HIGH-PRESSURE STRUCTURAL TRANSFORMATIONS IN SEMICONDUCTOR NANOCRYSTALS

Pressure-induced structural transformations in semiconductor nanocryst als are examined. High-pressure Raman spectroscopy, EXAFS, X-ray diffr action, and optical absorption are discussed as methods for studying t hese transformations in CdSe, CdS, and Si nanocrystals. In these nanoc rystal systems, each technique shows an elevation in solid-solid struc tural transformation pressure as crystallite size decreases. By analog y with melting in nanocrystals, this elevation in transformation press ure is explained in terms of an increase in surface energy in the newl y formed high-pressure phase crystallites. The increase in surface ene rgy is in turn the result of transition path-induced changes in the sh ape of the nanocrystals. These changes convert spherical nanocrystals with low-index, low-energy surfaces into oblate or prolate crystallite s with higher-index, higher-energy surfaces. The elevation in structur al transformation pressure in nanocrystals is thus a kinetic rather th an a thermodynamic phenomenon.