Showing papers on "Electronic band structure published in 2009"

Design of narrow-gap TiO2: a passivated codoping approach for enhanced photoelectrochemical activity.

Abstract: To improve the photoelectrochemical activity of TiO2 for hydrogen production through water splitting, the band edges of TiO2 should be tailored to match with visible light absorption and the hydrogen or oxygen production levels. By analyzing the band structure of TiO2 and the chemical potentials of the dopants, we propose that the band edges of TiO2 can be modified by passivated codopants such as (Mo+C) to shift the valence band edge up significantly, while leaving the conduction band edge almost unchanged, thus satisfying the stringent requirements. The design principle for the band-edge modification should be applicable to other wide-band-gap semiconductors.

Crystal and electronic band structure of Cu2ZnSnX4 (X=S and Se) photovoltaic absorbers: First-principles insights

Abstract: The structural and electronic properties of Cu2ZnSnS4 and Cu2ZnSnSe4 are studied using first-principles calculations. We find that the low energy crystal structure obeys the octet rule and is the kesterite (KS) structure. However, the stannite or partially disordered KS structures can also exist in synthesized samples due to the small energy cost. We find that the dependence of the band structure on the (Cu,Zn) cation ordering is weak and predict that the band gap of Cu2ZnSnSe4 should be on the order of 1.0 eV and not 1.5 eV as was reported in previous absorption measurements.

Hexagonal warping effects in the surface states of the topological insulator Bi2Te3.

Abstract: A single two-dimensional Dirac fermion state has been recently observed on the surface of the topological insulator Bi2Te3 by angle-resolved photoemission spectroscopy. We study the surface band structure using k x p theory and find an unconventional hexagonal warping term, which is the counterpart of cubic Dresselhaus spin-orbit coupling in rhombohedral structures. We show that this hexagonal warping term naturally explains the observed hexagonal snowflake Fermi surface. The strength of hexagonal warping is characterized by a single parameter, which is extracted from the size of the Fermi surface. We predict a number of testable signatures of hexagonal warping in spectroscopy experiments on Bi2Te3. We also explore the possibility of a spin-density wave due to strong nesting of the Fermi surface.

Excitonic Effects on the Optical Response of Graphene and Bilayer Graphene

Abstract: We present first-principles calculations of many-electron effects on the optical response of graphene, bilayer graphene, and graphite employing the GW-Bethe Salpeter equation approach We find that resonant excitons are formed in these two-dimensional semimetals The resonant excitons give rise to a prominent peak in the absorption spectrum near 45 eV with a different line shape and significantly redshifted peak position from those of an absorption peak arising from interband transitions in an independent quasiparticle picture In the infrared regime, our calculated optical absorbance per graphene layer is approximately a constant, 24%, in agreement with recent experiments; additional low frequency features are found for bilayer graphene because of band structure effects

First-principles study of two- and one-dimensional honeycomb structures of boron nitride

Abstract: This paper presents a systematic study of two- and one-dimensional honeycomb structures of boron nitride (BN) using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of all allotropic forms of three-dimensional (3D) BN crystals and truly one-dimensional atomic BN chains are also included. Two-dimensional (2D) graphenelike BN is a wide band-gap semiconductor with ionic bonding through significant charge transfer from B to N. Phonon-dispersion curves demonstrate the stability of 2D BN flakes. Quasi-one-dimensional (1D) armchair BN nanoribbons are nonmagnetic semiconductors with edge states. Upon passivation of B and N with hydrogen atoms these edge states disappear and the band gap increases. Bare zigzag BN nanoribbons are metallic but become a ferromagnetic semiconductor when both their edges are passivated with hydrogen. However, their magnetic ground state, electronic band structure, and band gap are found to be strongly dependent on whether B or N edge of the ribbon is saturated with hydrogen. Vacancy defects in armchair and zigzag nanoribbons affect also the magnetic state and electronic structure. Harmonic, anharmonic, and plastic regions are deduced in the variation in the total energy of armchair and zigzag nanoribbons as a function of strain. The calculated force constants display a Hookian behavior. In the plastic region the nanoribbon is stretched, whereby the honeycomb structure of hexagons change into different polygons through sequential structural transformations. In order to reveal dimensionality effects these properties are contrasted with those of various 3D BN crystals and 1D BN atomic chain.

Photocatalytic water splitting by RuO2-loaded metal oxides and nitrides with d0- and d10 -related electronic configurations

Abstract: This article reviews photocatalysis for water splitting by various kinds of metal oxides and nitrides such as ferroelectric metal oxides, different kinds of titanates with d0 (Ti4+) electronic configuration as a core metal ion, and various typical metal oxides with d10 (In3+, Ga3+, Ge4+, Sn4+, Sb5+) configuration, and d10 (Ga3+) metal nitride, together with d10s2 (Pb2+) and d0f0 (Ce4+) metal oxides. Ferroelectric metal oxides with single domain structure showed anomalous photovoltaic effects that controlled the behavior of photoexcited electrons. Various metal oxides involving ionic alkaline metal/alkaline earth metal ions showed good correlation between photocatalytic activity and the distortion of octahedral XO6/tetrahedral XO4 (X = core metal ion). When the metal oxides involved covalent metal ions such as Zn2+ and Pb2+, the activity was invoked even in distortion-free structures: this is due to strong electronic effects that affect the band structure. The activity of d10 metal oxides and nitrides is associated with conduction bands of hybridized sp orbitals with large dispersion that are able to generate photoexcited electrons with large mobility. A feature of the photocatalytically active metal oxides and nitrides discovered so far, which is their closed shell electronic structures is discussed.

Reconstruction of molecular orbital densities from photoemission data.

Abstract: Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.

Electronic structures of zigzag graphene nanoribbons with edge hydrogenation and oxidation

Abstract: Using the ab initio density-functional theory method and local spin-density approximation, we calculated the electronic band structures of H or H2 edge-hydrogenated zigzag graphene nanoribbons (ZGNRs) as well as COH, CO, or C2O edge-oxidized ZGNRs. We found that the OH group yields almost the same band structure as the $s{p}^{2}$ hybridization of H edge, and that the ketone (CO) and ether (C2O) groups result in band structures similar to those of $s{p}^{3}$ hybridization of H2 edge. Compared to H passivation, edge oxidation by the ketone or the ether group is energetically more favorable, suggesting that the GNR's edges will be oxidized in the presence of oxidizing species. Edge oxidized GNRs show metallic band structures caused by the larger electronegativity of oxygen relative to carbon, and these findings raise a question about the physical origins of the experimental observations of semiconducting GNRs. Such discrepancy suggests that more realistic modeling of GNR edge structures will be necessary to understand the experimental findings.

First Principles Calculations of Electronic Band Structure and Optical Properties of Cr-Doped ZnO

Abstract: Electronic band structure and optical properties of Cr-doped ZnO were studied using the density functional method within the generalized-gradient approximation. Three configurations with the substitution of Zn by one and two Cr atoms in different positions were considered. For the pure ZnO, the Fermi level locates at the valence band maximum, while it shifts to the conduction band and exhibits metal-like characteristic after Cr atoms are introduced into the ZnO supercell. The calculated optical properties indicate that the optical energy gap is increased after Cr doping. More importantly, strong absorption in the visible-light region is found, which originates from the intraband transition of the Cr 3d bands and the conduction bands. Our calculations provide electronic structure evidence that, in addition to usage as short-wavelength optoelectronic devices, the Cr-doped ZnO system could be a potential candidate for photoelectrochemical application due to the increase in its photocatalytic activity.

Turning a nickelate Fermi surface into a cupratelike one through heterostructuring.

Abstract: Using the local density approximation and its combination with dynamical mean-field theory, we show that electronic correlations induce a single-sheet, cupratelike Fermi surface for hole-doped 1=1 LaNiO3=LaAlO3 heterostructures, even though both eg orbitals contribute to it. The Ni 3d 3z 2 � 1 orbital plays the role of the axial Cu 4s-like orbital in the cuprates. These two results indicate that ‘‘orbital engineering’’ by means of heterostructuring should be possible. As we also find strong antiferromagnetic correlations, the low-energy electronic and spin excitations in nickelate heterostructures resemble those of high-temperature cuprate superconductors. The discovery of high-temperature superconductivity (HTSC) in hole-doped cuprates [1] initiated the quest for finding related transition-metal oxides with comparable or even higher transition temperatures. In some systems, such as ruthenates [2] and cobaltates [3], superconductivity has been found. However, in these t2g systems superconductivity is very different from that in cuprates, and transition temperatures (Tc’s) are considerably lower. As it became possible to grow transition-metal oxides in heterostructures, this quest got a new direction: Novel effectively two-dimensional (2D) systems could be engineered. But which oxides, besides cuprates, are most promising for getting high Tc’s? The basic band structure of the hole-doped cuprates is that of a single 2D Cu 3d x 2 � y 2-like band which is less than half filled (configuration d 9� h ). In this situation, antiferromagnetic fluctuations prevail and are often believed to mediate the superconductivity. The Fermi surface (FS) from this x 2 � y 2 band has been observed in many overdoped cuprates and found to agree with the predictions of local density-functional (LDA) band theory. Recently the following idea for arriving at a cupratelike situation in nickelates was presented [4]: Bulk LaNiO3 (d 7 ) has one electron in two degenerate eg bands, but sandwiching a LaNiO3 layer between layers of an insulating oxide such as LaAlO3 will confine the 3z 2 � 1 orbital in the z direction and may remove this band from the Fermi level, thus leaving the electron in the x 2 � y 2 band. The possibility of finding bulk nickelates with an electronic structure analogous to that of cuprates was discarded awhile ago [5], but heterostructures offer new perspectives. Indeed, a major reconstruction of orbital states at oxide interfaces may recently have been observed [6], and this kind of phenomenon could lead to novel phases not present in the bulk. Extensive theoretical studies of mechanisms for orbital selection in correlated systems [7] have revealed the complexity of this problem, where details of the electronic structure and lattice distortions play decisive roles. It is therefore crucial to examine nickelate heterostructures by means of state-of-the-art theoretical methods and find the optimal conditions for x 2 � y 2 orbital selection. In this Letter we present results of electronic-structure calculations using the merger [8] of LDA band theory, which provides an ab initio description of the materials chemistry, and the dynamical mean-field theory (DMFT) [9], which includes electronic correlations. We find that the hopping between the x 2 � y 2 and 3z 2 � 1 orbitals substantially reduces the effects of correlations in the 3z 2 � 1 orbital. In this respect, eg electrons behave very differently than the t2g electrons, which have no interorbital hopping on a square lattice. Nevertheless, we do find that the correlations may sufficiently shift the bottom of the hybridizing e g bands relatively to each other to yield a FS with only one sheet. This sheet has predominantly x 2 � y 2 character and a shape like in the cuprates with the highest Tc max (Tc at optimum hole doping) [10], but even more extreme. Moreover, stretching the in-plane lattice constants by suitable choice of substrate reduces the correlation strength needed to produce a single-sheet FS. Since we also find strong antiferromagnetic fluctuations, somewhat larger than in the cuprates, nickelate heterostructures hold the basic ingredients for high-temperature superconductivity.

Nodal Spin Density Wave and band topology of the FeAs based materials

Abstract: The recently discovered FeAs-based materials exhibit a $(\ensuremath{\pi},0)$ spin density wave (SDW) in the undoped state, which gives way to superconductivity upon doping. Here we show that due to an interesting topological feature of the band structure, the SDW state cannot acquire a full gap. This is demonstrated within the SDW mean-field theory of both a simplified two-band model and a more realistic five-band model. The positions of the nodes are different in the two models and can be used to detect the validity of each model.

Band Structure Asymmetry of Bilayer Graphene Revealed by Infrared Spectroscopy

Abstract: We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observe a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the next-nearest-neighbor interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined.

Strained graphene: tight-binding and density functional calculations

Abstract: We determine the band structure of graphene under strain using density functional calculations. The ab initio band structure is then used to extract the best fit to the tight-binding hopping parameters used in a recent microscopic model of strained graphene. It is found that the hopping parameters may increase or decrease upon increasing strain, depending on the orientation of the applied stress. The fitted values are compared with an available parameterization for the dependence of the orbital overlap on the distance separating the two carbon atoms. It is also found that strain does not induce a gap in graphene, at least for deformations up to 10%.

Reduced Bloch mode expansion for periodic media band structure calculations

Abstract: Reduced Bloch mode expansion (RBME) is presented for fast periodic media band structure calculations. The expansion employs a natural basis composed of a selected reduced set of Bloch eigenfunctions. The reduced basis is selected within the irreducible Brillouin zone at high symmetry points determined by the medium’s crystal structure and group theory (and possibly at additional related points). At each of the reciprocal lattice selection points, a number of Bloch eigenfunctions are selected up to the frequency/energy range of interest for the band structure calculations. As it is common to initially discretize the periodic unit cell and solution field using some choice of basis, RBME is practically a secondary expansion that uses a selected set of Bloch eigenvectors. Such expansion therefore keeps, and builds on, any favourable attributes a primary expansion approach might exhibit. Being in line with the well-known concept of modal analysis, the proposed approach maintains accuracy while reducing the computation time by up to two orders of magnitudes or more depending on the size and extent of the calculations. Results are presented for phononic, photonic and electronic band structures.

Trigonal warping and Berry's phase Nπ in ABC-stacked multilayer graphene

Abstract: The electronic band structure of ABC-stacked multilayer graphene is studied within an effective mass approximation. The electron and hole bands touching at zero energy support chiral quasiparticles characterized by Berry's phase $N\ensuremath{\pi}$ for $N$ layers, generalizing the low-energy band structure of monolayer and bilayer graphene. We investigate the trigonal-warping deformation of the energy bands and show that the Lifshitz transition, in which the Fermi circle breaks up into separate parts at low energy, reflects Berry's phase $N\ensuremath{\pi}$. It is particularly prominent in trilayers, $N=3$, with the Fermi circle breaking into three parts at a relatively large energy that is related to next-nearest-layer coupling. For $N=3$, we study the effects of electrostatic potentials which vary in the stacking direction, and find that a perpendicular electric field, as well as opening an energy gap, strongly enhances the trigonal-warping effect. In magnetic fields, the $N=3$ Lifshitz transition is manifested as a coalescence of Landau levels into triply degenerate levels.

Evanescent Bloch waves and the complex band structure of phononic crystals

Abstract: The complex band structure of a phononic crystal is composed of both propagating and evanescent Bloch waves. Evanescent Bloch waves are involved in the diffraction of acoustic phonons at the interfaces of finite phononic crystal structures. They are shown to arise both because of band gaps, where they directly measure the exponential decrease upon transmission, and because of the frustrated nature of higher-order diffracted waves at low frequencies. These diffracted evanescent Bloch waves become propagative as the frequency increases thus populating higher frequency bands. These results should apply as well to any periodic medium supporting the propagation of waves.

Branch-point energies and band discontinuities of III-nitrides and III-/II-oxides from quasiparticle band-structure calculations

Abstract: Using quasiparticle band structures based on modern electronic-structure theory, we calculate the branch-point energies for zinc blende (GaN, InN), rocksalt (MgO, CdO), wurtzite (AlN, GaN, InN, ZnO), and rhombohedral crystals (In2O3). For InN, CdO, ZnO, and also In2O3 the branch-point energies are located within the lowest conduction band. These predictions are in agreement with observations of surface electron accumulation (InN, CdO) or conducting behavior of the oxides (ZnO, In2O3). The results are used to predict natural band offsets for the materials investigated.

Electronic structure and band-gap modulation of graphene via substrate surface chemistry

Abstract: We have studied the electronic structure of graphene deposited on a SiO2 surface using density functional methods. The band structure of the graphene monolayer strongly depends on surface characteristics of the underlying SiO2 surface; for an oxygen-terminated surface, the monolayer exhibits a finite energy band gap while the band gap is closed when the oxygen atoms on the substrate are passivated with hydrogen atoms. We find that at least a graphene bilayer is required for a near zero energy gap when deposited on a substrate without H-passivation. Our results are discussed in the light of recent experiments.

Red Shift in Manganese-and Iron-Doped TiO2 : A DFT+U Analysis

Abstract: DFT (density functional theory) and DFT+U (DFT with Hubbard U correction for the on-site Coulomb repulsion) calculations have been carried out to study the effects of Mn or Fe doping to the energy band structures and optical properties of rutile TiO2. Both Fe and Mn doping reduces the overall energy band gap and introduces intermediate states/bands into the forbidden gap. It is observed that the Hubbard U correction to the 3d electrons of the host Ti atoms helps open up the energy gap of TiO2. On the other hand, the Hubbard U correction to the 3d electrons of the doping atom dictates the positions of intermediate states/bands with respect to the top of the valence band (VBM). The Hubbard correction to the 3d orbitals of a dopant, either Mn or Fe, shifts the up-spin states due to doping down to deeper levels, while it raises the down-spin states to higher energy levels. The doping induced up-spin states/bands are attributed to the hybridization between the dopant’s 3d and the oxygen 2p orbitals, while the ...

(pi, pi) electronic order in iron arsenide superconductors.

Abstract: The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba(1-x)K(x)Fe(2)As(2) is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (pi, pi) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

Hall effect and resistivity study of the magnetic transition, carrier content, and Fermi-Liquid Behavior in Ba(Fe(1-x) Co(x))(2)As(2).

Abstract: The negative Hall constant R(H) measured all over the phase diagram of Ba(Fe(1-x) Co(x))(2)As(2) allows us to show that electron carriers always dominate the transport properties. The evolution of R(H) with x at low doping (x<2%) indicates that important band structure changes happen for x<2% prior to the emergence of superconductivity. For higher x, a change with T of the electron concentration is required to explain the low T variations of R(H), while the electron scattering rate displays the T(2) law expected for a Fermi liquid. The T=0 residual scattering is affected by Co disorder in the magnetic phase, but is rather dominated by incipient disorder in the paramagnetic state.

Surface-stress-induced Mott transition and nature of associated spatial phase transition in single crystalline VO2 nanowires.

Abstract: We demonstrate that the Mott metal-insulator transition (MIT) in single crystalline VO(2) nanowires is strongly mediated by surface stress as a consequence of the high surface area to volume ratio of individual nanowires. Further, we show that the stress-induced antiferromagnetic Mott insulating phase is critical in controlling the spatial extent and distribution of the insulating monoclinic and metallic rutile phases as well as the electrical characteristics of the Mott transition. This affords an understanding of the relationship between the structural phase transition and the Mott MIT.

Comment on ``Band structure engineering of graphene by strain: First-principles calculations''

Abstract: In their first-principles calculations of the electronic band structure of graphene under uniaxial strain, Gui, Li, and Zhong, [Phys. Rev. B 78, 075435 (2008)] have found opening of band gaps at the Fermi level. This finding is in conflict with the tight-binding description of graphene which is closed gap for small strains. In this Comment, we present first-principles calculations which refute the claim that strain opens band gaps in graphene.

Electronic properties of graphene antidot lattices

Abstract: Graphene antidot lattices constitute a novel class of nano-engineered graphene devices with controllable electronic and optical properties. An antidot lattice consists of a periodic array of holes that causes a band gap to open up around the Fermi level, turning graphene from a semimetal into a semiconductor. We calculate the electronic band structure of graphene antidot lattices using three numerical approaches with different levels of computational complexity, efficiency and accuracy. Fast finite-element solutions of the Dirac equation capture qualitative features of the band structure, while full tight-binding calculations and density functional theory (DFT) are necessary for more reliable predictions of the band structure. We compare the three computational approaches and investigate the role of hydrogen passivation within our DFT scheme.

Band Formation from Coupled Quantum Dots Formed by a Nanoporous Network on a Copper Surface

Abstract: The properties of crystalline solids can to a large extent be derived from the scale and dimensionality of periodic arrays of coupled quantum systems such as atoms and molecules. Periodic quantum confinement in two dimensions has been elusive on surfaces, mainly because of the challenge to produce regular nanopatterned structures that can trap electronic states. We report that the two-dimensional free electron gas of the Cu(111) surface state can be trapped within the pores of an organic nanoporous network, which can be regarded as a regular array of quantum dots. Moreover, a shallow dispersive electronic band structure is formed, which is indicative of electronic coupling between neighboring pore states.

Image potential states in graphene

Abstract: Using combined ``$\text{LDA}+\text{image}$ potential'' calculations we show that below the vacuum level a single graphene layer possesses a double Rydberg series of even $({n}^{+})$ and odd $({n}^{\ensuremath{-}})$ symmetry image-potential states and argue that the widely discussed interlayer band in graphite is a consequence of the intersheet hybridization of the first even image-potential state. In light of the present results, the unoccupied electronic states with nearly-free-electron properties in carbon nanotubes, fullerenes, and fullerites can be understood to originate from the image-potential states of graphene.

Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy

Abstract: We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of ${\text{BaFe}}_{2}{\text{As}}_{2}$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\ensuremath{\Gamma}$ point and an electron pocket at each of the $X$ points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.