First-principles Studies Of Electronic And Mechanical Properties,and Crystal Structure Of Low-dimensional Materials

In the last few decades, both theoretical chemistry and computer science have undergone tremendous changes. In the field of theoretical chemistry, the theory of density functional theory makes it possible to simulate the properties of materials including mechanics, thermotics, optics, electrics and acoustics. In the computer science, with the development of computer science, especially due to the constantly updated and enhanced computer hardware, we can rely on large-scale parallel technology to rapidly calculate the properties of the complex system. The effective combination of the two, it can enable us to do screening, analysis materials in a short period of time, and thus reduce the costs for designing and synthesizing new materials,new catalysts from the theoretical level. Besides, a certain material can be designed by the use of reverse design method and with the help of high-throughput computing platform, which can provide guidance for the experiment. This work runs through the following three main lines: a) from one-dimensional (1D) systems to two-dimensional(2D) systems; b) from experimental systems to theoretical predictive systems; c) from the use of existing global optimization algorithms to design global optimization algorithm. At the same time, density functional theory is utilized for in-depth study of some low-dimensional materials, electronic structure, mechanical properties and potential applications. The main content is divided into the following six parts:The first chapter is the introduction of the first principle simulation and the global optimization method. Firstly, we give the introduction of the Hartree-Fock method which is the most basic in the calculation of the first principles simulation, and explain the advantages and demerits of the Hatree-Fock method. Secondly, we introduce the most widely used density functional theory (DFT) in the current first-principles calculation, including the core concepts of Honbeng-Kohn theorem, Kohn-Sham equation, exchange-correlation functional, basis set and pseudopotential, as well as software implementation of density functional theory using different basis set. We then briefly explain the basic idea of the global optimization algorithm and the widely used software package for crystal structure prediction. In the last part of this chapter, we briefly review the software used in this article.In the second chapter, we study the electronic structure, optical and mechanical properties of one-dimensional diamond nanowire (CNW) encapsulated in carbon nanotubes (CNW) (labeled as CNW@CNT). We firstly discuss the stability of CNW@CNTs formed by CNT with different sizes of radius, then study the interaction between CNT and CNW. After that, we analyze the basic electronic structure properties of CNW@(8,8)-CNT and strain engineering under the uniaxial stress. In terms of optical properties, we describe the absorption characteristics of CNW@(8,8)-CNT and the effect of nearly-free electron (NFE) state on the internal CNW. As for mechanical properties, we use the uniform definition to analyze the Young’s modulus of the 1D nanometer systems and propose to use the parallel spring model to correlate the Young’s modulus of CNT, CNW and CNW@CNT. At the end of this section, we propose the potential applications of CNW@CNT for designing nano-devices, such as UV photodetectors and etc.In the third chapter, we mainly focus on the study of prevalent 2D phosphene material.As a basic mechanical parameter, Poisson’s ratio (v) is used to measure the response of solids to external loads. Most materials have positive Poisson’s ratio. However, in some special cases, the material has a negative Poisson’s ratio (NPR), showing the so-called auxetic effect. In recent years, more and more researchers have begun to pay attention to the low-dimensional systems with auxetic effect, especially 2D materials with high NPR. In this work, based on first principles calculations, we restudy the previously proposed δ-phosphene (8-P). Firstly, the mechanical properties of δ-P are discussed. It is found that δ-P is a flexible material and shows strong in-plane auxetic effect.Secondly, we analyze the origin of its auxetic effect and put forward the idea of searching for new materials in the future. Finally, we analyze the effect of quantum size effect on the mechanical properties and the electronic structure properties of 8-P. If 8-P can be synthesized, these extraordinary properties would endow it with great potential application in designing low dimensional electromechanical devices.In the fourth chapter, we predict a new allotrope of 2D phosphorene by using particle swarm optimization (PSO) algorithm. Although some allotropes of phosphorene have been proposed, all of those structures are built from chemical or physical intuition,which is incomprehensive and biased. In this work, we perform an unbiased state-of-the-art theoretical crystal structure search on the basis of PSO, then a new allotrope of phosphorene named Ψ-phosphorene (Ψ-P) is obtained. According to our first-principles calculation, Ψ-P is predicted to have potential applications in gas purification membrane and solar cells. In order to guide future experimental work, we also propose the possible substrate for synthesizing Ψ-P monolayer.In the fifth chapter, we try to use the cuttlefish (CF) algorithm to predict 2D materials.It is found that all experiments give no clear structure information of layered B-P binary compounds. In theoretical aspect, all the studies are prone to use the graphitic structure of boron-phosphide (g-BP) as the most stable phase, which may be wrong in reality.What’s more,it is also not consistent with the use of high concentrations of phosphorous sources during the experimental synthesis. Therefore, we design CF algorithm to predict the 2D boron-phosphorus compounds with high concentrations of phosphorous.Based on CF algorithm, we put forward a stable 2D phase of boron phosphide with 1:5 stoichiometry, i.e. boron pentaphosphide (BP5) monolayer, which has a lower formation energy than that of the commonly believed graphitic phase g-BP. The results show that BP5 monolayer has multiferroics,negative Poisson’s ratio, good anisotropic optical absorption and high carrier mobility. At the end of the work, we discuss the potential applications of BP5 monolayer in designing nonvolatile memory devices.In chapter six, we focus on the implementation of the cuttlefish (CF) algorithm in crystal structure prediction. As discussed in chapter five, our MATLAB version implementation of CF algorithm has successfully predicted 2D BP5 monolayer, and the results are confirmed by the PSO algorithm. However, the initial version of CF has a poor performance. Thus, we revise the CF algorithm and propose an improved cuttlefish(ICF) algorithm. The ICF algorithm has implemented in pychmia software by using the object-oriented method. Finally, we use the Lennard Jones cluster, 3D SiC, Si and bulk phosphorus to test the reliability of ICF in structural prediction.