Ro

Figure 26. Single-source approach for preparing semiconductor nanocrystallites in TOPO. Reprinted with permission from [22], T. Trindade et al., Chem. Mater. 13, 3843 (2001). © 2001, American Chemical Society.

The importance of using small molecular clusters as syn-thons to extended solid-state structures is especially evident in those examples where a structurally characterized molecular species reveals the metal-ligand interactions and coordination present in solid-state material. Several examples of well-defined polynuclear molecular clusters, possessing the elemental ratio suitable for the formation of a solidstate phase, are known; these represent the small (molecular) size regime where the properties are more like those of molecules than like those of bulk [461]. Farneth et al. have investigated the mechanism of the solid-state conversion of a series of II-VI precursors of general formula (R4N+)4[S4M10(SPh)16]4- (R = Me, Et; M = Cd, Zn) to the bulk metal sulphide structure [17]. The transformation as followed by combined TGA and mass spectroscopy proceeds in two discrete reaction steps: the first reaction, occuring around 200 °C, is characterized as a nucleophilic substitution or elimination reaction of a fragment (NMe4SPh) of the anion cluster with the tetraalkylammonium counterion. The loss of the cluster countercations produces a new molecular solid of stoichiometry Cd10S16Ph12, which has been isolated and characterized. This intermediate composition gave a broad XRD pattern that indicated very small (<25 A) sphalerite-phase (cubic) crystals of CdS. It undergoes a second reaction around 350 °C, whereupon S6Ph12 is eliminated, leaving a solid residue that was shown to be phase-pure CdS in the Wurtzite structure (Eq. (17)). The compound Cd10S16Ph12 is highly soluble in pyridine and was recrystallized by the addition of dimethylformamide (DMF). The solid-state structure determined by X-ray crystallography confirmed the formation

Pyridine/DMF

Cd32S14(SPh)36-(DMF)4 ^ CdS

of a cluster with Cd32S14(SPh)36-(DMF)4 (Fig. 27). This molecule is much larger than the original Cd10S16Ph12 precursor-cluster, suggesting that the Cd32 cluster results from the assembly of CdS and SC6H5 species of various nuclearities, which were shown by 113 Cd NMR spectroscopy

Figure 27. Molecular structure of Cd32S14(SPh)36-(DMF)4 cluster (drawn after [461]).

to be in rapid exchange [17]. The structural details reveal a chunk of sphalerite form of bulk CdS whose dangling surface bonds have been terminated by wurtzite-like (hexagonal) CdS units at four tetrahedral corners (Fig. 27). This corresponds to an overall cluster size of ~15 A.

The lowest absorption band of Cd32S14(SPh)36-(DMF)4 located at 358 nm shows a significant "blue-shift" compared with the bulk CdS (Fig. 28).

The two main implications of this work are (i) the formation of molecular cluster solids as tractable intermediates, which display the features of nanocrystalline solids, and (ii) the possibility of exploiting the ligand chemistry to control the microstructure of the final nanocrystalline solids. Similarly, CdSe (2-9 nm) and ZnSe (2-5 nm) nanoparticles have been prepared from the corresponding selenides. Although the molecular precursors (R4N+)4-[S4M10(SPh)16]4- and the resulting nanocrystals have been the subject of intensive investigations, the chemical reaction associated with this transformation, which could uncover the nucleation mechanism, has been not well studied. Strouse et al. [110] have investigated the decomposition of (R4N+)4- [Se4M10(SPh)16]4- to form MSe (M = Zn, Cd). The Cd:Se ratio in [Se4Cd10(SPh)16]4- is significantly higher

Figure 28. Absorption spectrum of Cd32S14(SPh)36-(DMF)4 in THF at room temperature compared with the bulk CdS absorption spectrum. Reprinted with permission from [461], N. Herron et al., Science 259, 1426 (1993). © 1993, American Association for the Advancement of Science.

(2.5:1) than required for the final product. Cd(SPh)- has been suggested as a labile fragment that would yield a Cd:Se stoichiometry of 1.5:1 based on the Cd6Se4 core of the cluster. Since the CdSe nanomaterial grown in hex-adecylamine is strictly stoichiometric, unidentified Cd byproducts are required to balance the reaction stoichiometry. Several mechanisms for nucleus formation have been put forward, for instance, the fragmentation of the M10 cluster into M2+ and Se2- or (CdSe)„ species that can reassemble to form the nucleus or the alternative possibility that the cluster remains intact, acting as partial nuclei, and that the nanomaterial growth proceeds through scavenging of free M and Se atoms. Moreover, a combination of fragmentation via ring opening and subsequent ligand exchange, proposed for other metal chalcogenide clusters [462, 463], seems to be probable as well (Fig. 29). Apparently the Cd10 cluster grows (to Cd32) by ring opening followed by attack of the exposed Cd atom at the trigonally passivated chalco-genide ions and subsequent loss of the apical Cd(SPh)-caps (Fig. 29). As a consequence, nucleus formation occurs through ligand exchange, and in effect the cluster acts as a template for the formation of larger-nuclearity clusters. The transformation of molecular metal-chalcogenide systems into semiconductor nanocrystals with narrow size and shape dispersions demonstrates the versatility of templat-ing nanomaterial growth by providing molecular inorganic clusters as intact nuclei. In view of the above, the use of SSPs in the deposition of thin film semiconductors by CVD techniques has been extensively studied [106, 464, 465]. Cadmium bis(diethylmonothiocarbamate), Cd(Et2mtc)2, was shown to be a suitable source for the deposition of nano-crystalline and transparent CdS films at low temperatures (300-450 °C) (Fig. 30) [104].

Gallium nitride (GaN) has promising applications for blue and ultraviolet optoelectronic devices and has attracted much attention recently after the successful fabrication of high-efficiency blue light-emitting diodes [466]. GaN possesses a range of interesting properties, such as a wide bandgap (3.45 eV), high chemical inertness, radiation resistance, and capability of working at high temperatures, which make it an interesting candidate for optoelectronic devices.

Figure 29. Proposed reaction mechanism for the formation of CdSe nanocrystals from cluster precursors. The thiol and amine ligands for the clusters and nanomaterials are not shown. Reprinted with permission from [110], S. L. Cumberland et al., Chem. Mater. 14, 1576 (2002). © 2002, American Chemical Society.

Figure 30. SEM image of CdS thin film deposited on the GaAs(100) substrate. Reprinted with permission from [104], M. Chunggaze et al., Adv. Mater. Opt. Electron. 7, 311 (1998). © 1998, John Wiley & Sons.

Figure 29. Proposed reaction mechanism for the formation of CdSe nanocrystals from cluster precursors. The thiol and amine ligands for the clusters and nanomaterials are not shown. Reprinted with permission from [110], S. L. Cumberland et al., Chem. Mater. 14, 1576 (2002). © 2002, American Chemical Society.

Figure 30. SEM image of CdS thin film deposited on the GaAs(100) substrate. Reprinted with permission from [104], M. Chunggaze et al., Adv. Mater. Opt. Electron. 7, 311 (1998). © 1998, John Wiley & Sons.

For the above reasons, there is growing interest in the synthesis of GaN nanocrystals and nanostructured films. The established method of depositing device-quality GaN by MOCVD utilizes high-temperature interaction of (CH3)3Ga, with a large excess of NH3. However, the high stability of the N-H bond in NH3 requires reaction temperatures in excess of 1000 °C and the inefficient use of toxic NH3. Therefore, the choice of substrate material is limited by a high substrate temperature, which may result in residual strains in the deposited layers. To overcome the limitations of this process, alternative methods based on SSPs that incorporate direct Ga-N bonds and do not contain any strong N-H bonds, or even organic groups, offer the potential for significant improvements in the growth process and film quality [137, 467-469]. The major advantages of using molecular precursors with pre-formed Ga-N bonds include lower deposition temperatures, exclusion of NH3 from the process, reduction in N vacancies, and elimination or drastic reduction of carbon impurities [470, 471].

Recently, a large number of routes for synthesizing freestanding nanocrystals and films (nanostructured and epitaxial) of GaN have been reported. For example, the pyrolyses of [Ga(NH2)3/2]„ (450 °C) [472], [H2GaNH2L (600 0C) [473], [Ga{N(CH3)2>3]2 (600 0C) [474], [H2Ga^] (350 °C) [467], and [Ga(N3)3]„ (280 0C) [475] gave nano-crystalline GaN powders. Although all of the precursors produced chemically homogeneous GaN, the temperature of crystallization and the amount of organic contamination depend on the organic content in the precursor molecule. In the case of large organic groups, high temperatures are required for their dissociation and complete decomposition. Despite several promises, the single-source route implies problems, such as low volatility of precursors and carbon contamination of the nitride films from undecomposed organic fragments. Therefore, a single precursor should be carefully designed so that it follows a clean decomposition pathway to produce the desired phase with a minimum level of impurities and the lowest possible decomposition temperature.

In a nonconventional approach, Fischer et al. have used detonation of gallium azides, (R3N)-Ga(N3)3, to obtain phase-pure nanocrystals of hexagonal GaN [137]. Kouvetakis et al. have reported several related routes for GaN synthesis utilizing a carbon- and hydrogen-free SSP, Cl2GaN3, to grow thin oriented films of high-quality GaN on sapphire at 700 0C. The main limitation of this approach is the extremely low vapor pressure of Cl2GaN3, which is an involatile polymeric solid at room temperature, and the inherent loss of substantial quantities of GaCl3 (Eq. (18)) at the growth temperature:

0 0

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