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FOUNDER’FOUNDER’FOUNDER’FOUNDER’S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015

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DEVELOPMENT OF SINGLE SOURCE MOLECULAR PRECURSORS

FOR ADVANCED SEMICONDUCTING METAL CHALCOGENIDE MATERIALS

G. Kedarnath and Vimal K. Jain Chemistry Division

DrDrDrDr. G. Kedarnath is the recipient of the DAE Scientific & Technical . G. Kedarnath is the recipient of the DAE Scientific & Technical . G. Kedarnath is the recipient of the DAE Scientific & Technical . G. Kedarnath is the recipient of the DAE Scientific & Technical

ExcExcExcExcellence Award for the yeaellence Award for the yeaellence Award for the yeaellence Award for the year 2013r 2013r 2013r 2013

Growing energy demand worldwide has directed research in renewable energy sources. Semiconducting materials both in bulk and nanoform showed immense potential in photovoltaic and thermoelectric applications [1,2]. However, properties of bulk materials can be improved by tailoring the size and shape of the material either in the form of colloidal solution or thin films. Controlling the size of materials facilitates tuning electronic, optical, magnetic properties, etc. [3]. Such types of tunable semiconducting materials are of great interest in technological advances. A wide range of methods for the preparation of nanomaterials and for deposition of thin films have been evolved over the years. The synthetic ways for the preparation of nanomaterials involve physical (top-down, e.g. ion sputtering), chemical (bottom-up e.g. solvothermal synthesis) or hybrid methods. Although there are number of routes available for the preparation of nanomaterials and thin films, single source molecular precursor route has emerged as a versatile method which can be used not only for the synthesis of phase pure and narrowly distributed materials but also for the deposition of thin films using aerosol assisted chemical vapor deposition (AACVD) method.

Single source molecular precursors for I-VI nanomaterials

For I-VI materials, novel tetrameric copper, [Cu{EC5H3(R-3)N}]4 (E/R = Se/Me or Te/R; R = H or Me) [4] and hexameric, [M{SeC4H(Me-4,6)2N2}] 6 (M = Cu, Ag) complexes using pyridyl/pyrimidyl chalcogenolate ligands have been synthesized and characterized structurally [5]. Structural analysis revealed that the complexes, [Cu{EC5H3(R-3)N}]4 are tetrameric in nature where each copper atom lies at the vertex of the tetrahedron and each face of the tetrahedron is capped by the bridging pyridylchalcogenolate ligand. Similarly, the structures of [Cu{SeC4H(Me-4,6)2N2}] 6.H2O and [Ag{SeC4H(Me-4,6)2N2}] 6.6MeOH.H2O (Figure 1) revealed that the respective metal centre adopt distorted tetrahedral and trigonal geometries.



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Fig. 1: Crystal structures of a) [Cu{SeC4H(Me-4,6)2N2}]6.H2O and b) [Ag{SeC4H(Me-4,6)2N2}]6.6MeOH.H2O.

The tetrameric copper complexes have been used for the synthesis of copper chalcogenide (CuE) nanoparticles and for deposition of thin films by AACVD. Depending on reaction conditions, thermolysis gave both stoichiometric and non-stoichiometric copper chalcogenides. For instance, thermolysis of [Cu{SeC5H3(Me-3)N}]4 in TOPO and HDA/TOPO at 170 and 150 oC, respectively afforded cubic phase of Cu7Se4 spherical (average diameter 125 nm) and cubic phase of Cu1.8Se polygon shaped nanoparticles, respectively. The former precursor has also been used for the deposition of orthorhombic phase of Cu5Se4 thin films at 400 oC using AACVD. Thermolysis of [M{SeC4H(Me-4,6)2N2}] 6 (M = Cu or Ag) in DDT (1-dodecanethiol) at 150 oC afforded cubic phase of Cu7Se4 and orthorhombic phase of Ag2Se, respectively.

Single source molecular precursors for II-VI nanomaterials

Synthesis, characterization and properties of II-VI semiconductor nanostructures have been probed and reviewed extensively [6,7]. Accordingly, a wide range of synthetic routes have been developed for utilizing their potential. Of them, single source molecular precursor route delivers monodispersed and phase pure materials. Several single source molecular precursors for II-VI materials have been designed and studied by spectroscopic techniques using different ligand systems. Dithiocarboxylates of zinc triad, [M(S2CAr)2] (M = Zn, Cd, Hg; Ar = Phenyl or Tolyl) and [M(S2CAr)2(tmeda)] have been synthesized and used for the preparation of phase pure metal sulfide quantum dots under different pyrolytic conditions [8]. HgS nanoparticles could be isolated at low temperature (57 ºC). Different phases (cubic/hexagonal) can be obtained under different experimental conditions. Similarly, a range of precursors for metal selenides have been developed. Among them monomeric metal selenocarboxylates, [M(SeCOAr)2(tmeda)] containing easily cleavable C-Se bond help in the formation of metal selenides at low temperatures [9]. For instance, HgSe nanoparticles of uniform size and shape have been prepared in good yields at room temperature. Another family of single source precursors of the type, [M(Se(CH2)nNMe2)2] (M = Zn or Cd for



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n =2; M = Zn, Cd and Hg for n =3) have been developed using internally functionalized selenolate ligand, [Se(CH2)nNMe2)2] [10]. Phase pure and luminescent CdSe (Figure 2) quantum dots have been prepared by pyrolysis of [Cd(SeCH2CH2NMe2)2] in coordinating solvents.

Fig.2.a) Absorption and b) emission spectra of CdSe nanoparticles obtained by thermolysis of [Cd(SeCH2CH2NMe2)2] at 187 oC in HDA/TOPO recorded at 4, 6 and 30 minutes of preparation

In addition to above complexes, a number of precursors without M-E linkages have also been developed as chalcogenolates of zinc and cadmium often hydrolyze by atmospheric moisture or polymerize on ageing. Accordingly, heterocyclic diselenides and ditellurides with nitrogen donors such as (EC5H4N)2 (E = Se or Te) have been utilized to synthesize zinc and cadmium complexes where nitrogen atom of the ligands coordinate to the metal atom [11]. Thermolyses of the resulting complexes yield metal chalcogenides. Unlike group II metal sulfide and selenide nanomaterials, synthetic routes for analogous tellurides are relatively unexplored due to the difficulty in obtaining phase pure products. Recently, we have prepared [Hg(TeCH2CH2NMe2)2] and utilized for the preparation of undoped and doped HgTe quantum dots by pyrolysis of [Hg(TeCH2CH2NMe2)2] in HDA at ~100◦C in the size range of 5-10 nm [12]. Although doping of HgTe is a challenging problem due to labile nature of Hg-Te bond, a paramagnetic ion (Mn2+) in the HgTe lattice could be introduced successfully. The Mn doped HgTe nanoparticles show ferromagnetism at room temperature.

Single source molecular precursors for III-VI nanomaterials The III-VI chalcogenides find applications in solar cell absorbers, opto-electronics, photodetectors and photovoltaic devices. Among III-VI materials, indium chalcogenide have received considerable attention due to their photovoltaic properties. Thus for indium chalcogenides, indium complexes with 2-seleno- and -telluro pyridines, [In(EpyR)3] (E = Se or Te; R = H and Me) have been synthesized which on pyrolysis/solvothermolysis gave In2E3. CuInSe2 nanomaterials have also been prepared by decomposing precursors, [In(SepyR)3] and



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[CuSepyR]4 together in high boiling coordinating solvents and have been characterized by electron microscopic techniques [14]. Single source molecular precursors for IV-VI nanomaterials The IV-VI binary semiconductors like SnE, SnTe, PbE (E = S, Se, Te) show distinctive properties such as larger bulk Bohr radius and narrow band gaps which is helpful in thorough understanding of quantum confinement and their applications IR detectors and thermoelectrics [2,15]. Therefore, a number of methods have been explored both in solution and in gas phase leading to the preparation of different morphologies of IV-VI materials. Of them, the least investigated method is single source precursor route and hence, a number of thio- and seleno-pyridyl/pyrimidyl derivatives of organotin(IV) have been synthesized and structurally characterized.

Fig. 3: a) Crystal and b) schematic structures of [Et2SnCl{SC4H(Me-4,6)2N2}] and [Et2Sn{2-SeC5H3(Me-3)N}2], respectively.

Fig. 4: SEM images of a) SnS sheets and b) SnSe hexagons obtained by thermolysis of [Et2SnCl{SC4H(Me-4,6)2N2}] and [Et2Sn{2-SeC5H3(Me-3)N}2] in OLA at 300 and 215 oC for 5 and 25 min, respectively



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Further, the diorganotin complexes, [Et2Sn(2-SC5H4N)2], [Et2SnCl{SC4H(Me-4,6)2N2}] and [R2Sn{2-SeC5H3(R’-3)N)2}] (R = Me, Et or tBu; R’ = H or Me) [16,17] on thermolysis in hot oleylamine produced different morphologies tin chalcogenide nanostructures at different temperatures. For instance, thermolysis of [Et2SnCl{SC4H(Me-4,6)2N2}] (Figure 3a) and [Et2Sn{2-SeC5H3(Me-3)N}2] (Figure 3b) in oleylamine (OLA) at 300 and 215 oC afforded rectangular SnS and hexagonal shaped SnSe sheets, respectively (Figures 4a and 4b). SnSe thin films have been deposited on glass and silicon substrates by AACVD of [tBu2Sn(2-SeC5H4N)2]. The former have showed photo response. Other than tin chalcogenides, difficult to prepare PbE (E = S or Se) quantum dots have been synthesized by employing monomeric complexes, [Pb(ECH2CH2NMe2)2] [18]. PbSe nanoparticles with an average diameter of 10 nm have been isolated. The complexes [Pb(S2CAr)2] (Ar = Ph or Tol) in refluxing HDA afforded PbS nanoparticles at fairly low temperatures. Single source molecular precursors for V-VI nanomaterials

The anisotropic and layered structured V-VI semiconducting materials, M2E3 (M = Sb or Bi, E = S, Se, Te) having direct band gap have drawn significant interest due to their potential applications in a variety of thermo-electric and optoelectronic devices, optical recording systems, television cameras and X-ray computed tomography [19-22].

Fig. 5: Crystal structures of a) [Sb{SeC5H3(Me-3)N}3].1.5H2O and b) [Bi{SeC5H3(Me-3)N}3].0.5H2O

Fig. 6: SEM images of a) Sb2Se3 nanorods and b) BiSe nanoflowers obtained by pyrolysis of [Sb{Se-C5H3(Me-3)N}3] and [Bi{Se-C5H3(Me-3)N}3] in a furnace at 400 and 450 oC for 1 h.



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Therefore, for V-VI materials, pyridylselenolate complexes of antimony and bismuth, [M{SeC5H3(R-3)N}3] (M = Sb or Bi; R = H or Me) have been developed and characterized structurally (Figure 5) [23]. The complexes either have been pyrolyzed in a furnace or thermolyzed in hexadecylamine (HDA) afforded a variety of M2Se3/MSe nanostructures [nanorods (Figure 6a) and nanoflowers (Figure 6b), etc.]. The precursors ave also been employed to deposit films of M2Se3 (M = Sb or Bi) by AACVD. Acknowledgements

We thank Dr. B. N. Jagatap for encouragement of this work. We are grateful to all our collaborators, whose names appear in references for their sustained interest.

References

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