High pressure studies on nanometer sized clusters: Structural, optical, and cooperative properties

摘要

High-pressure Se EXAFS is used to study pressure-induced structural transformations in CdSe nanocrystals. The transformation is wurtzite to rock salt, at a pressure much higher than in bulk. High-pressure XRD is used to confirm the EXAFS results. Diffraction peak widths indicate that nanocrystals do not fragment upon transformation. Optical absorption correlates with structural transformations and is used to measure transition pressures; transformation pressure increases smoothly as nanocrystal size decreases. Thermodynamics of transformation is modeled using an elevated surface energy in the high-pressure phase. High-pressure study of Si nanocrystals show large increases in transformation pressure in crystallites to 500(angstrom) diameter, and an overall change in crystallite shape upon transformation is seen from XRD line widths. C(sub 60) single crystals were studied using Raman scattering; results provide information about the clusters' rotational state. Optical properties of high-pressure phase CdSe clusters were studied.