Showing papers on "Electronic structure published in 2006"

Controlling the Electronic Structure of Bilayer Graphene

Abstract: We describe the synthesis of bilayer graphene thin films deposited on insulating silicon carbide and report the characterization of their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic-scale electronic devices.

Electronic structure calculations with dynamical mean-field theory

Abstract: We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

Handbook of Photochemistry

Abstract: Photophysics of Organic Molecules in Solution Introduction Electronic States Radiative Transitions Nonradiative Transitions Excited State Kinetics Acknowledgments Bibliography Transition Metal Complexes Electronic Structure Types of Excited States and Electronic Transitions Absorption and Emission Bands Jablonski Diagram Photochemical Reactivity Electrochemical Behavior Polynuclear Metal Complexes Photophysical Properties of Organic Compounds Photophysics of Organic Molecules in Solution Triplet-State Energies: Ordered Flash Photolysis: Designing Experiments Low-Temperature Photophysics of Organic Molecules Absorption and Luminescence Spectra of Organic Compounds ESR and ODMR Parameters of the Triplet State Photophysical Properties of Transition Metal Complexes Photophysical Parameters Absorption and Emission Spectra Abbreviations Rate Constants of Excited-State Quenching Ionization Energies, Electron Affinities, and Reduction Potentials Ionization Energies and Electron Affinities Reduction Potentials Bond Dissociation Energies Solvent Properties Donor Number Luminescence Spectroscopy Measurements Correction of Luminescence Intensity Measurements in Solution Fluorescence Quantum Yield Standards Phosphorescence Quantum Yield Standards Luminescence Lifetime Standards Light Sources and Filters Spectral Distribution of Photochemical Sources Transmission Characteristics of Light Filters and Glasses Chemical Actinometry Ferrioxalate Actinometer Photochromic Actinometers Reinecke's Salt Actinometer Uranyl Oxalate Actinometer Other Actinometers Miscellaneous Spin-Orbit Coupling Hammett ? Constants Fundamental Constants and Conversion Factors Index *References included in each chapter

Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces

Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

Calculated electronic and magnetic properties of the half-metallic, transition metal based Heusler compounds

Abstract: In this work, results of {\it ab-initio} band structure calculations for $A_2BC$ Heusler compounds that have $A$ and $B$ sites occupied by transition metals and $C$ by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap and the peculiar magnetic behavior found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both $A$ and $B$ atoms to states near the Fermi energy and to the total magnetic moment. The magnitude of the total magnetic moment, which depends as well on the kind of $C$ atoms, shows a trend consistent with the Slater-Pauling type behavior in several classes of these compounds. The localized moment in these magnetic compounds resides at the $B$ site. Other than in the classical Cu$_2$-based Heusler compounds, the $A$ atoms in Co$_2$, Fe$_2$, and Mn$_2$ based compounds may contribute pronounced to the total magnetic moment.

Nitrogen-vacancy center in diamond: Model of the electronic structure and associated dynamics

Abstract: Symmetry considerations are used in presenting a model of the electronic structure and the associated dynamics of the nitrogen-vacancy center in diamond. The model accounts for the occurrence of optically induced spin polarization, for the change of emission level with spin polarization and for new experimental measurements of transient emission. The rate constants given are in variance to those reported previously.

Electronic states and Landau levels in graphene stacks

Abstract: We analyze, within a minimal model that allows analytical calculations, the electronic structure and Landau levels of graphene multi-layers with different stacking orders. We find, among other results, that electrostatic effects can induce a strongly divergent density of states in bi- and tri-layers, reminiscent of one-dimensional systems. The density of states at the surface of semi-infinite stacks, on the other hand, may vanish at low energies, or show a band of surface states, depending on the stacking order.

Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces

Abstract: The electronic structure of benzene on graphite (0001) is computed using the $GW$ approximation for the electron self-energy. The benzene quasiparticle energy gap is predicted to be 7.2 eV on graphite, substantially reduced from its calculated gas-phase value of 10.5 eV. This decrease is caused by a change in electronic correlation energy, an effect completely absent from the corresponding Kohn-Sham gap. For weakly coupled molecules, this correlation energy change can be described as a surface polarization effect. A classical image potential model illustrates the impact for other conjugated molecules on graphite.

Disorder induced localized States in graphene.

Abstract: We consider the electronic structure near vacancies in the half-filled honeycomb lattice. It is shown that vacancies induce the formation of localized states. When particle-hole symmetry is broken, localized states become resonances close to the Fermi level. We also study the problem of a finite density of vacancies, obtaining the electronic density of states, and discussing the issue o f electronic localization in these systems. Our results have also relevance for the problem of disorder in d-wave superconductors. Introduction. The problem of disorder in systems with Dirac fermions has been studied extensively in the last few years in the context of dirty d-wave superconductors (1). Dirac fermions are also the elementary excitations of the hon- eycomb lattice at half-filling, equally known as graphene, which is realized in two-dimensional (2D) Carbon based ma- terials with sp 2 bonding. It is well-known that disorder is ubiquitous in graphene and graphite (which is produced by stacking graphene sheets) and its effect on the electronic s truc- ture has been studied extensively (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13). It has been shown recently (14) that the interplay of disorder and electron-electron interactions is fundame ntal for the understanding of recent experiments in graphene de- vices (15). Furthermore, experiments reveal that ferromag- netism is generated in heavily disordered graphite samples (16, 17, 18, 19, 20), but the understanding of the interplay of strong disorder and electron-electron interactions in t hese systems is still in its infancy. Different mechanisms for fe rro- magnetism in graphite have been proposed and they are either based on the nucleation of ferromagnetism around extended defects such as edges (2, 8, 12, 13) or due to exchange in- teractions originating from unscreened Coulomb interactions (21). Therefore, the understanding of the nature of the elec- tronic states in Dirac fermion systems with strong disorder is of the utmost interest. In the following, we analyze in detail states near the Fermi energy induced by vacancies in a tight-binding model for the electronic states of graphene planes. We show that single va- cancies in a graphene plane generate localized states which are sensitive to the presence of particle-hole symmetry break- ing. Moreover, a finite density of such defects leads to stron g changes in the local and averaged electronic Density Of States (DOS) with the creation of localized states at the Dirac point. The model. We consider a single band model described by the Hamiltonian:

Time evolution of the electronic structure of 1T-TaS2 through the insulator-metal transition.

Abstract: Femtosecond time-resolved photoemission is used to investigate the time evolution of electronic structure in the Mott insulator 1T-TaS2. A collapse of the electronic gap is observed within 100 femtoseconds after optical excitation. The photoemission spectra and the spectral function calculated by dynamical mean field theory show that this insulator-metal transition is driven solely by hot electrons. A coherently excited lattice displacement results in a periodic shift of the spectra lasting for 20 ps without perturbing the insulating phase. This capability to disentangle electronic and phononic excitations opens new directions to study electron correlation in solids.

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

Abstract: The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

Spin-flip equation-of-motion coupled-cluster electronic structure method for a description of excited states, bond breaking, diradicals, and triradicals.

Abstract: The spin-flip (SF) approach to multireference situations (e.g., bond breaking, diradicals, and triradicals) is described. Both closed- and open-shell low-spin states are described within a single reference formalism as spin-flipping, e.g., α → β, excitations from a high-spin reference state for which both dynamical and nondynamical correlation effects are much smaller than for the corresponding low-spin state. Formally, the SF approach can be viewed as an equation-of-motion model, where target states are sought on the basis of determinants conserving the total number of electrons but changing the number of α and β electrons.

Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides.

Abstract: Understanding electron transport in metal−molecule−metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal−molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (−S) or isocyanide (−CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au−CN contact is more resistive than Au−S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyani...

Special Sites at Noble and Late Transition Metal Catalysts

Abstract: An overview of recent advancements in density functional theory modeling of particularly reactive sites at noble and late transition metal surfaces is given. Such special sites include sites at the flat surfaces of thin metal films, sites at stepped surfaces, sites at the metal/oxide interface boundary for oxide-supported metal clusters, and sites at the perimeter of oxide islands grown on metal surfaces. The Newns–Anderson model of the electronic interaction underlying chemisorption is described. This provides the grounds for introducing the Hammer–Norskov d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. A reactivity change described by this model is characterized as an electronic structure effect. Bronsted plots of energy barriers versus reaction energies are discussed from the surface reaction perspective and are used to analyze the trends in the calculated changes. Deviations in the relation between energy barriers and reaction energies in Bronsted plots are identified as due to atomic structure effects. The reactivity change from pure Pd surfaces to Pd thin films supported on MgO can be assigned to an electronic effect. Likewise for the reactivity change from flat Au surfaces, over Au thin films to Au edges and the Au/MgO interface boundary. The reactivity enhancement at atomic step sites is of both electronic and atomic structure nature for NO dissociation at Ru, Rh and Pd surfaces. The enhancement of the CO oxidation reactivity when moving from a CO+O coadsorption structure on Pt(111) to the PtO2 oxide island edges supported by Pt(111) is, however, identified as mainly an atomic structure effect. As such, it is linked to the occurrence of favorable pathways at the oxide island edges and is occurring despite of stronger adsorbate binding of the oxygen within the oxide edge, i.e. despite of an opposing electronic effect. As a final topic, a discussion is given of the accuracy of density functional theory in conjunction with surface reactions; adsorption, desorption, diffusion, and dissociation. Energy barriers are concluded to be more robust with respect to changes in the exchange-correlation functional than are molecular bond and adsorption energies.

Role of spin-orbit coupling and hybridization effects in the electronic structure of ultrathin Bi films

Abstract: The electronic structure of Bi(001) ultrathin films (thickness > or =7 bilayers) on Si(111)-7x7 was studied by angle-resolved photoemission spectroscopy and first-principles calculations. In contrast with the semimetallic nature of bulk Bi, both the experiment and theory demonstrate the metallic character of the films with the Fermi surface formed by spin-orbit-split surface states (SSs) showing little thickness dependence. Below the Fermi level, we clearly detected quantum well states (QWSs) at the M point, which were surprisingly found to be non-spin-orbit split; the films are "electronically symmetric" despite the obvious structural nonequivalence of the top and bottom interfaces. We found that the SSs hybridize with the QWSs near M and lose their spin-orbit-split character.

Theoretical study of the electronic structure and stability of titanium dioxide clusters (TiO2)n with n = 1-9.

Abstract: The electronic structure and the stability of both neutral and singly charged (TiO2)n clusters with n = 1−9 have been investigated using the density functional B3LYP/LANL2DZ method. The lowest-lying singlet clusters tend to form some compact structures with one or two terminal Ti−O bonds, which are about 1.4−2.5 eV more stable than the corresponding triplet structures. For the lowest-lying structures, strong infrared absorption lines at 988−1020 cm-1 due to terminal Ti−O bonds and below 930 cm-1 due to Ti−O−Ti bridging bonds may be observed, with some characteristic lines at 530−760 cm-1 due to 3-fold coordinated O-atoms that are comparable with the spectra of rutile and anatase bulk. The holes and excited electrons within triplet structures tend to be localized on the least coordinated O- and Ti-atoms, respectively, with some exceptions possibly due to the electron−hole interaction. The extra electrons within (TiO2)n- clusters and the holes within (TiO2)n+ clusters show a clearer preference of location o...

Introduction to Half‐Metallic Heusler Alloys: Electronic Structure and Magnetic Properties

Abstract: Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the the full-Heusler alloys like Co2MnGe. Ab-initio results suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment Mt scales linearly with the number of the valence electrons Zt, such that Mt = Zt − 24 for the full-Heusler and Mt = Zt − 18 for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

Electronic Structure, Defect Chemistry, and Transport Properties of SrTi1-xFexO3-y Solid Solutions

Abstract: The electronic structure, defect chemistry, and transport properties of members of the mixed ionic electronic conducting SrTi1-xFexO3-y (STF) solid-solution system are revisited, and an improved defect chemical model is proposed in which Fe is considered to be one of the main constituents that shape the energy-band structure of STF, rather than an impurity dopant with acceptor-like character. As a consequence of the high inherent deficiency in the oxygen sublattice, introduced by the mixed-valence states of the B-site cations Ti4+ and Fe3+, oxygen vacancies and interstitials generated by the anion Frenkel reaction dominate the defect equilibria, leading to predominant ionic conductivity at intermediate partial pressures of oxygen. Increasing Fe content results in both a systematic decrease in band-gap energy, Eg0 = 3.2 − 1.9x + 0.5x2 eV, and reduction enthalpy, ΔHred = 5.8 − 3.4x + 1.7x2 eV. The decrease in band gap is explained on the basis of the systematic broadening of the Fe-derived 3d band lying abo...

Time-of-flight photoelectron spectromicroscopy of single MoS2 nanotubes

Abstract: There is a recent interest in nanoscale materials, in particular, nanotubes based not only on carbon. In this study, photoemission spectra of single MoS2 nanotubes deposited on a Si surface were recorded in order to explain their electronic structure. The photoelectrons were excited by a femtosecond laser oscillator resulting in two-photon photoemission. A spectromicroscopic technique based on imaging time-of-flight detection was used to record the spatially resolved photoelectron spectra. Self-consistent electronic structure calculations for MoS2 slabs using the full potential linear augmented plane wave method are used to explain the peculiarities of the observed spectra. It turns out that the MoS2 nanotubes are semiconducting with a band gap of about 1eV. The two-photon transitions proceed through intermediate states in a region with high density of states; this gives rise to a high photoemission intensity.

Electronic structure of semiconductor nanowires

Abstract: We compute the subband structure of several group IV and III-V 001-, 110-, and 111-oriented nanowires using sp 3 and sp 3 d 5 s * tight-binding models. In particular, we provide the band gap energy of the nanowires as a function of their radius R in the range R =1–2 0 nm. Wethen discuss the self-energy corrections to the tight-binding subband structure, that arise from the dielectric mismatch between the nanowires with dielectric constant in and their environment with dielectric constant out. These self-energy corrections substantially open the band gap of the nanowires when in out, and decrease slower 1/ R than quantum confinement with increasing R. They are thus far from negligible in most experimental setups. We introduce a semi-analytical model for practical use. This semianalytical model is found in very good agreement with

Electronic transitions involved in the absorption spectrum and dual luminescence of tetranuclear cubane [Cu4I4(pyridine)4] cluster: a density functional theory/time-dependent density functional theory investigation.

Abstract: We present a combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) study of the geometry, electronic structure, and absorption and emission properties of the tetranuclear "cubane" Cu4I4py4 (py = pyridine) system. The geometry of the singlet ground state and of the two lowest triplet states of the title complex were optimized, followed by TDDFT excited-state calculations. This procedure allowed us to characterize the nature of the excited states involved in the absorption spectrum and those responsible for the dual emission bands observed for this complex. In agreement with earlier experimental proposals, we find that while in absorption the halide-to-pyridine charge-transfer excited state (XLCT*) has a lower energy than the cluster-centered excited state (CC*), a strong geometrical relaxation on the triplet cluster-centered state surface leads to a reverse order of the excited states in emission.

Influence of the Al distribution on the structure, elastic properties, and phase stability of supersaturated Ti1- xAlxN

Abstract: Ti1−xAlxN films and/or their alloys are employed in many industrial applications due to their excellent mechanical and thermal properties. Synthesized by plasma-assisted vapor deposition, Ti1−xAlxN is reported to crystallize in the cubic NaCl (c) structure for AlN mole fractions below 0.4–0.91, whereas at larger Al contents the hexagonal ZnS-wurtzite (w) structure is observed. Here we use ab initio calculations to analyze the effect of composition and Al distribution on the metal sublattice on phase stability, structure, and elastic properties of c-Ti1−xAlxN and w-Ti1−xAlxN. We show that the phase stability of supersaturated c-Ti1−xAlxN not only depends on the chemical composition but also on the Al distribution of the metal sublattice. An increase of the metastable solubility limit of AlN in c-Ti1−xAlxN from 0.64 to 0.74 is obtained by decreasing the number of Ti–Al bonds. This can be understood by considering the Al distribution induced changes of the electronic structure, bond energy, and configuration...

Nanoscopic Materials: Size-Dependent Phenomena

Abstract: 1: Introduction 1.1: Clusters and nanoparticles 1.2: Feynman's vision 2: Bulk and interface 2.1: Gradients near surfaces 2.2: The coordination number rules the game 2.3: Surface science, a source of information for nanoscience 2.4: Particle size and microstrain 2.5: Biomimetics: nature as a source of inspiration for strategies in nanotechnology 3: Geometric structure, magic numbers, and coordination numbers of small clusters 3.1: The consequences of the range of the radial potential energy function 3.2: Magic numbers by geometric shells closing 3.3: Magic numbers by electronic shells closing 3.4: Cohesive energy and coordination number 4: Electronic structure 4.1: Discrete states versus band structure 4.2: The effects of dimensionality and symmetry in quantum structures 4.3: The nonmetal-to-metal transition 4.4: Work function, ionisation potential and electron affinity 4.5: Electronic structure of semiconductor and metal clusters 4.6: A semiconductor quantum dot electronic device 5: Magnetic properties 5.1: A brief primer on magnetism 5.2: The concept of frustration 5.3: Magnetic properties of small clusters 5.4: Ferromagnetic order in thin films and monoatomic chains 5.5: Finite size effects in magnetic resonance detection 6: Thermodynamics for finite size systems 6.1: Limitations of macroscopic thermodynamics 6.2: The basics of capillarity 6.3: Phase transitions of free liquid droplets 6.4: The Lotus effect 6.5: Classical nucleation theory 6.6: Shape control of nanocrystals 6.7: Size effects on ion conduction in solids 6.8: Principles of self-assembly 7: Adsorption, phase behaviour and dynamics of surface layers and in pores 7.1: Surface adsorption and pore condensation 7.2: Adsorption hysteresis and pore criticality 7.3: The melting point of pore-confined matter 7.4: Layering transitions 7.5: Liquid coexistence and ionic solutions in pores 7.6: The effect of pressure 7.7: Dynamics in pores 8: Phase transitions and dynamics of clusters 8.1: Melting point and melting enthalpy 8.2: Dynamics of metal clusters 9: Phase transitions of two-dimensional systems 9.1: Melting of thin layers 9.2: Structural phase transitions in thin layers 9.3: Glass transition of a polymer thin film 9.4: Surface alloy phases 10: Catalysis by metallic nanoparticles 10.1: Some general principles of catalysis by nanoparticles 10.2: Size-controlled catalytic clusters 10.3: Shape dependent catalytic activity 10.4: The effect of strain 10.5: The effect of alloying 10.6: Metal-support interaction 10.7: The influence of external bias voltage 11: Applications: facts and fictions 11.1: Nanomaterials 11.2: Nanotechnology 11.3: Hopes, hazards and hype

Phase Stability of Nickel Hydroxides and Oxyhydroxides

Abstract: In this paper, we investigate the phase stability of nickel hydroxides from first-principles. We predict that the previously uncharacterized crystal structure of III‐NiOOH is actually derived from the P3 host. Furthermore, we identify a plausible crystal structure for the -NiO2H2O0.67K0.33Hx phase that is consistent with available experimental observations. The proposed crystal structure has a P3 host and the K ions reside exactly between adjacent trigonal prismatic sites of the intercalation layer. We have also calculated the topotactic voltage curves for the and phases, and predict the existence of a large step in voltage at -NiOOH, which effectively limits the capacity of the Ni-hydroxide compound to one electron per Ni ion. Methodology We combine first-principles electronic structure calculations with a cluster expansion approach for the disorder of protons in the materials to calculate phase stability and thermodynamic properties of the nickel hydroxide system. The electronic structure calculations

Electronic structures of (Zn, TM)O (TM: V, Cr, Mn, Fe, Co, and Ni) in the self-interaction-corrected calculations

Abstract: We calculate the electronic structures of ZnO-based dilute magnetic semiconductors within the self-interaction-corrected local density approximation. The results are compared with those calculated within the standard local density approximation. We find the differences in the band gap energy, the energetic position of the Zn 3d bands, and the description of the transition-metal d bands.