期刊
ACS NANO
卷 6, 期 4, 页码 3269-3277出版社
AMER CHEMICAL SOC
DOI: 10.1021/nn300184n
关键词
quantum dots; multiexciton generation; carrier multiplication; ultrafast spectroscopy; exciton cooling
类别
资金
- James Franck program for laser matter interactions
- Israel Science Foundation
- Minerva Gesellschaft, GmbH, Munich, Germany
Hyperspectral femtosecond transient absorption spectroscopy is employed to record exciton relaxation and recombination in colloidal lead selenide (PbSe) nanocrystals in unprecedented detail. Results obtained with different pump wavelengths and fluences are scrutinized with regard to three Issues: (1) early subpicosecond spectral features due to hot excitons are analyzed in terms of suggested underlying mechanisms; (2) global kinetic analysis facilitates separation of the transient difference spectra into single, double, and triple exciton state contributions, from which Individual band assignments can be tested; and (3) the transient spectra are screened for signatures of multiexciton generation (MEG) by comparing experiments with excitation pulses both below and well above the theoretical threshold for multiplication. For the latter, a recently devised ultrafast pump-probe spectroscopic approach is employed. Scaling sample concentrations and pump pulse intensities inversely with the extinction coeffident at each excitation wavelength overcomes ambiguities due to direct multiphoton excitation, uncertainties of absolute absorption cross sections, and low signal levels. As observed in a recent application of this method to InAs core/shell/shell nanodots, no sign of MEG was detected in this sample up to photon energy 3.7 times the band gap. Accordingly, numerous reports of efficient MEG in other samples of PbSe suggest that the efficiency of this process varies from sample to sample and depends on factors yet to be determined.
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