| Semiconductor colloidal quantum dots are chemmically synthesized nanocrystals (NCs), whose size is in the range of several to tens of nanometers. The size-dependent tunable optical properties have made them ideally suitable for a varirty of optical devices. Due to the spectral overlap, energy transfer (ET) can be realized from semiconductor nanocrystals to other adjacent fluorophores. Because of the strong quantum confinement of electrons and holes in all three spatial dimensions, the discrete structure of energy levels appears and the carrier-carrier interactions are greatly enhanced in the nanocrystals. These have led to the efficient enhancement of carrier multiplication (CM) and nonradiative Auger recombination processes in semiconductor NCs, which would be otherwise ignored in their bulk counterparts. In this thesis, we focus on the optical studies of several novel optical phenomena of semiconductor nanocrystals, such as energy transfer, photoluminescence (PL) blinking and carrier multiplication. The main contents of this thesis are as follows:In an energy transfer process, it is the optical responses of donor and acceptor materials on the single-particle level that ultimately determine its overall performance. In chapter2, we performed time-tagged, time-resolved optical measurements to correlate the photoluminescence intensities and lifetimes of a donor semiconductor NCs and acceptor dye molecules linked to its surface. The results reveal that the PL intensity of dye molecules follows exactly the blinking properties of the donor NCs and shows a step-like quenching behavior due to the photobleaching effect. The corresponding recovery of the NCs PL intensity has allowed us to realize the textbook definition of PL quantum efficiency measurement in dye molecules upon absorbing a single exciton. The theoretical fitting of the lifetime data demonstrates that the buildup time of acceptor PL could be solely determined by the radiative lifetime of dye molecules when it is any shorter than the NCs lifetime, thus confirming the long-existing Forster theory on ET dynamics.Carrier multiplication describes an interesting optical phenomenon in semiconductors whereby more than one electron-hole pair, or exciton, can be simultaneously generated upon absorption of a single high-energy photon. So far, it has been highly debated whether the carrier multiplication efficiency is enhanced in semiconductor nanocrystals as compared with their bulk counterpart. The controversy arises from the fact that the ultrafast optical methods currently used need to correctly account for the false contribution of charged excitons to the carrier multiplication signals. In chapter3, we show that this charged exciton issue can be resolved in an energy transfer system, where biexcitons generated in the donor nanocrystals are transferred to the acceptor dyes, leading to an enhanced fluorescence from the latter. With the biexciton Auger and energy transfer lifetime measurements, an average carrier multiplication efficiency of～17.1%can be roughly estimated in CdSe nanocrystals when the excitation photon energy is～2.46times of their energy gap.Multiple excitons (MEs) of a single semiconductor nanocrystals describe an intriguing electronic configuration with two or more electron-hole pairs present simultaneously within its excitation volume. Owing to the Auger recombination effect, MEs are dissipated nonradiatively on the sub-nanosecond time scale, which sets a stringent limit on the time window within which one can operate with them. In chapter4, we show that this issue can be resolved utilizing an intrinsic energy transfer system in CdSe NCs, where MEs created in the donor quantized states can be effectively extracted to the acceptor trap states. This was evidenced by the step-like change in the intensity and the apparent shortening in the rise time of the trap-state fluorescence with the increasing excitation laser power. With the radiative lifetime being tens of nanoseconds for the trap states, the extended storage of MEs has been achieved and marks a crucial step towards flexible manipulations of their optoelectronic properties.In chapter5, the photoluminescence quantum yield (QY) dynamics of single semiconductor nanocrystals exhibiting fluorescence "blinking" has been studied, whereby two kinds of behaviors were observed. In normal blinking, the "on" state QY is higher and remains constant, while in bleached blinking for the same NCs, it either stays at a lower value or decreases continuously until the fluorescence is completely quenched. By analyzing power-law statistics of the same single NCs during both normal and bleached blinking periods, we found that the bleached blinking is associated with truncated "on" and "off" time durations, as compared to those measured for normal blinking. The above observations can be well explained when a clear distinction was made between surface carrier traps and defect sites, which mainly affect the PL blinking statistics and the "on" state QY, respectively. |
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