Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Abstract

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots (QDs) were investigated by two-colour femtosecond (fs) pump-probe (400/800 nm) and picosecond time-resolved photoluminescence (ps TRPL) experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.

Signatures of Exciton Dynamics and Carrier Trapping in the Time-Resolved Photoluminescence of Colloidal CdSe Nanocrystals

“…The influence of trap states on CdSe NC PL dynamics has recently been the subject of several studies; [38][39][40][41][42][43][44][45] however, to date, little consensus has emerged on the nature of exciton-trap interactions or on how the trap sites might be distributed either spatially about the quantum dots or energetically relative to exciton states. This is largely because of the inherent inhomogeneity of NC size, shape, and composition, which enforces a choice upon those who try to understand their photoexcitation dynamics: we can either use a microscopic approach in which we infer ensemble properties from a representative number of individual NCs; or a macroscopic approach, which requires us to deduce individual NC properties from a knowledge of ensemble photophysics.…”

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

“…Nonetheless, there are 2 major advantages of using PL to examine trap states: (i) the well documented (5-12) sensitivity of NC PL to interface and environmental changes and (ii) the ability of time-resolved PL measurements to resolve processes that occur over a large dynamic range of timescales. Several groups (11,(31)(32)(33)(34)(35)(36)(37) have studied the influence of trap states on PL transients in CdSe NCs, and stochastic models have been used in some of these studies to extract radiative decay rates or trapping rates. Although such models successfully reproduce exciton-only dynamics, they have been unable to capture satisfactorily the influence of interface states.…”

Hybrid Complexes of Photosensitizers with Luminescent Nanoparticles: Design of the Structure

“…Particle size nonuniformity can be an alternative reason for the emergence of several time components in the decay curves of QD luminescence. Increasing the QD size not only leads to a bathochromic shift in the luminescence spectrum, but also causes a corresponding shift of the exciton band in the absorption spectrum [ 48 ], reduces the luminescence lifetime [ 49 ], and causes nonlinear changes in the quantum yield φ [ 50 ]. Consequently, a broadened luminescence spectrum will be observed for a sample containing several fractions of QDs of different sizes.…”