Ab initio modelling of boron and nitrogen in diamond nanowires

In this study an analysis is presented of the bonding and structural properties of dehydrogenated and hydrogenated doped cylindrical diamond nanowires calculated using the Vienna Ab Initio Simulation Package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally along the axis of an infinite one-dimensional diamond nanowire and include the single-electron acceptor boron and the single-electron donor nitrogen. The doped nanowires have then been re-relaxed, and properties compared with the undoped structures. The structural properties of relaxed nanowires considered here include an examination bonding via the electron charge density, with the aim of providing a better understanding of the effects of dopants on the stability of diamond nanostructures and nanodevices.     

Ab initio modelling of dopants in diamond nanowires: Ii

In this study an analysis is presented of the bonding and structural properties of dehydrogenated and hydrogenated doped cylindrical diamond nanowires calculated using the Vienna Ab Initio Simulation Package, employing density functional theory within the generalized-gradient approximation. The dopants studied here have been inserted substitutionally, equidistant along the axis of an infinite (periodic) diamond nanowire. These dopants include aluminium, phosphorus, oxygen and sulphur. The doped nanowires have then been re-relaxed, and properties compared with previously calculated results for undoped, boron-doped and nitrogen-doped structures. Structural properties of relaxed nanowires considered here include an examination bonding via the electron charge density, with the aim of providing a better understanding of the effects of dopants on the stability of diamond nanostructures and nanodevices.     

Understanding the Effects of Point Defects on the Electronic Properties of Titania Nanoparticles: An Ab Initio Study

First-principles calculations are used to study the electronic structure of the TiO₂(101) surface. The effect of oxygen vacancies and interstitial titanium ions on the electronic structure is investigated, and models for optical single-electron charge transfer transitions in the structure are proposed. We found that the addition of an uncharged oxygen vacancy leads to a shift of the total density of states toward lower energy, and the bandgap increases. Therefore, interstitial titanium ion incorporation induces donor states above the valence band and increases the bandgap. These results can be used to explain the observed blueshift in nanoscale TiO_2.     

Raman spectroscopy of the BC3 phase obtained under high pressure and high temperature

A direct transformation of the g‐BC₃ phase to a new diamond‐like d‐BC₃ phase was observed in a diamond‐anvil cell (DAC) at high temperature, 2033 ± 241 K, and high pressure, 50 GPa. Analysis of the peak positions of the d‐BC₃, B₄C, α‐boron, and the boron‐doped diamond leads to the conclusion that the positions of the peaks of the d‐BC₃ are more similar to the peak pattern of the boron‐doped diamond rather than that of boron carbide, α‐boron. Copyright © 2007 John Wiley & Sons, Ltd.     

分层掺B和吸附H2O碳纳米管的结构稳定性及电子场发射性能

运用基于第一性原理的密度泛函理论,系统研究了处于外电场中分层掺B并吸附不同数目H₂O碳纳米管体系的结构稳定性和电子场发射性能. 研究表明:第3层掺B并吸附5个H₂O的B₃CNT+5H₂O体系结构最稳定,管帽处Mulliken电荷最密集,尤其与单独掺B的B₃CNT和单独吸附H₂O的B₃CNT+5H₂O相比,其Fermi能级处态密度分别     

纤锌矿BeO掺Cd的电子结构与能带特性研究

采用基于密度泛函理论平面波赝势方法, 对纤锌矿BeO掺Cd的Be_1-xCd_xO合金进行电子结构与能带特性研究. 结果表明: Be_1-xCd_xO的价带顶始终由O 2p电子态决定, 而导带底由Be 2s和Cd 5s的电子态决定.随着Be_1-xCd_xO合金的Cd掺杂量增加, Cd 4d与O 2p的排斥效应逐渐加强, 同时Be_1-xCd_xO的带隙逐渐变小, 出现"直接-间接-直接"的带隙转变. 为了使理论值与实验值相一致, 对Be_1-xCd_xO带隙进行修正, 并分析了纤锌矿BeO-ZnO-CdO三元合金的带隙和弯曲系数与晶格常数的关系.     

Superconductivity in impurity bands

We present a theory of superconductivity in doped insulators. In the magnetic metal state of the compound we obtain the self-consistency equations for the superconducting state in the spin-dependent impurity bands of both extended and localized states in the initial insulator gap. A BCS-type triplet pairing field is considered. We show that the superconducting gap in which single-electron extended states do not exist is overlapped by the distribution of the localized states. The formation of a latent superconducting gap is discussed in connection with the unusual properties of high-T _c compounds. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 419–424 (10 March 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

First principles study the ferromagnetic properties and electronic structure of boron doped ZnSe

Using the full-potential linearized augmented plane wave method with generalized gradient approximation, the magnetic properties and the electronic structure of the boron-doped ZnSe (zinc blende phase) are investigated. Spin polarization calculations show the magnetic moment of the 64-atoms supercell containing one B_Se (B_Zn) is 3.00 (0.015) μ_B. The density of states indicates the magnetic moments of the B_Se doped configuration mainly come from the doped boron atoms and a few from its neighboring zinc atoms. The ferromagnetic and antiferromagnetic calculations for several doped configurations suggest B_Se could induce stable ferromagnetic ground state in ZnSe hosts and ferromagnetic couplings exist between the doped boron atoms. Electronic structures show that B_Se is p-type ferromagnetic semiconductor and hole-mediated double exchange is responsible for the ferromagnetism, while the B_Zn doped configuration is n-type semiconductor. Relative shallow acceptor and donor levels indicate boron-doped ZnSe is ionized easily at working temperatures.     

Effect of La doping on the electronic structure and optical properties of ZnO

The electronic structure and optical properties of ZnO doped with La have been investigated using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the La doping increases the bandgap of ZnO, in agreement with the experimental results; while the Fermi level shifts into the conduction band, revealing the so-called Burstein-Moss effect. In comparison to pure ZnO, a new peak appears in the imaginary part of dielectric function in the system doped with La and the optical absorption edge has been obviously changed. Moreover, the covalent property of Zn_1−xLa_xO is found to weaken with the increase of La concentration.     

掺稀土元素(Y，La)的γ-Si3N4的电子结构和光学性质

采用基于密度泛函的平面赝势方法(PWP)和广义梯度近似(GGA)，计算了未掺杂和掺杂稀土(Y，La)的γ-Si₃N₄中N-Y(La)键的布居值和它们的键长、掺杂后能带结构和态密度.发现掺杂后的带隙要减小，并且可能形成新的半导体，这将为找到新的半导体提供一个方向.还进一步研究了掺杂稀土(Y，La)后的光学性质，掺杂后有更高的静态介电常数，可以作为新的介电材料和好的折射材料，这对于一定的光学元件有潜在的应用前景.     

Role of zinc and nickel impurities in high-temperature superconductors

The local changes produced in the electronic structure and their effect on the physical properties of the superconducting and normal phases when zinc and nickel are substituted for copper are examined on the basis of a multiband p-d model. It is shown that strong electronic correlations suppress the S=1 configuration of Ni²⁺ and cause the superposition of the S=1/2 and S=0 states of nickel. The change in the density of states in p-and n-type systems is studied, and the peculiarity of Zn impurity for p-type systems and Ni impurity for n-type systems is shown. The universal dependence of the T _c on the residual resistance in lightly doped superconductors and deviations from it in optimally doped systems are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 596–600 (April 1999)     

Doping level and environment dependence of structural stability and magnetic properties in Mn-doped WS2 bilayer in first principles

We carried out first-principles electronic structure calculation to study the structural stability and magnetic properties of Mn-doped WS₂ ultra-thin films within the density functional theory. Adopting various configurations of Mn doping into WS₂ bilayer, we find that the magnetic phase can be manipulated among the ferromagnetic, antiferromagnetic, or ferrimagnetic phases by altering doping level and growth environment. Magnetic phase and strength are determined by magnetic coupling of Mn dopants 3d electrons which can be attributed crucially to the exchange interaction mediated by neighboring S atoms 3p electrons. Accompanying to the magnetic phase transition, the electronic structure reveals that transport properties switch from semiconducting with various bandgap to half-metallic states. This result implicates possible way to develop magnetic semiconductors based on Mn doped 2D WS₂ ultra-thin films for spintronics applications.     

Pressure studies of band structure,defects and impurities in group III nitrides

Abstract

This paper reviews high pressure investigations of the physical properties of group III nitrides. After presenting the most important results of high pressure research in this field, we focus on the problem of donors and acceptors in GaN and AlGaN with the special emphasis on the highly localized electronic states. Oxygen and silicon donors and their resonant localized states are discussed in detail. Finally the anomaly of the pressure coefficient of the energy gap of InGaN, will be considered in the relation to the peculiarities of these mixed crystals band structure.     

Defects and localized states in MBE-grown GaAs1−x N x solid solutions prepared by molecular-beam epitaxy

Defects and localized states have been studied for molecular-beam-epitaxy (MBE)-grown high-resistivity and undoped GaAs_1?x N _x films with a N concentration not exceeding approximately 1.0 at.%. The crystalline quality of the films and hence the defects and localized states were determined by high-resolution X-ray diffraction, photoluminescence spectra, capacitance versus voltage measurements and photoinduced current transient spectra of GaAs and GaAs_1?x N _x layers. It was concluded that incorporation of low concentrations of N into MBE-grown GaAs_1?x N _x films promotes the formation of high densities of deep centres similar to EL2 donors, leading to heavy compensation of the films by some unidentified acceptors. GaAs antisite acceptors were believed to be responsible for the said compensation. A prominent defect band near 1.33-1.38 eV also appeared to be associated with these defects. The most prominent centres in dilute GaAs_1?x N _x films with N content less than 0.35 at.% seem to be the EL2 donors and the hole traps located near E _v +0.3 eV.     

Doping induced electronic structure and estimated thermoelectric properties of CaMnO3 system

The electronic properties of Sr doped CaMnO₃ are studied using the first principle density functional theory calculation based on a plane wave basis and pseudopotentials. The thermoelectric properties are analyzed on the basis of electronic properties. The band structure results show that the doped system undergoes a semiconductor-to-conductor transition and the bands near Fermi level experience a significant distortion; the density of states results show that the density of states near Fermi level is increased. The combination of Mnd and Op orbitals exhibits enhanced covalence nature. It is estimated that the thermopower and carrier conduction capability should be enhanced, and the phonon conduction should be depressed, indicating the improved thermoelectric properties for Sr doped CaMnO₃ system.     

Structural and electronic properties of sulphur-doped boron nitride nanotubes

First-principles calculations based on density functional theory were performed to study the structural and electronic properties of sulphur substitution-doped boron nitride (BN) nanotubes, using the theory as implemented in SIESTA code, which uses non-conserving pseudo-potentials in fully non-local form and atomic orbitals as the basis set. The generalized gradient approximation (GGA) was used for the exchange–correlation (XC) potential. The tube selected was a (10, 0) BN nanotube that fell in the range of energy gap independent of the tube diameter. The electronic and structural properties for sulphur substitution in the boron and the nitrogen sites were studied. The structural arrangement in equilibrium conditions for S shows an outward radial deformation around the sulphur atom in the tube. The bandgap of the pristine BN nanotubes was found to be significantly modified on doping.     

Nanometric diamond delta doping with boron

Diamond is desired for active semiconducting device because of it high carrier mobility, high voltage breakdown resistance, and high thermal diffusivity. Exploiting diamond as a semiconductor is hampered by the lack of shallow dopants to create sufficient electronic carriers at room temperature. In this work, nanometer thick, heavily boron doped epitaxial diamond ‘delta doped’ layers have been grown on ultra smooth diamond surfaces which demonstrate p type conduction with enhanced Hall mobilities of up to 120 cm²/Vs and sheet carrier concentrations to 6 × 10¹³ cm^–2, thus enabling a new class of active diamond electronic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

Modified cascade model of resonant Raman scattering: a case study of UV Raman scattering in Zn1−xMnxO thin films

The cascade model of inelastic resonant Raman scattering considers real electronic states in the conduction band (CB) as intermediaries to explain multiple longitudinal optical (LO) Stokes‐shifted lines in the emission spectra. In this study, we report modification in the cascade model under conditions where the scattered photons after multiple transitions have energy lower than the bandgap (E_g) and give rise to higher order n‐LO lines. The higher order n‐LO lines involve electron transition between the trap levels, which are created by impurities or defects in the forbidden energy gap, and are analogous to the real electronic states in CB, depending on the density of defects or impurities in the lattice. The presence of traps in the forbidden gap (1) acts as virtual intermediate states giving rise to higher order n‐LO modes and (2) tends to decrease the radiative recombination probability leading to quenching of the luminescence emission and line width (full‐width at half‐maximum) broadening. Ultraviolet Raman scattering in Mn‐doped ZnO (Zn_1−xMn_xO) thin films were investigated and the experimental observations analyzed in the domain of the modified cascade model. Copyright © 2011 John Wiley & Sons, Ltd.