The Studies On Blinking Behavior And Mechanism Of Quantum Dots

With the development of nanotechnology,semiconductor quantum dots(QDs)are widely applied in solar energy,biological cell tracking,luminescent materials and displays,due to their unique physical and chemical characteristics,especially in their biological and optical aspects,which have great academic value and business prospects.However,with the development of spectroscopy technology at singlemolecule level,it is found that colloidal quantum dot exhibit blinking behavior.So far,most studies focused on the type-I structural QDs,despite type-II QDs demonstrated relatively wider wavelength ranges in absorption and emission spectra.Here we report a shell dependence of blinking dynamics of fully type-II ZnSe/CdS core/shell quantum dots with a range of shell thickness(1-6 monolayers of CdS),and the suppressed blinking can be realized in single-exciton regime by artificially controlling the thicknesses of shells.In this paper,the fluorescence blinking and intensity resolved fluorescence lifetime of ZnSe/CdS quantum dots with different shell thickness are studied by using single molecule fluorescence Lifetime Imaging Microsystem(FLIM)and time dependent single photon counter(TCSPC).The intensity resolved fluorescence lifetime revealed the existence of various fluorescence decay channels.We use a physical model to explain the fluorescence blinking kinetic mechanism of single-particle type-II ZnSe/CdS quantum dots with shell thickness.The details are as follows:1.we firstly prepared the ZnSe/CdS Core-shell Quantum Dots with different Shell thickness,and the ensemble optical measurements were conducted to study the optical properties of quantum dot with different shell thickness,including UV-vis absorption spectra,fluorescence spectra,quantum yields,bulk lifetime and so on.2.The fluorescence blinking and lifetime of quantum dots with different shell thickness were measured with single molecule fluorescence lifetime imaging system(FLIM)and time-dependent single photon counter(TCSPC)at the single-particle level.The fluorescence blinking behavior of a large number of single particle quantum dots were statistically analyzed.It is found that the fluorescence blinking behavior is closely related to the shell thickness.The fluorescence blinking behavior was divided into on-state,grey state and off-state.It was found that the fraction of on states(fraction-on)varied with the shell thickness.And the grey state is generated as the shell thickness increases.We found that when the shell thickness is 4 layers,the photoluminescence(PL)quantum yield(QY)and fraction-on will reach the maximum.In other words,in this case,the "blinking" will be suppressed.Furthermore,we speculate that the fluorescence blinking behavior of quantum dots with different shell thickness is related to the decay pathways of different excitons.3.After intensity-resolved fluorescence lifetime analysis of single-particle quantum dots,the decay pathways of excitons were further determined We extracted the PL decay rates of on-states and grey states from single-dot intensity-resolved fluorescence lifetime traces and investigated the effect of shell thickness on exciton recombination pathway in ZnSe/CdS QDs.Single-dot fluorescence lifetime analysis indicates the fluorescence intensity fluctuation originates from three components: trion emission,bandedge emission and trap-state emission.The relative amplitudes of these components demonstrate a dependence on the shell thickness of CdS,and thus the values of QY and fraction-on states.We observe that the contribution of the band-edge emission is the main factor to improve photoluminescence quantum yields and fraction-on.In a word,we first introduce a brief introduction to quantum dots,including the confinement effect of quantum dots,the physical process of the luminescent of quantum dots.The fluorescence blinking behavior and the model of fluorescence blinking mechanism are also introduced.In addition,the principle,measurement process and method of fluorescence lifetime imaging microscopy system are briefly introduced.Finally,we investigate the optical properties of quantum dots from solution aggregates and single particles,respectively.This study may provide a strategy to better control blinking and interpret exciton decay pathways in various type-II core/shell QD architectures.