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Colloidal semiconductor nanocrystals (NCs) or quantum dots (QDs) based on semiconductors open possibilities for applications in new areas (Bawendi et al., 1990; Alivisatos, 1996; Markovich et al., 1999). Since hot injection synthesis of mono-dispersed spherical nanocrystals using organometallic precursors was introduced by an MIT group (Murray et al., 1993), extensive works have been devoted to make variation of their size and shape, compositions. In this chapter, we will focus on the effect of the alloying or impurity doping on the shape and the optical properties of the NCs. In the first section, we will introduce and discuss the effect of alloying on the morphology of semiconductor NCs. The effects of Pb2+ addition on the morphological development of CdSe nanocrystals was discussed. The addition of Pb ions in the initial precursor solution for the synthesis of alloyed CdSe NCs changed the morphology of CdSe nanocrystals to branched rod with high aspect ratio. The branched nanocrystals are mainly composed of wurzite phase grown along [001] direction and the length of rods in each branched nanocrystal can be increased by increasing amount of Pb2+ addition to accelerate anisotropic growth of nanocrystals. The luminescence, however, mostly arises from trap-related recombination and significantly red-shifted by Pb2+ addition. Moreover, it will be shown that the zinc blend-wurzite polymorphism was popularly observed in these branched CdSe NCs and the twinned structure gave rise to highly asymmetric line shape of photoluminescence spectra. Based on the known optical properties, we will further discuss the application of the branched CdSe NCs to the photovoltaic devices. In the second section, we discussed the effect of the alloying on the optical properties of Cdfree I-III-VI2 group semiconductor nanocrystals. Although, many works have been focused on II-VI semiconductors such as CdSe, CdS, CdTe, etc. due to their high quantum yield of fluorescence with tunable emission wavelength, incorporation of toxic heavy metals such as Cd, Pb, Hg in the II-VI semiconductors have limited their public application. As alternatives to II-VI semiconductors, several materials system including III-V semiconductor NCs (e.g. InP) and impurity-doped ZnS NCs have been developed. However, each material system have some drawbacks such as requirement of expensive pyrophoric compounds as precursors, chemical stability, tunability of emission wavelength, etc. Ternary chalcopyritetype I-III-VI2 group semiconductor NCs were also synthesized as less-toxic new candidates

Tailoring the Morphology and the Optical Properties of Semiconductor Nanocrystals by Alloying