Systematic investigation of the structure and photophysical properties of CdSe, CdSe/ZnS QDs and their hybrid with beta-carotene

Abstract

A systematic investigation was conducted to determine the excitation energy dependent emission characteristics of CdSe quantum dot-beta-carotene and CdSe/ZnS core-shell quantum dot-beta-carotene hybrid samples. The emission intensity was recorded at different excitation energies by continuously varying the excitation energy from the band edge value of the QDs to a maximum value. In both the hybrids, the emission intensity increased with an increase of the excitation energy and then a sudden quenching occurred, which is in contrast with the results from bare samples, where the emission intensity showed a decreasing trend at all excitation energies. The quantum yields corresponding to the maximum emission are 47.12% and 69%, for CdSe QD-beta C and CdSe/ZnS core-shell QD-beta C hybrids, respectively. At higher excitation energy, electrons were transferred to the molecule's LUMO level, leaving behind holes in the valance band of the QD, and thus the produced charge separated state became responsible for the PL quenching in the hybrid sample. We have confirmed this charge separated state by lifetime measurements. It is because of this PL quenching behaviour of the hybrids that the nature of the interface band structure was deduced as type I. Furthermore, because of the involvement of higher energy photons in the quenching process, the transferred electrons in the LUMO level of the molecule were known as hot electrons. The present paper discusses the excited state electron dynamics across the interface between the two hybrid materials in detail.