Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

The synthesis was conducted in rigorously air-free and water-free atmospheres.One important aspect in the synthesis of GaN is the purity of thefinal product. Carbon can be left on the QD surfaces after pyrolysis, and it isdifficult to remove. TOA/HDA decreases carbon adsorption and, after purification,yields a white colloidal solution. Repetitive flocculation and redispersionin a solution of hexane containing 1% HDA leads to the isolationof clean, white samples. This powder was redispersed in 2,2,4-trimethypentane,which contained 1% HDA. After that, the solution was sonicated in ahigh-intensity ultrasonic processor and filtered to produce an optically clear,nonscattering organic glass at 10 K. The QD surface is derivatized withTOA/HDA and ensures that the QD particles are isolated from each other insolution.The colloidal GaN solution shows an absorption spectrum with a weakshoulder at 330 nm and a structureless emission spectrum [32]; this againindicates a broad size distribution of particles. The absorption and emissionspectra are shifted to higher energies (3.65 eV) compared with bulk GaN(E g = 3.2–3.3 eV for zinc-blende structure) [33], confirming that the GaNparticles are in the quantum-confinement regime.6. Lattice-Matched Core–Shell InP/ZnCdSe 2 Quantum DotsCore–shell quantum dots with a zinc-blende structure consisting of InP coresand lattice-matched ZnCdSe 2 shells have been successfully prepared bycolloidal chemistry [34]. The core InP QDs, with an average size of 25–45A˚ , were synthesized by colloidal chemistry methods using InCl 3 and tris-(trimethylsilyl)phosphine ([P(SiMe 3 ) 3 ] as described earlier. Fractionation ofthe QD particles into different sizes was obtained by selective precipitation,collected as powders, and then redispersed in pyridine.The CdZnSe 2 precursor was prepared by mixing dimethylzinc (ZnMe 2 ),dimethylcadmium (CdMe 2 ), and tributylphosphineselenium (TBPSe) in tributylphosphine(TBP) solution in a molar ratio 1:1:4, respectively. Freshprecursor solutions were always prepared before use. Excess Se was used toensure complete formation of CdZnSe 2 . TBPSe was prepared by dissolving Se(1 M) in TBP.The InP QDs were dispersed in pyridine and then overcoated withCdZnSe 2 in pyridine by reacting the precursors at 100jC. Successful overcoatingof QDs in pyridine at 100jC had been previously used for (CdSe)CdSQDs [35]. The ratio of the ZnCdSe 2 precursor to InP necessary to form a shellof a desired thickness was based on the ratio of the volume of the shell to thatof the core, assuming that spherical cores and annular shells are formed.High-resolution TEM (HR-TEM) images of the QDs show wellresolvedlattice fringes that extend in a straight line through the whole QDCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.