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

Bulk GaP is an indirect semiconductor with an indirect bandgap of 2.22eV (559 nm) and a direct bandgap of 2.78 eV (446 nm). Theoreticalcalculations [15] on GaP QDs show that the increase of the indirect bandgapwith decreasing QD size is much less pronounced than that for the direct gap;for 30-A˚ -diameter GaP QDs, the direct and indirect bandgaps are predictedto be 3.35 eV and 2.4 eV, respectively. Below 30 A˚ , the direct bandgap ispredicted to decrease with decreasing size while the indirect bandgap continuesto increase. As a result, GaP is expected to undergo a transition from anindirect semiconductor to a direct semiconductor below about 20 A˚ .The steep rise in absorption and the shoulder at 420 nm in theabsorption spectrum of 30-A˚ GaP QDs [5] is attributed to a direct transitionin the GaP QDs; the shallow tail region above 500 nm is attributed to theindirect transition. Also, the absorption tail extends below the indirectbandgap of bulk GaP [5]. The origin of this subgap absorption could becaused either by a high density of subgap states in the GaP QDs, by impuritiescreated by the high decomposition temperature, or by Urbach-type bandtailing produced by unintentional doping in the QDs [16]. We note that suchsubgap absorption below the bandgap was also observed in GaP nanocrystalsthat were prepared in zeolite cavities by the gas-phase reaction of trimethylgalliumand phosphine at temperatures above 225jC [17]. This latter resultimplies that the subgap absorption in GaP QDs is intrinsic and is not causedby synthetic by-products or impurities.3. Colloidal GaInP 2 Quantum DotsQuantum dots of GaInP 2 can be synthesized by mixing chlorogallium oxalateand chloroindium oxalate complexes and P(Si(CH 3 ) 3 ) 3 in the molar ratio ofGa:In:P of 1:1:2.6 in toluene at room temperature, followed by heating inTOPO [5]. Heating at 400jC for 3 days is required to form 25-A˚ QDs. X-raydiffraction for a 65 A˚ sample shows that the lattice spacings of GaInP 2 QDs isapproximately the average of that for GaP and InP [5].The ternary Ga–In–P system forms solid solutions which can exhibitdirect bandgaps ranging from 1.7 to 2.2 eV, depending on composition andgrowth temperature [18–24]. At the composition Ga 0.5 In 0.5 P, the structurecan be either atomically ordered or disordered (random alloy) [18–24]; thebandgap is direct, but it can range from about 1.8 to 2.0 eV, depending on thedegree of atomic ordering. An open issue of interest is how size quantizationwill affect the dependence of bandgap on atomic ordering.The absorption spectrum of 25-A˚ GaInP 2 QDs does not show anyexcitonic structure [5]; this is caused by an exceptionally wide size distributionwhich masks the excitonic peaks. An estimate of the direct bandgap of theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.