Role of Core–Shell Interfaces on Exciton Recombination in CdSe–Cd_xZn_1–xS Quantum Dots

Abstract

The shell thickness and composition of CdSe–Cd x Zn1–x S core–shell quantum dots (QDs) are defining parameters for the efficiency of such materials as light emitters. In this work we present a detailed study into the optical absorption and fluorescence properties of CdSe–CdS, CdSe–Cd0.5Zn0.5S, and CdSe–ZnS QDs as a function of shell thickness. Moreover, the single-exciton recombination dynamics of these systems are analyzed by means of a time-correlated single-photon counting technique and directly related to the specific core–shell interfaces of the various QDs studied using a phenomenological kinetic model. The findings from this model highlight the strong role of the core–shell interface on both steady state photoluminescence and exciton recombination dynamics in these systems.

Bridging Energetics and Dynamics of Exciton Trapping in Core–Shell Quantum Dots

“…The accumulation of charges at HT levels explains, according to the scheme in Figure 2, the drastic changes in PLQY values that cannot be justified starting from the trapping and de-trapping rates extracted from TRPL measurements. 28 Microscopically, this is consistent with theoretical predictions relating the excitation energy dependence of PLQY to energy dependent carrier trapping rates. [30][31] In addition to this, the dependence of PLQY on excitation energy and shell thickness provides further information on the nature of these traps.…”

“…We explained these findings in terms of charge trapping dynamics. By using the information obtained by applying a kinetic model to the analysis of TRPL data, 28 we identified a correlation between the trapping and de-trapping rates and the PL efficiency.…”

“…28 Notably, we found that the exciton de-trapping plays a pivotal role also in determining the steady-state optical properties. Within this framework, we interpret the excitation wavelength dependence of PLQY and the mismatch between the measured PLQY and the estimated one as a steady-state signature of "hot-carrier" trapping.…”

“…We and others 25,[28][29] The importance of the exciton de-trapping processes in influencing steady-state properties is dependent on the energy distribution of the trap states. Excitations at energies higher than the bandgap generate "hot" excitons with different carrier distribution, that are subject to charge transfer with fast rates to high-lying trap levels (HTs).…”

“…[25][26][27] Recently, our group reported on the effect of trapping on PL dynamics in core/shell CdSe/CdxZn1-xS QDs. 28 We interpreted transient PL (TRPL) data formulating a phenomenological model, comprising the balance between charge trapping and de-trapping processes. The former involves the formation of surface or interface trapped charge states, whereas de-trapping regenerates bound exciton states.…”