CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals: synthesis and characterization
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Semiconductor nanocrystals are interesting candidates as new light-absorbing materials for photovoltaic (PV) devices. They can be dispersed in solvents and cheaply deposited at low-temperature on various substrates. Also, the nanocrystals have unique optical properties depending on their size due to the quantum size effect and moreover it is easy to uniformly control their stoichiometry. CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals were selected to synthesize and investigate in order to utilize the benefits of colloidal nanocrystals described above. Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS). The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model. The synthetic method of monodisperse chalcopyrite (tetragonal) CuInSe₂ nanocrystals was also developed. The nanocrystals have trigonal pyramidal shape with one polar and three non-polar surface facets. When drop-cast onto carbon substrates, the nanocrystals self-assemble into close-packed monolayers with triangular (honeycomb) lattice structure. Moreover, the effect of excess Cu precursor (CuCl) was studied for the formation of monodisperse trigonal pyramidal CuInSe₂ nanocrystals. The formation mechanism of monodisperse trigonal pyramidal CuInSe₂ nanocrystals was suggested with regard to excess amount of CuCl precursor, based on the nucleationgrowth model of colloidal nanocrystal formation. A new wurtzite phase of CuInS₂, CuInSe₂, and Cu(InxGa1-x)Se₂ (CIGS) was observed in nanocrystals synthesized by heating metal precursors and Se-(or S-)urea in alkylamine. X-ray diffraction (XRD) showed the predominant phase to be wurtzite (hexagonal) instead of chalcopyrite (tetragonal). High resolution transmission electron microscopy (TEM), however, revealed polytypism in the nanocrystals, with the wurtzite phase interfaced with significant chalcopyrite domains.