Precursor Chemistry – Main Group Metal Chalcogenides

1.32

Precursor Chemistry – Main Group Metal Chalcogenides

M Afzaal, Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia P O’Brien, The University of Manchester, Manchester, UK ã 2013 Elsevier Ltd. All rights reserved.

1.32.1 Introduction 1.32.2 Group II–VI Thin Films 1.32.3 Group II–VI Nanocrystals 1.32.4 Group III–VI Thin Films 1.32.5 Group III–VI Nanocrystals 1.32.6 Group IV–VI Thin Films 1.32.7 Group IV–VI Nanocrystals 1.32.8 Group V–VI Thin Films 1.32.9 Group V–VI Nanocrystals 1.32.10 Conclusion Acknowledgments References

Abbreviations 2,6-dipic 2D 3D AA-CVD AP-CVD bipy Bu Bz CVD DDT depe DFT EG Et EtPy GaAs HDA Hex HR-TEM InCl LI-CVD LP-CVD Me

1.32.1

2,6-Dipicolinic Two-dimensional Three-dimensional Aerosol-assisted CVD Atmospheric pressure CVD Bipyridine Butyl Benzyl Chemical vapor deposition Dodecanethiol 1,2-bis(diethylphosphino)ethane Density functional theory Ethylene glycol Ethyl Ethylpyridine Gallium arsenide Hexadecylamine Hexyl High-resolution TEM Indium (I) chloride Liquid injection CVD Low-pressure CVD Methyl

Introduction

Compound semiconductors, based on combinations of elements from the classical groups II and VI (2,6), III and V (3,5), III and VI (3,6), or IV and VI (4,4), have had a significant impact on our everyday lives. They have found applications as diverse as in satellite TV receivers, optical fiber communications, compact disc players, bar-code readers, full color advertising displays to solar cells, etc. These semiconductors are at

Comprehensive Inorganic Chemistry II

1001 1002 1006 1008 1010 1011 1013 1015 1017 1018 1018 1018

MO NMR OA Ph Phen pic PL PMDETA PVP SAED sal SEM Si SSPs TEM TGA THF tipt TMEDA TOA TOP TOPO VLS VS

Molecular organic Nuclear magnetic resonance Oleylamine Phenyl 1,10-Phenanthroline Picolinic Photoluminescence Pentamethyldiethylenetriamine Poly(vinylpyrrolidone) Selected area electron diffraction Salicylic Scanning electron microscope Silicon Single-source precursors Transmission electron microscope Thermogravimetric analysis Tetrahydrofuran 2,4,6-Triisopropylbenzenethiol Tetramethylethylenediamine Trioctylamine Trioctylphosphane Trioctylphosphane oxide Vapor–liquid–solid Vapor–solid

present generally mostly utilized as thin films, but solids have a great potential for development especially as nanoparticles, most notably as quantum dots. Chemical vapor deposition (CVD) is the one most commonly used for depositing thin films. Manasevit first demonstrated the use of molecular organic (MO)-CVD for gallium arsenide (GaAs) thin films.1 The most important factor which affects the composition and structure of the deposited material is the nature and the purity of the precursors. In CVD processes, molecular precursors

http://dx.doi.org/10.1016/B978-0-08-097774-4.00133-9

1001

1002

Precursor Chemistry – Main Group Metal Chalcogenides

must shed the ligands that contributed to their volatility, to produce a film of a very pure solid-state material. Thermal decomposition of a precursor is the key step which produces the thin films and ideally the ligands associated with the precursor are cleanly lost into the gas phase. Often the ligands undergo complex fragmentation and contamination of the deposited material can result. A rational development of precursors for a specific application is only possible with some insight into the molecular decomposition pathways. Study on precursor decomposition mechanism in principle needs to be carried out in the gas phase and subsequently of the surface reactions of the precursors or relevant fragments. Several groups have been actively looking into the development of so-called single-source precursors (SSPs) for semiconducting thin films and nanoparticles. The SSPs can have numerous advantages over conventional precursors, including air and moisture stability, ease of handling, comparatively low toxicity, and often lower growth temperatures. The presence of only one precursor molecule in the supply stream reduces the likelihood and extent of pre-reaction and the associated contamination of deposited film, and permits intrinsic control of film stoichiometry. Ligand design can also allow for a degree of control over both the type and the level of impurities incorporated into the films such as carbon from an alkyl group. Furthermore, molecular clusters may be able to control from their core and/or stoichiometry the nature of deposited material. Despite the numerous potential advantages of SSPs, these compounds are not without drawbacks. Their generally high molecular weights tend to lead to lower volatility, which can be problematic in conventional atmospheric pressure (AP)-CVD. This problem is exacerbated in systems requiring two or more metals, since the presence of an additional metal tends to increase the molecular weight. However, these problems can be mitigated by using a lowpressure environment, or by employing a solution of the precursor as in aerosol-assisted (AA)-CVD or liquid injection (LI)-CVD. A further problem associated with SSPs is the difficulty in depositing materials with defined nonintegral stoichiometries or when dopants are required. The route used to prepare or deposit a material can profoundly affect the phase composition, thermal stability, and morphology, which in turn can influence the functional behavior of the materials. The use of SSPs has been actively explored for the preparation of nanocrystals. The strength of ligand in the SSP can be easily modified to tune the decomposition kinetics of the precursor.2 Upon decomposition of the precursor, the growth process takes place by two opposing factors: (1) crystal growth (or faceting) and (2) crystal dissolution such as Ostwald ripening. In general, the morphology of a nanocrystal is often controlled by delicately achieving a mixture of thermodynamic and kinetic approaches.3,4 The thermodynamic control is achieved by using a surfactant or a capping ligand to minimize the total surface energy of a system, whereas growth kinetics is attained at a rate at which atoms are generated and added to the surface of a seed. The growth of anisotropic structures is achieved under kinetic conditions to achieve favorable directions with low energy barriers. In this chapter, we highlight some recent relevant examples of relationships between precursors and materials deposited

and discuss the advances made in the design and preparation of precursors involving main group elements and their use for the synthesis of semiconducting materials in the form of thin films and nanocrystals.

1.32.2

Group II–VI Thin Films

Zinc and cadmium complexes are the simplest candidates as SSPs for zinc or cadmium chalcogenides and have been recently reviewed.5 The research received a real impetus in the early 1990s and still continues to be in full swing. The chemistry of divalent zinc and cadmium with chalcogen-containing ligands such as thiolates generally results in the formation of polymeric structures with a tetrahedral metal center; however, such compounds proved to be practically involatile.6–8 This lack of volatility means that they are not generally useful as precursors for low-pressure (LP)-CVD studies. Precursors based on 1,2 bis(diethylphosphino)ethane (depe), M(ER)2 (M ¼ Zn, Cd, Hg; E ¼ S, Se, Te) developed by Steigerwald and coworkers show that complexes containing one or two mole equivalents of the phosphane can be isolated.9 The 1:2 species are polymers and the 1:1 complexes are dimers. One successful approach to reduce the molecularity of the complexes involves the introduction of bulky chalcogenide ligands such as 2,4,6-triisopropylbenzenethiol (tipt). A series of low-coordination metal complexes were prepared with tipt by Dilworth et al.10 Bochmann et al. produced a range of precursors for II–VI materials based on 2,4,6-tri-tertbutylphenylchalcogenolate (Figure 1). The general synthesis of precursors is highlighted in Scheme 1.11–14 The thermally stable, sublimable complexes (M ¼ Zn, Cd) have been used to grow thin films of group 12 tellurides free from phosphorus contamination. These compounds have been used to deposit thin films of metal sulfides or selenides, in preliminary low-pressure growth experiments.15–17 One problem with such ligands is that steric bulk is achieved by the incorporation of large numbers of aryl carbon atoms. The compounds are often dimeric, even in the

Figure 1 Structure of [{Cd(SeC6H2But3-2,4,6)2}2].

Precursor Chemistry – Main Group Metal Chalcogenides

vapor phase and can be used in LP-MOCVD studies. However, the mercury analogs readily decompose via a reductive elimination path to form atomic mercury and diaryldichalcogenides. Park et al.18 have employed the tris(trifluoromethyl)substituted derivative [Cd{SeC6H2(CF3)3}2], which sublimes at lower temperatures (160  C under vacuum). Films of cubic CdSe were grown on silicon (Si) substrates in the temperature range 425–475  C with growth rates ranging from 0.5 to 1 mm h1. Another series of precursors involve bulky siliconbased systems of stoichiometry M[ESi(SiMe)3]2 (M ¼ Zn, Cd, Hg; E ¼ S, Se, Te)19,20 (Figure 2) devised by Arnold and coworkers to deposit a range of chalcogenides.21 The most detailed work has been reported on thin films of tellurides deposited by LP-MOCVD.22–24 Other classes of molecules, which have proved useful for the deposition of thin films, include coordination complexes such as dialkyl dichalcogenocarbamates25–31 or dithiophosphinates.32,33,28 The dithiocarbamate ligand, R2NCS2 is a formal three-electron donor and has the ability to stabilize metal centers in a variety of oxidation states. There are several methods for the preparation of such carbamates and the most popular is the reaction of a metal salt with CE2 (E ¼ S, Se) in the presence of a secondary amine or the formal insertion of CE2 across an M–NR2 bond in an alkylamide. These compounds have proved to be useful SSPs for the thin films. One striking observation has concerned differences

But2PNHR (R =

Pr i or

Te

Te

But2P

C6H11)

3

LiBun

Li[But2P(Te)NR]

NHR M[N(SiMe3)2]2 (M = Cr, Mn, Fe, Co, Zn or Cd)

R N But

2P

Te MCI2(PMe3)2 PBut2 M (M = Fe or Ni) Te N R 4

Scheme 1 Synthesis of tert-butylphenylchalcogenolates.

Figure 2 Structure of Zn[TeSi(SiMe)3]2.

1003

between the behavior of the simple diethyldithio- and diethyldiseleno-carbamates of zinc and cadmium. The bis-diethyldithiocarbamates are effective precursors for materials such as CdS and films of good quality can be deposited. Recent developments in deposition techniques have led to growth of optoelectronic-quality hexagonal CdS films. The use of [Cd(S2CNHexyl2)2] in supercritical chemical fluid experiments results in a narrow band-edge photoluminescence (PL) peak.34 The reason for the choice of n-hexyl terminal groups is to impart solubility in nonpolar solvents. While carbon dioxide (CO2) is a convenient and environmentally acceptable solvent and chosen as the supercritical carrier fluid, it was found to be straightforward to control concentrations in this case by introducing the precursor in n-pentane solution, which was mixed with the flowing SC-CO2 before introduction into the reactor chamber. n-Pentane/CO2 mixtures were held in temperature/pressure regimes where they form a single homogeneous phase. Initial experiments resulted in sulfur-deficient films (up to 10%). The production of stoichiometric CdS films needed an additional sulfur source, and butane-1-thiol was included. In general, films produced with thiol have a PL efficiency that is two orders of magnitude greater than those produced without the thiol. The use of carbamates in CVD, for example, [Cd(SeCNEt2)2]2 (Figure 3) leads to films of elemental selenium or metal selenide films heavily contaminated with selenium.35 In efforts to increase the volatility of this family of precursors, bulkier substituents on the amine parent of the carbamates were introduced. Asymmetric compounds such as [M(Se2CNMenHex)2] (M ¼ Cd, Zn) proved to be most useful precursors for metal selenide thin films by LP-CVD.36 This unexpected result can be understood in terms of quite subtle changes in the mechanism of thermal decomposition of the complexes. On changing the alkyl groups on the parent amine of the diselenocarbamates from the symmetric diethyl to the asymmetric methyl(n-hexyl) derivative, the formation of diethyl diselenide is hindered and thus the deposition of selenium during growth is inhibited. Mixed alkyl diselenocarbamates have also been prepared by either insertion or comproportionation reaction; the latter provides the most convenient route.37 One interesting application of the comproportionation reaction is the preparation of a mixed species such as dimethylcadmium/dimethylzinc diethyldiselenocarbamate, which is used for the deposition of thin films of ternary Cd0.5Zn0.5Se on glass.37 Thus, the reaction of dimethyl zinc with [Cd(SeCNEt2)2] gave [Me2CdZn(Se2CNEt2)2] where

Figure 3 Structure of [Cd(SeCNEt2)2]2.

1004

Precursor Chemistry – Main Group Metal Chalcogenides

R2P(Se)CI

R2PCI + Se

+2NaSeH

R2PSe2−Na+ + NaCI + H2Se

Scheme 2 Synthesis of sodium selenophosphinates.

R2PH + nBuLi

R2PLi

2Se

(Scheme 4).42 However, upon descending the group 16 elements, phosphorus–chalcogen bond becomes weaker and more susceptible to cleavage which results in the formation of the asymmetric mixed dichalcogenophosphinate complex being in equilibria in the solution with diseleno and ditelluro complexes. Phosphorus-31 nuclear magnetic resonance (NMR) studies seem to suggest the formation of asymmetric species in tetrahydrofuran (THF) at room temperature and above. At low temperatures, equilibrium is shifted toward the symmetric species. In 2002, we showed that the metal complexes of the monoanionic imino-bis(di-iso-propylphosphane chalcogenide) ligands [N(PPh2E)2] (Scheme 2, 1a, E ¼ S; 2a, E ¼ Se) are suitable SSPs for the group 12 metal selenides.45 The neutral ligands in theory can exist in two different tautomeric states: an N–H tautomer (a) and a E–H tautomer (b). However, studies in both solution and the solid state have shown that in all cases, the N–H tautomer predominates.46 Structural studies have revealed the N(PE)2 group to be planar or close to planar,

−Li+

R2PSe2

Scheme 3 Facile route for lithium selenophosphinates.

cadmium and zinc atoms were modeled in crystallography as randomly occupying the metal sites. In the case of dichalcogenophosphinato complexes, sulfur derivatives have been investigated extensively38 while studies on the seleno analog have been somewhat limited due to the difficulties associated with their preparation.39 Diselenophosphinates were formed from R2PCl in a two-step procedure.40 Initially the chlorophosphane is oxidized to the selenophosphinic chloride using elemental selenium which is followed by the addition of sodium hydroselenide, giving the sodium diselenophosphinate ligand (Scheme 2). Another route to diselenophosphinates, which does not involve the emission of any noxious hydrogen selenide by-products, involves metallation of a secondary phosphane using an alkali-metal reagent, such as H

R

R P

P

PH

R

R E

(a)

N

R

N

R

P

PH

R

EH

R E

(b)

n-butyllithium, followed by addition of two equivalents of elemental selenium to give the alkali-metal diselenophosphinate salt (Scheme 3).41 Similarly, lithium diphenyltellurophosphinate, [Ph2PTe2]Liþ, can also be prepared from the lithium diphenylphosphide with elemental tellurium.42 In order to find a facile and suitable synthetic method, we proposed the formation of an intermediate R2PSiMe3, followed by selenium insertion into the P–Si bond.43 The efficiency of the synthesis relies on the process of making R2PSiMe3, prepared by reaction of lithium with R2PCl to form R2PLi, followed by exchange reaction between R2PLi and R0 3SiCl at low temperature. The disadvantage of this process is the extreme reaction conditions. Utilizing the Benkeser reaction, a similar R2PSiCl3 intermediate can be easily prepared with high yield under normal reaction conditions.44 The synthetic process includes two steps: (a) making the R2PSiCl3 intermediate by the reaction of R2PCl with HSiCl3 and NEt3 in toluene under nitrogen at room temperature, followed by (b) inserting Se into the P–Si bond of the intermediate by refluxing the intermediate with Se powder in toluene. In step (b), by controlling the Se:R2PSiCl3 ratio, two different products can be prepared. Mixed chalcogen species of the form [R2P(E)E0 ] (E¼ S, Se, Te) are also accessible from lithium chalcogenophosphinite with the elemental chalcogen. For example, a highly air-sensitive mixed seleno-telluro-diphenylphosphinate can be from elemental tellurium and lithium selenodiphenylphosphinate

R N

R

E

EH

P -

R

R E

(c)

with P–N–P bond angles in the range 122–133 , indicative of substantial sp2 character at the nitrogen center (c). Since a wide range of metal complexes of these bidentate inorganic ligands are known and easily prepared,47 this discovery paved the way for the generation of thin films of a wide variety of metal chalcogenides from homoleptic complexes. In CVD applications, complexes of the iso-propyl derivatives 1b and 2b are preferred in view of their higher volatility compared to that of the phenyl-substituted analogs 1a and 2a. One of the most convenient and high-yielding routes for the preparation of symmetric R2P(E)NHP(E)R2 (E ¼ S, Se) is from reaction of diorganochlorophosphane with hexamethyldisilazane to give the iminobis(diorganophosphane), followed by oxidative addition of two equivalents of elemental sulfur or selenium47 (Scheme 5). The oxidation can also be carried out in two steps to give first the monochalcogenide, followed by further oxidation with a different chalcogen to give the asymmetric mixed dichalcogenide. These neutral ligands are readily deprotonated to the monoanions 2a and 2b by treatment with bases such as sodium methoxide.48 Subsequent metathetical reactions of 2a or 2b with metal halides produce homoleptic complexes in good yields.44 However, ditelluro analogs of the neutral ligands 3 and 4 are not available by the route shown in Scheme 5. The oxidation of the phosphorus (III) systems R2PN(H)PR2 does not occur to a significant extent in the case of R ¼ Ph

Precursor Chemistry – Main Group Metal Chalcogenides

Se

Ph

− Li

P

Ph2PSeLi + Te

+

P

1/2 Ph

Te

Ph

Se

Ph

− Li+ Se

Te

Ph + 1/2

P Ph

1005

− Li+ Te

Scheme 4 Alternative route of lithium chalcogenophosphinites.

R2P

H N

2E PR2

R2P

H N

PR2

NaOMe R P 2

E E 3a, E = S, R = Ph 3b, E = S, R = Pri 4a, E = Se, R = Ph 4b, E = Se, R = Pri

N PR2

E

E

1a, E = S, R = Ph 1b, E = S, R = Pri 2a, E = Se, R = Ph 2b, E = Se, R = Pri

Scheme 5 General synthesis of dithio- and diselenoimidophosphinates.

R2P

2Na[R2PNPR2] + 4Te + 2TMEDA

N

PR2

Te (tmeda) Te Na Na (tmeda) Te Te R 2P

N

PR2

6a, R = Ph 6b, R = Pr i

Scheme 6 Synthesis of tellurium derivatives.

even in boiling toluene.49 The iso-propyl derivative Pri2PN(H) PPri2 is oxidized by tellurium at room temperature to the monotelluride, which is isolated in 81% yield as the P–H tautomer Pri2P(H)NPPri2Te.50 However, further oxidation of 5 with tellurium is not possible. In 2002 Chivers et al. developed an alternative strategy for the synthesis of the desired ditelluro PNP ligands that involves metallation of the phosphorus (III) reagents R2PN(H)PR2 with sodium hydride ‘prior to reaction with elemental tellurium.’ The installation of a formal negative charge renders the phosphorus (III) centers more nucleophilic toward tellurium. As depicted in Scheme 6, this methodology facilitated the synthesis of the ditelluro PNP ligand as the sodium salt (6a, R ¼ Ph), which forms a centrosymmetric dimer in the solid state.50 This new synthetic protocol was later shown to be applicable to the sodium salt of the iso-propyl-substituted ligand 6b, which is of more interest in SSP work (vide supra).51 The availability of the ditelluro PNP ligand as the sodium salt 6b has paved the way for the preparation of a wide variety of homoleptic metal complexes for investigations of their suitability as SSPs to metal tellurides. For example, the Zn, Cd, and Hg complexes with distorted tetrahedral structures are obtained in good yields from metathetical reactions of 6b with metal dihalides.50 The AA-CVD of Cd[N(PPri2Te)2]2 on glass substrates produces CdTe films whose composition is temperaturedependent.52 Thermogravimetric analysis (TGA) of the Cd complex under nitrogen showed a single-step weight loss at  360  C with a residue of 22% corresponding to bulk CdTe (calculated

21%). At lower temperatures a mixture of cubic CdTe and hexagonal Te is obtained, whereas in the temperature range 425–475  C pure CdTe films are produced. By contrast, the AA-CVD of Hg[N(PPri2Te)2]2 under similar conditions produces hexagonal tellurium. This observation has been tentatively explained, as resulting from the reductive elimination of the ligand from the metal center, with consequent formation of the ditelluride dimer (TePPri2NPPri2Te)2 and elemental Hg. Under the experimental conditions, the dimeric species decomposes to give the observed elemental Te. Interestingly, AP-CVD of Cd[N(PPri2Te)2]2 on Si/SiO2 substrates leads to the growth of anisotropic CdTe structures at 700–900  C.53 Scanning electron microscope (SEM) studies reveal rods with diameters ranging from 0.25 to 0.65 mm and lengths varying from 0.5 to 2.2 mm, together with bipodal and tripodal structures. The horizontally grown rods and pods were randomly distributed over the substrate surface, with a complete absence of significant two-dimensional (2D) planar growth. The Si/SiO2 substrates, sputtered with gold layers, were also investigated for the growth of CdTe structures which ultimately lead to formation of Au/Si islands on the substrate surface. The formation of Au/Si alloy islands serves as a preferential site for the epitaxial growth of rods on a substrate due to the high sticking coefficient of the molten alloy. One parameter of crucial importance in this context, especially for epitaxially grown rods, is the crystallographic growth direction. Further investigation into the angle of growth confirms epitaxial growth at 90 , 54 , and 45 , being consistent with the growth angle expected of rods growing out of the Si (1 0 0) plane along the (1 1 0) axis (45 ), (1 1 1) axis (54.75 ), and (1 0 0) axis (90 ).54 As depicted in Figure 4, the CdTe rods are unambiguously (1 1 0)-oriented, with further evidence of growth along the (1 1 1) and (1 0 0) planes being identified due to the orientation visible in SEM. Although Au-sputtered substrates were used for deposition studies, there was no evidence of Au-capped particles at the tips of the crystallites. Growth of the CdTe structures is therefore unlikely to have taken place by a vapor–liquid– solid (VLS) mechanism. It is more likely that the growth is induced by a vapor–solid (VS) process. The resulting Au/Si islands have a large accommodation coefficient and are therefore a preferred deposition site for incoming CdTe atoms from the vapor phase.55 O’Brien and coworkers have established that different deposition techniques can lead to growth of different materials. For example, we have shown that AA-CVD of Cd[N(PPri2S)2]2 can lead to the growth of cadmium sulfide (at 500  C) and/or phosphide (at 525  C) thin films on glass.56 Preliminary mass spectrometry and density functional theory (DFT) methods suggest a plethora of pathways which could easily lead to a reaction that switches over a short range of temperature, and could lead to either direct deposition or metathesis to the phosphide, the former being held to be, on balance, more likely.

1006

Precursor Chemistry – Main Group Metal Chalcogenides

3.4

d(111) or (002)

3.35 3.3 3.25 3.2 3.15 3.1 3.05

0

0.25 0.5 0.75 Molar fraction of cadmium precursors

1

Figure 5 d(1 1 1) or (002) versus molar fraction of cadmium precursor (a) ZnxCd1xS films prepared using (♦) complexes (1) and (3), (▪) ZnxCd1xS films prepared using complexes (2) and (4), and (▲) ZnxCd1xS films prepared using complexes (5) and (6).59

(a)

45º

54º

ZnS at 400  C, the diffraction peak of the hexagonal (0 0 2) plane shifts toward a lower angle with an increased concentration of cadmium precursors. Hence, the lattice expands on (0 0 2) as the bulkier cadmium substitutes for zinc. Moreover, the shift in diffraction angle clearly indicates the formation of a ZnxCd1xS solid-state solution. Figure 5 shows the increase in lattice distances proportional to the increasing concentration of cadmium precursors. Their resulting bandgaps were found to be intermediate between bulk CdS and ZnS.

1.32.3

(b)

Figure 4 SEM images of rods grown on (a) Au/Si islands; (b) along (1 1 1) plane. Inset shows growth along the (1 1 0) plane.53

Complexes such as xanthates,57 monothiocarbamates,58 mixed alkyl/dithio-, or diselenocarbamates36 have also been studied for thin films. More recently, thio- and dithio-biuret complexes of cadmium and zinc, [M(N(SCNR2)2)2] [M ¼ Zn, R ¼ isopropyl (1), ethyl (2) and M ¼ Cd, R ¼ methyl (3), ethyl (4)] and [M(SON(CNiPr2)2)2] [M ¼ Zn, R ¼ isopropyl (5) and M ¼ Cd, R ¼ isopropyl (6)] have been synthesized and utilized for ZnS, CdS, and ZnxCd1xS thin films by AA-CVD.59 The complexes were synthesized by reacting excess dialkylamine with in situ generated dialkyl carbamoylthiocyanate in actonitrile. All the complexes are air and moisture stable. The zinc complexes (1) and (5) deposited cubic ZnS films by AA-CVD with small rods and granular crystallites at 300 and 350  C, whereas at 400 and 450  C hexagonal ZnS with granular crystallites were dominant. Complex (2) gave granular hexagonal ZnS films at all deposition temperatures. Cadmium complexes (3), (4), and (6) gave granular hexagonal CdS films at all deposition temperatures. ZnxCd1xS films were deposited on glass substrates by varying the relative concentrations of precursors (1) and (3), (2) and (4), and (3) and (6) at 400  C by AA-CVD. As zinc complexes give predominantly hexagonal

Group II–VI Nanocrystals

Following the success of the use of dithio-/diseleno carbamato complexes in CVD experiments, we used similar compounds for the preparation of nanoparticles, and we extensively investigated the synthesis of ME (M ¼ Cd, Zn, Pb; E ¼ S, Se) quantum dots and related core/shell structures from bis(dialkyldithio or diseleno)-carbamato) metal(II) complexes.60,61 A typical synthesis involves the dissolution of the precursor in trioctylphosphane (TOP) followed by decomposition in a suitable high-boiling coordinating solvent (trioctylphosphane oxide (TOPO) or ethylpyridine (EtPy)) usually at temperatures above 200  C.62,63 The size of as-synthesized nanoparticles strongly depends on the reaction time, temperature, and precursor/capping agent ratio along with alkyl groups. In the case of zinc, symmetrical or unsymmetrical alkyl groups lead to ZnS64 and ZnSe63 quantum dots, whereas65 for CdSe66,67 and PbSe,68,69 mainly precursors with unsymmetrical groups produce high-quality nanoparticles. The cadmium complex of thiosemicarbazide in TOPO leads to good-quality CdS nanorods 10–24 nm in length.70 No shapeinducing reagents were used during the experiments.71,72 In order to find a correlation between the pattern of thermal decomposition and morphology of resulting nanoparticles from dithioureas, xanthate, and thio-/seleno-semicarbazides, a systematic approach has been applied.71–73 It is discovered that only cadmium complexes of thio-/seleno-semicarbazides yield CdE (E ¼ S, Se) nanorods while others yield only spherical nanoparticles. This observation is thought to be based on a kinetic effect – the semicarbazide precursors having a lower activation barrier for primary decomposition as providing favorable conditions for the growth of nanorods. It has also been

Precursor Chemistry – Main Group Metal Chalcogenides

suggested that the semicarbazide precursors release a structuredirecting agent upon decomposition that coordinates to the nanorod surfaces.73 The factors controlling the formation of nanodimensional rods are complex. The first report emphasized the role of hexylphosphonic acid as a growth modifier.74 However, later studies have shown an effect on the rate of nucleation and monomer concentration on the formation of the nonspherical morphologies.75 A key factor is whether the reaction takes place under thermodynamic or kinetic control.76,77 Li et al. have varied reaction conditions to synthesize spherical particles or rods from xanthates (Figure 6).78,79 Lewis base-catalyzed, low-temperature thermal decomposition of metal alkyl xanthates is also reported for CdS, ZnS, and HgS nanoparticles.80 It was suggested that nanocrystals prepared using dithiocarbamates in the synthesis were of lower quality as compared to their xanthate counterparts. The difference was attributed to the higher decomposition temperature of the dithiocarbamate precursors. The tuning of nanoparticle size can be achieved by controlling parameters including reaction temperature, reaction time, concentration of the precursors, and the alkyl chain length. Thermal decomposition of cadmium hexadecylxanthate in hexadecylamine (HDA) leads to one-step synthesis of luminescent, 80–150-nm-long CdS wires with ultra-narrow width (1.7 nm).81 Ligand exchange of the HDA by ethanethiol or 1-hexadecanethiol in toluene, or mercaptoethylamine in water, breaks them into rods ( 15–

(a)

(d)

100 nm

50 nm

(e)

40 nm long). This preparative approach relies on the different adsorption and desorption rates of the two ligands with cadmium, along with the higher ligand strength of thiols compared to that of primary amines such as HDA. The thiols are covalently bound and may have a stronger influence on the structure of the rods. Other types of SSPs studied for the nanoparticle forselenoureas,83 mation include selenophosphinates,82 84 dithioacetylacetonate, thio-/seleno-carboxylates,85 and diselenoimidodiphosphinates.86 In the case of cadmium selenophosphinate,79 [Cd(iPr2PSe2)2], a micro-fluidic reactor (microcapillary tube) has been used to prepare CdSe nanoparticles. The precursor dissolved in TOP and oleylamine (OA) was injected into a reactor composed of fused silica microcapillary at 200  C under a flow rate of 50 ml min1. For the purpose of obtaining highly luminescent nanoparticles, the resulting crude CdSe nanoparticles were mixed with [Cd(S2CNMenHex)2] and re-injected into the capillary layer to obtain CdSe/CdS core–shell nanoparticles. Overall, the result was that quantum yield increased from 12% for CdSe to 33% for CdSe/CdS. Inorganic clusters of the formula [(X)4[M10Se4(SPh)16] (X¼ Li, (CH3)3NH) have been employed by Cumberland et al. for the synthesis of CdSe and CdSe/ZnS nanoparticles in HDA.87 Later, decomposition of (Me4N)4[Cd10S4(SPh)16] has been reported in HDA to produce CdS nanoparticles.88 It is believed that the clusters act as nuclei capable of structural rearrangement

100 nm

(b)

4 nm

1007

(c)

50 nm

4 nm

Figure 6 Transmission electron microscope (TEM) images of the ZnS nanorods prepared in the solution of hexadecylamine þ octylamine with different reaction conditions: (a) at 150  C, 0.5 g of [Zn(C2H5OCS2)2] in the solution, and for 10 h; (b) at 200  C, 0.2 g of [Zn(C2H5OCS2)2] in the solution, and for 6 h; (c) at 250  C, 0.35 g of [Zn(C2H5OCS2)2] in the solution, and for 4 h; (d) at 250  C, 0.5 g of [Zn(C2H5OCS2)2] in the solution, and for 4 h; (e) large area HR (high-resolution) TEM images for (d) (the inset (left side) is a magnified HR-TEM image).79

1008

Precursor Chemistry – Main Group Metal Chalcogenides

without dissolution and separate the nucleation and growth steps. Based on the well-established chemistry of amines, it is expected that HDA acts as a ‘nucleation initiator’ which helps to separate the nucleation and growth steps.89 By selecting the decomposition temperature between 100 and 300  C particle growth can be tuned to give monodispersed particles with sizes in the 2–10-nm range. In a related study, Thoma et al. have synthesized blue-emitting CdSe nanorods at 100  C in HDA using Li4[Cd10Se4(SPh16)].90 The anisotropic growth of cubic CdSe nanorods seems to occur via the assembly of originally isotropic clusters, an observation of nonclassical crystal growth supported by TEM and dynamic light scattering studies. Depending on the reaction conditions, it is also possible to observe S2 incorporation into CdSe nanoparticles from [Cd10Se4(SPh16)]4þ, the parentage of which can be traced back to phenylthiolate decomposition at the quantum dot surface.91– 93 The compositional control and presence of S2 in CdSSe alloys from Li4[Cd10Se4(SPh16)] are also described.92 In the alloy, pure CdSe nuclei of  1.5 nm are generated but reaches 4 nm in size as S2 incorporation is increased, at prolonged growth time. NMR studies have indicated that the rapid decomposition of phenylthiolate arises with subsequent enhanced S2 incorporation. CdTe nanocrystals from the Li2[Cd4(SPh)10] cluster and elemental tellurium can also be prepared at relatively low temperatures and have quantum yields of up to 37%.94 Subsequent surface passivation of the CdTe nanocrystals with a wider bandgap CdS shell leads to nanocrystals exhibiting quantum yields of up to 52%. More complex structures based on ZnS/CdSe/ZnS quantum wells can also be efficiently prepared by the use of cadmium and zinc clusters.95 Preparation of ternary II–II0 –VI nanoparticles is an attractive option for the manipulation of the bandgap by changing both particle size and composition (i.e., the ratio of M to M0 ). These ‘pre-mixed’ ternary cluster molecules are exceptional candidates for the synthesis of ternary nanoparticles in which the Zn/Cd ratio of the final products can be modulated by the adjustable stoichiometry of the clusters. The development of ternary II–II0 –VI clusters via the silylated chalcogenolate complexes is a viable route for the preparation of ZnxCd1xE (E¼ Se, Te). For the degradation of the adamantoid clusters [ZnxCd10x E4(EPh)12(PnPr3)4] (E¼ Se, Te; x ¼ 0.18, 0.26), both the composition and metal-ion distribution of the clusters are reflected in the resultant nanocrystalline products, demonstrating that these clusters are efficient precursors for the formation of ternary materials.96 Thermolysis of these ternary nanoclusters in HDA provides an efficient system in which the pre-defined metalion stoichiometry of the clusters is retained in the formed nanoparticles. Spectroscopic and crystallographic data suggested an inhomogeneous distribution of the metal centers, with Zn ions evidently concentrated near the surfaces of the nanoparticles, whereas [Zn5Cd11E13(EPh)6(thf)(N,N0 -tmeda) (E ¼ Se, Te) clusters undergo a continual decrease in the Zn composition of the isolated materials with increasing growth temperature.97 Such differences probably arise due to their different structural motifs.

1.32.4

Group III–VI Thin Films

The group III and VI elements form compounds with several stoichiometries and in a number of phases: M2E3 (M ¼ Ga, In;

E ¼ S, Se, Te), a zinc-blende type (M ¼ Ga) and a defect spinel (M ¼ In), and ME (with the same layered structure as in M2E3).98 The III–VI materials are alternatives to those in the II–VI series, especially in photovoltaic applications. However, their polytypism and the variety of stoichiometries accessible are a problem as compared to the II–VI materials, and the availability of some of the elements, notably indium, is somewhat limited. Unlike II–VI, preparation of III–VI thin films started with the use of SSPs. Nomura et al. successfully prepared volatile alkylindium alkyl thiolates and used them for the preparation of two different kinds of indium sulfide thin films, depending on the number of thiolate ligands bound to the indium atom.99 Dialkyl indium monothiolates give InS, and monoalkylindium dithiolates produce In2S3 at 300  C under static pyrolysis conditions. The compound [InnBu(SiPr)2], prepared by the reaction of tri-n-butylindium and two equivalents of 2-propanethiol, was used to grow films by LP-MOCVD at 300–400  C on Si (11 1) and quartz substrates.100 The films were shown to be tetragonal b-In2S3 with a preferred orientation along the (10 3) plane. However, at higher growth temperature (450  C), a sulfur-deficient phase, In6S7 was deposited. The compound, [IniBu2(SnPr)], was also used to deposit conductive and transparent sulfur-doped indium oxide films under a slightly oxygenated carrier gas system.101 Sulfur-deficient phases such as In6S7 have different electronic properties from either InS or In2S3. Therefore, it is essential to prepare the required stoichiometry. The stoichiometry of the compound could be controlled by changing the growth temperature in this case. Barron et al. have suggested that molecular design can play an important and direct role in determining the nature of thin films deposited from SSPs.102 A comparison of the films grown from [IniBu2(StBu)]2 and [InMe2(StBu)]2, as well as from the bisthiolate complex, [InMe(StBu)2]2, shows the importance of deposition temperature, decomposition mechanism, and the choice of precursor used. At 400  C the methyl-substituted complex deposited an indium-rich amorphous phase and In2S3, whereas the tert-butyl-substituted complex deposited highly orientated tetragonal InS. This observation suggests that the stronger In–C bond in the former results in fragmentation of the precursor on decomposition, whereas the latter exhibits clean ligand loss. However, the bis-thiolate complex leads to an amorphous phase, though annealing does yield crystalline b-In2S3. In related studies, indium selenide thin films have been prepared from the analogous selenolate complexes.103 Barron and coworkers also prepared a number of dialkylindium selenolates and alkylindium selenides,103 and deposited indium selenide films by LP-MOCVD.104 While [IntBu2(SetBu)]2 deposited indium-rich films at temperatures between 230 and 420  C, [In (CEtMe2)(m3-Se)]4 gave crystalline films of InSe; however, the film quality depended on the growth temperature. Gysling et al. have also prepared thin films of indium selenide from [InMe2(SePh)] or [In(SePh)3], by a spray-assisted MOCVD technique, on GaAs (10 0).105 Different phases of InSe were grown at different substrate temperatures, with a cubic phase observed at deposition temperatures between 310 and 365  C. Pyrolysis of [In (SePh)3] gave hexagonal films of In2Se3 at temperatures from 470 to 530  C. In addition to these, a number of other potential precursors for the growth of indium selenide have been prepared with more bulky alkyl substituents.106

Precursor Chemistry – Main Group Metal Chalcogenides

Barron and coworkers have prepared a number of metal chalcogenide complexes with interesting cubane structures; the first was [tBuGaS]4 (Figure 7), prepared by thermolyzing the dimer [GatBu2(m-SH)]2, which was synthesized by the addition of an excess of H2S to [GatBu3].107,108 Much of the impetus for making the complex was that many of the dimeric thiolate complexes, as described above, were prone to yield sulfurdeficient films. The cubane has no direct carbon–sulfur bonds and is supported by metal–sulfur interactions. [tBuGaS]4 is an air-stable white solid and was used to grow gallium sulfide by AP-CVD at 380–400  C. The GaS films were deposited on both potassium bromide and GaAs (1 0 0) and on the latter a degree of epitaxial growth was observed.109 The films were of a novel cubic phase of GaS, with good properties as a passivating material for GaAs.110 The hypothesis that molecular structure can influence the phase of the as-deposited films was further investigated with the cubane selenide or telluride precursors [GaR(m3-E)]4 (E ¼ S, Se, Te; R ¼ CMe3, CEtMe2, CEt2Me, or Et3C) (e.g., Figure 8).111,112 However, in contrast to the sulfides, the hexagonal phase of GaSe was deposited by AP-CVD ( 350  C) and, similarly, metastable hexagonal GaTe was deposited by LP-MOCVD.113 It is predicted that the formation of hexagonal GaSe and GaTe is as a consequence of the ‘controlled’ cleavage of the appropriate Ga4E4 core during MOCVD. Another series of complexes that have been studied as precursors for the deposition of III–VI thin films are those containing dialkyldithio- and dialkyldiseleno-carbamato ligands. The tris(dialkyldithiocarbamates) of gallium or indium are monomeric solids,114 in contrast to the bis(dialkyldithiocarbamates) of zinc or cadmium that are usually dimers. The symmetrical tris(dialkyldithiocarbamates) are of quite low volatility. The use of secondary amines in their preparation was originally to improve volatility and some indium tris(dialkyldithiocarbamates) are prepared.115 Compounds with

various asymmetric alkyl groups (R ¼ Me and R0 ¼ Et, nBu, or Hex) were prepared, with the structure of the methyl(ethyl) derivative determined.115 The other two complexes were used as SSPs to grow In2S3 by LP-MOCVD on glass and InP (1 1 1). The analogous diselenocarbamate, [In(Se2CNMenHex)3], has been used to deposit thin films of cubic In2Se3.116 At a growth temperature of 450  C, films of cubic In2Se3 were deposited on glass substrates with a preferred (1 1 1) orientation. The results are similar to those of Arnold and coworkers who prepared cubic In2Se3 films from [In{SeC(SiMe3)3}3].117 In addition to studies involving group 13 metal tris(dialkyldichalcogenocarbamate) complexes, there has also been some investigation into thin-film deposition from mixed alkyl group 13 metal dialkyldichalcogenocarbamates. A series of mixed alkylindium and gallium diethyldithiocarbamates of the formula [MR2(S2CNEt2)] (R ¼ Me, Et or CH2tBu) have been prepared,118 and were found to be monomeric solids in the case of indium complexes, and liquids in the case of gallium. The compounds can be prepared by either the comproportionation of a metal trialkyl and the appropriate metal tris(dialkyldithiocarbamate) or the reaction of the sodium diethyldithiocarbamate salt with chlorodiethylindium in n

Figure 8 Structure of [Ga(CEt3)(m3-S]4.

Figure 7 Structure of [tBuGaS]4.

1009

Figure 9 Structure of [InEt2(S2CNEt2)].

1010

Precursor Chemistry – Main Group Metal Chalcogenides

diethyl ether.118 The molecular structure appears to affect the nature of the as-deposited films, sulfur evaporation and b-hydrogen elimination being possibly important. For example, [InEt2(S2CNEt2)] (Figure 9) was different from the other two compounds, [InMe2(S2CNEt2)] and [In(CH2tBu)2(S2CNEt2)], in that a single-phase, cubic b-In2S3 was deposited over the temperature range between 325 and 400  C. In this case, only the ethyl compound contains b-hydrogen atoms, and the ready elimination of the alkyl fragment may influence the film composition in many ways, including by affecting the amount of carbon incorporation. Monothiocarbamato complexes [In(SOCNEt2)3] (Figure 10),119 [In(SOCNiPr2)3],120 and [Ga(SOCNEt2)3]121 have been used in deposition. The films grown from both of the indium precursors were found to be of b-In2S3. Using the diisopropyl compound, deposition occurred at temperatures as low as 300  C. However, the gallium monothiocarbamato precursor deposited cubic GaS at 450  C, and the film was comparable to those obtained by Barron and coworkers from [tBuGaS]4.107 It was orientated in the (2 0 0) direction even though deposited

onto glass. The sulfur-deficient phase deposited may be due to the Ga:S ratio in the precursor. Organometallic complexes of indium and gallium based on imidodiphosphinato ligands also proved to be useful compounds for MOCVD studies.122 The acidic nature of the imino proton in [NH(SePiPr2)2] makes it a good candidate for alkane elimination reactions with group 13 metal alkyls. Thermodynamically stable cubic Ga2Se3 could be grown on glass substrates by LP-MOCVD and AA-CVD. However, the quality of films in terms of morphology is somewhat better in lowpressure studies in comparison with AA-CVD and higher growth rates  1.25 mm h1 could be achieved at 500  C. In an attempt to prepare an indium complex of [N(TePiPr2)2] anion, indium (I) chloride (InCl) in the presence of elemental tellurium is used to give a trimeric complex, In(m-Te)[N(iPr2PTe)2]}3, comprised of a central In2Te3 ring instead of an expected octahedral complex (Scheme 7).123 The gallium analog can be obtained in a similar manner by using gallium iodide instead of InCl. AA-CVD studies of indium complex deposited cubic In2Te3 films, whereas gallium complexes yielded a mixture of Ga2Te3, GaTe, and elemental tellurium on silicon substrates.124 Mass spectrometric studies indicate that the indium complex deposits In2Te3 by the fragmentation of the central In3Te3 ring, with the remaining species In[(TePiPr2P)2N]2þ and [N(TePiPr2)2N]2 staying intact. The gallium complex undergoes a similar fragmentation process; however, it is significantly more susceptible to hydrolysis and oxidation than its indium counterpart. This is a possible explanation of the formation of Te during the AA-CVD deposition of the Ga complex.

1.32.5

Group III–VI Nanocrystals

Barron et al. reported the preparation of nanoparticles of GaSe and InSe by MOCVD using the cubane precursors [(tBu)GaSe]4 and [(EtMe2C)InSe]4.125 The GaSe particles had a mean diameter of 42 nm with a standard deviation of 13 nm, whereas the spherical InSe particles were 88 nm in diameter with a standard deviation of 30 nm. The particle size was determined by TEM, and no optical data were reported. O’Brien et al.126 reported the synthesis of InS and InSe nanoparticles capped with TOPO and nanoparticles of InSe capped with 4-EtPy from [In(E2CNEt2)3] (E ¼ S, Se). GaSe has a hexagonal layered

Figure 10 Structure of [In(SOCNEt2)3].

N iPr P 2

3

Te

Te

Te

N iPr P 2

PiPr2

M

PiPr2

3MX, 3Te

Te

-3NaX

Na (TMEDA)

Te

Te

M

M

Te iPr

2P

Te

N

Te

N

Te

P

P

iPr 2

iPr 2

M = In, X = CI M = Ga, X = I Scheme 7 General synthesis of [M(m-Te)[N(iPr2PTe)2]3 (M ¼ In, Ga).

PiPr2

Te

Precursor Chemistry – Main Group Metal Chalcogenides structure127 consisting of Se–Ga–Ga–Se sheets which are only weakly attracted by van der Waals interactions. GaSe can be b-, g-, or e-phase depending upon the arrangement of layers. GaSe nanoparticles (88 nm) were also synthesized from (RGaSe)4 cubanes by the MOCVD method.111,112 TEM images showed nanowire-type structures. Dutta et al. reported the preparation of In2S3 nanoparticles from SSPs such as indium xanthates.128 The same authors previously used methylindium thiolate complexes but produced indium sulfide nanoparticles of poor quality.129 Recently, they used polymeric indium and gallium precursors synthesized by the reaction of the trimethyl gallium/indium ether adduct (Me3Ga/InOEt2) with 1,2ethanedithiol (HSCH2CH2SH) in 1:1 stoichiometry. Ga2S3 and In2S3 nanoparticles were prepared by pyrolysis of these precursors [MeM(SCH2CH2S)]n (M ¼ In, Ga) in a tube furnace between 300 and 500  C.130

1.32.6

Group IV–VI Thin Films

The tin, lead, and germanium chalcogenides are narrowbandgap semiconductor materials which possess unique optical and electronic properties. In particular, structural diversity is seen in the case of tin chalcogenides where both layered 2D (e.g., SnS and SnSe) and 3D (e.g., SnTe) crystal structures are observed. This is partially a result of the Sn2þ oxidation state in these materials (vide infra).131,132 Deposition of PbS thin films has been mainly dominated by lead dithiocarbamates. Initially PbS thin films were deposited from [Pb(S2CNEt2)2] in the temperature range 425–500  C with a source temperature of 210  C, and in the temperature range 230–300  C by remote plasmaenhanced MOCVD.133 More recently, a comparative study has been carried out looking at a series of symmetrical and unsymmetrical [Pb(S2CNR1R2)2] (R1 ¼ Me, R2 ¼ benzyl, heptyl, octadecyl, hex, Me; R1 ¼ R2 ¼ Et; R1 ¼ R2 ¼ octyl) in AA-CVD experiments.134,135 The TGA curves as shown in Figure 11 reveal that all complexes showed clean, one-step decomposition except compounds with R1 ¼ Me, R2 ¼ octadecyl, and R1 ¼ R2 ¼ dioctyl

100

Weight loss (%)

90 80

4

70

3

60

5 1 2 6

50

7

40 30 20 10 100

200

300

400

500

Temperature (ºC) Figure 11 Thermogravimetric analysis of complexes [Pb (S2CNMeBenzyl)2] (1), [Pb(S2CNMeHep)2] (2), [Pb(S2CNMeOctdec)2] (3), [Pb(S2CNOct2)2] (4), [Pb(S2CNMeHex)2] (5), [Pb(S2CNMe2)2] (6), and [Pb (S2CNEt2)2] (7) at a heating rate of 10  C min1 under nitrogen.134

1011

which showed two-step decomposition. Deposition temperature of 350  C was found to be too low to initiate deposition, whereas above 350  C, films deposited on glass were nonadherent and had poor film coverage. A considerable difference could be noticed when the chain length reaches around six carbon atoms. The films obtained from longer chain alkyl derivatives are more crystalline and uniform than those from smaller chain alkyl derivatives. Also the lower deposition temperature for longer alkyl derivatives causes less defaults and contamination in the deposited films. This may be due to the fabrication of more stable and volatile species by longer chain alkyl groups which act as better leaving entities. Until now, little was known about the decomposition behavior of lead xanthates [Pb(S2COR)2] and their adducts in the vapor phase. Initially, we reported the low-temperature (150  C) growth of crystalline PbS films on plastic substrates by AA-CVD of [Pb(S2COBu)2].136 DFT calculations were carried out to explore the energetics of the Chugaev elimination reaction suggested to underlie the CVD process. Metal xanthates generally undergo Chugaev rearrangement, which provides a clean, low-temperature decomposition pathway which is generally lower than dithiocarbamates. The following reaction steps (at 425 K) lead to the formation of PbS, which are in line with this elimination mechanism: PbðS2 COBuÞ2 ! ðS2 COBuÞPbðS2 HCOÞ þ C4 H8

[1]

PbðS2 HCOÞ2 ! ðS2 HCOÞPbðSHÞ þ OCS

[2]

PbðSHÞ2 ! PbS þ H2 S

[3]

In the initial reaction [1] butene is lost via hydrogen abstraction by sulfur from the butyl moiety, with a barrier of 160 kJ mol1. The transition structure involves a sixmembered ring. In the second step [2] OCS is lost from the bixanthate ligand with a barrier of 3 kJ mol1. Thus, once butene has been eliminated there is an extremely facile loss of OCS. In the final step [3], the PbS product is formed, with an activation barrier of 112 kJ mol1. Calculations thus predict that the initial reaction [1] is rate limiting and the magnitude of the barriers suggests that this sequence of reactions is indeed feasible, with the barrier for step [1] being only a little larger than the experimental estimate. A series of N-donor adducts of lead xanthates [Pb(S2COR)2L] [R ¼ Et, nBu, L ¼ bipyridine (bipy), tetramethylethylenediamine (TMEDA), and pentamethyldiethylenetriamine (PMDETA)] have also been reported recently.137 [Pb(S2COEt)2TMEDA] is sevencoordinated at lead through three chelating ligands and one weak intermolecular Pb  S interaction. Both [Pb(S2COR)2bipy] (R ¼ Et, nBu) are dimers in which one xanthate is terminal, and the other, m2 bridging at each sulfur, generating an eightcoordinate lead when the bipy donor is included. Both [Pb (S2COR)2 PMDETA] (R ¼ Et, nBu] are seven-coordinated at lead by virtue of two bidentate chelating xanthate ligands and a tridentate PMDETA. [Pb(S2COEt)2TMEDA] has been used to deposit PbS films on glass, Mo-coated glass, and Si by AA-CVD. Deposition in all three films results in highly reflective, pin-hole free films but the quality of film on glass was somewhat less uniform compared to others. Recently, O’Brien and coworkers have reported a mixed lead phosphinato compound: [Pb{(C6H5)2PSSe}2] in

1012

Precursor Chemistry – Main Group Metal Chalcogenides

AA-CVD experiments to yield only PbSe thin films.138 The absence of sulfur in the films was confirmed by energydispersive x-ray analysis. DFT calculations were carried out to study the formation of PbSe from [Pb{(C6H5)2PSSe}2] which is observed during the thermal CVD process. A number of different fragmentation patterns have been investigated which can yield the observed product, PbSe, rather than PbS. The first two mechanisms involve the initial loss of a phenyl radical (C6H5), this process being highly endergonic. The next step is the loss of either SP(C6H5) or SeP(C6H5), the former reaction being somewhat less endergonic of the two. The loss of a second phenyl radical is postulated, which is only slightly less endergonic than the initial step. However, the subsequent decomposition of (C6H5)PSSe–Pb–Se to give the observed product, PbSe, is slightly exergonic. Thus, both reaction sequences point to the favorable formation of PbSe.

iPr P 2

2

N

E

iPr

PiPr2

Pbl2

E

Et2O

Na (TMEDA)

2P

E

N

PiPr2

E

N

Pb

i

E

Pr2P

PiPr2

E

E = Se, Te + 2TMEDA + 2NaI i

Scheme 8 Synthesis of Pb[(EP Pr2)2N]2 (E ¼ Se, Te).

Se

Se P Ph

Ph P

Se

Se

Se RONa 0 °C – RT

P Ph

Dichalcogenoimidodiphosphinato complexes Pb [(EPiPr2)2N]2 (E ¼ S, Se, Te) are another class of precursors for CVD thin films.135,139 All compounds are air- and moisture-stable except the tellurium analog. Both the selenium- and tellurium-based complexes were found to be associated in the solid state via M  E interactions with adjacent molecules to give C2-symmetric dimers. Taking these interactions into account, the geometry at the metal centers was best described as distorted square pyramidal. The metalbased lone pair of electrons was thus occupying the vacant octahedral coordination site. Attempts to prepare thin films of PbTe involved the AA-CVD of Pb[(TePiPr2)2N]2 (Scheme 8) onto glass substrates. The solubility of the complex was low in common organic solvents such as THF and toluene, but it was better in CH2Cl2 and MeCN. Sufficient solubility of the compound was achieved in THF/CH2Cl2 (1:1), MeCN, and MeCN/ CH2Cl2 (1:1). The AA-CVD of Pb[(TePiPr2)2N]2 using substrate temperatures below 475  C resulted in films composed of PbTe and Te, as determined by x-ray powder diffraction studies. Lead phosphonodiselenoato compounds Pb[Ph(RO) PSe2]2, (R ¼ Me, Et) have been demonstrated to deposit submicrometer cubes, truncated octahedrons, and octahedrons of PbSe in AA-CVD.140 Both complexes can be prepared by the reaction of lead acetate with two equivalents of sodium phosphonodiselenoate in high yield; the latter was derived from 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide

SeNa Pb(CH COO) .3H O 3 2 2 OR

RO

Se P

Se

Ph P

Pb

ROH, 60 °C Ph

Se

Se

OR

Scheme 9 Synthesis of Pb[Ph(RO)PSe2]2 (R ¼ Me, Et).

(c) (a)

(e)

m 0.35 nm

0.35 n

0.3

5n

(111)

m

0.21 nm (220) (b)

(d)

0.21 nm (220)

5 nm 51/nm

Figure 12 SEM images of (a) aggregation of truncated octahedron crystals through their (1 1 1) facets after 5 min and (b) truncated octahedrons exposing their (1 0 0) facets after 10 min on Si/SiO2 (1 0 0) surfaces from Pb[Ph(MeO)PSe2]2. (c) TEM and (e) HRTEM images of the octahedron from compound Pb[Ph(EtO)PSe2]2 on Si/SiO2 (1 0 0). (d) The corresponding selected area electron diffraction (SAED) pattern.140 Reproduced with permission from Ahmad, K.; Afzaal, M.; O’Brien, P.; Hua, G.; Woollins, J. D. Chem. Mater. 2010, 22, 4619. American Chemical Society.

Precursor Chemistry – Main Group Metal Chalcogenides

[PhP(Se)(m-Se)]2 (Woollins, reagent) with the corresponding sodium alkoxide (Scheme 9).141 During the course of experiments, it was determined that the shapes of the PbSe crystals can be controlled by choosing different ligands, and the ratio of (1 1 1) to (1 0 0) facets of the nanocrystals that are exposed can also be manipulated (Figure 12). Furthermore, it is also shown that variation in deposition temperatures can have a profound effect on the growth of PbSe crystals. Initial studies on the deposition of tin sulfide films from precursors such as [Sn(SCH2CF3)4] or [Sn(SPh)4] utilized hydrogen sulfide (H2S) as an additional sulfur source.142,143 These precursors on their own do not result in SnS. This was attributed to facile disulfide (RS–SR) elimination, which is due to the presence of noncovalently bonded S  S interactions and cis-annular interactions in the molecules like [M(SR)4], where M is a group IV element.144 Tin and lead compounds with pyridineselenolates, [Sn(Sepy)2]2, [Sn(Sepy)4], and [Pb(Sepy) [Sn (Sepy)2]2] can be used for the vapor phase deposition of MSe.145 Mass spectrometry and TGA analysis showed that [Sn(SR)4] results in RS–SR in gas phase, supporting the formation of a disulfide during CVD process.144 However, in the presence of minimal flow of H2S it was possible to deposit SnS films. It is predicted that the sulfur in tin sulfide may be from H2S gas and not from the thiolate precursors.146 Even unsymmetric tin dithiocarbamates146 and tin complexes containing chelating ligands144 possessing direct Sn–S bond may or may not form SnS in the absence of H2S gas. It was established that chelating thiolate ligands could hinder disulfide elimination.144 Thus, Parkin et al. have reported the use of chelating dithiolato ligand complex, [Sn(SCH2CH2S)2], for tin sulfide coatings on glass using AA-CVD.144 Orthorhombic SnS thin films, with extended close-packed stacking sequence (c ¼ 53–55 A˚), from simple tin thiosemicarbazone complexes of the type Bz3SnCl(L) (L ¼ thiosemicarbazones of salicylaldehye and 4-chlorobenzaldehyde) can be deposited.147 No H2S was employed during the course of CVD studies. In thiosemicarbazone complexes, disulfide elimination is hindered due to the absence of any RS ligand as well as of more than one sulfurcontaining ligand of any sort. In the present compounds, both molecules contain only one thio ligand and, hence, no S  S interaction is possible, that is, disulfide elimination is not feasible. The first report on the growth of SnSe and SnTe has been described by Dahmen and coworkers who tested bis[bis(trimethylsilyl)methyl]tin chalcogenides, [Sn{CH(SiMe3)2}2 (m-E)]2 (E ¼ Se, Te), in the MOCVD studies.148,149 The compounds are found to be dimeric both in the solution and in the solid state.150 During the course of growth experiments, it was determined that both compounds show selectivity in the decomposition reaction toward the metallic substrates, thus demonstrating the need for a seeding metallic layer to initiate the growth. In the absence of metallic substrates, no films were deposited under any experimental conditions. Reid and coworkers have reported the use of Sn (IV) compounds, [SnCl4{o-C6H4(CH2SeMe)2}] and [SnCl4(Et2Se)2], in LP-CVD studies.151 Both compounds can be obtained in good yield as moisture-sensitive yellow powders from direct reaction of SnCl4 with one mole equivalent of dichalcogenoether or two mole equivalents of Et2Se in anhydrous CH2Cl2 solution. Both compounds deposit SnSe2 films on silica substrates. However the Et2Se derivative, [SnCl4(Et2Se)2], leads to a more uniform

1013

deposition of SnSe2 with growth perpendicular to the substrate surface. Poor surface coverage is seen in the case of the bidentate selenoether complex, reflecting the low volatility of the compound. Reports on the deposition of germanium chalcogenides are rare. Only recently, germanium telluride thin films were reported from a single-source route.152 Insertion reaction of a germylene and dialkyl telluride leads to a stable alkyl tellurolato compound which has been used to deposit GeTe films in MOCVD studies.

1.32.7

Group IV–VI Nanocrystals

One of the earliest reports of the deposition of PbS involved the use of the diethyldithiocarbamate to produce well-defined cubes.69,68 It was subsequently shown that shape control was possible with changes in the reaction conditions.153 Many precursors have been used to deposit PbS or PbSe.69,68,80,153–155,156 Decomposition of lead hexadecylxanthate in trioctylamine (TOA) has been reported to yield ultranarrow rods with a diameter of 1.7 nm and lengths of 12–15 nm.156 Strong quantum confinement is observed in the absorption and PL spectra. The absorption band is strongly blue shifted and has a sharp excitonic band at 278 nm and a shoulder at 365 nm. These bands are of excitonic origin corresponding to the 1Pe to 1Ph, 1Se to 1Ph, and 1Se to 1Sh transitions.157–159 The PL spectra show strong and sharp band-edge emission at 410 and 434 nm along with a shoulder at 465 nm and a weak band at 500 nm. In another report, Acharya and coworkers have reported the preparation of rods (1.7 and 2.5 nm in diameter) from lead hexadecylxanthate, and demonstrated that the control over the aspect ratio is achieved simply by changing the temperature of injection of the precursor.160 Both the nanorods exhibited molecule-like discrete narrow optical behavior with highfluorescence quantum yield and are also found to be robust. It is also determined that the initial crystallographic phase of seeds at the nucleating stage is critical in determining the shape of the nanorods. Injection of the precursor creates seeds that are terminated by {1 0 0}, {1 1 0}, and {1 1 1} faces. Due to the mixed nature of {1 0 0} and {1 1 0} facets, the ligand adsorption kinetics are different from the {1 1 1} facets. Typically {11 1} faces have higher energy in comparison to {1 0 0} and {1 1 0} faces, which favors stronger binding of TOA of {1 1 1} faces.76 As the {1 1 1} faces are terminated by either Pb or S atoms, the ligand binds strongly to the metal atoms facilitating asymmetric growth. The surface energy of {1 1 1} face is lower than {1 0 0} or {1 1 0} faces upon TOA binding, which facilitates the growth along the (11 1) direction. Compared to {1 1 0} facets, {1 0 0} facets have higher kinetic energy barrier, facilitating the growth along the (1 1 0) direction. The kinetic rate of surfactant adsorption and desorption is dominant in determining the shape of the nanorods in order to reach the thermodynamic equilibrium. Alivisatos and coworkers recently reported the conversion of Cu2S rods to PbS simply by cation-exchange reactions.161 This reaction proved to be much more favorable than converting CdS rods to PbS. This effect is possibly due to fact that the soft base, tri-n-butylphosphane, binds strongly to the Cuþ cation as compared to Cd2þ or Pb2þ and provides more control. Vittal et al. synthesized PbS particles by employing a Lewis base-catalyzed approach to decompose metal alkyl xanthates,

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by using alkyl amines as a solvent to promote decomposition as well as capping ligand for the particles formed.155 Spherical PbS nanoparticles of diameter 5–10 nm were obtained when long-chain alkylamines were used. When ethylenediamine was used instead, PbS dendrites were isolated from the same precursor at room temperature. Uniform six- and four-armed dendrites were observed, with regular branches of 20 nm in diameter growing in a parallel order. These kinds of precursors were stable for periods of months, easy to synthesize, and decomposed with high yields. Cheon and coworkers provided some insight into factors underlying the ripening observed with the PbS nanoparticles from an SSP.153 By varying the ratio injection of temperature, the shape of the resulting particles evolved from rods to multipods to cubes. A novel PbS hierarchical superstructure, labeled as octapodal dendrites with a cubic center (Figure 13), has been produced by solvothermal decomposition of lead hydroxyl thiocyanate in ethylene glycol (EG).162 This superstructure is a result of the delicate balance between kinetic growth and thermodynamic growth regimes. It is evident that a cubic center with short arms grow along the {1 1 1} directions from the eight corners of a cubic core with {1 0 0} terminating. With prolonged reaction time, arms continue to grow along the {1 1 1} directions. It is worth pointing out here that the central cube of the octapodal dendrites is protected (at early stages of crystal growth) by the chemisorption of OH ions (from

(a1)

(a2)

(b1)

(a3)

(b2)

(c1)

(c2)

the precursor) which prevents the addition of PbS nuclei to the {1 0 0} planes of a growing seed. The net result is that the corners of PbS nuclei continue to grow along the {1 1 1} directions to form the final octapodal dendrites. During the later stages of crystal growth, the etching of OH ions on the {1 0 0} planes is also evident owing to the slow growth rate. Lead precursors of salicylic (sal), picolinic (pic), and 2,6dipicolinic (2,6-dipic) compounds form stable adducts with thiourea or thiosemicarbazide and have been decomposed in both aqueous and nonaqueous media to yield a variety of structures.163 The sal compounds were found to be the most soluble followed by the pic and 2,6-dipic derivatives. All the precursors seem to undergo a single decomposition step around 170  C, yielding pure PbS. The presence of dodecanethiol (DDT) in the surfactant system results in a preferential and uniform growth of cubic structures. The growth ratio between {1 0 0} and {1 1 1} faces can be adjusted by varying the polarity of the surfactants and the addition of stabilizers such as cetyltrimethylammonium bromide and sodium dodecylsulfonate. The dissolution growth at the atomic scale leads to crystal splitting and regrowth, while faceted growth occurs near equilibrium dissolution through the linear and planar defects at the atomic scale. SSPs have been used by a hydrothermal process to grow a range of PbS nanocrystals.164 More recently, Bi nanoparticles have been used to catalyze solution–liquid–solid (SLS) growth

(a4)

(b3)

(a5)

(b4)

(c3)

(c4)

Figure 13 TEM images of PbS octapodal dendrites obtained at different reaction times: 1, 4, 8, 10, and 12 h for columns 1–5, respectively; (b) scheme for the shape evolution of octapodal dendrites with a cubic center; (c) the selective protection and etching process of the cubic surface (inset of c3 and c4 are corresponding 3D structural models). The scale bar is 1 mm.162 Reproduced with permission from Duan, X.; Ma, J.; Shen, Y.; Zheng, W. Inorg. Chem. 2012, 51, 914. American Chemical Society.

Precursor Chemistry – Main Group Metal Chalcogenides of PbS nanowires from [Pb(S2CNEt2)2].165 The minimum controlled nanowire diameters that can be typically achieved from Bi catalysts is  5 nm, because the small Bi nanoparticles (d < 5 nm) required to produce smaller nanowire diameters are very unstable with respect to agglomeration. Jen-La Plante has used dibutyldithiocarbamate compound of lead to produce rectangular sheets and branched dendritic stars of PbS.166 The same growth mechanism has been explored in a lead selenourea compound to produce PbSe nanowires.167 The use of tin dithiocarbamate [Sn(S2CNEt2)2] in OA has been reported to yield SnS nanocrystals, and reaction conditions do not require the use of hazardous materials such as phosphanes and volatile organometallic compounds.168 The crystalline SnS nanocrystals display strong optical absorption in the visible and near-infrared spectral regions. Recently, 2D nanosheets of SnS have been reported from [Sn(S2CNEt2)2(phen)] (phen ¼ 1,10-phenanthroline) in a mixture of OA/octadecene which exhibited excellent electrochemical properties with a capacity of 350 mAh g1 around 1.2 V.169 In OA, ultralarge SnS nanosheets are prepared. Uniform SnS2 nanoplates can be easily obtained by carrying thermal decomposition of the precursor in OA and oleic acid.

1.32.8

Group V–VI Thin Films

Binary metal chalcogenides M2VE3VI (M ¼ Sb, Bi; E ¼ S, Se, Te) have drawn extensive interest as they are an important class of semiconductors that are already used in numerous applications, e.g., thermoelectronic devices,170 sensors,171 photoelectrochemical (PEC) solar cells,172 photoconducting targets,173 etc. Among different chalcogenide material, bismuth sulfide (Bi2S3) is known to be an attractive material for PEC applications as it has a reasonably low bandgap (Eg ¼ 1.7 eV), an absorption coefficient in the order of 104–105 cm1, and a reasonable incident photon to electron conversion efficiency (5).174 There are several reports on the use of SSPs for the growth of Bi2S3 films by CVD. A single-source route was carried out by Monteiro et al. on the deposition of thin films of Bi2S3 from bismuth dithiocarbamates, [Bi(S2CNRR0 )3] (R ¼ Et, Me, R0 ¼ hexyl) by LP-MOCVD at 400–450  C.175 The resulting films are found to be composed of nanofibers of orthorhombic Bi2S3 with direct bandgaps of 1.29 eV. In another study, bismuth dithiocarbamates and related bismuth xanthate species were used in CVD studies to deposit Bi2S3 films in the form of the nanorods that vary only in length.176 A more recent example includes the deposition of Bi2S3 nanotubes from [Bi (S2CNEt2)3] on fluorine-doped, SnO2-coated conducting substrates by AA-CVD.177 Deposited films displayed a reasonable photoactivity under illumination and indicated a photocurrent density of 1.9 mA cm2. TEM analysis (Figure 14) of a single nanorod from [Bi(S2COEt2)3] shows a lattice spacing of 0.4 nm in the (0 0 1) direction which agrees with the c axis of the orthorhombic structure of Bi2S3.176 Therefore, the c axis is parallel to the long axis of the rod. In another HRTEM image, the lattice spacing between two planes is 0.56 nm, which is one-half of the lattice constant along the (1 0 0) direction. Formation of the rods is ascribed to two reasons. First, the stronger covalent bond, between the planes perpendicular to the c axis, facilitates a higher growth rate along the c axis

1015

combined with weak van der Waals bonding between the planes, perpendicular to the a axis, limiting growth of the rods in the horizontal direction and facilitates their cleavage. Bismuth chalcogenide films have been reported from CVD studies of air-stable complexes of Bi[(EPR2)2N]3 (E ¼ S, Se; R ¼ Ph, iPr).178 Crystalline thin films of rhombohedral Bi2Se3 (from Bi[(SePiPr2)2N]3), hexagonal BiSe (from Bi [(SePPh2)2N]3), and orthorhombic Bi2S3 (from Bi[(SPR2)2N]3) have been deposited on glass substrates. The suggested reason for deposition of monophasic Bi2Se3 and BiSe from isopropyland phenyl-substituted precursors, respectively, is highly speculative. It may be that the difference in the electron-donating character of the alkyl group on the P atoms affects the relative bond strengths within the structures and hence the decomposition profiles of the parent molecules. This effect could potentially have a significant impact on the relative amounts of bismuth and selenium reaching, or being retained at, the surface. Thin films composed of hexagonal Bi2Se3 nanoplates have also been deposited from MOCVD of the bismuth diselenophosphato complex, [Bi(Se2P{OiPr)2}3], on modified and unmodified Si substrates.179 The deposited Bi2Se3 nanoplates have a superior thermoelectric property over bulk Bi2Se3. A series of unsymmetrical antimony dithiocarbamates [Sb (S2CNMeR)3] (R ¼ nBu, nHex, Bz) have been used in AA-CVD to deposit Sb2S3 films; the film purity depended on the substrate temperature.180 Despite their relatively low melting points, none of the compounds proved to be a viable precursor for LP-CVD. Transport of the precursors, even at relatively elevated temperatures (90 < T < 300  C), leads to poor substrate coverage and extensive decomposition of the precursor. It is plausible that the dimeric nature of these compounds is responsible for the lack of volatility. Films grown by AA-CVD at temperatures in excess of  300  C become susceptible to oxidation, and deposition of oxides and mixed oxide/sulfides becomes more prevalent. The origin of the oxygen within the film is uncertain and probably due to traces of oxygen either in the nitrogen carrier gas and/or from the solvent. Sb2S3 films seem especially susceptible to oxidation, even at modest temperatures in CVD; LP-CVD as a deposition technique would appear to be more attractive than AA-CVD in this case. Other monomeric compounds such as antimony thiolates Sb (SR)3(R ¼ tBu, CH2CF3) have been investigated for deposition of Sb2S3 thin films by LP-CVD.181 In both cases, only orthorhombic Sb2S3 films are deposited at temperatures of 300 and 450  C. The morphologies of the films show a strong dependence on both precursor and substrate. The film deposited from the tert-butyl derivative on conducting glass had a bandgap of 1.6 eV and was found to be photoactive. The same group has also reported phase-pure Sb2S3 films from antimony xanthates [Sb(S2 COR)3] (R ¼ Me, ethyl, iPr) in AA-CVD studies.182 These precursors lacked sufficient volatility for either AP- or LP-CVD. The advantages of xanthate precursors are rationalized in terms of a facile, low-temperature Chugaev rearrangement. In contrast, dithiocarbamates do not undergo Chugaev elimination, since formation of the weaker CdN bond in RNCS (analogous to formation of OCS) works against this. Application of the AA-CVD technique to the homoleptic antimony (III) complex, Sb[N(PPri2Te)2]3, in the temperature range 375–475  C produces hexagonal-shaped nanoplates of pure rhombohedral Sb2Te3.183

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Precursor Chemistry – Main Group Metal Chalcogenides

0.5 mm

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110

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Figure 14 (a, b) TEM images of Bi2S3 nanorods from [Bi(S2COEt2)3]. (c, e) HRTEM images and (d, f) corresponding FFTs of two Bi2S3 nanorods. (g) SAED pattern of a typical Bi2S3 nanorod.176 Reproduced with permission from Koh, Y. W.; Lai, C. S.; Du, A. Y.; Tiekink, E. R. T.; Loh, K. P. Chem. Mater. 2003, 15, 4544. American Chemical Society.

Precursor Chemistry – Main Group Metal Chalcogenides

1.32.9

Group V–VI Nanocrystals

Several morphologies such as nanorods, nanocrystals, and nanotapes of Bi2S3 are obtained from solvent thermolysis of bismuth trisxanthates or bismuth dithiocarbamates in EG.176 The morphology of the Bi2S3 crystals depended on the solvent used, the thermolysis temperature, and the nature of the precursor. The bismuth xanthates have a lower decomposition temperature (<150  C) than the dithiocarbamates (>200  C). Precursors adopting the polymeric structures in solution decompose at a higher temperature than those with dimeric structures. The outcome is that polymeric [Bi(S2COiPr)3] gives long Bi2S3 nanofibers, whereas dimeric [Bi(S2COR)3] (R¼ alkyl) produces nanocrystals. Bi2S3 has the tendency to form nanorods relatively easily due to the inherent Bi–S chain-type structure. It is known that Bi2S3 crystallizes with a lamellar structure with linked Bi2S3 units forming infinite chains, which, in turn, are connected via van der Waals interactions.184 The formation of Bi2S3 nanorods originates from the preferential growth of Bi2S3 crystallites along its c axis. For example, nanorods of Bi2S3 and Sb2S3 were also reported from hydrothermal decomposition of [M(S2CNEt2)3] (M ¼ Bi, Sb).185 Water exerts an influence on the formation of M2S3 nanocrystals. A proton and hydroxide anion from H2O may promote cleavage of one of the C–S bonds and S–M coordination bonds in the precursors to form M2S3. Adducts of [Bi(S2COsbutyl)3] with pyridine and 1,10-phenanthroline have also been hydrothermally treated to deposit rods with diameters in the range of 20–35 nm and lengths up to several hundreds of nanometers.186 When poly(vinylpyrrolidone) (PVP) is used instead of neutral ligands, highly uniform Bi2S3 nanorods with an average diameter of 20 nm are produced. The optical band edge of Bi2S3 nanorods from the 1,10phenanthroline derivative is blue shifted with a bandgap of

1017

1.67 eV. High-quality M2S3 nanorods are also reported by solvothermal treatment of metal dithiophosphates [M(S2PnOctyl2)3] in the presence of OA.187 The conditions such as reaction time, temperature, and concentration of precursors all play an important role in determining the size and aspect ratio of the product. Vittal and coworkers reported various morphologies of Bi2S3 nanomaterials including nanorods, nanoleaves, nanoflowers, and dandelion-like structures through colloidal solution of [Bi(SCOPh)3].188 In the presence of both DDT and TOPO, Bi2S3 nanorods with an average diameter of 21.3 nm and a length of 300 nm are reported. When the surfactant is changed to DDT and OA, Bi2S3 nanorods with grooves are deposited. The OA might have played a key role in making grooves on the surface of Bi2S3 nanorods due to its corrosive nature. Previously amines have also been used solely for etching purposes.189 In the presence of PVP and EG, dandelion-shaped Bi2S3 nanostructures are obtained. It is possible for PVP to adsorb onto the surface of Bi2S3 branches through nitrogen and oxygen atoms in the pyrrolidone groups and prompt formation of dandelion-like structures from individual Bi2S3 wires due to its cross-linking ability.190 Production of Sb2Se3 nanowires from [Sb{Se2P(OiPr)2}3] under solvothermal conditions has been reported.191 In a later study the authors investigated the electrical and optical properties of a single Sb2Se3 nanorod with an average size of 70 nm in diameter and a length of  1–2 mm.192 Schulz et al. have reported the size-selective synthesis of Sb2Te3 nanoplates by decomposition of bis(diethylstibino) telluride (Et2Sb)2Te in freshly prepared solution of 2,6diisopropylbenzene and poly(1-vinylpyrolidone)-graft-(1triacontene) at 180  C.193 The liquid compound is prepared by insertion reaction of elemental tellurium into the Sb–Sb bond of tetraethyldistibine at ambient temperature. By increasing the

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Figure 15 SEM photographs of Sb2Te3 nanoplates obtained with polymer concentrations of (a) 3, (b) 10, (c) 20, and (d) 50 wt% at 170  C.193 Reproduced with permission from Schulz, S.; Heimann, S.; Friedrich, J.; Engenhorst, M.; Schiernng, G.; Assenmacher, W. Chem. Mater. 2012, 24, 2228. American Chemical Society.

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polymer concentration (3, 10, 20, and 50 wt%), the average diameter of the hexagons decreases from about 500 nm to roughly 200 nm (Figure 15). The growth of resulting Sb2Te3 nanoplates seems highly influenced by its inherent rhombohedral crystal structure with infinite –Te1–Sb–Te2–Sb–Te1 chains along the c axis and van der Waals bonds between adjacent Te1 layers.194 The formation of uniform nanoplates with narrow-size distribution is a result of fast nucleation followed by slow crystal growth.195

10. 11. 12. 13. 14.

15.

1.32.10 Conclusion This chapter summarizes the use of different families of SSPs based on main group metal complexes that have been used for the preparation of functional materials as thin films or nanoparticles. Several research groups have been actively designing and developing new compounds to overcome some of the problems associated with dual-source routes. Most of the molecular precursors have low volatilities but improvements in CVD techniques mean this is no longer an issue. Furthermore, preparation and thin-film studies of mixed chalcogenide compounds means that it is now possible to deposit heterogeneous thin films by altering the growth parameters. Detailed investigations on the decomposition of the precursors have resulted in marked improvements in the properties of nanocrystals. It is possible to generate and stabilize various types of anisotropic structures, without the need of any growth modifier. Recent work has also indicated that compounds can be used for in situ growth of nanoparticles in polymer blends and have the potential to overcome some of the issues (solubility, aggregation, and conductivity) often associated with organic-capped nanocrystals. It is expected that chemists will continue with their efforts to offer lowtemperature processing, higher growth rates, and more reproducible optical/electronic properties. As the demand for sustainable energy and cost-effective process technologies continue to grow, this area still requires further development and growth. For related chapters in this Comprehensive, we refer to Chapters 1.06, 1.07, and 4.09.

16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.

35. 36. 37.

Acknowledgments 38.

MA and POB would to thank CORERE, KFUPM, and EPSRC, respectively for the support. 39.

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