Size Dependent Spectroscopy of CdSe Nanocrystallites

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