Spintronics Materials And Water Itting Photocatalyst Materials:a First-Principles Design

Materials are closely related to people’s daily life and promote the development of human society. Nearly every big change in economy and society is more or less caused by materials. As an example, the discovery and wide application of semiconductors brought people into the microelectronics era, from which we still benefit nowadays. However, the number of materials in nature is quite limited, which can not meet the ever-growing demand of people. Thus, it is urgently needed to design and synthetize new functional materials through various physical or chemical means. For experimentalists, it is a trial and error job, consuming much time and energy, and inevitably causes a waste of experimental resources.An effective solution is the computational quantum chemistry. By first-princples calculations, the properties of materials can be routinely predicted, based on which one selects those ones with required properties, and then confirms them by a few experiments. Such a procedure can largely improve the productivity of experimentalists and reduce the period of materials design. The aim of this dissertation is to introduce our works on designing spintronics materials and water splitting photocatalyst materials by employing first-principles calculations. Spintronics and photocatalytic water splitting are both devoted to the solution of nowadays serious energy and environmental problems:the former is for high speed at low energy cost, and the latter is to use clean and renewable energy, i.e. hydrogen energy from photocatalytic water splitiing, to replace the widely used fossil energy.Spintronics, which exploits the spin of electrons for information processing, holds great potential advantages of speeding up data processing, high circuit integration density, and low energy consumption. To obtain high performance spintronics devices, there are still several challenges ahead, including spin-polarized carrier injection, long-distance spin transport, and effective manipulation and detection of the carriers’ spin orientation. The solutions to these problems lie in finding new materials with specific spintronics properties, such as magnetic semiconductors and half metals. Although lots of spintronics materials have been proposed previously, they are far from practical applications partly due to the destruction of half-metallicity by spin-flip transitions, low magnetic ordering temperature, difficulty in synthesis, or bad controllability. Our works are devoted to solving these problems, including designing novel spintronics materials with certain functions, and exploring practical spintronics materials that can work at room temperature.On the other hand, as an ultimate clean energy, hydrogen produced from photocatalytic water splitting using solar light is regarded as the holy grail to chemists. The key of photocatalytic water splitting is to develop photocatalysts with high solar energy conversion efficiency. However, traditional photocatalysts such as transition metal oxides are mostly active only under UV irradiation, which constitute7%of the solar energy, while other photocatalysts that can absorb visible light are not stable during the reaction process, or the quantum yield under visible light is relatively low, leading to a very inefficient usage of sunlight. Thus it is very desirable to develop new photocatalysts which can efficiently utilize the visible or even near-infrared part of the solar light, while possessing good stability. This is what we have done in the dissertation.The dissertation includes three chapters. In chapter1, we briefly introduce the theoretical basis for materials design, i.e. computational quantum chemistry. According to the different choice of fundamental variables, quantum chemistry can be classified into wavefunction based and electron density based methods. The former possesses the advantage of high accuracy, while it is time-consuming, thus can only afford a molecule or cluster type system with tens of atoms. Moreover, for chemists, it is hard to imagine the shape of wavefunctions since they are3N dimensional functions (N is the number of electrons in the system). Contrarily, electron density is only a three dimentional function and is much more intuitive. Thus the electron density based method, i.e. density functional theory (DFT), is preferred by chemists. More importantly, using electron density as the fundamental variable makes the Schrodinger equation easy to solve, resulting fast calculations while keeping a moderate accuracy. DFT can deal with relatively large systems with hundreds of atoms, and has been broadly applied to bulk solids, surfaces and nanomaterials. Since our calculations in this dissertation are all based on DFT, we will provide an overview of its origin, developments and basic framework.In chapter2, we focus on the theoretical design of diverse spintronics materials. First, to realize electrical control of carriers’spin orientation, we propose a new concept of bipolar magnetic semiconductors (BMS), in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. Based on first-principles calculations, a serial of BMS materials are designed in one dimensional chemically modified carbon nanotubes, quasi two dimensional La(Mn,Zn)AsO alloy, MnPSe3nanosheets, surface-doped SiC nanofilms, and three dimensional quaternary Heusler alloy FeVXSi (X=Ti, Zr). Second, to obtain magnetic semiconductors with both room-temperature magnetic ordering and large spin-polarization, we propose a general scheme of asymmetric antiferromagnetic semiconductors (AAMS), in which magnetic moments are designed to be carried by different transition-metal ions and antiferromagnetically coupled with each other. The strong antiferromagnetic super-exchange interaction ensures a high magnetic ordering temperature, while valence and conduction bands around the Fermi level are highly spin-polarized as a result of d orbital mismatch among different transition-metal ions. The concept of AAMS is verified by the first-principles calculations on a serial of double perovskites A2CrMO6(A=Ca, Sr, Ba; M=Ru, Os), which have antiferromagnetic orders above room temperature and exhibit diverse spin-polarization patterns. The third work is the design of two dimensional intrinsic ferromagnetic semiconductors, i.e. the CrXTe3(X=Si, Ge) nanosheets, which possess the same spin-polarization in their valence and conduction bands, and is promising for spin-polarized carrier injection and detection at nanoscale. Last, we move our attention to another kind of spintronics materials, the half metals. To develop practicable spintronic devices, half metals should have high Curie temperature, wide half-metallic gap, and large magnetic anisotropy energy. However, there is still no candidate to fulfill all these requirements. Using first-principles calculations, we design a practicable half-metal based on the previously proposed La(Mn,Zn)AsO alloy, which shares the same "1111" crystal structure as superconducting LaFeAsO. Via element substitutions, the alloy changes from an antiferromagnetic semiconductor to ferromagnetic half metals. The half-metallic La(Mn,Zn)AsO with element substitutions possesses both wide half-metallic gaps (up to0.74eV) and high Curie temperature ranging from475to600K. Moreover, the quasi two-dimensional structure endows the doped La(Mn,Zn)AsO alloy a sizable magnetic anisotropy energy with the magnitude of at least one order larger than those of bulk magnetic metals.In chapter3, we focus on the theoretical design of novel water splitting photocatalysts. One is the prediction of semihydrogenated BN sheet as a promising metal-free visible-light driven photocatalyst for water splitting. The other is the proposal of a new mechanism for water splitting in which near-infrared light can be used to produce hydrogen. This ability is a result of the unique electronic structure of the photocatalyst, in which the valence band and conduction band are distributed on two opposite surfaces with a large electrostatic potential difference produced by the intrinsic dipole of the photocatalyst. This surface potential difference, acting as an auxiliary booster for photoexcited electrons, can effectively reduce the photocatalyst’s band gap required for water splitting in the infrared region. Our first-principles calculations on a surface-functionalized hexagonal boron-nitride bilayer confirm the existence of such photocatalysts and verify the reaction mechanism.