Electron relaxation and electron transfer dynamics in semiconductor quantum dots studied by femtosecond time-resolved spectroscopy.

摘要

In semiconductor quantum dots (QDs), three-dimensional confinement of electronic excitations transforms the broad density of states characteristic of the bulk into discrete atom-like transitions, and leads to a size-dependent blue shift of the semiconductor band gap. Colloidal QDs are ideal candidates for electro-optic applications which can utilize their size-tunable absorption and emission properties, such as photovoltaic cells, light-emitting diodes, and QD lasers. The successful incorporation of QDs into devices requires a detailed understanding of the ultra-fast electronic processes that will determine the efficiency of each particular device. For example, some key processes for QD photovoltaic cells include intraband and interband electronic relaxation, as well as homogeneous (QD-QD) and heterogeneous (QD-acceptor) electron and hole transfer.; The detailed relaxation pathways in Inp QDs have been studied with time-resolved TA spectroscopy, with a focus on the relaxation of "hot" electrons within the QD conduction band (intraband relaxation). The possibility of inhibited hot electron relaxation in QDs due to the discrete nature of the excitonic states has been termed the "phonon bottleneck" effect. Our results suggest that alternate relaxation pathways provide efficient relaxation for hot carriers, inhibiting the observation of a true phonon bottleneck. However, studies on QDs where electrons are chemically injected into the conduction band show that intraband relaxation is slowed by the absence of the hole. Also, low temperature measurements suggest that thermal effects mask some relaxation pathways that occur on time scales of hundreds of picoseconds, which are more consistent with what is expected from a "phonon bottleneck".; The photo-sensitization of nanocrystalline titanium dioxide (TiO 2) with QDs is also studied. The direct nucleation and growth of SdS QDs on TiO2 is studied, as well as separately prepared and adsorbed InP QDs. The results suggest that electron transfer from the QD to the TiO 2 is sensitive to the degree of intimate contact of the two materials. Electron transfer from CdS QDs is the most easily quantifiable and is found to occur on the time scale of tens of picoseconds. Electron transfer from spherical InP QDs occurs primarily from surface-localized trap states, and not from the delocalized core states.