Gan Nanotubes And Nanobelts Structural Stability And Electronic Properties From First Principles Of Research

Gallium Nitride (GaN), as the representation of the third semiconductor material, is an important wide band gap semiconductor material and is also one of the most advanced semiconductors in the world due to its excellent optical, electrical and physical properties. It can be applied to blue-green light-emitting devices, blue light lasers, ultraviolet detectors and high temperature/high power integrate circuit, which make them promising candidates as functional materials in microelectronics and photoelectron fields of application. With the rapid development of microelectronic and optoelectronic technology, the integrated degree is higher and higher and the size of the devices is smaller and smaller. Therefore, it is of great significance to fabricate nanodevices using nano-size materials with excellent and unique properties. The experiments and theories have proved that one-dimension GaN nanostructures can significantly improve the properties of the blue/green/ultraviolet optical and electrical devices. So one-dimension GaN materials are considered to be a kind of promising material.Although a lot of pioneering works have been done on the synthesis, doping, microstructure, and physical properties of the GaN nanostructures, it is still in the initial research stage. It is very difficult to explore the characteristics of its novel and reasonably use of its excellent performance. Therefore there is necessary to study one-dimensional GaN materials in detail. Up to now, GaN bulk structures and GaN films have been studied extensively. However, the investigation of the one-dimensional GaN nano-materials is at the initial stage. As typical one-dimensional nano-materials, GaN nanotubes (GaNNTs) and GaN nanoribbons (GaNNRs) possess the unique mechanical, magnetic and optical properties. In this thesis, the stability and the electronic properties as well as the magnetic properties of GaNNTs and GaNNRs have been investigated by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. The principal results are concluded as:(1) The study of transition-metal atoms adsorption on GaN nanotube. The geometric structures, stability, electronic and magnetic properties of the transition-metal atoms adsorption on GaN nanotube have been investigated by using the first-principles method. The most stable adsorption site for Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Pd, and Pt is the nitrogen site. In contrast, Ni prefers the hexagonal site. The transition-metal (TM) atoms with a small number of d electrons form strong bonds. Cu and Zn atoms with fully filled 3d electrons of 10 have relatively lower binding energies. Most TM atoms can be chemically adsorbed on the surface of the GaNNT, the adsorption process is typically exothermic. The binding energy of Cr is negative, indicating the adsorption process is endothermal. The adsorption of TM atoms generally induces some impurity states within the band gap of the pristine GaNNT, except for adatom Pd which has filled 4d orbitals with 10 electrons. The results of projected density of states (PDOS) show that these impurity states are mainly due to the d and s electrons of the TM atoms. For Cu-adsorbed (8,0) GaNNT, only one type of bands (minority spin) crosses the Fermi level implying the Cu-adsorbed (8,0) GaNNT is half-metal and can be used in the field of spintronics for producing nearly 100%. spin polarized currents. The amount of charge transfer from the TM atom to the (8,0) GaNNT varies with the metal and is primarily located on the nearest N atom with a larger electronegativity. The Sc-, V-, Cr-, Mn-, Fe-, Co- and Cu- adsorbed (8,0) GaNNT systems have a net magnetic moment from 0.60μB to 4.98μB. These net magnetic moments are located mainly on the TM atoms.(2) The study of hydrogen atoms adsorption on GaN nanotube. The geometric structures, stability, electronic and magnetic properties of the hydrogen atoms adsorption on GaN nanotube have been investigated by using the first-principles method. Hydrogen atom adsorbed on the top site of gallium introduces an impurity band near the Fermi level; the impurity band corresponds to the acceptor energy level. Hydrogen atom adsorbed on the top site of nitrogen introduces an impurity band near the Fermi level; the impurity band corresponds to the donor energy level. There are the impurity bands inside the nanotube band gap for a carbon atom substitution for a gallium atom or a nitrogen atom. The impurity bands disappear upon the adsorption of a hydrogen atom on the carbon-doped GaNNT. Compared with pristine (8,0) GaNNT, carbon-doping enhances the binding energy of a hydrogen atom adsorbed on the similar site. The energetically and dynamically favorable site is top site of the carbon atom in carbon-doped GaNNT. The results show that the adsorption of a hydrogen atom on the carbon atom of the carbon-doped GaNNT almost compensates the doping effect of carbon atom in carbon-doped GaNNT.(3) The study of 3d transition metal-doped GaN nanotubes. The geometric structures, stability, electronic and magnetic properties of the 3d transition metal-doped GaN nanotubes have been investigated by using the first-principles method. The calculated local atomic bond lengths around 3d TM impurity indicate that all the N-TM bond lengths in 3d TM-doped (5,5) and (8,0) GaNNTs are shorter than those of N-Ga in pristine (5,5) and (8,0) GaNNTs, respectively. The N-Ga bond lengths around 3d TM impurity in 3d TM-doped (5,5) and (8,0) GaNNTs are different from the N-Ga bond lengths in pristine (5,5) and (8,0) GaNNTs, respectively. Obviously, there are distortion around 3d TM impurity with respect to the pristine GaNNTs for both 3d TM-doped (5,5) and (8,0) GaNNTs. The trends in the variation of the total magnetic moments versus the atomic numbers are similar to the 3d TM-doped (5,5) and (8,0) GaNNTs. The magnetic moment distribution reaches its maximum and then the trend decreases with the increasing atomic number. The change of total magnetic moment follows Hund's rule for 3d TM-doped (5,5) and (8,0) GaNNTs, respectively. The analyses of total density of states indicate Cr-, Mn-, Fe- and Ni-doped (5,5) GaNNT as well as Cr-, Mn-, Ni- and Cu-doped (8,0) GaNNT are all half-metals and magnetic with 100% spin polarization and seem to be good candidates as The diluted magnetic semiconductors for spintronic applications.(4) The study of GaN nanotubes filled with nickel nanowires. The geometric structures, stability, electronic and magnetic properties of the GaN nanotubes filled with nickel nanowires have been investigated by using the first-principles method. For both Ni5@(8,8) and Ni9@(8,8) systems, the initial shapes are preserved without any visible changes after optimization, while for the Ni13@(8,8) system not only a quadraticlike cross-section shape is formed for the nanotube but also an anticlockwise rotation about common axis is taken place for the nanotube with respect to the nanowire. From minimization of the formation energy, the Ni13@(8,8) system is most easily formative. The negative formation energies are obtained for all three Nin@(8,8) systems implying the formation processes of these systems are all exothermic. All three Nin@(8,8) systems have metallic character, there are more bands crossing the Fermi level for the minority spin than those of the majority spin. The magnetic moments analyses show that no magnetization is found on the N and Ga atoms. For both the free-standing Nin nanowires and Nin@(8,8) systems, the magnetic moment increases with decreasing the number n of the Ni atoms in per unit cell. Especially for thin nanowires encapsulated into (8,8) GaNNTs due to very weak influence of outer nanotubes leading to their magnetic moments close to those of the frees-standing nanowires. The charge density analyses show that the spin polarization and the magnetic moment of Nin@(8,8) systems come solely from the Ni nanowires. Especially for the Ni5@(8,8) and Ni9@(8,8) systems, imply the Nin@(8,8) systems can be applied to the circuits that demand preferential transport of electrons with a specific spin.(5) The study of GaN nanoribbons. The geometric structures, stability, electronic and magnetic properties of the perfect and defect GaN nanoribbons have been investigated by using the first-principles method. The ZGaNNRs have an indirect gap, whereas AGaNNRs have a direct gap. As the nanoribbon widths increase both band gaps of ZGaNNRs and AGaNNRs decrease gradually and become close to their asymptotic limit of a single layer of GaN sheet. The structural and electronic properties of GaNNRs are shown to be tunable by an external transverse electric field. When a positive electric field is applied, the band gap of the 6-ZGaNNR decreases with increasing the strength of electric filed and is eventually closed at a strength of electric field around 7.5 eV/A. The formation of either N or Ga vacancy in 8-ZGaNNR is endothermic. Furthermore, at each equivalent geometrical site, the formation of the N vacancy is easier than that of the Ga vacancy. An inward relaxation of the three nearest Ga atoms around N vacancy occurs while for the three nearest neighbor N atoms around the Ga vacancy, an outward relaxation occurs. One readily identifies the asymmetry in the proximity of the Fermi level between the minority spin and the majority spin for 8-ZGaNNR with or defect (i= 1,4 and 7), indicating a spin polarization character. The magnetic moments of defects are dependent on defect position, while for the defects, these properties are less dependent on the defect position. Furthermore, the magnetic moments of the 8-ZGaNNR with are larger than those of the 8-ZGaNNR with (i= 1-7).