First-principles studies on stabilities and electronic properties of AlN nanostructures

Under the generalized gradient approximation (GGA), the stabilities and electronic properties of semiconductor AlN nanostructures have been investigated by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. The single-walled faceted AlNNTs present an obvious structural modification. The larger the diameter, the more stable the nanowire, and the wires without internal surfaces are more stable than the multiwalled tubes or the SWNT. Therefore, the large-size nanowires are easier to be synthesized than the corresponding multiwalled tubes or single-walled nanotube in experiment. The dangling bonds of surface atoms cause the “localized edge-induced states”. These two nanostructures C and F are still wide band gap semiconductors accompanied by a few surface states located in the band gap of bulk AlN and thus extremely suitable for application in flexible pulse wave sensors, nanomechanical resonators and light-emitting diodes.     

First-principles study of ferromagnetism in Ti-doped ZnO with oxygen vacancy

Using first-principles density functional calculations, we have investigated the electronic structures of Ti-doped ZnO (Ti is in 4+ oxidation state) with and without oxygen vacancy. The ferromagnetic property is identified in the presence of oxygen vacancy despite Ti being nonmagnetic in its natural phase. The ferromagnetism originates from the charge transferring from donor derived-defect band to unoccupied Ti-3d states and the hybridization between Ti-3d and O-2p band near the Fermi level. On increasing the oxygen vacancy concentration, a transition from a long-ranged magnetic order to a short-ranged interaction is found and the oxygen vacancies prefer to distribute non-uniformly in Ti-doped ZnO.     

Size effect of half-metallic properties of BN/C hybrid nanoribbons

Based on the first-principle calculations performed by Vienna Ab initio simulation package (VASP), we report the size limitation of half-metallic properties in hybrid zigzag BCN nanoribbons. Both boron–carbon (B–C) and nitrogen–carbon (N–C) interfacial hybrid zigzag BCN nanoribbons are considered. We find that all hybrid systems establish antiferromagnetic ground states. Moreover their electronic properties are mainly determined by the carbon rather than boron nitride segments. Transitions between semiconductor, half-metal and metal can be realized in both systems as the width of the carbon segment increases. However, the half-metallic property can only exist in the systems for which the zigzag carbon chain is less than 6 and 9 for B–C and N–C interfacial systems, respectively. As long as the carbon segment is wider than the above sizes, the systems behave as metals. This effect derives from the electron or hole doping of carbon on the BN segment.     

NO在氧预吸附Ir(100)表面吸附和解离的第一性原理研究

采用第一性原理密度泛函理论和周期性平板模型研究了NO在O预吸附Ir(100)表面的吸附和解离, 并考察了预吸附的O对可能产物N₂, N₂O和NO₂的选择性的影响. 优化得到反应过程中初态、 过渡态和末态的吸附构型, 并获得反应的势能面信息. 计算结果表明, NO在O预吸附表面最稳定的吸附位是桥位, 其次是顶位. 桥位和顶位的NO在表面存在两条解离通道, 即直接解离通道和由桥位和顶位扩散到平行空位, 继而发生N-O键断裂生成N原子和O原子的解离通道. 此分离机理与洁净表面上NO解离机理相同, 但后一种解离方式优于前一种, 是NO在表面上解离的主要通道. 预吸附的O原子在不同程度上抑制了NO的解离, 导致桥位和顶位NO解离互相竞争. 在O预吸附Ir(100)表面, N₂气是唯一的产物, 不会有副产物N₂O和NO₂的生成, 与实验结果一致. 预吸附的O在N/O低覆盖度下几乎不影响N₂气的生成, 但在较高覆盖度下则促进了N₂气的生成.     

Electronic structure of p-type (Ga,Fe)N diluted magnetic semiconductors

By ab-initio calculation we show that the (Ga,Fe)N ground state may be changed from anti-ferromagnetic to ferromagnetic by acceptor defect like Ga vacancies. The electronic structures are calculated by using the Korringa-Kohn-Rostoker (KKR) method combined with coherent potential approximation (CPA). We show that we can increase the magnetic moment of Fe in p-type GaN by oxygen co-doping. Mechanism of exchange interactions between magnetic ions in p-type (Ga,Fe)N is also studied. The effect of external magnetic field on the electronic structure of (Ga, Fe)N and p-type (Ga, Fe)N is investigated.     

First-principles identification of two- and four-membered-ring hybrid structures of silica nanorings

We performed first-principles calculations to study the energetics, IR spectra, and electronic structures of silica nanorings (NR) consisting of two- and four-membered ring (2-4MR) units. A comparison study of other silica clusters, such as nanochains (NC) and nanorings formed by two-membered rings (2MRs) was made. The results show that for small-size (SiO₂)_n clusters with n<24, the nanochains composed of 2-4MRs (2-4MR-NCs) are more stable than other kinds of NRs and NCs. When n>24 the 2-4MR-NRs structures become energetically favorable. 2-4MR-NRs have the narrowest HOMO–LUMO gaps which increase with increasing cluster size, distinctive IR spectra characterized by several peaks at the 1000–1150 cm⁻¹ region.     

First-principles studies of Al-Ni intermetallic compounds

The structural properties, heats of formation, elastic properties, and electronic structures of Al-Ni intermetallic compounds are analyzed here in detail by using density functional theory. Higher calculated absolute values of heats of formation indicate a very strong chemical interaction between Al and Ni for all Al-Ni intermetallic compounds. According to the computational single crystal elastic constants, all the Al-Ni intermetallic compounds considered here are mechanically stable. The polycrystalline elastic modulus and Poisson's ratio have been deduced by using Voigt, Reuss, and Hill (VRH) approximations, and the calculated ratio of shear modulus to bulk modulus indicated that AlNi, Al₃Ni, AlNi₃ and Al₃Ni₅ compounds are ductile materials, but Al₄Ni₃ and Al₃Ni₂ are brittle materials. With increasing Ni concentration, the bulk modulus of Al-Ni intermetallic compounds increases in a linear manner. The electronic energy band structures confirm that all Al-Ni intermetallic compounds are conductors.     

Surface structure for Au on (0 0 1) SrTiO3

First-principle calculations are performed to study the crystal structure, formation energies, and electronic structures of (0 0 1) SrTiO₃ surfaces with/without Au covered. The initiative Au additive layer is crystallized in a fcc structure with (0 0 1) face on SrO-terminated surface. The bimodal growth trend of Au on TiO₂-terminated surface is qualitatively consistent with the experimental observations. The defect structure of Au occupying the oxygen (O) vacancies of TiO₂-terminated surface is energetically favorable under oxygen-poor conditions, and a feature corresponding to gap states appear and the occupied Ti 3d states disappear.     

Cooperative activation effect on H2O adsorption in MnO–Co catalyzed steam methane reforming

Steam methane reforming is a very important chemical process in hydrogen production and solid oxide fuel cells (SOFCs). Cobalt (Co) is an important catalyst for dry and steam methane reforming. However, previous studies have confirmed that metal Co surfaces only have weak adsorption activity for H₂O, which is evidently unfavorable for steam reforming. In this work we used first-principles simulations to study the activity of MnO–Co catalysts for the adsorption of H₂O. Compared with the Co (111) surface and pristine Co clusters, the MnO–Co catalytic layer has a stronger adsorption capability for H₂O because of the introduction of the MnO substrate, which is crucial for improving the steam reforming reaction and inhibiting carbon disposition in SOFCs. The cooperation mechanism between MnO and Co is discussed based on the analysis of electronic structures. The conclusions from this work are universal for other metal-oxide composite catalyst layers.