ABSTRACT
Pressure-induced structural transformations in semiconductor nanocrystals are examined. High-pressure Raman spectroscopy, EXAFS, X-ray diffraction, and optical absorption are discussed as methods for studying these transformations in CdSe, CdS, and Si nanocrystals. In these nanocrystal systems, each technique shows an elevation in solid-solid structural transformation pressure as crystallite size decreases. By analogy with melting in nanocrystals, this elevation in transformation pressure is explained in terms of an increase in surface energy in the newly formed high-pressure phase crystallites. The increase in surface energy is in turn the result of transition path-induced changes in the shape of the nanocrystals. These changes convert spherical nanocrystals with low-index, low-energy surfaces into oblate or prolate crystallites with higher-index, higher-energy surfaces. The elevation in structural transformation pressure in nanocrystals is thus a kinetic rather than a thermodynamic phenomenon.
ABSTRACT
Measurements of the size dependence of a solid-solid phase transition are presented. High-pressure x-ray diffraction and optical absorption are used to study the wurtzite to rock salt structural transformation in CdSe nanocrystals. These experiments show that both the thermodynamics and kinetics of this transformation are altered in finite size, as compared to bulk CdSe. An explanation of these results in the context of transformations in bulk systems is presented. Insight into the kinetics of transformations in both bulk and nanocrystal systems can be gained.
