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

narrower in samples with larger mean diameters; this is why higher-energytransitions can be resolved for the larger-sized QD ensembles. All of theprepared QD nanocrystallites are in the strong confinement regime becausethe Bohr radius of bulk InP is about 100 A˚ .Figure 3 also shows typical room-temperature global emission spectraof the InP colloids as a function of QD diameters. We define global PL as thatobserved when the excitation energy is much higher than the energy of theabsorption threshold exhibited in the absorption spectrum produced by theensemble of QDs in the sample; that is, the excitation wavelength is well tothe blue of the first absorption peak for the QD ensemble and, therefore, alarge fraction of all the QDs in the sample are excited. The particle diametersthat are excited range from the largest in the ensemble to the smallest, whichhas a diameter that produces a blue-shifted bandgap equal to the energy of theexciting photons. In Fig. 3, the excitation energy for all QD sample ensembleswas 2.48 eV, well above their absorption onset in each case. The global PLemission peaks (‘‘nonresonant’’) in Fig. 3 are very broad (linewidth of 175–225 meV) and are red-shifted by 100–300 meV as the QD size decreases from60 to 26 A˚ [10]. The broad PL linewidth is caused by the inhomogeneous linebroadening arising from the f10% size distribution. We attribute the largeglobal red shift and its increase with decreasing QD size to the volumedominance of the larger particles in the size distribution; the larger QDs willabsorb a disproportionally larger fraction of the incident photons relative totheir number fraction and will show large red shifts (because the PL excitationenergy is well above their lowest transition energy) that will magnify theoverall red shift of the QD ensemble.2. Colloidal GaP Quantum DotsQuantum dots of GaP can be synthesized by mixing GaCl 3 (or the chlorogalliumoxalate complex) and P[Si(CH 3 ) 3 ] 3 in a molar ratio of Ga:P of 1:1 intoluene at room temperature to form a GaP precursor species, and thenheating this precursor in TOPO at 400jC for 3 days [5]. Wells et al. [13,14] firstsynthesized and characterized the yellow GaP precursor, [Cl 2 GaP(SiMe 3 ) 2 ] 2 ,formed from GaCl 3 and P(SiMe 3 ) 3 . The mean particle diameters of GaP QDpreparations can be estimated from the line broadening of their x-raydiffraction patterns and from TEM.The absorption spectrum of a 30-A˚ -diameter GaP QD colloid (heated at400jC) exhibits a shoulder at 420 nm (2.95 eV) and a shallow tail that extendsout to about 650 nm (1.91 eV) [5]. For 20-A˚ -diameter GaP QDs (heated at370jC), the shoulder is at 390 nm (3.17 eV) and the tail extends to about 550nm [5].Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.