First Principles Studies On The Effects Of Impurities And Electric Field Of Low Dimensional BN Nanomaterials

Density functional theory (DFT) is a quantum mechanics method for studying electronic structures of many bodies. DFT has wide range of applications in physics and chemistry, especially for the properties of molecular and condensed matter materials, which is a commonly used method in the fields of condensed matter physics and computational chemistry. Using DFT to solve the Schrodinger equation has guiding significance for predicting and synthesizing materials, and this is also named first principles method. Using first principles method for solving Schrodinger equation, there is no any empirical parameters, only need nuclear charge numbers and some environmental parameters. With the rapid development of computer hardware and large clusters, the calculating accuracy is improved quickly, thus the results calculated by first principles are comparable with the results obtained by experiments in many aspects, and the computational method also can obtain some results which are difficult to acquire for experiments. Generally speaking, the computational method based on DFT maybe is not perfect, while it is indeed a powerful method.It is well known that nanomaterials have many special and attractive performances, such as optical, electrical, magnetic, thermal, mechanical, etc. Nanotechnology has penetrated into various fields of materials rapidly and attracts the interesting around the world. Nanotechnology is still in the initial stage, but from the obtained properties, the rapid development of nanotechnology has profound impact on economic and social development in the 21th century. While some nanomaterials have high requirements on method and technology for synthesizing, thus the researching and predicting properties of nanomaterials using the first principles calculations is very effective. In this paper, we focus on the low-dimensional BN nanomaterials to study, including nanotubes, nanowires and two-dimensional nanosheets structures. There are two main research topics:(1) predicting nano-scale half metallic ferromagnetic materials and (2) tuning the electronic structures by applying external electric field, stress and doping.In chapter 1, the development and application of nanomaterials are described firstly. The half metallic ferromagnets due to spin up electrons and spin down electrons have different nature around the Fermi level, and have special applications. Then the development and types of half metallic ferromagnets are introduced briefly. The introduction for several low dimensional nanomaterials based on BN are included (such as nanotube, nanowire and nanosheet) in this chapter.In chapter 2, we have introduced the basic concept of DFT and reviewed its recent progress. In electron density based DFT, all ground state properties can be derived from the electron density. The functions and features of the packages used in the current work are introduced briefly (CASTEP and DMol3).In chapter 3, the geometric structure, electronic structure, and ferromagnetism of transition metal (TM) doped BN(5,5) are investigated by the first-principles polarized spin calculations within generalized gradient approximating (GGA). The optimized structure shows that the TM atom moves outwards and the hexagonal rings which include the V atom have serious distortion with respect to the pure nanotube. When doping with the TM atom, the peaks of DOS shift to lower energy levels and there is significant spin splitting for TM 3d, B 2p, and N 2p electrons around the Fermi level. There is a peak that locating at the Fermi level for the majority spin electronic band while there is a band gap for the minority spin electronic band, which indicates that V-, Cr-, and Mn-doped BN(5,5) nanotube show half-metallic behaviour. The calculated ferromagnetic moments are 2μB,3μB, and 4μB for V-, Cr-, and Mn-doped BN(5,5) respectively, and the main part comes from the TM atom. The B atoms near the V atom have a little contribution to magnetic moment, while the N atoms provide a little negative magnetic moment.In chapter 4, we focus on the properties of BN nanowires and the effect of vanadium doping. The total energy dependence on the relaxation of periodic length along wire axis was tested firstly, and we found the total energy has obvious change. At the minimal energy, the periodic lengths of nanowires are 0.427 nm, which is almost equivalent with that of bulk structure. The optimized structure of pure nanowires indicate that B atoms move inwards whereas N atoms move outwards, and the electronic structures exhibits that BN nanowires with larger diameters have a constant band gap about 4.08 eV. These features are similar with those of BN nanotubes. The stability of nanowires has been discussed and we found that the pure nanowires become more and more stable with increasing diameter. The optimized structures show that V atoms move outwards and the hexagonal rings which include V atom have serious distortion with respect to pure nanowire. For pair V-doped BN nanowires, the ferromagnetism (FM) and antiferromagnetism (AFM) configurations are performed, and the total energies indicate that FM state is more stable than AFM state. The electronic structures reveal that both pair and single V doped BN nanowires show half-metallic behaviour. The calculated ferromagnetic moments are 4μB for pair V atoms doped BN nanowires and 2μB for single V doping. The main part of magnetic moment comes from V atom, and B atoms near V atom have a little contribution, while N atoms provide a little negative magnetic moment.In chapter 5, we have performed first principles calculations on BN nanotubes (BNNTs) under strain and electric field along tube axis. The band gap of isolated BNNTs increases under compressive strain while decreases under tensile strain. The atomic charges in every layer and interlayer distances change uniformly under strain. With increasing electric field, the conduction band minimum (CBM) splits because of the Stark effect and downshifts rapidly, the valence band maximum (VBM) has no shift, thus the band gap decreases strongly for both armchair and zigzag nanotubes. In the stage, the band gap of armchair nanotube transits from indirect to direct, however, it is always direct for zigzag nanotube. The charge population indicates that the atomic charge changes dramatically in the middle layers, which even result in that some B (N) atoms show negative (positive) charge. Under electric field, the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unocupied Molecular Orbital) distribute separately along tube axis which has a key implication that it can be realized in a pure material PN junction without doping. In chapter 6, we studied the electronic structures of single layer and bilayer BN nanosheets, and the effects of doping, applying strain and external electric field are also studied. For the single layer BN nanosheet, the band gap increases linearly with compressing the two periodic directions, but with 10% strain, the gap changes is not very obvious. The effects for geometric structure and electronic structure by doping with C are obvious. The C atom shifts outwards and the electronic structure indicates halfmetallicity. The pair C atoms doped BN sheets have been studied. We found that the half-metallic ferromagnet can be obtained by controling the positions of pair C dopants. For the bilayer BN sheets, we considered Bernal stacked and Hexagonal stacked styles. The band structures show the bilayer BN nanosheets are direct band gap by complete relaxing, and the gap decreases with decreasing the interlayer space. In this stage, the band gaps of Bernal stacked BN nanosheets become indirect with decreasing the interlayer space while is always direct for hexagonal stacked, and the gaps for both stacked sheets become saturated with increasing the interlayer space. The electric field is applied along the z axis which is vertical to the sheet. The electric field leads the electrons transferring from the underlayer to upperlayer, thus, the B atoms of upperlayer (N atoms of underlayer) present negative charges (positive charges). When the electric field is weak, the gaps of BN nanosheets have minor change, while the gaps increase quickly with the increasing of the electric field.