First-principles Studies Of Low-Dimensional Materials

With the development of density functional theory(DFT)and computer science,the simulation accuracy and efficiency of first principles caculation are greatly improved.As a result,first principles study has become an indispensable means of research in the field of physical sciences.Especially,compared with the experimental "trial and error"approach of material design,the method based on first-principles can greatly reduce the cost of new material design and shorten the corresponding development period.On the other hand,first-principles study provides not only structures and properties of materials but also interactions between different systems.It can thus finally provide theoretical explanation and guidance to improve material performance and optimize material preparation methods.In this dissertation,based on the first-principles calculations at the level of DFT,we investigate the geometric configurations and electronic structures of some low-dimensional materials.In the first chapter,the theoretical framework of DFT,especially its exchange-correlation functionals and approximations to describ weak interactions,is introduced.In addition,we introduce some first-principles software packages,including corresponding technical details and post-processing softwares.At the end of this chapter,we briefly introduce the STM simulation methods which will be used later in this dissertation.In chapter 2,we focus on the theoretical design of compound material with two-dimensional electron gas in free space(2DEG-FS).This part of study is motivated by recent progress in the field of a kind of novel material named electride,where electrons serve as the anions.Distributed in the free space away from the cationic framework,the anionic electrons provide a nuclear-scattering-free transport channel.Inspired by a new two-dimensional electride material Ca2N,we first demonstrate the stability and exfoliation possibility of Ca2N monolayer,and suggest that 2DEG-FS exists on its surface.Considering the strong chemical activity of 2DEG-FS,we propose to use graphane for the van der Waals encapsulation of Ca2N monolayer,which finally leads to a stable 2DEG-FS system in the atmospheric environment.In chapter 3,we study the electronic structures of 0-dimensional metal nanoparticles.We demonstrate that the twin-induced tensile strain of Au75Pd25 icosahedral nanocrystals shifts up the surface d-band center,and the charge donation from Pd atoms to Au atoms increases the negative charge density of Au atoms on the surface.These two factors enhance the catalytic activity of aerobic oxidation of cyclohexane.In chapter 4,we focus on the electronic structures of copper sulfides(Cu2-xS)heterointerfaces.We show both Cu1.94S and CuS have desirable band gap as a photoelectrochemical response material.In the heteronanostructure of Cu1.94S(100)-CuS(10O),its band alignment is helpful for electron-hole separation,where the photogenerated electrons move to Cu1.94S,and photogenerated holes are collected in CuS.Moreover,due to the dumbbell-like architecture of the heteronanostructure,a better contact between high conductive planes of CuS and the electrode is built to promote the flow of photogenerated holes.Therefore,both the electronic structure and the geometric configuration of such a self-coupled polymorphs improve the photoresponse properties compared with the individual Cu1.94S nanocrystals and CuS nanoplates.In chapter 5,we discuss the interaction between phosphorus and the substrates.The current focus of the phosphorus study is selecting an appropriate substrate for the growth of a few layers of two-dimensional(2D)phosphorus.The experimental group first synthesize a honeycomb 2D phosphorus structure on the Au(111).Based on DFT calculation,we identify that the synthesized two-dimensional phosphorus is a reconstructed struture of blue phosphorus.And when Cu(l 10),a more active metal than Au,is used as the substrate,only a ordered phosphorus nanostripe arrays can be found.We show that the geometry corresponding to the nanostripes on Cu(110)is a adsorption structure of discrete phosphorus atoms.At last,tellurium functionalized Au(111)is considering as the substrate to grow blue phosphorus.We find that the adsorption energy and charge transfer of blue phosphorus on tellurium functionalized Au(111)become less than those on Au(111).In general,due to the fact that the 2D phosphorus itself is not as robust as graphene,its epitaxial growth requires low active substrates such as gold or other suitable semiconductors.