Observation of biexcitons in nanocrystal solids in the presence of photocharging
Date
2013Source Title
ACS Nano
Print ISSN
1936-0851
Publisher
American Chemical Society
Volume
7
Issue
6
Pages
4799 - 4809
Language
English
Type
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Abstract
T In nanocrystal quantum dots (NQDs), generating multiexcitons
offers an enabling tool for enhancing NQD-based devices. However, the photocharging
effect makes understanding multiexciton kinetics in NQD solids fundamentally
challenging, which is critically important for solid-state devices. To date,
this lack of understanding and the spectral temporal aspects of the multiexciton
recombination still remain unresolved in solid NQD ensembles, which is mainly due
to the confusion with recombination of carriers in charged NQDs. In this work, we
reveal the spectral temporal behavior of biexcitons (BXs) in the presence of
photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of
biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon
counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with
different core sizes, revealing the strength tunability of the X X interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion
kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further
experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the
photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings
fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the
way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.
Keywords
Semiconductor Quantum DotsMultiexciton Generation
Multiexciton Recombination
Biexciton
Trion
Time-resolved Fluorescence