| Quantum dots(QDs),as a new type of luminescent material,have been applied for various applications due to their unique optical properties such as broad absorption band,narrow and tunable emission peak,high photoluminescence(PL)quantum yield and high photo-chemical stability.However,the photoluminescence intermittency or blinking of a single QD-PL intensity randomly switching between distinguishable brightness states under constant excitation-was discovered by single molecule spectroscopy.For the past two decades,PL blinking of single quantum dots has attracted tremendous efforts in the field but the physical chemistry mechanism is still obscure.In this thesis,high quality zinc-blende CdSe/CdS core/shell QDs as a model system,coupling with improved measuring and analysis methods for single QD spectroscopy,were used to study the PL blinking mechanism of single QDs.In the first part,we systematically studied single-dot spectroscopy of CdSe/CdS core/shell QDs basing on their outstanding ensemble optical properties.Measuring methods for single molecule spectroscopiy were improved to correspond to ensemble measurements.Experimental results showed that the PL spectra and PL decay curves of single QDs were consistent with the corresponding ensembles.The consistency of optical properties at single-dot and ensemble levels confirmed the QD system we studied was an ideal model to explore single QD spectroscopy.On this basis,we systematically studied the optical properties of neutral single exciton state in single QDs.Experimental results revealed that CdSe/CdS core/shell QDs with different thicknesses of CdS shell,down to 4 monolayers of shell,were almost all non-blinking.This meant that the nonblinking threshold volume of QDs was less than 100 nm3,which was far below the values of 1000 nm3 suggested in literatures.In the second part,we developed a new analysis method for single molecule spectroscopy of treating the PL blinking of single QDs as reversible photochemical redox processes,and proposed a blinking mechanism.By varying the power density,repetition frequency and wavelength of excitation as well as the CdS shell thickness of QDs,the rates and rate constants of charging(from bright to dim state)and discharging(from dim to bright state)processes were calculated.Experimental results revealed that both charging and discharging rates linearly depended on excitation power density.The single channel photo-charging and photo-discharging processes were in rate constants of 10-6 to 10-5 events per photon absorbed respectively.An additional spontaneous discharging channel was exist in the rate of 2 events per second.The widely claimed Auger ionization of multi-exciton channel was negligible in our experiments.Instead,photo generated hot-carrier ionization was proposed for the two linear channels.In the third part of this thesis,we focused on the optical properties of charged excitons of single QDs.With single molecule spectroscopy,we systematically studied the optical properties of charged exciton state and multi-exciton state of single QDs.Understanding the charging and discharging channels in PL blinking helped us to control the blinking behavior of single QDs.We obtained stable emission of neutral exciton state or charged exciton state by adjusting the charging and discharging rates.By comparing the PL spectra,the second-order photon correlation,and the PL quantum yield of charged exciton state,the basic optical properties of charged exciton in single QDs were evaluated.In addition,multiple dim states with different emission quantum yields were observed for a single QD.By calculating the charging and discharging rates,we inferred that the multiple dim states were attributed to emission of charged states with various numbers of charge carriers. |
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