凝聚态物理
半导体技术半导体技术
    V2O5是一种极其重要的无机材料，在化工和电子工业领域有着广泛的应用。尤其是在电存储和电致变色器件中的应用深受研究者的关注。目前，影响其未来发展的难题主要有：Li嵌入对V2O5电子结构的影响及其嵌入/脱出的机理缺少理论依据，此外，本征氧空位缺陷对材料特性的影响机制也尚不明确。本论文通过理论计算和实验研究，较深入地考察了V2O5体材料的电子结构及其分别受Li离子嵌入和氧空位的影响。理论上，我们分析了Li离子的嵌入机理以及氧空位对体材料的电子结构所产生的影响。实验上，我们分别利用物理汽相沉积和射频磁控溅射技术制备了V2O5薄膜，采用包括电子显微镜、X射线光电子能谱（XPS）、紫外光电子能谱（UPS）等技术，表征薄膜样品的各项性质。并将实验结果与理论计算相比较，得到了一些较为有意义的结论：
    在Li离子的嵌入机理研究方面，通过对Li离子的不同嵌入位置的理论模拟，发现两种模型下的Li0.5V2O5总能相差不是很大，表明在常温下Li离子嵌入后可沿着b 轴方向在一平台上进行扩散。当结构变为LiV2O5时，发现Li位于层间两个桥氧的间隙位时，体系最为稳定。但不管嵌入多少量的Li离子，体系的电子跃迁方式都没有改变，仍为间接带隙。但是体系的费米能级随着Li离子浓度的增加而上升，并进入导带中，这极有可能是实验上发现Li离子嵌入后导致吸收边发生蓝移的主要原因。
    在氧空位的研究方面，通过XPS测量出具有不同空位浓度的薄膜样品，发现随着空位浓度的增加，V的氧化态逐渐降低；UPS表明体系的功函数随着氧空位浓度的增加而降低；紫外可见光吸收谱发现随着氧空位浓度的增加，吸收边向高能方向移动。理论计算同样发现V的价态随着O的缺失而降低，功函数也随之减小。此外，氧空位的形成将转移一部分的电子给近邻的V离子，使V的电子结构发生了明显的变化。同时，光学带隙也伴随氧空位的形成而增大。
    根据以上对V2O5体材料的深入研究，为Li离子的嵌入/脱出反应提供了理论的依据，并为高质量V2O5薄膜的生长积累了宝贵的经验。 [英文摘要]：     
      Because it is a promising material in many technological applications, vanadium pentoxide (V2O5) has been the subject of intense study. Especially, the applications of V2O5 in electrochemical charge storage and electrochromic devices have been paid the most attention. However, several critical factors have restricted the development of its applications: 1) the mechanism which changes the electron properties of V2O5 due to the intercalation/de-intercalation of Li is still not clear; 2) how does the oxygen vacancy affect the properties of V2O5 are still controversial. In this thesis, both theoretical calculations and experimental measurements are applied for studying these factors. For theoretical calculations, the influences on the electron properties of V2O5 by intercalation of Li and oxygen vacancy were investigated by using first principle method based on the density functional theory. For experimental studies, two kinds of V2O5-x have been prepared by physical vapor deposition and radio frequency (rf) magnetron sputtering. Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS) were employed to detect the properties of film samples. At the same time, some useful conclusions have been obtained by comparing experimental data with theoretical calculations.
      On the aspect of Li intercalation, different intercalation sites of Li in the V2O5 lattices have been considered. The total energies are similar for different Li intercalation sites when the chemical composition is Li0.5V2O5, which indicate there is a flat potential energy surface and Li ions can diffuse along the b direction. When it is LiV2O5, there is a lowest one when the intercalated Li ions only locate on the c axis between two bridging oxygen ions at sequential V2O5 layers. The intercalation of Li into V2O5 does not change the electron transition property of V2O5, which is indirect band gap semiconductor. The Fermi lever of LixV2O5 increases with the amount of Li, which maybe the main reason why blue shift has been found on the absorption of LixV2O5.
      On the aspect of oxygen (O) vacancies, different vacancy concentration samples were defined by XPS, and the vanadium ions were gradually reduced to lower oxidation states with the increase of O vacancy density. Simultaneously, the formation of O vacancy leads to decrease the work function of V2O5-x, which was confirmed by UPS. At the same time the absorption edge moved to higher energy due to the formation of O vacancies. Theoretical calculations further proved that the formation of O vacancies would cause the reduction of V ions and the decrease of work function. The electronic structures of V ions are strongly alerted by the removal of O ions nearby due the electron transfer to the V 3d orbitals. Additionally, the optical band-gap was broadened with the increase of O vacancies density.
      In summary, we prove the theoretical results on the Li intercalation and O vacancies on the effects of V2O5 electronic structures. The results will help us to better understand the intercalation reactions and prepare high quality V2O5 thin films.