HIGH PRESSURE STUDIES ON SEMICONDUCTOR NANOCRYSTALS: SIZE EFFECTS IN THE STRUCTURAL AND ELECTRONIC PROPERTIES OF SOLIDS. Sarah H. Tolbert(a) and A. P. Alivisatos(b) (a) Department of Chemistry, University of California, Santa Barbara, CA 93106-9510 (b) Department of Chemistry, University of California, Berkeley, CA 94720

In this talk we will explore the effects of finite size on the structural and electronic properties of solid systems. In particular, pressure induced structural transformations in CdSe, InP, and Si nanocrystals will be examined. These crystallites, ranging in size from 2 to 50 nm in diameter, exhibit phase behavior that is significantly different from the analogous bulk systems. Structural transformations are observed to be uniformly elevated in nanocrystals in comparison to bulk materials. High pressure X-ray diffraction and optical absorption are combined to understand this elevation in terms of the surface thermodynamics and kinetics of solid-solid structural transformations in nanocrystals. The results have implications for phase transition in both bulk and finite size systems. Optical properties of nanocrystals at high pressures are also examined The results are relevant for understanding the nature of quantum confinement in very small indirect gap solids.