Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton phonon coupling between the various excitonic states of the dot and the optical and acoustic phonons. We show that the divergence of prior results can be reconciled by adopting this multichannel picture of hot exciton relaxation dynamics. This picture establishes a framework for designing materials with relaxation properties targeted for specific applications. We conclude with connection to hot exciton surface trapping. The process of surface trapping is the key step in creation of the photoproduct which can obscure measurements of optical gain, multiexciton recombination, multiple exciton generation, and single dot blinking. We show that hot exciton surface trapping can effectively compete with hot exciton relaxation, thereby obfuscating these processes.