全势多重散射理论在纳米材料XANES中的应用

发布时间：2018-04-04 06:22

本文选题：X射线吸收谱近边结构(XANES)　切入点：多重散射方法(MST)　出处：《中国科学技术大学》2016年博士论文

【摘要】：X射线吸收近边结构(XANES)是研究材料中特定元素的原子的电子结构及其周围原子的空间分布的极具价值的技术。不过,要提取这些信息,我们需要有精确的且具有较好预测性的理论工具。多重散射方法(MST)正是这样一个已经被广泛应用于XANES计算的理论方法。然而,传统的MST由于采用了Muffin-Tin (MT)近似,往往不能正确描述很多体系,包括开放晶体,分子体系,表面和界面。为了克服MT近似所导致这些缺点,我们需要将传统的MST推广到全势多重散射方法(FP-MST)。本文中,作者总结了前人对FP-MST的一些研究,并详细介绍推导了一种最新发展起来基于space-filling胞概念的FP-MST (K. Hatada等,2007,2010)。另外,多重散射方法在其它方面的一些进展和该方法目前存在的一些局限性也在本文中进行了讨论。作者的工作着力于FP-MST的理论研究及其在纳米材料的XANES计算中的应用。作者的科研工作主要在以下几个方面展开：1.作者编写了一个名为ES2MS的接口程序。该程序读取基于平面波的从头计算程序VASP的计算结果,进而为由Hatada等编写的基于FP-MST的程序FPMS构建所需的全电子自洽电荷密度和势函数。ES2MS程序的核心功能是,己知VASP计算所得到的赝势和全电子电荷密度,通过求解泊松方程,构建以径向网格形式存储的全电子势函数。2.利用上述的ES2MS程序构建的自洽势和电荷密度以及FPMS程序,作者计算了石墨烯和氧化石墨烯的C和O的K边吸收谱。相比于传统的基于MT近似的多重散射方法计算(MS计算),我们得到的结果和实验谱的符合程度大大提高。我们的计算结果较好地重现了包括分子层的堆垛,边结构和氧吸附等各种结构调制对石墨烯吸收谱的谱形的影响。3.作者发展了一个高效的散射矩阵求逆的新方法,并实现于FPMS程序。该方法提高的计算速度能够达到20到30倍。一般而言,只要体系不是太小,散射矩阵的求逆是MS计算中耗时最多的一个步骤。因此,该方法大大减少了MS计算的时间,并且在精确度上只有极小的损失。该新方法的基本思想是,按照矩阵元对应的轨道角动量的大小,将散射矩阵划分为四个子矩阵,并对其中的若干子矩阵的矩阵元采用近似值。4.作者参与推导了基于实空间MST的考虑了”完全”多重散射效应的芯态电子能量损失谱的公式体系。所谓的完全多重散射效应包括了入射电子和出射电子受到的所研究材料的散射作用。芯态电子能量损失谱是一种以电子为激发源的探测入射电子能量损失的实验手段,其所能得到的信息类似于X射线吸收谱。
[Abstract]:X-ray absorption near edge structure (XANES) is a valuable technique for studying the electronic structure of the atoms of specific elements and the spatial distribution of the atoms around them.To extract this information, however, we need theoretical tools that are accurate and predictable.The multiple scattering method (MSTs) is such a theoretical method which has been widely used in XANES computation.However, because of the traditional MST approximation, many systems, including open crystal, molecular system, surface and interface, can not be described correctly.In order to overcome these shortcomings caused by MT approximation, we need to extend the traditional MST to full potential multiple scattering method (FP-MSTT).In this paper, the author summarizes some previous researches on FP-MST, and introduces in detail a new kind of FP-MST K. Hatada based on the concept of space-filling cell.In addition, some advances in other aspects of the multiple scattering method and some limitations of the method are also discussed in this paper.The author's work focuses on the theoretical study of FP-MST and its application in XANES calculation of nanomaterials.The author's research work is mainly carried out in the following aspects: 1.The author wrote an interface program called ES2MS.The program reads the calculation results of the ab initio program VASP based on plane wave, and then the core function of the whole electron self-consistent charge density and potential function .ES2MS program for the FPMS program based on FP-MST written by Hatada et al.It is known that the pseudopotential and total electron charge density obtained by VASP are calculated. By solving Poisson's equation, the full electron potential function .2stored in radial grid form is constructed.Using the self-consistent potential, charge density and FPMS program constructed by the above ES2MS program, the K edge absorption spectra of C and O of graphene and graphene oxide have been calculated.Compared with the traditional multiple scattering method based on the MT approximation, the agreement between the obtained results and the experimental spectra is greatly improved.Our results reproduce the effects of structural modulations, including molecular stacking, edge structure and oxygen adsorption, on the absorption spectra of graphene.A new method of inverse scattering matrix is developed and implemented in FPMS program.This method can increase the calculation speed by 20 to 30 times.Generally speaking, as long as the system is not too small, the inverse of scattering matrix is the most time-consuming step in MS calculation.Therefore, the time of MS calculation is greatly reduced, and the accuracy of the method is minimal.The basic idea of the new method is to divide the scattering matrix into four submatrices according to the magnitude of orbital angular momentum corresponding to matrix elements, and to use approximate value .4 for matrix elements of several submatrices.The authors have participated in the derivation of a formula system based on the real space MST for the energy loss spectrum of the core electron considering the "complete" multiple scattering effect.The so-called complete multiple scattering effect includes the scattering of incident and outgoing electrons from the materials studied.The core electron energy loss spectrum is an experimental means to detect the incident electron energy loss with the electron as the excitation source. The information it can get is similar to the X-ray absorption spectrum.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O434.1;TB383.1

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