Exciton Dynamics Of Single Quantum Dots Based On Time-Resolved Technique

Quantum dots(QDs)are nano-scale semiconductor particles with similar atomic characteristics.Their wideband absorption,narrowband emission,continuously adjustable emission spectrum,and high fluorescence quantum yield exciton dynamics opens exciting windows for solar cells,LEDs,single photon sources,and entangled photon sources.However,the reason why the single exciton quantum yield of QDs is randomly changed under the continuous excitation of light is whether the surface trap states or photoionization is still controversial.In addition,due to the low quantum yields of biexciton and multiexciton,the study of the dynamics of biexciton stays under ideal conditions without considering the effects of surface traps and photoionization on the biexciton dynamics.My Ph D research focused on the exciton dynamics of Cd-based and perovskite QDs at single particle level.Time tagged,time-resolved and time-correlated single photon counting(TTTR-TCSPC)technology is used to collect the arrival time of the photoluminescence(PL)photons of a single QD.Through self-programming,PL intensity trace,PL decay curve,second-order correlation function,PL lifetime-intensity distribution,and the lifetime and quantum yield of single exciton and biexciton were extracted to obtain exciton dynamics.The effect of the surface traps and photoionization on single and biexciton dynamics and the influence of the interface environment on the exciton dynamics are analyzed,and relevant theoretical models were established to explain the experimental phenomena.The research work in the thesis mainly contains: fast recognition of single QDs from high multi-exciton emission and clustering effects;exciton and biexciton dynamics of single QDs;the effect of interface environment on exciton dynamics of single QDs;the effect of surface charges on exciton fine structure of single QDs.The main innovations are:1.We demonstrate a method for fast recognizing single QDs based on the probabilities of detecting single-and two-photon events.The TTTR-TCSPC technique is applied to effectively remove multi-exciton emission and low-counting background.By this way,single QDs can be fastly recognized by the spatial coincidence-counting model.In addition,the fast recognition of single QDs by using the collected photons during the confocal scanning imaging process has been achieved synchronously.2.Biexciton dynamics of single colloidal QDs.This robust method can determine the absolute radiative and Auger recombination rates of both neutral and charged biexciton states,and the corresponding ratios between the two states are in agreement with the theoretical predictions of the asymmetric band structures of Cd Se based QDs.Furthermore,the surface traps are found to provide additional nonradiative recombination pathways for the biexcitons,and their contributions are quantified by the method.3.Exciton and biexciton dynamics of single perovskite QDs.It was found that while the rates of radiative recombination remain essentially constant,the overall relaxation process is dominated by nonradiative recombination of single excitons and biexcitons.The radiative lifetime scaling is determined to be ~4.4 for biexcitons.The blinking mechanism of perovskite QDs is addressed by considering the trion states under higher excitation powers.4.ITO nanoparticles can effectively suppress the PL blinking of Cd Se Te/Zn S single QDs,and can increase the proportion of on-state to more than 98%.We have found that Cs Pb Br3/ITO system with Fermi level alignment can effectively suppress Auger recombination and non-radiative recombination processes of valence band hole transfer.This can effectively improve the photoelectric conversion efficiency of photovoltaic devices based on perovskite QDs.5.By covering the single dot-in-rods with ITO nanoparticles,the surface of single dotin-rods is charged by ITO through interfacial electron transfer.It is found that the full width at half maximum of histogram of polarization degrees of the single dot-in-rods is broadened from 0.24(on glass)to 0.41(in ITO).In order to explain the exprimental results,the bandedge exciton fine structure of single dot-in-rods is calculated,and results show that the level ordering of the emitting states determines the polarization degrees tending to increase or decrease under the influence of surface electrons.