Low-Dimensional Magnetic Materials And Energy Conversion Materials:First Principles Design And Simulations

Information and energy are the two pillars of modern society,and low-dimensional materials play an important role in both fields.In the field of information,in order to design high-performance,low-energy-consumption spintronics devices,two-dimensional magnetic materials are indispensable.In the energy field,photoelectric catalysis,photovoltaic and lithium(sodium)battery technologies are important to solve energy problems,and the development of high-performance energy conversion and storage materials is the the main issue of these technologies.In this paper,through first-principles calculations,we study and design some two-dimensional magnetic materials,including two-dimensional magnetic ohmic contacts with spin switching functionality,and thickness/strain dependent magnetic transition materials;Besides,we analyze some lowdimensional conversion materials theoretically,and proposed some methods to improve their performance.Chapter 1 introduces low-dimensional materials and first-principles calculations,and is divided into two sections.In the first section,we briefly introduced the definition,classification,properties and applications of low-dimensional materials,and focused on low-dimensional magnetic materials and energy conversion materials which are expected to play an important role in the information and energy fields.The second section introduces some basic knowledge of first-principles calculation.First,we introduce the Born-Oppenheimer approximation,which can separate the nuclear coordinates and electronic coordinates in the many-body Schrodinger equation,and decompose the complex process of solving the Schrodinger equation of the entire system into solving the electron part Schrodinger equation that depends on the nuclear coordinates and solving the motion of nucleus these two relatively simple processes.Next,two methods for solving the ground state energy of the electron part are introduced.The first is the Hartree-Fock method obtained by the variational method assuming that the ground state wave function is a slater determinant.The second is the density functional theory method based on the Hohenberg-Kohn theorem.Finally,the wave function-dependent density functional theory method actually used in this article and several first-principles calculation software packages is introduced.In Chapter 2,we explore the application of magnetic materials in spintronics.In the first section,we design a kind of magnetic metal-magnetic semiconductor contacts and propose a mechanical method which can simply and effectively control the spin transport properties of these contacts.While previous studies are mostly focused on modulating the electron transport properties of nonmagnetic metal-semiconductor contacts,such as achieving ohmic conductivity,the properties of magnetic metal-magnetic semiconductor contacts and their controllability have been largely overlooked,which are however very crucial to the development of high-performance nanospintronic devices.Here we propose a mechanical means to selectively switch between spin up and spin down electron/hole transport channels by regulating the magnetic coupling between the magnetic metal and magnetic semiconductor via interfacial sliding.By first-principles calculations,two experimentally accessible 2D magnetic metal-magnetic semiconductor contacts with spin switching functionality and single spin channel ohmic conductivity(Fe3GeTe2/CrGeTe3 and Fe3GeTe2/CrI3)have been designed.In the second section,we uncover a highly thickness dependent magnetism in 2D1T-CrTe2 nanosheets by first-principles calculations.When the number of layers increases from 1 to 6,the CrTe2 nanosheet successively undergoes an intralayer striped AFM to FM transition,and interlayer AFM to FM transition.Concomitantly,the electronic structure changes from unpolarized metal to polarized metal.Theoretical analysis shows that the variation of the in-plane lattice is the main reason for the intralayer AFM-FM transition,and the intralayer coupling transition will induce the interlayer coupling transition.Our research indicates lattice strain and layer number can be potential routes to control and enrich the electronic and magnetic properties of 2D CrTe2.In Chapter 3,we analyze three energy conversion materials,which are divided into three sections.In the first section,we study the mechanism of C2N-based photocatalytic water splitting.Through first-principles calculations,we find that the photogenerated hole at the valence band maximum of the C2N transfer to the HOMO of adsorbed water molecule rapidly,and promote the dissociation of the adsorbed water molecule.Then,we analyze the change of the relative position of the HOMO of water molecule and the vbm of C2N.Finally,we find that similar to photo-generated hole,the p-type defect in C2N can also promote the dissociation of adsorbed water molecule.The second section introduces a work in cooperation with experiments.The experimental group observe that the electrochemical CO2 reduction activity of Pd icosahedral nanoparticles is much higher than that of Pd octahedral nanoparticles.We explore the above phenomenon theoretically.Through molecular dynamics simulation,we find that the surface of Pd octahedral nanoparticles has compressive strain,while the surface of Pd icosahedral nanoparticles has tensile strain.The results of first-principles calculations show that the tensile strain will increase the d-band center of the surface Pd,which can help stabilize the important intermediate COOH*in the electrochemical CO2 reduction reaction,thereby increasing the catalytic activity.Combining these two points,we explaine the phenomenon observed in the experiment.Another work in cooperation with the experiment is described in the third section.In this section,we introduce the application of black phosphorene in energy conversion and storage.Aiming at the problem of poor environmental stability of black phosphorene,the experimental group propose a covalent functionalization method,which can significantly improve the environmental stability of black phosphorene.Through theoretical analysis,we find that the origin of ambient stability of functionalized black phosphorene is that the chemically adsorbed polar groups reduce the absolute band positions of the black phosphorus nanosheets,making them difficult to be oxidized by the oxygen in the environment.Chapter 4 is a summary and prospect.My work is mainly divided into two parts:the design of low-dimensional magnetic materials and the theoretical characterization of low-dimensional energy conversion materials.In the future,I will continue to engage in these two directions,including thinking and designing new schemes for controlling the magnetization of materials with electric field,adopting modeling and theoretical methods which are closer to the experiment to simulate photocatalytic reactions,studying the effect of surface strain induced by different shapes or surface alloying on the catalytic performance of nanoparticles and further exploring the changes in the energy conversion and storage performance of black phosphorene after covalent functionalization.