; Wang, Y.; Russo,
S. P.; Kaner, R. B.; Kalantar‐zadeh, K., Small 2015, 11 (38), 5035-5040.
Patterned films from exfoliated two-dimensional transition metal
dichalcogenides assembled at a liquid–liquid interface.
Clark, R.; Berean, K.; Carey, B. J.; Pillai, N.; Daeneke, T.; Cole, I.; Latham, K.;
Kalantar-zadeh, K., J. Mater. Chem. C 2017, 5 (28), 6937-6944.
Two-step synthesis of luminescent MoS2–ZnS hybrid quantum dots.
Clark, R. M.; Carey, B. J.; Daeneke, T.; Atkin, P.; Bhaskaran, M.; Latham, K.;
Cole, I. S.; Kalantar-Zadeh, K., Nanoscale 2015, 7 (40), 16763-16772.
Exfoliation of quasi-stratified Bi2S3 crystals into micron-scale ultrathin
corrugated nanosheets.
Clark, R. M.; Kotsakidis, J. C.; Weber, B.; Berean, K. J.; Carey, B. J.; Field, M. R.;
Khan, H.; Ou, J. Z.; Ahmed, T.; Harrison, C. J., Chem. Mater. 2016.
Light driven growth of silver nanoplatelets on 2D MoS2 nanosheet
templates.
Daeneke, T.; Carey, B. J.; Chrimes, A.; Ou, J. Z.; Lau, D.; Gibson, B.; Bhaskaran,
M.; Kalantar-Zadeh, K., Journal of Materials Chemistry C 2015, 3 (18), 4771-
4778.
Reductive exfoliation of substoichiometric MoS2 bilayers using hydrazine
salts.
Daeneke, T.; Clark, R. M.; Carey, B. J.; Ou, J. Z.; Weber, B.; Fuhrer, M. S.;
Bhaskaran, M.; Kalantar-zadeh, K., Nanoscale 2016, 8 (33), 15252-15261.
Investigation of two-solvent grinding assisted liquid phase exfoliation of
layered MoS2.
Nguyen, E. P.; Carey, B. J.; Daeneke, T.; Ou, J. Z.; Latham, K.; Zhuiykov, S.;
Kalantar-Zadeh, K., Chem. Mater. 2015, 27 (1), 53-59.
122
Excitation dependent bidirectional electron transfer in phthalocyanine-
functionalised MoS2 nanosheets.
Nguyen, E. P.; Carey, B. J.; Harrison, C. J.; Atkin, P.; Berean, K. J.; Della Gaspera,
E.; Ou, J. Z.; Kaner, R. B.; Kalantar-Zadeh, K.; Daeneke, T., Nanoscale 2016, 8
(36), 16276-16283.
Electronic tuning of 2D MoS2 through surface functionalization.
Nguyen, E. P.; Carey, B. J.; Ou, J. Z.; van Embden, J.; Gaspera, E. D.; Chrimes,
A. F.; Spencer, M. J.; Zhuiykov, S.; Kalantar‐zadeh, K.; Daeneke, T., Advanced
Materials 2015, 27 (40), 6225-6229.
Physisorption-based charge transfer in two-dimensional SnS2 for selective
and reversible NO2 gas sensing.
Ou, J. Z.; Ge, W.; Carey, B. J.; Daeneke, T.; Rotbart, A.; Shan, W.; Wang, Y.; Fu,
Z.; Chrimes, A. F.; Wlodarski, W., ACS Nano 2015, 9 (10), 10313-10323.
Acoustically-driven trion and exciton modulation in piezoelectric two-
dimensional MoS2.
Rezk, A. R.; Carey, B. J.; Chrimes, A. F.; Lau, D. W.; Gibson, B. C.; Zheng, C.;
Fuhrer, M. S.; Yeo, L. Y.; Kalantar-zadeh, K., Nano letters 2016, 16 (2), 849-855.
Intercalated 2D MoS2 Utilizing a Simulated Sun Assisted Process: Reducing
the HER Overpotential.
Wang, Y.; Carey, B. J.; Zhang, W.; Chrimes, A. F.; Chen, L.; Kalantar-zadeh, K.;
Ou, J. Z.; Daeneke, T., The Journal of Physical Chemistry C 2016, 120 (4), 2447-
2455.
Enhanced quantum efficiency from a mosaic of two dimensional MoS2
formed onto aminosilane functionalised substrates.
Wang, Y.; Della Gaspera, E.; Carey, B. J.; Atkin, P.; Berean, K. J.; Clark, R. M.;
Cole, I. S.; Xu, Z.-Q.; Zhang, Y.; Bao, Q., Nanoscale 2016, 8 (24), 12258-12266.
Plasmon resonances of highly doped two-dimensional MoS2.
Wang, Y.; Ou, J. Z.; Chrimes, A. F.; Carey, B. J.; Daeneke, T.; Alsaif, M. M.;
Mortazavi, M.; Zhuiykov, S.; Medhekar, N.; Bhaskaran, M., Nano letters 2015,
15 (2), 883-890.
Liquid metal/metal oxide frameworks with incorporated Ga2O3 for
photocatalysis.
Zhang, W.; Naidu, B. S.; Ou, J. Z.; O’Mullane, A. P.; Chrimes, A. F.; Carey, B. J.;
Wang, Y.; Tang, S.-Y.; Sivan, V.; Mitchell, A., ACS applied materials & interfaces
2015, 7 (3), 1943-1948.
123
Proton intercalated two-dimensional WO 3 nano-flakes with enhanced
charge-carrier mobility at room temperature.
Zhuiykov, S.; Kats, E.; Carey, B. J.; Balendhran, S., Nanoscale 2014, 6 (24),
15029-15036.