典型光电功能材料的高压超快光谱研究

发布时间：2018-05-23 09:54

本文选题：高压 + 飞秒瞬态吸收　；参考：《吉林大学》2016年博士论文

【摘要】：金刚石对顶砧高压装置的发明使我们可以更方便的研究极端条件下物质的结构和性质,极大地促进了高压科学研究的发展。瞬态吸收光谱可以探测到光与物质相互作用的瞬态过程,是研究光物理和光化学过程中激发态动力学及瞬态产物的强有力的手段。将金刚石对顶砧高压技术与瞬态吸收光谱技术相结合,可以更好地研究物质在极端条件下,在飞秒和皮秒时间尺度内的光物理和光化学现象,例如电子跃迁、能量转移和新物种的生成及其随时间的变化情况等。我们在借鉴前人工作的基础上,利用金刚石对顶砧和瞬态吸收光谱技术,成功搭建了高压飞秒瞬态吸收光谱系统,这套系统的特点是采用共轴泵浦-探测方式来探测变压时不同能态上粒子数布局的变化;样品前使用显微物镜对泵浦光和探测光进行聚焦,使会聚后的泵浦光和探测光的光斑直径仅有几个微米大,保证了高压原位测量实验数据的可靠性;利用CCD实时观测压机内的情况;通过变换泵浦光和探测光之间的相对延时来实现时间分辨;利用自主编写的时间分辨控制软件和数据采集软件进行数据采集和分析。这套系统可以用来研究物质在不同压力条件下的一些超快光物理过程,为进一步研究极端条件下物质的结构和性质提供了一个有效的方法。高压飞秒瞬态吸收光谱系统搭建完成以后,我们利用这套系统对有机共轭聚合物PDPP-F薄膜和无机单、双层MoS_2这几种典型的光电功能材料进行了不同压力条件下的超快光物理过程的研究,从分子水平上揭示了材料的结构和物理特性的关系,以及高压极端条件对材料结构性质的影响。利用高压飞秒瞬态吸收系统对PDPP-F共轭聚合物薄膜在不同压力条件下的光物理特性进行了研究,探明了高压对PDPP-F共轭聚合物薄膜材料中的激子弛豫动力学的影响。我们发现常压时,PDPP-F薄膜中的激子只有一个较慢的弛豫过程,相应的激子弛豫寿命是172ps,表明常压时激子弛豫过程是自由激子弛豫。而高压时,PDPP-F薄膜中的激子在初始弛豫过程有一个非常快的弛豫过程,之后跟着一个比较慢的弛豫过程,这说明高压时,有多物种参与了弛豫。我们发现这个压力依赖的快的弛豫过程是激子-激子湮灭过程,而之后的慢弛豫过程是自由激子扩散过程。根据不同压力下的激子弛豫动力学实验数据,我们对PDPP-F薄膜中的激子离域程度进行了定量计算。发现,随着压力的增加,PDPP-F薄膜中的激子的离域程度明显增加,30万大气压时,PDPP-F薄膜中的激子的离域程度是常压时的3倍。而且我们发现随着压力的增加,PDPP-F薄膜中不仅激子的扩散速率增加了,而且激子的弛豫时间也增加了,由此我们认为激子的扩散长度是随着压力的增加而增加的。这个工作使我们更好的了解了聚合物的光物理特性,有益于聚合物太阳能电池效率的提高。利用飞秒瞬态吸收系统对单、双层MoS_2薄膜样品的结构和层数依赖的光物理特性及进行了对比性研究。首先我们用化学气相沉积方法在石英衬底上生长单、双层MoS_2薄膜,并通过拉曼光谱来确定了MoS_2样品的层数。我们对单、双层MoS_2薄膜样品分别进行了拉曼光谱、稳态吸收光谱、稳态荧光光谱和瞬态吸收光谱的测量和分析。发现单、双层MoS_2样品都存在A激子和B激子的吸收。双层MoS_2样品的两个吸收带较单层样品发生了一定程度的红移,这是由于随层数的增加,量子限域效应减弱导致的。将单、双层MoS_2样品的荧光光谱与吸收光谱进行比对,我们发现单、双层MoS_2样品都有的两个荧光峰与A、B激子的共振吸收对应,因此我们认为实验中测得的单、双层MoS_2样品的两个荧光带属于直接跃迁发光。而荧光信号的强度随MoS_2样品层数的减少而增强,这种层数依赖的荧光特性是源于从双层MoS_2样品到单层MoS_2样品发生了从间接带隙半导体向直接带隙半导体的转变,致使发光量子效率显著提高。另外,从单、双层MoS_2样品的瞬态吸收光谱中我们发现双层MoS_2样品的激子弛豫动力学曲线衰退的比单层MoS_2样品的快,这说明双层MoS_2样品中激子的复合更快。我们先从光物理特性差异的角度分析,认为这是由于双层MoS_2具有更快的非辐射跃迁速率。之后我们从半导体特性差异的角度即利用激子模型来分析,认为这是由于双层MoS_2样品具有更小的激子结合能;作为间接带隙半导体的双层MoS_2比单层MoS_2多一个从布里渊区内K点的导带底到Γ点的价带顶的辐射跃迁途径;此外双层MoS_2具有的层间耦合作用会使激子的振动加强,这些因素都会促进激子的快速复合,由此我们认为层间耦合作用对激子复合起到了促进的作用。此外,我们对单层MoS_2薄膜进行了高压下的激发态弛豫动力学研究,得到了压力调控的单层MoS_2薄膜的能带结构对超快光物理特性的影响。我们所研究的层数依赖的MoS_2的光物理特性和半导体特性可以为制备基于MoS_2的光电器件和半导体器件提供重要的指导和依据,并为其它过渡金属硫化物材料的结构及物理性质的研究提供重要指导。
[Abstract]:The invention of the high pressure device of the diamond to the anvil makes it more convenient for us to study the structure and properties of the material under extreme conditions, which greatly promotes the development of high pressure scientific research. Transient absorption spectra can detect the transient process of interaction between light and matter. It is a study of the dynamics and transient of excited states in photophysical and photochemical processes. The strong means of the product. The combination of the high pressure technology with the diamond anvil and the transient absorption spectroscopy can better study the photophysical and photochemical phenomena, such as the electron transition, the energy transfer and the generation of new species and their changes in time, in extreme conditions, in the extreme conditions of the femtosecond and picosecond time. On the basis of the predecessors' work, the high pressure femtosecond transient absorption spectrum system has been successfully constructed by using the diamond anvil and transient absorption spectroscopy. The system is characterized by the use of coaxial pump detection to detect the change of the particle number distribution on different states of energy in the variable pressure. The sample is used to pump the pump light and probe before the sample. The focus of the light measurement makes the pump light and the spot diameter of the probe light only a few microns in diameter. It ensures the reliability of the experimental data in the high pressure in situ measurement; the real-time observation of the pressure in the press by CCD is used to realize the time resolution by changing the relative delay between the pump light and the detection light, and the time resolved time resolved control written by ourselves is used. Software and data acquisition software are used for data acquisition and analysis. This system can be used to study some ultra fast light physical processes under different pressure conditions. It provides an effective method to further study the structure and properties of material under extreme conditions. After the completion of the high pressure femtosecond transient absorption spectrum system, we can benefit This system is used to study the ultra fast optical physical processes of organic conjugated polymer PDPP-F film and inorganic single, double layer MoS_2, the typical photoelectric functional materials under different pressure conditions. The relationship between the structure and physical properties of the material is revealed from the molecular level, and the influence of the extreme conditions of the high pressure on the structure and properties of the material is also revealed. The photophysical properties of PDPP-F conjugated polymer film under different pressure conditions were studied by high pressure femtosecond transient absorption system. The effect of high pressure on exciton relaxation kinetics in PDPP-F conjugated polymer film materials was explored. We found that only a slower relaxation process in the exciton in PDPP-F film at atmospheric pressure is corresponding. The exciton relaxation life of the exciton is 172ps, indicating that the exciton relaxation process at atmospheric pressure is free exciton relaxation. At high pressure, the exciton in the PDPP-F film has a very fast relaxation process in the initial relaxation process, followed by a slower relaxation process, which indicates that at high pressure, there are many species involved in relaxation. We find this pressure dependence. The fast relaxation process is the exciton exciton annihilation process, and the slow relaxation process after the exciton diffusion process is a free exciton diffusion process. According to the experimental data of exciton relaxation kinetics under different pressure, we calculate the exciton dissociation degree in the PDPP-F thin film quantitatively. It is found that the exciton dissociation degree in the PDPP-F film increases with the increase of pressure. It is obvious that the exciton in the PDPP-F film is 3 times that of the normal pressure at 300 thousand atmospheric pressure. Moreover, we find that the diffusion rate of the exciton in the PDPP-F film increases with the increase of pressure, and the relaxation time of the exciton increases, thus we think the diffusion length of the exciton increases with the increase of pressure. This work gives us a better understanding of the photophysical properties of the polymer and is beneficial to the improvement of the efficiency of the polymer solar cells. By using the femtosecond transient absorption system, the structure and number dependent photophysical properties of the single and double layer MoS_2 thin film samples are compared. First we use the chemical vapor deposition method in Shi Yingchen. A single, double layer MoS_2 film is grown on the bottom, and the number of MoS_2 samples is determined by Raman spectrum. We have measured and analyzed the Raman spectra, steady absorption spectra, steady-state fluorescence and transient absorption spectra of single and double layer MoS_2 thin films. It was found that the absorption of A exciton and B exciton in the single, double layer MoS_2 samples were both in the double layer M. The two absorption bands of the oS_2 sample have a certain degree of red shift compared with the single layer, which is due to the decrease of the quantum confinement effect as the number of layers increases. The comparison between the fluorescence spectra of the single and double layer MoS_2 samples and the absorption spectra shows that the two fluorescence peaks of the single, double layer MoS_2 samples are corresponding to the resonance absorption of the A and B excitons. Therefore, we think that the two fluorescent bands of the single, double layer MoS_2 samples measured in the experiment are direct transition luminescence. The intensity of the fluorescence signal is enhanced with the decrease of the number of MoS_2 samples. The fluorescence characteristics of this number dependence are derived from the indirect band gap semiconductors to the direct band gap semiconductors from the double layer MoS_2 sample to the single layer MoS_2 sample. In addition, in the transient absorption spectra of single and double MoS_2 samples, we found that the decay of the exciton relaxation kinetics curve of the double layer MoS_2 sample is faster than that of the single MoS_2 sample, which shows that the complex of the exciton in the double layer MoS_2 sample is faster. It is considered that this is due to the faster non radiation transition rate of the double layer MoS_2. Then we use the exciton model to analyze from the point of view of the difference of semiconductor properties, which is attributed to the smaller exciton binding energy of the double layer MoS_2 sample, and the double layer MoS_2 as the indirect band gap semiconductor is more than the single layer MoS_2 from the Brillouin region K point. In addition, the interlayer coupling effect of the double MoS_2 will enhance the vibration of the exciton, and these factors will promote the rapid recombination of the exciton. Therefore, we think the interlayer coupling plays an important role in promoting the exciton recombination. In addition, we have carried out the high pressure of the monolayer MoS_2 film under high pressure. The study of excited state relaxation kinetics has obtained the influence of the band structure of the single layer MoS_2 film controlled by pressure on the ultra fast optical physical properties. The photophysical and semiconductor properties of the MoS_2 based on the number of layers we have studied can provide important guidance and basis for the preparation of MoS_2 based optoelectronic devices and semiconductor devices. The study of the structure and physical properties of transition metal sulfides provides important guidance.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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