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Femtosecond transient absorption spectroscopy has been used to characterize charge carrier relaxation from the second excited state (1P) to the first excited state (1S) in colloidal indium phosphide (InP) quantum dots (QDs). A three pulse experiment consisting of a visible pump, infrared pump, and white light probe was used to characterize the relaxation of photogenerated excitons, and the roles of surface chemistry and size were investigated. A two pulse experiment consisting of only the infrared pump and white light probe was used to characterize the relaxation of chemically injected electrons in the absence of holes. In the case of photogenerated excitons, two subsets of QDs were probed in the experiment, corresponding to exciton-confined and charge-separated QDs. The relaxation rates for exciton-confined and charge-separated QDs increase with decreasing QD diameter. The relaxation rates obtained for both photogenerated excitons that become charge-separated and chemically injected electrons were slowed by approximately 1 order of magnitude as compared to excitons confined to the QD core.
Blackburn et al. (2002) studied this question.