Showing papers on "Electronic structure published in 1998"

Electronic structure of atomically resolved carbon nanotubes

Abstract: Carbon nanotubes can be thought of as graphitic sheets with a hexagonal lattice that have been wrapped up into a seamless cylinder. Since their discovery in 19911, the peculiar electronic properties of these structures have attracted much attention. Their electronic conductivity, for example, has been predicted2,3,4 to depend sensitively on tube diameter and wrapping angle (a measure of the helicity of the tube lattice), with only slight differences in these parameters causing a shift from a metallic to a semiconducting state. In other words, similarly shaped molecules consisting of only one element (carbon) may have very different electronic behaviour. Although the electronic properties of multi-walled and single-walled nanotubes5,6,7,8,9,10,11,12 have been probed experimentally, it has not yet been possible to relate these observations to the corresponding structure. Here we present the results of scanning tunnelling microscopy and spectroscopy on individual single-walled nanotubes from which atomically resolved images allow us to examine electronic properties as afunction of tube diameter and wrapping angle. We observe bothmetallic and semiconducting carbon nanotubes and find thatthe electronic properties indeed depend sensitively on thewrapping angle. The bandgaps of both tube types are consistent with theoretical predictions. We also observe van Hove singularities at the onset of one-dimensional energy bands, confirming the strongly one-dimensional nature of conduction within nanotubes.

Atomic structure and electronic properties of single-walled carbon nanotubes

Abstract: Carbon nanotubes1 are predicted to be metallic or semiconducting depending on their diameter and the helicity of the arrangement of graphitic rings in their walls2,3,4,5. Scanning tunnelling microscopy (STM) offers the potential to probe this prediction, as it can resolve simultaneously both atomic structure and the electronic density of states. Previous STM studies of multi-walled nanotubes6,7,8,9 and single-walled nanotubes (SWNTs)10 have provided indications of differing structures and diameter-dependent electronic properties, but have not revealed any explicit relationship between structure and electronic properties. Here we report STM measurements of the atomic structure and electronic properties of SWNTs. We are able to resolve the hexagonal-ring structure of the walls, and show that the electronic properties do indeed depend on diameter and helicity. We find that the SWNT samples exhibit many different structures, with no one species dominating.

Ab initio pseudopotentials for electronic structure calculations of poly-atomic systems using density-functional theory

Abstract: The package fhi98PP allows one to generate norm-conserving pseudopotentials adapted to density-functional theory total-energy calculations for a multitude of elements throughout the periodic table, including first-row and transition metal elements. The package also facilitates a first assessment of the pseudopotentials' transferability, either in semilocal or fully separable form, by means of simple tests carried out for the free atom. Various parameterizations of the local-density approximation and the generalized gradient approximation for exchange and correlation are implemented.

Heteroepitaxial graphite on 6h-sic(0001): interface formation through conduction-band electronic structure

Abstract: When annealed at elevated temperatures under vacuum, silicon carbide surfaces show a tendency towards graphitization. Using the sensitivity of empty conduction-band states dispersion towards the structural quality of the overlayer, we have used angular-resolved inverse photoemission spectroscopy (KRIPES) to monitor the progressive formation of crystalline graphite on $6H\ensuremath{-}\mathrm{SiC}(0001)$ surfaces. The KRIPES spectra obtained after annealing at 1400 \ifmmode^\circ\else\textdegree\fi{}C are characteristic of azimuthally oriented, graphite multilayers of very good single-crystalline quality. For lower annealing temperatures, the ordered interface already presents most of the fingerprints of graphite as soon as 1080 \ifmmode^\circ\else\textdegree\fi{}C. The observation of unshifted ${\ensuremath{\pi}}^{*}$ states, which reveals a very weak interaction with the substrate, is consistent with the growth of a van der Waals heteroepitaxial graphite lattice on top of silicon carbide, with a coincidence lattice of $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ symmetry. The growth of the first graphene sheet proceeds on top of adatoms characteristic of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction. These adatoms reduce the chemical reactivity of the substrate. A strong feature located at 6.5 eV above the Fermi level is attributed to states derived from Si vacancies in the C-rich subsurface layers of the SiC substrate. This strongly perturbed substrate can be viewed as a diamondlike phase which acts as a precursor to graphite formation by collapse of several layers. In this framework, previously published soft x-ray photoemission spectra find a natural explanation.

Eight-band calculations of strained InAs/GaAs quantum dots compared with one-, four-, and six-band approximations

Abstract: The electronic structure of pyramidal shaped InAs/GaAs quantum dots is calculated using an eight-band strain-dependent $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian. The influence of strain on band energies and the conduction-band effective mass are examined. Single-particle bound-state energies and exciton binding energies are computed as functions of island size. The eight-band results are compared with those for one, four, and six bands, and with results from a one-band approximation in which ${m}_{\mathrm{eff}}(\stackrel{\ensuremath{\rightarrow}}{r})$ is determined by the local value of the strain. The eight-band model predicts a lower ground-state energy and a larger number of excited states than the other approximations.

Changes in the Electronic Properties of Si Nanocrystals as a Function of Particle Size

Abstract: X-ray absorption and photoemission spectra have been used to measure the band edges of silicon nanocrystals with average diameters ranging from 1 to 5nm. We compare the experimentally measured band edges to recent electronic structure calculations and find that the experimentally measured band gap is smaller than that predicted by theory. {copyright} {ital 1998} {ital The American Physical Society}

Comparison of two methods for describing the strain profiles in quantum dots

Abstract: The electronic structure of interfaces between lattice-mismatched semiconductors is sensitive to the strain. We compare two approaches for calculating such inhomogeneous strain—continuum elasticity [(CE), treated as a finite difference problem] and atomistic elasticity. While for small strain the two methods must agree, for the large strains that exist between lattice-mismatched III-V semiconductors (e.g., 7% for InAs/GaAs outside the linearity regime of CE) there are discrepancies. We compare the strain profile obtained by both approaches (including the approximation of the correct C2 symmetry by the C4 symmetry in the CE method) when applied to C2-symmetric InAs pyramidal dots capped by GaAs.

Ab Initio Study Of Hydrogen-Bonded Complexes Of Small Organic Molecules With Water

Abstract: Hydrogen bonding between water and a series of small organic molecules was examined via electronic structure calculations. Several computational methods were examined, including both a hybrid density functional procedure (Becke3LYP) and second-order Moller−Plesset theory (MP2) coupled with a double-ζ basis set augmented by diffuse polarization functions on heteroatoms. The agreement between Becke3LYP and MP2 energies was generally good, as was the agreement with energies obtained using more sophisticated and costly methods. The energies and structures of 53 hydrogen-bonded complexes of water with various small organic molecules, including alcohols, thiols, ethers, thioethers, carboxylic acids, esters, amines, amides, nitriles, and nitro compounds, were then examined systematically using the Becke3LYP and MP2 procedures. The hydrogen bond geometries were generally linear, and acceptor sites corresponded closely to the positions of lone pairs as predicted by simple hybridization arguments. Structures with s...

Macroscopic polarization and band offsets at nitride heterojunctions

Abstract: Ab initio electronic structure studies of prototypical polar interfaces of wurtzite III-V nitrides show that large uniform electric fields exist in epitaxial nitride overlayers, due to the discontinuity across the interface of the macroscopic polarization of the constituent materials. Polarization fields require a nonstandard evaluation of band offsets and formation energies: we find a large strain-induced asymmetry of the offset [0.2 eV for AlN/GaN (0001), 0.85 eV for GaN/AlN (0001)], and tiny interface formation energies.

Effects of nanodomain formation on the electronic structure of doped carbon nanotubes

Abstract: Changes in the electronic structure of multiwalled nanotubes due to the introduction of boron in the lattice are identified using scanning tunneling spectroscopy. Doped tubes are metallic with no apparent band gap, in contrast to undoped tubes with varying electronic character. Combined with ab initio calculations, we show that changes in the local density of states, as determined from tunneling spectroscopy, must be interpreted in terms of nanodomains of dopant islands and not as isolated substitutional species.

Oscillatory Conductance of Carbon-Atom Wires

Abstract: The conductance of carbon-atom chains is found from first-principles calculations to vary in an oscillatory manner as the number of carbon atoms is increased, with odd-numbered chains having a lower resistance than even-numbered chains. This finding is explained in terms of the electronic structure of the free chains and its modification by interaction with the metal electrodes: the critical factor is the density of states at the Fermi level of the combined electrode\char21{}atomic-wire system. Stronger electrode\char21{}atomic-wire coupling, i.e., shorter metal-carbon distance, does not necessarily imply a higher conductance.

TheRate: Program for Ab Initio Direct Dynamics Calculations of Thermal and Vibrational-State-Selected Rate Constants

Abstract: We introduce TheRate THEoretical RATEs , a complete . application program with a graphical user interface GUI for calculating rate constants from first principles. It is based on canonical variational transition-state . theory CVT augmented by multidimensional semiclassical zero and small . curvature tunneling approximations. Conventional transition-state theory TST with one-dimensional Wigner or Eckart tunneling corrections is also available. Potential energy information needed for the rate calculations are obtained from ab initio molecular orbital andror density functional electronic structure theory. Vibrational-state-selected rate constants may be calculated using a diabetic model. TheRate also introduces several technical advancements, namely the focusing technique and energy interpolation procedure. The focusing technique minimizes the number of Hessian calculations required by distributing more Hessian grid points in regions that are critical to the CVT and tunneling calculations and fewer Hessian grid points elsewhere. The energy interpolation procedure allows the use of a computationally less demanding electronic structure theory such as DFT to calculate the Hessians and geometries, while the energetics can be improved by performing a small number of single-point energy calculations along the MEP at a more accurate level of theory. The CH q H l CH q H reaction is used as a model to demonstrate usage of the 43 2 program, and the convergence of the rate constants with respect to the number of electronic structure calculations. Q 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1039)1052, 1998

Crossover from semiconductor to magnetic metal in semi-Heusler phases as a function of valence electron concentration

Abstract: Experimental and theoretical investigations of intermetallic semi-Heusler compounds (CoTiSn, FeTiSb, CoTiSb, NiTiSn, CoNbSn, CoVSb, NiTiSb) and their solid solutions are presented. The physical properties of these systems are found to be mostly determined by the number of valence electrons. Resistivity experiments show that compounds with 18 valence electrons are either semiconductors (CoTiSb, NiTiSn) or semi-metals (CoNbSn). The electronic structure calculations performed on 18-valence-electron systems by the KKR method show nine valence bands below the Fermi level and a gap of order 0.4-0.9 eV. A decrease or increase of the number of valence electrons in CoTiSb, NiTiSn or CoNbSn leads in either case to a metallic state and either ferromagnetic (CoTiSn, CoVSb) or paramagnetic (FeTiSb, NiTiSb) properties. The KKR results concerning 17- and 19-valence-electron systems correspond well with experimental characteristics, except in the case of CoVSb which KKR calculations predict to be a half-metallic ferromagnet, which conflicts with experimental data. Magnetization and resistivity measurements indicate that semiconductor-metal crossovers occur together with the appearance of ferromagnetism in the and series, for x near 0.4. This behaviour is discussed in the context of the KKR-CPA results.

Resonant magnetization tunneling in the trigonal pyramidal mnivmniii3 complex mn4o3cl(o2cch3)3(dbm)3

Abstract: The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)3(dbm)3], where dbm- is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the MnIVMnIII3 complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (DŜz2) with D = −0.53 cm-1 and a quartic zero-field splitting (B40O40)with B40 = −7.3 × 10-5 cm-1. Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426−2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each MnIVMnIII3 complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by...

Radiation-induced point defects in simple oxides

Abstract: We present a survey of recent theoretical studies of radiation-induced point defects in simple oxides with emphasis on highly ionic MgO, partly-covalent corundum (Al2O3) and ferroelectric KNbO3. The atomic and electronic structure of the electronic (F a and F centers) and hole centers, as well as interstitial atoms therein are discussed in light of the available experimental data. Results for defect diAusion and photo-stimulated F a fi F center conversion are also ana

Scanning tunneling microscopy observation of an electronic superlattice at the surface of clean gold

Abstract: We have used scanning tunneling spectroscopy to spatially resolve the electronic structure of clean Au(111) at low temperature. We find that the long-range herringbone reconstruction on Au(111) acts as a superlattice for surface-state electrons, creating a new band structure and modulated electronic density. Low energy electrons respond to the superlattice by localizing in the hexagonal-close-packed (hcp) region of the reconstruction, while higher energy electrons reverse this trend, shifting density back to the adjacent face-centered-cubic (fcc) region. These observations are quantitatively explained by an extended Kronig-Penney model, from which we estimate the well-depth of the reconstruction-induced surface superlattice.

Crystal Structure, Electronic Structure, and Temperature-Dependent Raman Spectra of Tl[Ag(CN)2]: Evidence for Ligand-Unsupported Argentophilic Interactions

Abstract: The structure of thallium dicyanoargentate(I) has been determined crystallographically. The crystal structure shows an Ag−Ag distance of 3.11 A. This is the shortest Ag−Ag distance reported for any silver dicyanide salt whose crystal structure has been determined. Raman spectra of the compound show four νC-N peaks that are well-resolved in the 10−80 K temperature range. This result agrees well with group theory analysis. Extended Huckel calculations using relativistic wave functions have been carried out for two models which describe the interactions between the Ag(CN)2- ions within the crystal structure of Tl[Ag(CN)2]. The results of these calculations indicate the formation of potential wells at short Ag−Ag distances. The data in this study suggest the significance of ligand-unsupported silver−silver interactions (argentophilicity) in Tl[Ag(CN)2]. Tl−Ag interactions are determined to be insignificant in the compound. Tl[Ag(CN)2] crystallizes in the monoclinic space group P21/c (No. 14), with a = 7.798(1...

Electron-electron correlations in carbon nanotubes

Abstract: Single-wall carbon nanotubes1,2 are ideally suited for electron-transport experiments on single molecules because they have a very robust atomic and electronic structure and are sufficiently long to allow electrical connections to lithographically defined metallic electrodes The electrical transport properties of single nanotubes3 and bundles of nanotubes4 have so far been interpreted by assuming that individual electrons within the nanotube do not interact, an approximation that is often well justified for artificial mesoscopic devices such as semiconductor quantum dots5 Here we present transport spectroscopy data on an individual carbon nanotube that cannot be explained by using independent-particle models and simple shell-filling schemes For example, electrons entering the nanotube in a low magnetic field are observed to all have the same spin direction, indicating spin polarization of the nanotube Furthermore, even when the number of electrons on the nanotube is fixed, we find that variation of an applied gate voltage can significantly change the electronic spectrum of the nanotube and can induce spin flips The experimental observations point to significant electron–electron correlations We explain our results phenomenologically using a model that assumes that the capacitance of the nanotube depends on its many-body quantum state

Covalency in semiconductor quantum dots

Abstract: Chemical schemes for the preparation of direct band-gap semiconductor quantum dots have advanced rapidly over the past few years. It is now possible to prepare a variety of III–V semiconductors with a finite size (InP, InAs, GaAs, etc.) and compare their size-dependent properties with the well studied II–VI class of quantum dots (ZnS, CdS, CdSe, etc.). In this Reviews, various physical properties of semiconductor quantum dots are presented within a discussion framework of lattice covalency. Included in the Reviews are discussions of the various chemical synthetic routes for making the particles, as well the electronic structure and the electronic dynamics of nanocrystals.

Electronic Structure and Elastic Properties of Strongly Correlated Metal Oxides from First Principles: LSDA + U, SIC‐LSDA and EELS Study of UO2 and NiO

Abstract: We compare experimentally observed electron energy loss spectra (EELS) of uranium dioxide UO 2 and nickel monoxide NiO with the results of ab-initio calculations carried out by using a method combining the local spin density approximation and the Hubbard U term (the LSDA + U method). We show that by taking better account of strong Coulomb correlations between electrons in the 5f shell of uranium ions in UO 2 and in the 3d shell of nickel ions in NiO it is possible to arrive at a better description of electron energy loss spectra, cohesive energies and elastic constants of both oxides compared with local spin density functional theory. For NiO we also compare the LSDA + U results and EELS spectra with a self-interaction corrected LSDA calculation.

Canonical purification of the density matrix in electronic-structure theory

Abstract: Two methods are suggested for performing ground-state electronic-structure calculations within the independent-electron approximation. Both methods involve the purification of a specified initial density matrix, which can be done either canonically (at a fixed electron count ${N}_{e})$ or grand canonically (at a fixed chemical potential \ensuremath{\mu}). Linear system-size scaling is achieved either way, as we illustrate in example tight-binding calculations on carbon nanotubes.

Electric-field-gradient calculations using the projector augmented wave method

Abstract: The first application of the projector augmented wave method to calculate electric-field gradients is presented. The projector augmented wave method is an all-electron electronic structure method that provides an accurate description of the wave function near the nucleus, and thus is well suited to the prediction of hyperfine parameters. Electric-field gradients have been evaluated for a variety of molecules and crystals containing main-group and transition-metal elements. Our results compare well with experiment and previous calculations based on the linear augmented plane-wave method.

Opacity of TiO from a coupled electronic state calculation parametrized by ab initio and experimental data

Abstract: We have carried out calculations of ro-vibrational energy levels of the 13 lowest electronic states of TiO. The dominant couplings between the various states are included, with the coupling parameters and potential parameters optimized to match experimental energy levels. We have also performed abinitio electronic structure calculations of the spin–orbit and rotation–orbit couplings, to verify that the physical results are obtained from the optimization. Our wavefunctions were used to predict the intensities of both well characterized and unobserved forbidden bands.

Connections between the electron-energy-loss spectra, the local electronic structure, and the physical properties of a material: A study of nickel aluminum alloys

Abstract: The local electronic structure of a material can be determined from the energy-loss spectrum of a swift electron beam scattered through it. When the electron beam is focused down to the width of an atomic column, the electronic density of states (DOS) at an interface, grain boundary, or impurity site can be decomposed by site, chemical species and angular momentum. Here we discuss the use of electron-energy-loss spectroscopy (EELS) fine structure to provide insight into the origin of grain boundary and interfacial properties reported earlier [D. A. Muller et al., Phys. Rev. Lett. 75, 4744 (1995)] for Ni${}_{3}$Al. We examine the electronic structure trends in Ni-Al compounds, both experimentally with the EELS measurements and theoretically, using ab initio band-structure calculations. The conditions under which the band-structure calculations can quantitatively reproduce the EELS measurements (and in particular, the question of just which local DOS is being measured) are addressed. Cyrot-Lackmann's moments theorem provides a framework to explain the systematic changes in the local DOS on alloying. The shape changes in the near-edge fine structure of both the Ni and Al $L$ edges are readily understood by the sensitivity of the fourth moment of the local DOS to the angular character of the Ni-Al bonding. The language of bond-order potentials proved useful in linking shape changes in the DOS to changes in cohesion. The consequences for formation energies and ordering trends in the transition-metal--aluminum alloys are also discussed.

Electronic structure of CoSb 3 : A narrow-band-gap semiconductor

Abstract: We report calculations which show that the band structure of CoSb{sub 3} is typical of a narrow-band-gap semiconductor. The gap is strongly dependent on the relative position of the Sb atoms inside the unit cell. We obtain a band gap of 0.22 eV after minimization of these positions. This value is more than four times larger than the result of a previous calculation, which reported that the energy bands near the Fermi surface are unusual. The electronic states close to the Fermi level are properly described by a two-band Kane model. The calculated effective masses and band gap are in excellent agreement with Shubnikov{endash}de Haas and Hall effect measurements. Recent measurements of the transport coefficients of this compound can be understood assuming it is a narrow-band-gap semiconductor, in agreement with our results. {copyright} {ital 1998} {ital The American Physical Society}

Atom-resolved electronic spectra for alq3 from theory and experiment

Abstract: The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.

The Effects of Finite Length on the Electronic Structure of Carbon Nanotubes

Abstract: The electronic structure of finite-length armchair carbon nanotubes has been studied using several ab-initio and semi-empirical quantum computational techniques. The additional confinement of the electrons along the tube axis leads to the opening of a band-gap in short armchair tubes. The value of the band-gap decreases with increasing tube length, however, the decrease is not monotonic but shows a well defined oscillation in short tubes. This oscillation can be explained in terms of periodic changes in the bonding characteristics of the HOMO and LUMO orbitals of the tubes. Finite size graphene sheets are also found to have a finite band-gap, but no clear oscillation is observed. As the length of the tube increases the density of states (DOS) spectrum evolves from that characteristic of a zero-dimensional (0-D) system to that characteristic of a delocalized one-dimensional (1-D) system. This transformation appears to be complete already for tubes 5-10 nm long. The chemical stability of the nanotubes, expressed by the binding energy of a carbon atom, increases in a similar manner.