Quickest single-step mechanosynthesis of CdS quantum dots and their microstructure characterization.

CdS quantum dots (QDs) with a mixture of both cubic (Zinc-blende) and hexagonal (Wurtzite) phases have been prepared within 50 min by mechanical alloying the stoichiometric mixture of Cd and S powders at room temperature in a planetary ball mill under Ar. The Rietveld analysis of X-ray powder diffraction data collected from ball-milled samples estimates relative phase abundances of cubic and hexagonal phases and several microstructure parameters like lattice parameters, particle size, lattice strain, concentrations of different kinds of stacking faults etc. in both kinds of QDs. Initially, the hexagonal phase is formed and then simultaneously, the most dense plane (111) of minor cubic phase is formed coherently on the most dense (002) plane of hexagonal phase. In the course of milling up to 30 h, the hexagonal phase partially transforms slowly to cubic phase and the molar ratio of cubic and hexagonal phase becomes approximately 0.7:0.3 and particles sizes of both phases reduce to approximately 6 nm. The cubic phase contains a significant amount of lattice strain but the hexagonal phase is almost strain-free. The presence of different kinds of stacking faults is revealed clearly from the high resolution transmission electron microscope (HRTEM) images. The particle sizes of QDs and their distributions obtained from the Rietveld analysis agree well with the analysis of HRTEM images.