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 lightinduced 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 CHCl 3 , H 2 O, and [bmim][PF 6 ] 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 (λ max em ) in the early stages of the light irradiation, such enhancement could not be observed in the case of CHCl 3 solution of CdTe/HDA and [bmim][PF 6 ] solution of CdTe/SMIM. Instead, exposure to light leads to a rapid reduction in luminescence intensity and large blue shift of λ max em in the case of CHCl 3 solution of CdTe/HDA and a very slow decrease of luminescence intensity with negligible shift of λ max em 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.

Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

“…For a dot with R = 2.3 nm, the formation of CdO on its surface would bring the slow component of the bi-exponential fit into the ns range and increase τ f by about two orders of magnitude (see Figure 3), in agreement with experiment. 18 Interestingly we find that it is the lifetime relative to the efficient traps, Lifetime (ps)…”

“…Based on Figure 7 (the values at zero irradiation time are chosen here to remove all light-induced effects discussed in ref. 18 ).…”

“…This is the case of CdTe CQDs, where recent experimental studies have evidenced fluorescence decay curves that required at least a tri-exponential function to yield good agreement with the observed kinetics. [17][18][19] In addition to radiative decay, two nonradiative components were therefore extracted from the experimental data, suggesting the presence of (at least) two types of traps. However their nature (i.e., whether hole or electron traps) was not clear 18 and their location on the surface was not determined.…”

Impact of capping agent on the electron transfer dynamics of CdTe QDs with methyl viologen

“…There are reports found for the binding of thiol and carboxyl groups on the surface of QDs [16,17,27,28]. So in order to check the presence of functional groups and binding nature of QDs with capping agents, we have done the FT-IR analysis (Fig.…”

“…However, surface reagents such as organic molecules with long alkyl chains, polymers, or inorganic shells decelerate the rate of electron transfer (ET) between the excited QDs and electron acceptors because of the increased distance from the QD core to the adsorbates [13,14]. There are reports available for the effect of capping agent and size of QDs on the excited state electron transfer studies [15][16][17] which clearly indicates that the properties of molecular linkers can significantly influence the deactivation pathways of QDs and the efficiency of interfacial electron-transfer reactions between QDs and other molecules. For instance, Weiss reported the photoinduced electron transfer (PET) rate for a Cds absorption to simultaneously investigate the PET process of a single QD-ligand couple and quantify the affinity of the ligand for the QD surface under the conditions of the PET experiment [18].…”