Control of ferromagnetism via electron doping in In2O3:Cr

Carrier-induced ferromagnetism in wide-gap transparent conductive oxides has been widely discussed and debated, leading to confusion and skepticism regarding whether dilute magnetic oxides exist at all. We show from density-functional calculations within a band-gap corrected approach that ferromagnetic Cr-Cr coupling can be switched on and off via electron doping in the wide-gap transparent n-type conductive oxide In2O3. We show that (i) Cr does not produce in In2O3 any free electrons and renders the system an insulating paramagnet. (ii) Extrinsic n-type doping of In2O3:Cr via Sn produces free electrons, whose concentration is controllable via the oxygen partial pressure. Such additional carriers stabilize a strong long-range Cr-Cr ferromagnetic coupling.     

Ultrathin oxides: bulk-oxide-like model surfaces or unique films?

To better understand the electronic and chemical properties of wide-gap oxide surfaces at the atomic scale, experimental work has focused on epitaxial films on metal substrates. Recent findings show that these films are considerably thinner than previously thought. This raises doubts about the transferability of the results to surface properties of thicker films and bulk crystals. By means of density-functional theory and approximate GW corrections for the electronic spectra we demonstrate for three characteristic wide-gap oxides (silica, alumina, and hafnia) the influence of the substrate and highlight critical differences between the ultrathin films and surfaces of bulk materials. Our results imply that monolayer-thin oxide films have rather unique properties.     

Strain Effects on Electronic Properties of Boron Nitride Nanoribbons

We investigate the strain effects on the electronic properties of boron nitride nanoribbons （BNNFts） by using firstprinciples calculations. The results show that the energy gap of BNNRs with both armchair edges （A-BNNRs） and zigzag edges （Z-BNNFts） decreases as the strain increases. As strain increases, the energy gaps of Z-BNNRs decrease rapidly as the width increases and reduce significantly to small values, which makes Z-BNNRs change from wide-gap to narrow-gap semiconductors.     

Oxygen vacancy in N-doped Cu₂ O crystals:A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory.The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure.In N anion-doped Cu2O,some N 2p states overlap and mix with the O2p valence band,leading to a slight narrowing of band gap compared with the undoped Cu2O.However,it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase

Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.     

Oxygen vacancy in N-doped Cu2O crystals: A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

过渡金属(Ni)掺杂ZnV2O4的第一性原理计算

采用基于密度泛函理论下的MS软件模拟了过渡金属Ni掺杂ZnV₂O₄前后的能带结构、态密度以及光学性质.结果表明：ZnV₂O₄具有间接的光学跃迁且能带间隙为0.355 eV,Ni掺杂后能带间隙增加为0.785 eV,且带隙类型不变,引入的Ni-3d轨道电子对ZnV₂O₄的价带和导带组成提供了较大贡献.光学性质结果表明ZnV₂O₄为一种低介电材料,在可见光区的吸收系数和折射率较低,主要表现为紫外吸收.掺杂Ni后,在可见光区的吸收特性和光电导率均增大,有效改善了ZnV₂O₄在可见光区的光电性能.     

Luminescence properties of lanthanide(III) ions in concentrated carbonate solution

Luminescence properties of lanthanide(III) ions (Ln = Nd, Sm, Eu, Gd, Tb, Dy and Tm) were investigated by measuring the excitation and emission spectra, and emission lifetimes in H₂O and D₂O solutions of 3 moll^?1 K₂CO₃, where anionic tetra-carbonate complexes, [Ln(CO₃)₄]^5- were the predominant species.

Electronic transitions of the carbonato complex corresponding to both the excitation and emission spectra were assigned from the energy level diagrams of Ln(III) and compared with those of the aqua ion. Enhancement of emission intensity of the complex was observed at particular excitation transitions of Eu(III), Gd(III) and Tb(III), and at particular emission transitions of Sm(III), Eu(III), Dy(III) and Tm(III). The enhancement at the emission transition was estimated quantitatively as a branching ratio from the lowest emitting state of Ln(III), and discussed in terms of hypersensitivity.

Emission lifetimes of the carbonato complexes were all longer than those of aqua ions in H₂O solution, while the lifetimes of the complexes for Eu(III) and Tb(III) shorter than those in D₂O solution. The difference in non-radiative decay constants for the excited complex in H₂O and D₂O solutions was found to be proportional to an exponential of the energy gap of Ln(III). The lifetime ratio between the H₂O and D₂O solutions showed the order of Sm > Dy > Eu > Tb, corresponding to the opposite order of the energy gap. These were discussed in terms of an energy gap law, i.e. a relationship between the energy gap of Ln(III) and vibration energies of the ligand or water molecules.     

Sc2O3电子结构和光学性质的第一性原理研究

本文基于第一性原理平面波赝势(PWP)和广义梯度近似(GGA)方法,研究了Sc2O3的电子结构、态密度和光学性质. 计算结果表明：Sc2O3是一种直接带隙半导体,其能带宽度为3.79eV,价带顶部主要由O的2p和Sc的3p3d杂化而成,导带主要由Sc的3d和O的2p构成. 同时,文中也分析了Sc2O3的介电函数、折射率、光电导率和吸收谱等光学性质. 计算得到静态介电常数 ,折射率n0=1.25,在紫外区有较大的吸收系数.     

Creation of electronic excitations and defects by vuv radiation (6-40 eV) in wide-gap solids

The processes of multiplication of electronic excitations (MEE), connected with the creation of secondary excitons or electron-hole (e-h) pairs by hot conduction electrons, are realized in wide-gap metal halides and oxides. In oxides, secondary e-h pairs can be also formed by 27-40 v eV photons due to L 1 VV Auger transitions (with the participation of 2s oxygen holes). The excitation spectra of luminescence and the creation spectra of electron F centres or hole V centres have been measured for Na 6 Al 6 Si 6 O 24 (NaI) 2x sodalites and MgO:Be, respectively, at 8-80 v K. A high local density of excitations has been revealed under MEE conditions in KBr and Br sodalites with self-trapping excitons and holes.     

Electron localization and a confined electron gas in nanoporous inorganic electrides

The nanoporous main group oxide 12CaO.7Al(2)O3 (C12A7) can be transformed from a wide-gap insulator to an electride where electrons substitute anions in cages constituting a positive frame. Our ab initio calculations of the electronic structure of this novel material give a consistent explanation of its high conductivity and optical properties. They show that the electrons confined in the inert positive frame are localized in cages and undergo hopping between neighboring cages. The results are useful for the understanding of behavior of confined electron gas of different topology and electron-phonon coupling, and for designing new transparent conductors, electron emitters, and electrides.     

Structural,elastic and optoelectronic properties of Sr-based perovskite-type oxides SrXO3 (M = Th,Zr) via first-principles calculations

In this paper, we present a detailed theoretical investigation on the structural, elastic, electronic and optical properties of the perovskite oxides SrThO₃ and SrZrO₃ by using the pseudo-potential plane wave (PP-PW) method. The computed lattice constants of SrXO₃ (X = Th and Zr) are in excellent agreement with the available experimental data. SrThO₃ and SrZrO₃ are direct (Γ–Γ) and indirect (Γ–R) band gap semiconductors, respectively. Under pressure effect a crossover between the indirect band gap (R–Γ) and the direct band gap (Γ–Γ) curves occurs at about 35 GPa for SrZrO₃, resulting in the energy minimum of direct gap (Γ–Γ) for this compound. The covalence in the Zr–O and Th–O bonds arises due to the hybridization between O–p and Zr–d (Th–d) states. Under pressure effect, the threshold energy becomes slightly greater (smaller) for SrZrO₃ (SrTO₃) for 3.21 (2.28) eV and the main peaks are shifted towards higher energies. Although the positions of all peaks shifted under pressure, they still have the same type as those at zero pressure, with decreasing the intensity of the main peaks.     

Effect of nitrogen doping on optical properties and electronic structures of SrZrO3 films

The effect of nitrogen doping on optical properties and electronic structures of SrZrO₃ (SZO) films was investigated both experimentally and theoretically. A shift of the absorption edge to the longer wavelength, i.e., the narrowing of the band gap caused by nitrogen doping was observed. The first-principles calculations revealed that the band gap reduced in nitrogen-doped SZO because of the localized N 2p states above O 2p states and the bottom of conduction band of Zr 4d shifting to a lower energy.     

Highly stable two-dimensional graphene oxide:Electronic properties of its periodic structure and optical properties of its nanostructures

According to first principle simulations, we theoretically predict a type of stable single-layer graphene oxide(C_2O).Using density functional theory(DFT), C_2O is found to be a direct gap semiconductor. In addition, we obtain the absorption spectra of the periodic structure of C_2O, which show optical anisotropy. To study the optical properties of C_2O nanostructures, time-dependent density functional theory(TDDFT) is used. The C_2O nanostructure has a strong absorption near 7 eV when the incident light polarizes along the armchair-edge. Besides, we find that the optical properties can be controlled by the edge configuration and the size of the C_2O nanostructure. With the elongation strain increasing, the range of light absorption becomes wider and there is a red shift of absorption spectrum.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.     

铁电体SrBi2Nb2O2电子能带结构的第一性原理研究

采用第一性原理的方法计算了SrBi₂Nb_2O.9（SBN）的顺电相、铁电相的电子结构.顺电相是间接带隙, 铁电相是直接带隙，它们的大小分别为1.57和2.23 eV.顺电相和铁电相的价带顶均主要来自于O₂p态的贡献.而顺电相和铁电相的导带底则分别来自Nb₄d态和Bi₆p态的贡献.计算表明SBN铁电相的低的漏电流与Bi 6p轨道有关.由顺电相到铁电相时，Nb₄d和O_{2 关键词： 顺电相 铁电相 态密度 电子能带结构}     

氧空穴导致二氧化钒低温相带隙变窄

具有一定能量的光照导致低温绝缘二氧化钒(VO_2)发生绝缘体金属转变.本文通过密度泛函理论的Heyd-Scuseria-Ernzerhof杂化泛函方法对含氧空穴的低温绝缘VO_2非磁M1相进行第一性原理研究.研究发现,含氧空穴的M1的晶格参数几乎不变,但氧空穴附近的长的V—V键长却变短了.进一步研究发现,尽管纯的非磁M1的带隙是0.68 eV,但含O1和O2位的氧空穴非磁M1带隙分别为0.23 eV和0.20 eV,同时含有O1和O2位氧空穴非磁M1带隙为0.15 eV,这很好地解释了实验结果.     

Interface-induced states at the boundary between a 3D topological insulator and a normal insulator

We show that, when a three-dimensional (3D) narrow-gap semiconductor with inverted band gap (“topological insulator,” TI) is attached to a 3D wide-gap semiconductor with non-inverted band gap (“normal insulator,” NI), two types of bound electron states having different spatial distributions and spin textures arise at the TI/NI interface. Namely, the gapless (“topological”) bound state can be accompanied by the emergence of the gapped (“ordinary”) bound state. We describe these states in the framework of the envelope function method using a variational approach for the energy functional; their existence hinges on the ambivalent character of the constraint for the envelope functions that correspond to the “open” or “natural” boundary conditions at the interface. The properties of the ordinary state strongly depend on the effective interface potential, while the topological state is insensitive to the interface potential variation.     

Electronic and magnetic properties of Ti(2)O(3), Cr(2)O(3), and Fe(2)O(3) calculated by the screened exchange hybrid density functional

The electronic and magnetic properties of the transition metal sesqui-oxides Cr(2)O(3), Ti(2)O(3), and Fe(2)O(3) have been calculated using the screened exchange (sX) hybrid density functional. This functional is found to give a band structure, bandgap, and magnetic moment in better agreement with experiment than the local density approximation (LDA) or the LDA+U methods. Ti(2)O(3) is found to be a spin-paired insulator with a bandgap of 0.22?eV in the Ti d orbitals. Cr(2)O(3) in its anti-ferromagnetic phase is an intermediate charge transfer Mott-Hubbard insulator with an indirect bandgap of 3.31?eV. Fe(2)O(3), with anti-ferromagnetic order, is found to be a wide bandgap charge transfer semiconductor with a 2.41?eV gap. Interestingly sX outperforms the HSE functional for the bandgaps of these oxides.     

Non-stoichiometry effects on electrical and luminescence properties of the layered oxysulfide (LaO)CuS

The layered oxysulfide (LaO)CuS is a wide-gap semiconductor. In general, the wide-gap semiconductor has strong mono-polarity and non-stoichiometry which affect their physical properties. In this study, the relation between the non-stoichiometry of (LaO)CuS and their physical properties has been investigated. Several kinds of non-stoichiometric samples were prepared. In the Cu non-stoichiometry, (LaO)CuS can accommodate the excess Cu in preference to a deficiency. In the La and S non-stoichiometric samples, deficiencies are preferred. The Cu and S non-stoichiometry affects their electrical and emission properties as expected from the valence band structure. It should be noted that the La non-stoichiometry is also quite effective on the band edge emission and the resistivity as well as other non-stoichiometry. These results reveal that not only Cu but also La deficiency is important in controlling the physical properties.