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.

Excitons versus free charges in organo-lead tri-halide perovskites

“…We can exclude any confinement phenomena as in this case one would expect an opposite trend, that is, appearance of the exciton optical transition as the crystal size is reduced. On the other hand, the reduction in size, the enhancement of the surface area and the presence of a scaffold during the crystallization process is likely to increase the relative contribution of surface effects, which may result in structural strains and/or doping mechanisms, which notably reduce the exciton binding energy 21 .…”

Electronic Properties of Q-CdS Clusters Stabilized by Adenine

“…ernet.in. (17)(18)(19)(20)(21)(22)(23)(24). A variety of thio compounds (25)(26)(27)(28) and biological molecules (19,20,(29)(30)(31) have been tried as capping agents to produce nanoparticles of CdS with narrow size distribution and to obtain a bottlable solid material.…”

Optical and Dynamic Properties of Undoped and Doped Semiconductor Nanostructures

“…Nanoparticles have interesting nonlinear optical properties at high excitation intensities, including absorption saturation, shift of transient bleach, third and second harmonic generation, and up-conversion luminescence. The most commonly observed nonlinear effect in semiconductor nanoparticles is absorption saturation and transient bleach shift at high intensities [82,86,[99][100][101][102][103][104]. Similar nonlinear absorption have been observed for quantum wires of GaAs [105,106] and porous Si [107,108].…”