Linear- and nonlinear-optical properties of semiconductor clusters

Abstract

CdS and PbS clusters with sizes ranging from a few angstroms to 150 A can be synthesized in polymers. The dependence of the band gap on the cluster size deviates from the prediction of a simple particle-in-a-box model owing to the breakdown of the effective-mass approximation. Instead, the dependence can be described by a simple tight-binding cluster model. The optical nonlinearity, expressed as α2/α0, of 50-A CdS clusters in Nafion film is determined to be −6.1 × 10−7 cm2 W−1 at 480 nm. The nonlinearity originates from the bleaching of the excitonic absorption owing to the presence of trapped carriers on the cluster surfaces. By passivating the CdS surfaces with ammonia, we have shown that the nonlinearity can be controlled by surface chemistry. We have determined that the presence of one trapped electron–hole pair can bleach the excitonic absorption of the whole CdS cluster. This efficient bleaching can be understood by using a model that considers the shifting of the exciton resonance and weakening of its oscillator strength in the presence of a trapped electron or hole. We also discuss two new classes of material: superclusters in zeolites and surface-capped clusters. Both represent our first steps toward the systematic synthesis of clusters of controlled surfaces and sizes.