Optical transient bleaching of quantum-confined CdS clusters: The effects of surface-trapped electron–hole pairs

Abstract

We studied the optical transient bleaching of ∼40 Å, ammonia‐passivated CdS clusters in a polymer with nanosecond and picosecond pump‐probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30‐ps pump laser pulse width, we tentatively attribute the bleaching to the exciton‐exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron‐hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron‐hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron‐hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron‐hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red‐shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.