A Multi-Timescale Map of Radiative and Nonradiative Decay Pathways for Excitons in CdSe Quantum Dots

Abstract

A combination of transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopies performed on solution-phase samples of colloidal CdSe quantum dots (QDs) allows the construction of a time-resolved, charge carrier-resolved map of decay from the first excitonic state of the QD. Data from TA and TRPL yield the same six exponential components, with time constants ranging from ∼1 ps to 50 ns, for excitonic decay. Comparison of TA signals in the visible and near-infrared (NIR) spectral regions enables determination of the relative contributions of electron and hole dynamics to each decay component, and comparison of TA and TRPL reveals that each component represents a competition between radiative and nonradiative decay pathways. In total, these data suggest that the QD sample comprises at least three distinct populations that differ in both the radiative and nonradiative decay pathways available to the excitonic charge carriers, and provide evidence for multiple emissive excitonic states in which the hole is not in the valence band, but rather a relaxed or trapped state.

Stacking in Colloidal Nanoplatelets: Tuning Excitonic Properties

“…45À48 Hole trapping in CdSe nanocrystals was found to be related to poor surface passivation and Cd vacancies 36 and was shown to cause fast nonradiative recombination of the excitons with time constants on the order of 10 ps. 49,50 In the CdSe NPLs, it is expected that there might arise both surface traps and Cd vacancies, leading to interband hole trap states, which may rapidly quench the excitons nonradiatively. 15 Previously, blinking was also reported in the NPLs, suggesting that nonemissive or dark states arise in the NPL populations.…”

Platelet‐in‐Box Colloidal Quantum Wells: CdSe/CdS@CdS Core/Crown@Shell Heteronanoplatelets

“…[ 7,27 ] Although it is hard to interpret these distinct lifetime components one by one, the fastest lifetime component (≈0.15 ns) is attributed to the nonradiative decay pathway originating from the fast hole-trapping due to incomplete passivation of Cd atoms on the surface. [ 28,29 ] Also, with increasing the lateral size of CdSe core-only NPLs, the contribution of the fastest lifetime component has been shown to become more pronounced accompanied with decreased PL-QY. [ 7 ] This fi nding strongly supports the discussions above and shows the importance of peripheral passivation of sidewalls of the core-only NPLs.…”

Origins of Photoluminescence Decay Kinetics in CdTe Colloidal Quantum Dots

“…However the details of the trapping dynamics in many materials are still subject of debate and intense research. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, in some cases it is even unclear whether it is the trapping of the electron and/or that of the hole that affects the fluorescence efficiency, as several non-radiative decay components have been observed with different magnitudes (sometimes differing by several orders of magnitude for the same material), prompting the suggestion that different types of traps must be present. 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.…”