Showing papers on "Electronic band structure published in 2008"

Gate-Variable Optical Transitions in Graphene

Abstract: Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

Universal Optical Conductance of Graphite

Abstract: We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to $(\ensuremath{\pi}/2){e}^{2}/h$, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The $f$-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.

Band structure engineering of graphene by strain: First-principles calculations

Abstract: We have investigated the electronic structure of graphene under different planar strain distributions using the first-principles pseudopotential plane-wave method and the tight-binding approach. We found that graphene with a symmetrical strain distribution is always a zero band-gap semiconductor and its pseudogap decreases linearly with the strain strength in the elastic regime. However, asymmetrical strain distributions in graphene result in opening of band gaps at the Fermi level. For the graphene with a strain distribution parallel to C-C bonds, its band gap continuously increases to its maximum width of 0.486 eV as the strain increases up to 12.2%. For the graphene with a strain distribution perpendicular to C-C bonds, its band gap continuously increases only to its maximum width of 0.170 eV as the strain increases up to 7.3%. The anisotropic nature of graphene is also reflected by different Poisson ratios under large strains in different directions. We found that the Poisson ratio approaches to a constant of 0.1732 under small strains but decreases differently under large strains along different directions.

Strong Suppression of Electrical Noise in Bilayer Graphene Nanodevices

Abstract: Low-frequency 1/f noise is ubiquitous and dominates the signal-to-noise performance in nanodevices. Here we investigate the noise characteristics of single-layer and bilayer graphene nanodevices and uncover an unexpected 1/f noise behavior for bilayer devices. Graphene is a single layer of graphite, where carbon atoms form a two-dimensional (2D) honeycomb lattice. Despite the similar composition, bilayer graphene (two graphene monolayers stacked in the natural graphite order) is a distinct 2D system with a different band structure and electrical properties. 1,2 In graphene monolayers, the 1/f noise is found to follow Hooge’s empirical relation with a noise parameter comparable to that of bulk semiconductors. However, this 1/f noise is strongly suppressed in bilayer graphene devices and exhibits an unusual dependence on the carrier density, different from most other materials. The unexpected noise behavior in graphene bilayers is associated with its unique band structure that varies with the charge distribution among the two layers, resulting in an effective screening of potential fluctuations due to external impurity charges. The findings here point to exciting opportunities for graphene bilayers in low-noise applications.

Atomic Hole Doping of Graphene

Abstract: The application of graphene in nanoscale electronic devices requires the deliberate control of the density and character of its charge carriers. We show by angle-resolved photoemission spectroscopy that substantial hole doping in the conical band structure of epitaxial graphene monolayers can be achieved by the adsorption of bismuth, antimony, or gold. In the case of gold doping the Dirac point is shifted into the unoccupied states. Atomic doping presents excellent perspectives for large scale production.

Minimal two-band model of the superconducting iron oxypnictides

Abstract: Following the discovery of the Fe-pnictide superconductors, local-density approximation (LDA) band structure calculations showed that the dominant contributions to the spectral weight near the Fermi energy came from the $\text{Fe}\text{ }3d$ orbitals. The Fermi surface is characterized by two hole surfaces around the $\ensuremath{\Gamma}$ point and two electron surfaces around the $M$ point of the two Fe/cell Brillouin zone. Here, we describe a two-band model that reproduces the topology of the LDA Fermi surface and exhibits both ferromagnetic and $q=(\ensuremath{\pi},0)$ spin-density wave fluctuations. We argue that this minimal model contains the essential low energy physics of these materials.

Optical properties of rotationally twinned InP nanowire heterostructures.

Abstract: We have developed a technique so that both transmission electron microscopy and microphotoluminescence can be performed on the same semiconductor nanowire over a large range of optical power, thus allowing us to directly correlate structural and optical properties of rotationally twinned zinc blende InP nanowires. We have constructed the energy band diagram of the resulting multiquantum well heterostructure and have performed detailed quantum mechanical calculations of the electron and hole wave functions. The excitation power dependent blue-shift of the photoluminescence can be explained in terms of the predicted staggered band alignment of the rotationally twinned zinc blende/wurzite InP heterostructure and of the concomitant diagonal transitions between localized electron and hole states responsible for radiative recombination. The ability of rotational twinning to introduce a heterostructure in a chemically homogeneous nanowire material and alter in a major way its optical properties opens new possibilities for band-structure engineering.

Theoretical description of carrier mediated magnetism in cobalt doped ZnO.

Abstract: Substitutional cobalt in ZnO has a weak preference for antiferromagnetic ordering Stabilization of ferromagnetism is achieved through $n$-type doping, which can be understood through a band coupling model However, the description of the transition to a ferromagnetic ground state varies within different levels of band theory; issues arise due to the density functional theory underestimation of the band gap of ZnO, and the relative position of the nominally unfilled Co ${t}_{2d}$ states We examine these limitations, including approaches to overcome them, and explain the contradictions in previous studies, which drastically overestimate the doping threshold for magnetic ordering

Electronic structure of the iron-based superconductor LaOFeP

Abstract: The recent discovery of superconductivity in the so-called iron-oxypnictide family of compounds has generated intense interest. The layered crystal structure with transition metal ions in planar square lattice form and the discovery of spin-density-wave order near 130K seem to hint at a strong similarity with the copper oxide superconductors. A burning current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories have been put forward depending on the underlying band structures: local moment antiferromagnetic ground state for strong coupling approach and itinerant ground state for weak coupling approach. The local moment magnetism approach stresses on-site correlations and proximity to a Mott insulating state and thus the resemblance to cuprates; while the latter approach emphasizes the itinerant electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. Such a controversy is partly due to the lack of conclusive experimental information on the electronic structures. Here we report the first angle-resolved photoemission spectroscopy (ARPES) investigation of LaOFeP (T{sub c} = 5.9 K), the first reported iron-based superconductor. Our results favor the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper based superconductors.

Tight-binding description of the quasiparticle dispersion of graphite and few-layer graphene

Abstract: A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of $N$ stacked $s{p}^{2}$ graphene layers $(N=1\dots{}\ensuremath{\infty})$ with $AB$ stacking sequence. The present TB parameters are fit to ab initio calculations on the GW level and are universal, allowing to describe the whole $\ensuremath{\pi}$ ``experimental'' band structure with one set of parameters. This is important for describing both low-energy electronic transport and high-energy optical properties of graphene layers. The QP bands are strongly renormalized by electron-electron interactions, which results in a 20% increase in the nearest-neighbor in-plane and out-of-plane TB parameters when compared to band structure from density-functional theory. With the new set of TB parameters we determine the Fermi surface and evaluate exciton energies, charge carrier plasmon frequencies, and the conductivities which are relevant for recent angle-resolved photoemission, optical, electron energy loss, and transport measurements. A comparision of these quantitities to experiments yields an excellent agreement. Furthermore we discuss the transition from few-layer graphene to graphite and a semimetal to metal transition in a TB framework.

Role of Sn2+ in the Band Structure of SnM2O6 and Sn2M2O7(M = Nb and Ta) and Their Photocatalytic Properties

Abstract: The photocatalytic properties of metal oxides consisting of Sn2+ with a 5s2 configuration were studied to find new visible light responsive photocatalysts. The band gaps of Ca2Ta2O7, Sn2Ta2O7, and Sn2Nb2O7 were found to be 4.8, 3.0, and 2.3 eV, respectively. Density functional calculations revealed that the valence band levels formed with Sn 5s orbitals were more negative than those with O 2p orbitals. The result is that niobates and tantalates containing Sn2+ have narrow band gaps compared with typical niobates and tantalates. SnNb2O6 showed photocatalytic activity for H2 and O2 evolution from aqueous solutions containing sacrificial reagents (methanol and Ag+) under visible light irradiation (λ > 420 nm). Moreover, the photocatalytic activity of SnNb2O6 depended on the material used as the Sn source. SnNb2O6 synthesized from Sn3O2(OH)2 showed higher activity for H2 and O2 evolution than that synthesized from commercial SnO, which included a small amount of SnO2. SnNb2O6 synthesized from Sn3O2(OH)2 showe...

Mechanism of charge transport in self-organizing organic materials

Abstract: Currently there is great interest in the use of organic materials as the active component in opto-electronic devices such as field-effect transistors, light-emitting diodes, solar cells and in nanoscale molecular electronics. Device performance is to a large extent determined by the mobility of charge carriers, which strongly depends on material morphology. Therefore, a fundamental understanding of the relation between the mechanism of charge transport and chemical composition and supramolecular organization of the active organic material is essential for improvement of device performance. Self-assembling materials are of specific interest, since they have the potential to form well defined structures in which molecular ordering facilitates efficient charge transport. This review gives an overview of theoretical models that can be used to describe the mobility of charge carriers, including band theory for structurally ordered materials, tight-binding models for weakly disordered systems and hopping models...

Determination of band structure parameters and the quasi-particle gap of CdSe quantum dots by cyclic voltammetry.

Abstract: Band structure parameters such as the conduction band edge, the valence band edge and the quasi-particle gap of diffusing CdSe quantum dots (Q-dots) of various sizes were determined using cyclic voltammetry. These parameters are strongly dependent on the size of the Q-dots. The results obtained from voltammetric measurements are compared to spectroscopic and theoretical data. The fit obtained to the reported calculations based on the semi-empirical pseudopotential method (SEPM)-especially in the strong size-confinement region, is the best reported so far, according to our knowledge. For the smallest CdSe Q-dots, the difference between the quasi-particle gap and the optical band gap gives the electron-hole Coulombic interaction energy (J(e1,h1)). Interband states seen in the photoluminescence spectra were verified with cyclic voltammetry measurements.

Conserving GW scheme for nonequilibrium quantum transport in molecular contacts

Abstract: quasiparticle self-energy, but for practical reasons, we mainly specialize to the so-called GW self-energy, widely used to describe the quasiparticle band structures and spectroscopic properties of extended and lowdimensional systems. We restrict the GW self-energy to a finite, central region containing the molecule, and we describe the leads by density functional theory DFT. A minimal basis of maximally localized Wannier functions is applied both in the central GW region and the leads. The importance of using a conserving, i.e., fully self-consistent, GW self-energy is demonstrated both analytically and numerically. We introduce an effective spin-dependent interaction which automatically reduces self-interaction errors to all orders in the interaction. The scheme is applied to the Anderson model in and out of equilibrium. In equilibrium at zero temperature, we find that GW describes the Kondo resonance fairly well for intermediate interaction strengths. Out of equilibrium, we demonstrate that the one-shot G0W0 approximation can produce severe errors, in particular, at high bias. Finally, we consider a benzene molecule between featureless leads. It is found that the molecule’s highest occupied molecular orbital‐lowest unoccupied molecular orbital gap as calculated in GW is significantly reduced as the coupling to the leads is increased, reflecting the more efficient screening in the strongly coupled junction. For the I-V characteristics of the junction, we find that Hartree‐Fock HF and G0W0GHF yield results closer to GW than does DFT and G0W0GDFT. This is explained in terms of selfinteraction effects and lifetime reduction due to electron-electron interactions.

Fractional spins and static correlation error in density functional theory

Abstract: Electronic states with fractional spins arise in systems with large static correlation (strongly correlated systems). Such fractional-spin states are shown to be ensembles of degenerate ground states with normal spins. It is proven here that the energy of the exact functional for fractional-spin states is a constant, equal to the energy of the comprising degenerate pure spin states. Dramatic deviations from this exact constancy condition exist with all approximate functionals, leading to large static correlation errors for strongly correlated systems, such as chemical bond dissociation and band structure of Mott insulators. This is demonstrated with numerical calculations for several molecular systems. Approximating the constancy behavior for fractional spins should be a major aim in functional constructions and should open the frontier for DFT to describe strongly correlated systems. The key results are also shown to apply in reduced density-matrix functional theory.

Bandwidth and Fermi surface of iron oxypnictides: Covalency and sensitivity to structural changes

Abstract: Some important aspects of the electronic structure of the iron oxypnictides depend very sensitively on small changes in interatomic distances and bond angles within the iron-pnictogen subunit. Using first-principles full-potential electronic structure calculations, we investigate this sensitive dependence, contrasting in particular LaFeAsO and LaFePO. The width of the Fe bands is significantly larger for LaFePO, indicating a better metal and weaker electronic correlations. When calculated at their experimental crystal structures, these two materials have significantly different low-energy band structures. The topology of the Fermi surface changes when going from LaFePO to LaFeAsO, with a three-dimensional hole pocket present in the former case transforming into a tube with two-dimensional dispersion. We show that the low-energy band structure of LaFeAsO evolves toward that of LaFePO as the As atom is lowered closer to the Fe plane with respect to its experimental position. The physical origin of this sensitivity to the iron-pnictogen distance is the covalency of the iron-pnictogen bond, leading to strong hybridization effects. To illustrate this, we construct Wannier functions, which are found to have a large spatial extension when the energy window is restricted to the bands with dominant iron character. Finally, we show that the Fe bandwidth slightly increases as one moves along the rare-earth series in REFeAsO and we discuss the physical origin of this effect.

First-principles investigation of structural and electronic properties of ultrathin Bi films

Abstract: Employing first-principles calculations, we perform a systematic study of the electronic properties of thin (one to six bilayers) films of the semimetal bismuth in (111) and (110) orientation. Due to the different coordination of the surface atoms in these two cases, we find a large variation of the conducting properties of the films, ranging from small-band-gap semiconducting to semimetallic and metallic. The evolution of the Bi(111) and Bi(110) surface states can be monitored as a function of the film thickness. Another interesting feature is provided by the strong spin-orbit effects in Bi and the resulting Rashba-type spin splitting of the surface states. The relaxations, band structures, Fermi surfaces, and densities of states are presented and discussed with respect to possible applications in the field of spintronics.

Strain-driven electronic band structure modulation of si nanowires.

Abstract: One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.

Gate-Variable Optical Transitions in Graphene

Abstract: Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

Electronic Structures of Manganese-Doped Rutile TiO2 from First Principles

Abstract: DFT (density functional theory) calculations have been carried out to determine the effects of Mn doping to the energy band structures of rutile TiO2, using the latest improved exchange correlation functional of Wu and Cohen. According to calculations performed on pure rutile, the use of the Wu−Cohen functional leads to improved accuracy in terms of energetic and crystallographic data and energy gap. The Mn effect on energy band structure depends on its occupancy sites in the rutile lattice. Mn substitution of the Ti sites induces significant spin polarization and effective reduction of the energy gap of rutile, with the energy gap being continuously reduced with increasing Mn doping level. Also, quality intermediate energy bands are introduced by moderate levels of substitutional Mn doping, indicating the promise to make use of a wide spectral range of solar irradiance in the visible light regime. On the other hand, interstitial Mn doping results in localized state in the forbidden band, without inducing...

Calculations of strain-modified anatase TiO 2 band structures

Abstract: In order to theoretically quantify a strain-inducing method that we use to engineer the material properties of anatase titania, we studied its electronic band structure over a range of biaxial strain by utilizing both the density functional theory within the generalized gradient approximation (GGA) and quasiparticle theory calculations within the GW approximation. This strain-modified material is suitable for use as a high efficiency photoanode in a photoelectrochemical cell. We track the changes to the band gap and the charge carrier effective masses versus the total pressure associated with the strained lattice. Both the GGA and the GW approximation predict a linear relationship between the change in band gap and the total pressure. However, the GGA underestimates the slope by more than 57% with respect to the GW approximation result of 0.0685 eV/GPa. We also compare our predicted band gap shift to a reported experimental result.

Electronic Band Structure of Transparent Conductor: Nb-Doped Anatase TiO2

Abstract: We have investigated electronic band structure of a transparent conducting oxide, Nb-doped anatase TiO2 (TNO), by means of first-principles band calculations and photoemission measurements. The band calculations revealed that Nb 4d orbitals are strongly hybridized with Ti 3d ones to form a d-nature conduction band, without impurity states in the in-gap region, resulting in high carrier density exceeding 1021 cm-3 and excellent optical transparency in the visible region. Furthermore, we confirmed that the results of valence band and core-level photoemission measurements are consistent with prediction by the present band calculations.

Topology analysis and nonlinear-optical-active properties of luminescent metal-organic framework materials based on zinc/lead isophthalates.

Abstract: Two 3D Zn(II) and Pb(II) isophthalates, [Zn(ip)]n (1) and [Pb4(μ4-O)(ip)3(H2O)]n (2) (H2ip = isophthalic acid), have been prepared under hydro(solvo)thermal conditions and characterized by single-crystal X-ray diffraction. The two complexes crystallize in different space groups (P43212 for 1 and P21/c for 2) and have different bridging modes of the ip ligand. The 3D framework of 1 is constructed by the interconnection of ZnO4 polyhedra via ip ligands, which represents a chiral net with PtS-type topology. In contrast, complex 2 is formed by the combination of Pb4O-cluster secondary building units and has a novel (3.4.5)(32.45.56.67.72) topology, which is the first ever example of a (3,7)-connected net. Complex 1 displays a second harmonic generation efficiency of about 1.5 times that of KH2PO4. Optical properties and thermal stabilities of the two complexes have been studied. Additionally, the calculations of band structure and density of states of 1 have also been performed with the density functional the...

Angle-resolved photoemission study of the evolution of band structure and charge density wave properties in RTe3 (R=Y, La, Ce, Sm, Gd, Tb, and Dy)

Abstract: We present a detailed angle-resolved photoemission spectroscopy (ARPES) investigation of the $R{\text{Te}}_{3}$ family, which sets this system as an ideal ``textbook'' example for the formation of a nesting driven charge density wave (CDW). This family indeed exhibits the full range of phenomena that can be associated to CDW instabilities, from the opening of large gaps on the best nested parts of Fermi surface (up to 0.4 eV), to the existence of residual metallic pockets. ARPES is the best suited technique to characterize these features, thanks to its unique ability to resolve the electronic structure in $k$ space. An additional advantage of $R{\text{Te}}_{3}$ is that the band structure can be very accurately described by a simple two dimensional tight-binding (TB) model, which allows one to understand and easily reproduce many characteristics of the CDW. In this paper, we first establish the main features of the electronic structure by comparing our ARPES measurements with the linear muffin-tin orbital band calculations. We use this to define the validity and limits of the TB model. We then present a complete description of the CDW properties and of their strong evolution as a function of $R$. Using simple models, we are able to reproduce perfectly the evolution of gaps in $k$ space, the evolution of the CDW wave vector with $R$, and the shape of the residual metallic pockets. Finally, we give an estimation of the CDW interaction parameters and find that the change in the electronic density of states $n({E}_{F})$, due to lattice expansion when different $R$ ions are inserted, has the correct order of magnitude to explain the evolution of the CDW properties.

Structural, elastic, and electronic properties of Fe3C from first principles

Abstract: Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010], and [001] directions. The three methods agree well with each other; the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

Transition-Metal-Substituted Indium Thiospinels as Novel Intermediate-Band Materials : Prediction and Understanding of Their Electronic Properties

Abstract: Results of density-functional calculations for indium thiospinel semiconductors substituted at octahedral sites with isolated transition metals (M=Ti,V) show an isolated partially filled narrow band containing three t2g-type states per M atom inside the usual semiconductor band gap. Thanks to this electronic structure feature, these materials will allow the absorption of photons with energy below the band gap, in addition to the normal light absorption of a semiconductor. To our knowledge, we demonstrate for the first time the formation of an isolated intermediate electronic band structure through M substitution at octahedral sites in a semiconductor, leading to an enhancement of the absorption coefficient in both infrared and visible ranges of the solar spectrum. This electronic structure feature could be applied for developing a new third-generation photovoltaic cell.

Electronic structure and charge transport properties of amorphous Ta2O5 films

Abstract: Amorphous Ta2O5 films were deposited by sputtering Ta onto silicon substrates with reactive ion beam. Electron energy loss spectroscopy measurements on the film found that the plasma oscillation energy is 23.1 eV. The refractive index and the extinction coefficient were measured with spectroscopic ellipsometry over the spectral range of 1.9–4.9 eV. The optical band gap is found to be 4.2 ± 0.05 eV. The valence band consists of three bands separated by ionic gaps. The values of electron effective masses were estimated with DFT quantum-chemical calculation. Experiments on injection of minority carriers from silicon into oxide were also conducted and we found that the electron component of conduction current governed by the electron current in the amorphous Ta2O5.