Electronic States of Semiconductor Clusters: Homogeneous and Inhomogeneous Broadening of the Optical Spectrum.

The homogeneous (single cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35- 50angstroms diameter CdSe clusters are separated using transient photophysical hole-burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum-confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140cm sup (-1) (17meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron-vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940cm sup (-1) , or seven times the homogeneous width, in the most monodisperse sample.