First-Principles Calculation And Design Of Some 2D Materials

Material plays a very important role in our daily life, and it also promotes the progress of human civilization. Human civilization has experienced from the stone age, the bronze age and the iron age. These developments can not process without the devel-opment and improvement of materials. Now, the discovery of semiconductor materials, as well as large-scale investment in production and application, so that human society has entered a more advanced microelectronics era, greatly enriched and improved the human production and life. However, due to the material in the nature is very limited, it has long been unable to meet the growing demand for materials. This makes that the human must design new types of materials with specific functions, based on the existing materials through the science and technology we have already handled. However, if we use the conventional experimental means, a simple combination of existing materials, no doubt need to invest a lot of manpower, material resources and financial resources, this is a very unrealistic approach, but also relatively low efficiency.On the other hand, along with the development and improvement of quantum me-chanics, quantum chemistry, with high performance computing devices, we can use numerical method for solving the Schrodinger equations of some complex systems, and theoretically obtain some physical and chemical properties. In this way, we can make a preliminary screening of the materials, and then processing the experimental verifi-cation, which will greatly improve the efficiency of the experimental work, and save unnecessary expenses. The purpose of this thesis is introduced through using the first principles calculations, conducts the research to the nowadays very popular new ma-terial, the two-dimensional materials, and we hope that through the improvement and design of these materials we can obtain the materials we need. Through the study of these functional materials, we hope that can solve the energy shortage, the improvement of electronic devices and so on. This thesis consists of six chapters as follows.In the first chapter, we briefly introduce the electron density functional theory, which is the computational quantum chemistry. Which contains the commonly used Schrodinger equation, adiabatic, single electron approximation, Hohenberg-Kohn the-orem, Kohn-Sham equation, and the use of various exchange correlation functionals. Based on the density functional theory established by Kohn, the electron density of the ground state of the system as a variable can describe any property of a system, that is, all the properties of the system are the function of the electron density. For a system with many body interactions, the problem of solving this system is transformed into the search for appropriate exchange correlation energy, which makes the many body prob-lem become an effective single particle problem. So the main direction of the develop- ment of density functional theory is to find a suitable exchange correlation functional to describe the relevant system. At last, the paper briefly introduces several commonly used computational software package based on density functional theory.The second chapter describes some of the current very popular two-dimensional nano materials. Among them, the graphene is a single layer of two-dimensional crys-tal composed of carbon atoms, can be stripped from the graphite material. Because graphene has many unique properties, such as the dispersion relation near the Fermi distribution is linear, that is, there is the massless Dirac particles. In addition, it also has very special anomalous quantum Hall effect, and many other excellent physical and chemical properties, attracting a large number of theoretical and experimental research. In addition, its derivatives, such as graphene nano ribbons also have a lot of very special properties, such as a strong quantum confinement effect, also attracted the attention of many researchers. We also introduce the synthesis and the basic physical and chemi-cal properties of the hexagonal boron nitride; transition metal sulfides present research status, and in practical application; black phosphorus as the newly discovered a two-dimensional material, once found, caused great sensation, known as the band gap of the most appropriate two-dimensional material, more than the trend of graphene, we also briefly on its nature and characteristics are introduced.In the third chapter, a new kind of two-dimensional material - Germanane, GeH was studied. Germanane is a new kind of two-dimensional material, which has a very good physical and chemical properties, and has great potential in practical applications. Experimental and theoretical work show that the material has 1.56 eV direct band gap, indicating that the photolysis of water can be used; on the other hand, the carrier mobility rate is 5 times of the bulk germanium material that it can be developed in the electronic devices. And we hope that the material can be chemically modified, such as the method of substitution, to continue to improve the properties of this material, so that it has a better application prospects. Usually, the fluorine doping or replace are the common methods, so here is what we choose to fluorine on our system to replace the different proportion of doping, and the doping positions may affect the properties of the system, too. So we calculate the structures by the first principles, considering the different concentration and substitution positions, the properties of the electronic structure of the material, such as the main band width, energy level position and so on, are calculated. Through these properties, we can improve the properties of this material, get the material that is suitable for the water splitting in theory, and provide a feasible solution for the experimental work.In the fourth chapter, we mainly introduce a method which developed by Tsinghua University, Prof. Zhigang Shuai, using first principles calculations with the Boltzmann transport equation and the relaxation time approximation theory to predict the carrier mobility of some carbon and organic materials. And it can give successful calculation of many materials, such as graphene monolayer, GNRs and so on. We mainly study their method, and apply it to the calculation of the two-dimensional materials we care about, and predict the carrier mobility of a new type of two-dimensional material C5N.In the fifth chapter, we introduced a kind of two layers of materials, which is based on the weak interaction named van der Waals (vdW) interaction. High precision mea-surement in nano-scale is a very valuable research topic. Almost all of the measurements are concentrated in the micro-scale, but now a lot of materials are already nano-scale, so we need to develop a new method to measure the nano-scale material. Conven-tional measurement methods are optical, piezoelectric and so on. These methods are more or less defective and inadequate, so we need to develop a new method to replace these conventional methods. So we designed a new method, based on the vdW in-teraction of nano-scale measurement method. We envision that constructing a bilayer two-dimensional materials through the vdW interaction, fixed one layer, and move the other layer, if during the moving process, the other properties of the material, for ex-ample the variation of the gap is great, so we can through the band gap variation to reflect this tiny bit shift, so as to achieve the measurement of nano-scale. Because, vd-W interaction is a very weak interaction, moving one layer, does not require a lot of energy or force, so it can also be used as a method of measuring the displacement of the micro-force displacement. Based on the above ideas, we design and search through the first principles, found that blue phosphorus is very suitable with our requirements for this kind of material. And we also compared with other two layers of two-dimensional materials, got a more general law, to find the material to meet the conditions.In the sixth chapter, we have found a kind of new two-dimensional material, which is a kind of metal, and has very good physical and chemical properties, so it may be used as a very good spin electronics device. First of all, low dimensional materials play a very important role in modern nanoscience and nanotechnology. Among them, the discovery and preparation of graphene as a milestone in the development of two-dimensional materials. In recent years, other two-dimensional materials have also made considerable progress, in which the silicon and germanium also showed a good physical and chemical properties similar to graphene. On the other hand, in recent years, there is a very significant development of the spin electronics, usually we use ferromagnetic materials as the main material of the spin electronics. Recently, antiferromagnetic spin electronics has attracted the attention of more and more researchers, mainly lies in the antiferromagnetic has a lot of ferromagnetic with excellent properties. These properties as long as we try to use, can exhibit the characteristics of ferromagnetic better compared to. Therefore, we hope to find some low dimensional (such as 2D materials), with excellent properties of antiferromagnetic material. Such a material, which can work under normal circumstances, is almost no. So we hope that through the first principles, with the global search method, to find a kind of material with these excellent properties. This is not only the need of theory, but also can provide the direction of synthesis for the experiment.In the seventh chapter, we study the magnetoresistance effect of graphene/WTe2 heterostructure. The magnetoresistance effect exists in some metals and semiconduc-tors, specifically refers to the resistance of these materials in the external magnetic field. If a material has a larger magnetoresistance effect, the external magnetic field corre-sponding is very obvious, the materials in electronics and magnetic field will likely have very important applications, for example, can be used as magnetic sensors, mag-netic storage devices and so on. Recently, the synthesis of a new crystal materials-two telluride tungsten (WTe2), this material has great magnetoresistance effect, far higher than other magnetoresistive materials found before, caused a great sensation. Here, we design a graphene/WTe2 composed of heterojunction structure, between the two monolayers together through van der Waals interaction. We calculated by the first principles, to study the geometry, the material properties and electronic structure, mag-netism of this heterostructure. We found this heterojunction materials, compared to the two-dimensional WTe2 and magnetoresistance effect is greatly improved; on the other hand, in comparison to pure graphene and the carrier concentration is increased, in-dicating that in practical applications, the conductivity may larger than pure graphene. Overall, the heterojunction properties in some areas is better than any single component, which also reached the purpose of our material design.