First-principles Calculation Of Low Dimensional Hexagonal Boron Nitride

With unique optical, electrical and magnetic properties, as well as great potential value in microelectronics, hexagonal boron nitride(h-BN) has become the research hotspot of materials field. By means of Materials Studio and first-principles based on density functional theory, optical, electrical and magnetic properties of h-BN with vacancy defects or substitutional doped defects, and boron nitride nanoribbon with different boundary conditions are studied to provide guidance for the experiment in this paper. By analyzing band structure, density of states, electron orbits and optical properties, following conclusions are obtained:1. When there is a vacancy in the monolayer h-BN, B vacancy(VB) compared with N vacancy(VN) is formed more easily. Total magnetic moments of VB and VN are 2.56μB and 1.18μB respectively. Different properties of atoms at vacancy lead to different magnetic: N maintains sp2 hybrid orbits, electronics are not mutually paired; while B comes closer to each other, making electrons of sp2 hybrid orbits jump to Pz orbits and form a π bond resulting in the pairing of electrons and decreasing of magnetic. Diatomic vacancies with vacancies adjacent have a σ bond of B-B and its magnetic is provided by N; when vacancies are separated, then electronic exchange occurs between them leading to its magnetic moments to be 2.03μB. In terms of optical properties, B vacancy enhances the utilization of deep ultraviolet light while N vacancy is responsible for the increasing of absorption rate in visible light band.2. When h-BN is substitutional doped by C, the most stable way is to dope N as well as its adjacent B with C. Since C has four valence electrons, single C doped system will generate spin polarization and the spin electron will stay in C-Pz orbit. If the number of impurity atoms is upgraded to 2, same sites doping will increase the magnetic moments, while diverse sites doping systems have no magnetism due to the movement of electrons between carbons. By contrast with Si, Ge-doped h-BN, it can be found that under the same circumstances, doping site B can gain lower formation energy and better stability. It can also be found that, due to the lattice distortion, there are more impurity levels in the band gap of N-substituted systems, and the change of electrical structure is relevant to deformation degree.3. After doping B site of h-BN with 3d transition metal(TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni), the system exhibits large magnetic moments. The magnetism alters parabolicly with valence electrons of TM increasing and reaches 5μB as maximum when doped by Fe. Density of states of impurity atoms and charge distribution can prove that magnetism of TM doped h-BN is relevant with the electrons transfer caused by the iconicity of TM-N.4. By studying the effect of width and boundary condition of boron nitride nanoribbon(BNNR) on its electronic and optical properties, several conclusions are found: Armchair boron nitride nanoribbon(ABNNR) and zigzag boron nitride nanoribbon(ZBNNR) are direct and indirect band gap semiconductors separately, and band gaps of ZBNNR are inversely proportional to its width. Calculation results show that different boundary leads to a large difference between the optical properties of two types of BNNRs. When the width of ZBNNR reaches 7 or ABNNR reaches 8, optical coefficient will change extraordinarily. That means by altering boundary condition and width, electronic structure and optical properties of BNNR can be controlled.