Study Of Electron Excitation In Solid And Nanomaterials Under Electron Beam Irradiation

The inner-shell electrons of atoms in solids retain element-dependent characteristic energy level information and can be used as an identification of the elemental species of the material.In contrast,valence electrons are often present throughout the material system by communalization,and the electronic structure they eventually form is not only dependent on the type of atom,but also related to the combined morphology of atoms within the material.These complex and abundant valence electron structures in materials confer different physicochemical properties,such as electrical conductivity,optical properties,and catalytic active sites,among others.How to detect,characterize and apply the valence electrons in materials is one of the current research focuses in the field of condensed matter physics.The research of this PhD thesis consists of two main parts:one is the theoreti-cal development of a set of computational simulations of the inelastic interaction of microbeam electron probes with materials with position dependence.The other part investigates the search for its key information from experimental secondary electron energy spectra in material identification and long-range order characterization of amor-phous carbon materials.The details are as follows:Chapter 1 summarizes and outlines several theoretical frameworks for studying the inelastic interaction of electrons with materials,describes the scope of application of these approaches as well as their advantages and disadvantages,briefly introduces the semiclassical theoretical framework developed in this paper,and finally reviews the development of secondary electron excitation theory.Chapter 2 introduces the theoretical basis of density functional theory,including the ground state density functional theory and the time-dependent density functional theory dealing with excited states.First-principles calculations are an important tool in this paper regarding the simulation of nanocluster electron energy loss spectra to characterize the local excitation properties.The classical Hartree-Fock method is first reviewed,and then the importance of charge density in the multi-electron interaction regime is derived.The ideological basis of density function theory is depicted,and the idea of solving a system of interaction-free particles to correspond to the real interaction system is established through the Hohenberg-Kohn first and second theorems.The clas-sification of exchange-correlation functionals,the construction of pseudopotentials and three types of expressions of basis are also described.After that,the development of the time-dependent density functional theroy is introduced,and the one-to-one mapping be-tween the time-dependent external field and the density evolution of the system through the Runge-Gross theorem is highlighted,so as to establish the correlation between the real system and the density response of the non-interaction system.Finally,we present a method for calculating the light absorption of materials using linear response techniques in the framework of time-dependent density functional theroy.Chapter 3 presents a semi-classical theoretical model that we have developed inde-pendently and applied it to study the excitation behavior of electrons within nanomate-rials under convergent electron beam irradiation.This approach can effectively include the position information of the electron beam,thus solving the difficulty of consider-ing the position-dependent information in previous studies.The origin of the excitation contribution of the plasmon in the zigzag nanographene system is investigated using this method.It is shown by electron energy loss spectroscopy that the method can charac-terize the electron excitation within the material on the sub-A scale,and by comparing the electron energy loss spectra at different incident positions,it is pointed out that the influence of the excitation of the edge state is only in the range of a few A.In addition,for the simulation of the electron energy loss spectrum of the very large system,the dif-ficulty is solved by using the extended tight-binding Hamiltonian model combined with the calculation technique of the projection potential to extend the position-dependent electron energy loss spectrum simulation to defective graphene supercells containing thousands of carbon atoms.The formation of localized impurity states under a single nitrogen atom,a single boron atom,and a single carbon vacancy is carefully investi-gated,and the results show that the presence of defects leads to the reconfiguration of the? electronic states of the system.Through electron energy loss spectrum simulations,it is found that nitrogen(boron)doping effectively enhances(suppresses)the intensity of the main peak of the electronic excitation when the electron beam is incident at the defect position,while carbon vacancy defects lead to enhancement of the low-energy excitation.Simulations at large spatial scales show that the localized effect of electron excitation from a single defect in graphene is typically within 1 nm.Chapter 4 investigates the effect of changing the hybridization mode of individual carbon atoms on plasmon excitations in the nano-graphene system is carefully examined.By introducing additional hydrogen atom adsorption,it is found that the transition from sp2 to sp3 hybridization of internal or boundary carbon atoms significantly suppresses the excitation of both bulk and edge plasmon.Further analysis shows that the transition of to hybridization of individual carbon atoms causes a reconfiguration of the induced charge and induced electric field,and that this phenomenon remains significant even for systems up to 2 nm in size.In Chapter 5,we propose the idea of extracting the characteristic signal from the secondary electron spectrum for material characterization based on the correlation be-tween the secondary electron generation and the electronic structure of solids.A more scientific method of double logarithmic differential spectroscopy is proposed to success-fully extract the fingerprint information of the material hidden in the secondary electron spectrum.In combination with the support vector machine method in machine learning,the recognition accuracy of more than 50 materials is 98%.This research has laid a solid foundation for the construction of a multi-analysis tool based on secondary electrons for surfaces.In Chapter 6 we further apply the double logarithmic differential spectrum of sec-ondary electrons to the study of sp2 hybridized carbon allotropes,comparing it with the Auger electron energy spectrum and the reflected electron energy loss spectrum,show-ing that information related to the long-range order of the material can be extracted from the secondary electron energy spectrum.The results of the study reveal that the secondary electron excitations in these materials are homologous,and the similarity criterion is applied to establish a numerical characterization technique based on dou-ble logarithmic differential spectroscopy for the long-range order of amorphous carbon materials.A systematic summary and outlook of this doctoral thesis is presented in Chapter 7.