Abstract

Semiconductor crystallites which are 10's of Angstroms show a striking evolution of electronic properties with size.¹ These particles (quantum dots) are large enough to exhibit a crystalline core, but small enough that solid state electronic and vibrational band structure is not yet developed. We use a recently developed synthetic method for the synthesis of high quality nanometer size (1-10 nm) II-VI semiconductor crystallites with narrow size distributions (σ <5%), emphasizing CdSe.² Optical characterization of their electronic structure using pump-probe techniques, luminescence, and DC Stark techniques reveals both molecular and bulk-like characteristics as well as properties which are unique to nanometer size crystallites. We observe a number of discrete electronic transitions, assign them as coming from the creation of delocalized "particle-in-a-sphere" states using the theory of Ref. ³, and study their dependence on crystallite diameter.⁴ The Stark experiments are also compatible with the absorbing states as delocalized symmetric states. We use time resolved fluoresence line narrowing spectroscopy to study the dynamics of electron-hole recombination. We observe significant changes in electron-LO phonon coupling with time, temperature, and crystallite size and suggest that the electron-hole pair dynamics following photoexcitation are dominated by surface effects which are especially important in the smaller crystallites where a large fraction of the atoms are "surface" atoms.⁵

© 1994 Optical Society of America