Effect of Capping Agent and Medium on Light-Induced Variation of the Luminescence Properties of CdTe Quantum Dots: A Study Based on Fluorescence Correlation Spectroscopy, Steady State and Time-Resolved Fluorescence Techniques

Abstract

The influence of ligand and solvent on light-induced modulation of the emission behavior of the quantum dots (QDs) has been studied for CdTe QDs capped with hexadecylamine (HDA), mercaptopropionic acid (MPA), and 1-(1-undecanethiol)-3-methyl imidazolium bromide (SMIM) in CHCl3, H2O, and [bmim][PF6] ionic liquid, respectively, using steady state and time-resolved fluorescence and fluorescence correlation spectroscopy techniques. While an aqueous solution of CdTe/MPA QDs exhibits fluorescence enhancement and a small blue shift of the emission peak (λmaxem) in the early stages of the light irradiation, such enhancement could not be observed in the case of CHCl3 solution of CdTe/HDA and [bmim][PF6] solution of CdTe/SMIM. Instead, exposure to light leads to a rapid reduction in luminescence intensity and large blue shift of λmaxem in the case of CHCl3 solution of CdTe/HDA and a very slow decrease of luminescence intensity with negligible shift of λmaxem in the case of an ionic liquid solution of CdTe/SMIM. The time-resolved fluorescence behavior of the QDs is found to be consistent with the steady state results. Fluorescence correlation spectroscopy measurements on the other hand reveal a large decrease of the amplitude of correlation at time zero [G(0)] for the aqueous solution of CdTe/MPA and negligible change in the G(0) value for the ionic liquid solution of CdTe/SMIM with increasing excitation power. The mechanism of these light-induced changes of the luminescence behavior of the QDs is investigated.