Theoretical Properties’ Regulation Of Low-dimensional Nanomaterials

Nanomaterials are those in which the size of the geometry in at least one dimension is between 0.1nm～100nm. Due to the size effect and surface effect, nanomaterials often show unique and excellent properties, rendering vigorous development in many fields. The application of nanomaterials extends all over physics, chemistry and materials science, specifically including magnetic nanomaterials, semiconducting nanomaterials, sensors, catalysis, medical science, environmental science and mechanical engineering, etc. In a word, the technology of nanomaterials becomes one of the important driving forces for the development of economic and society in the future.Low-dimensional material is the material of which the dimension is less than three. Due to the limited size, the quantum effect of the system becomes more obvious and some peculiar physical phenomena can be achieved. Because the structures of low-dimensional nanomaterials can be well understood and the properties are usually adjustable, the low-dimensional nanomaterials are expected to be the most promising candidates in the field of energy conversion, spintronic devices and so on. Additionally, how to well control the performance of low-dimensional nanomaterials also becomes one of the important issues in nanoscience and nanotechnology.With the development of technology, the experimental researches on the low-dimensional nanomaterials have made many breakthroughs. However, the structures and the properties of low-dimensional materials highly depend on the quantum effect and the coupling with the outer field, so the preparation and the controlling of low-dimensional nanomaterials in experiment greatly challenge us. Recently, theoretical simulations are widely used to explore the structures and properties of nanomaterials. And they always provide reliable results. So basing on the theoretical calculations, we want to investigate the behaviors of low-dimensional nanomaterials and their tunable properties. The thesis here consists of four chapters. They can be divided into two parts. Part one is the theoretical background, and part two is the projects that we have performed.In the first chapter, we introduce the background of theoretical simulation. First of all, we introduce the theoretical basis of quantum chemisty, including Born-Oppenheimer approximation, one-electron approximation and Hatree-Fock equation. Then we introduce the theoretical framework of density functional theory and its development and applications, including Thomas-Fermi-Dirac model, Hohenberg-Kohn theorem, Kohn-Sham equation, exchange correlation energy functional, as well as the commonly used calculation software basing on first-principle calculation. Finally, we introduce the NEB (Elastic Band Method Nudged) method, which is used to search the minimum energy path and transition state in the chemical reaction or phase transition process,hi the second chapter, the influence of defects on the structure and properties of low-dimensional nanomaterials is introduced. The defects is inevitable in the formation and preparation of materials, which can make an important effect on the properties of the material, including mechanical properties, thermal properties, electrical properties, optical properties and magnetic properties. Understanding and controling of defects at atomic level have become one of the important research fields of materials science.In the first project, we perform a study of the effects of line defects in monolayer boron-nitride nanosheets, nanoribbons, and BN nanotubes basing on first-principles calculations and Born-Oppenheimer quantum MD simulation. We show that certain line defects can give rise to tailor-made edges on BN nanosheets (or imperfect nanotube), so that can significantly reduce the band gap of systems. Particularly, we find that the zigzag BN nanoribbons with chemically homogeneous edges can be achieved by introducing a B2, N2, or C2 pentagon-octagon-pentagon line defect or through the antisite line defect. The LD-zBNNRs with B-terminated edges are predicted to be antiferromagnetic semiconductors, whereas the LD-zBNNRs with N-terminated edges are metallic with degenerated antiferromagnetic and ferromagnetic states. Additionally, we find that the hydrogen-passivated LD-zBNNRs are nonmagnetic semiconductors with markedly reduced band gap. The band gap reduction is attributed to the impurity states. Potential applications of line-defect-embedded BN nanomaterials include nanoelectronic and spintronic devices. In the second project, twelve types of line-defect structures in single crystalline phosphorene are examined. These line defects are formed via migration and aggregation of intrinsic point defects. Our calculations demonstrate that the migration of point defects in phosphorene is anisotropic, for example, the lowest migration energy barriers are 1.39 and 2.58 for Stone-Wales defect in zigzag and armchair direction, respectively. Furthermore, the aggregation of point defects into line is energetically favorable compared with their separated ones. In particular, the axis of line defects in phosphorene is direction-selective which depends on the composed point defects. The presence of line defects can effectively modulate the electronic properties of phosphorene, leading to the defect-containing phosphorene either semiconducting with tunable band gap or metallic. With particular interest is that the SV-based line defect behaves as a metallic wire. It means that it is possible to fabricate circuit with subnanometer widths in phosphorene for nanoscale electronic application.In the third chapter, the influence of strain on the structure and properties of low dimensional nanomaterials is introduced. Using VS2 nanosheet as an example, we explore the strain on it. We find that not only the biaxial strain but also the vertical strain can adjust VS2 nanosheet from semiconductor to metallic material.In the fourth chapter, the influence of chemical modification on low-dimensional nanomaterials is introduced. This chapter consists of two works. In the first work, we study the magnetic properties of layered g-C3N4 with hydrogen adsorption. The experiment and the calculated results both demonstrate that this system exhibits ferromagnetic properties. In the second work, we explore the layered WS2 in an ammonia atmosphere. We find that the ammonia-intercalated WS2 is an ionic compound in the formula of [NH4+][WS2]x-. Different from the ideal phase of WS2 sheets, the WS2 layers with NH4+ undergoes an H-T phase transition, and the most stable configuration of it becomes a zigzag-chain phase, in which the W atoms lie in a distorted octahedral coordination field. The calculated density of states show that this kind of compound exhibits metallic behavior that is in accordance with the experimental results.