单量子点荧光闪烁机理研究

发布时间：2018-03-20 02:35

本文选题：量子点　切入点：单分子光谱　出处：《浙江大学》2017年博士论文　论文类型：学位论文

【摘要】：量子点具有吸收带宽、发射峰窄、发射波长连续可调、荧光量子效率高、光化学稳定性高等优异的光学性质。因此,量子点作为一类新型荧光和光电材料被广泛用于诸多领域。然而,在单分子水平上,量子点的发光却存在荧光闪烁的行为:在持续激发下,荧光强度随机呈现"亮态"和"暗态"。过去20年里,人们对该现象做了大量研究,但依然无法确定其物理化学机理。本论文利用高质量闪锌矿结构的CdSe/CdS核/壳量子点作为模型,系统研究单粒子荧光闪烁、发展单量子点光谱测量与分析方法。首先,利用闪锌矿CdSe/CdS核/壳量子点优异的集合体性质,我们对单分子光谱实验方法进行了系统研究,建立了能够与集合体荧光性质相互参比的单分子光谱测量方法。实验证明,单量子点的荧光发射光谱、荧光衰减曲线与集合体一致,意味着该量子点系统是目前研究单分子光谱和荧光闪烁很好的模型。在此基础上,对单粒子CdSe/CdS核壳量子点在中性单激子态的基本光学性质进行了系统研究。实验揭示,不同CdS壳层厚度的CdSe/CdS核壳单量子点均表现为荧光非闪烁行为。其中最小尺寸的CdSe/CdS核/壳量子点只有4个单层的CdS。这意味着,荧光非闪烁的临界体积阈值小于100 nm3,低于目前文献所建议的1000 nm3的阈值。第二部分,发展新的单分子光谱分析手段,将量子点单颗荧光闪烁行为看作可逆光氧化还原反应,提出量子点荧光闪烁机理模型。通过改变激发光功率密度、重复频率、波长等激发条件和量子点壳层厚度等条件,计算其电离与去电离速率以及速率常数。实验结果揭示,单量子点荧光闪烁行为中的电离和去电离速率均与激发光功率密度呈线性关系,证明了单一通道的光致电离/去电离的过程,它们各自对应的电离速率常数约为10-6至10-5次/光子。而且去电离过程还存在速率约为2次/秒的自发过程。过去文献报道中普遍认为的多激子俄歇电离过程在我们的系统中并未被观测到。实验结果还表明,我们得到了量子点电离和去电离过程均由光生"热"载流子引发。第三部分,研究单量子点带电激子态的光学性质。借助单分子单光子技术,我们对单量子点的带电激子态和多激子态的光学性质进行了系统的研究。在理解量子点电离和去电离过程的基础上,通过调控量子点荧光闪烁行为,我们得到了稳定的量子点中性激子态或者是稳定的带电激子态的荧光发射。通过研究带电激子态的荧光光谱、光子二阶互相关计数与其荧光量子产率的联系,获得量子点带电态的基本光谱学性质。同时,我们还观察到了同一个单量子点的多个不同发光效率的"暗态"的现象。利用电离和去电离速率的计算方法,我们推测具有不同荧光效率的"暗态"对应了带有不同数目电荷的带电激子态的发射。
[Abstract]:Quantum dots have excellent optical properties such as absorption bandwidth, narrow emission peak, continuously adjustable emission wavelength, high fluorescence quantum efficiency and high photochemical stability. As a new kind of fluorescent and optoelectronic materials, quantum dots are widely used in many fields. However, at the single molecular level, the luminescence of quantum dots has the behavior of fluorescence flicker: under the continuous excitation, Fluorescence intensity presents "bright state" and "dark state" at random. In the past 20 years, a great deal of research has been done on this phenomenon, but it is still impossible to determine its physicochemical mechanism. In this paper, CdSe/CdS core / shell quantum dots with high quality sphalerite structure are used as the model. Single particle fluorescence scintillation has been studied systematically, and single quantum dot spectral measurement and analysis methods have been developed. Firstly, we have systematically studied the single molecule spectroscopic method by using the excellent aggregate properties of sphalerite CdSe/CdS core / shell quantum dots. A method for measuring the fluorescence properties of single quantum dots is established, which can be compared with the fluorescence properties of the aggregates. The experimental results show that the fluorescence emission spectra of single quantum dots and the fluorescence attenuation curves are consistent with those of the aggregates. It means that the quantum dot system is a good model for the study of single molecule spectrum and fluorescence scintillation. Based on this, the basic optical properties of single particle CdSe/CdS core shell quantum dots in neutral monoexcitonic states are systematically studied. CdSe/CdS core-shell single quantum dots with different CdS shell thickness all exhibit fluorescence non-scintillation behavior. The smallest size of CdSe/CdS core-shell quantum dots is only four monolayer CdSs, which means that, The critical volume threshold of fluorescence non-scintillation is less than 100nm ~ 3, which is lower than the threshold of 1, 000 nm3 suggested in the literature. In the second part, a new method of single-molecule spectroscopic analysis is developed, in which the fluorescence scintillation behavior of quantum dots is regarded as a reversible photoredox reaction. A model of fluorescence scintillation mechanism of quantum dots is proposed. The ionization and deionization rates and rate constants are calculated by changing the excitation conditions such as excitation power density, repetition rate, wavelength and the shell thickness of quantum dots. The rate of ionization and deionization of single quantum dot fluorescence scintillation is linearly related to the power density of excited light, which proves the process of photoionization / deionization of single channel. Their corresponding ionization rate constants range from 10-6 to 10-5 times / photon, and there are spontaneous processes with a rate of about 2 times per second in the deionization process. The hyperexciton Auger ionization process is generally considered in the past literature. Our system has not been observed. The experimental results also show that, We have obtained that both the ionization and deionization processes of quantum dots are initiated by photogenerated "hot" carriers. In the third part, we study the optical properties of charged exciton states in single quantum dots. We have systematically studied the optical properties of charged exciton states and multiexciton states of single quantum dots. Based on the understanding of the ionization and deionization processes of quantum dots, the fluorescence scintillation behavior of quantum dots is regulated. We have obtained the fluorescence emission of the stable quantum-dot neutral exciton state or the stable charged exciton state. By studying the fluorescence spectra of the charged exciton state, the relation between the photon second-order cross-correlation counting and the fluorescence quantum yield is obtained. The basic spectral properties of charged state of quantum dots are obtained. At the same time, we also observe the phenomenon of "dark states" with different luminous efficiency in the same single quantum dot. We speculate that the "dark states" with different fluorescence efficiency correspond to the emission of charged exciton states with different number of charges.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O657.3

【相似文献】