Showing papers on "Electronic structure published in 2001"

Bulk electronic structure of SrTiO3: Experiment and theory

Abstract: Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3 Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states The experimentally determined indirect band gap energy is 325 eV, while the direct band gap energy is 375 eV The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 325 eV up to 15 eV The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV Interband transitions from the O 2s band to the conduction bands appear at 26 eV A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons

Visualizing the Role of Bi 6s “Lone Pairs” in the Off-Center Distortion in Ferromagnetic BiMnO3

Abstract: Results of first-principles electronic structure calculations on the low-temperature monoclinic phase of the ferromagnetic perovskite BiMnO3 [Atou et al. J. Solid State Chem. 1999, 145, 639] are presented. In agreement with experiments, the calculations obtain an insulating ferromagnetic ground state for this material. The role of Bi 6s “lone pairs” in stabilizing the highly distorted perovskite structure is examined using real-space visualization of the electronic structure. Comparisons are drawn with the electronic structures of hypothetical cubic BiMnO3 and with the electronic structure of the prototypical perovskite manganite, LaMnO3. The exploitation of s electron lone pairs in the design of new ferroic materials is suggested.

Observation of all-metal aromatic molecules.

Abstract: Aromaticity is a concept invented to account for the unusual stability of an important class of organic molecules: the aromatic compounds. Here we report experimental and theoretical evidence of aromaticity in all-metal systems. A series of bimetallic clusters with chemical composition MAl4– (M = Li, Na, or Cu), was created and studied with photoelectron spectroscopy and ab initio calculations. All the MAl4– species possess a pyramidal structure containing an M+ cation interacting with a square Al42– unit. Ab initio studies indicate that Al42– exhibits characteristics of aromaticity with two delocalized π electrons (thus following the 4n + 2 electron counting rule) and a square planar structure and maintains its structural and electronic features in all the MAl4– complexes. These findings expand the aromaticity concept into the arena of all-metal species.

Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping

Abstract: TiO2-based powder, including 0.1 at% of N doped in the rutile lattice, has been synthesized by oxidation of TiN. As a result, a significant shift of the absorption edge to a lower energy in the visible-light region has been observed. The substitutional doping of N into the TiO2 lattice is found to be effective; its 2p states contribute to the band-gap narrowing by mixing with O 2p as shown in ab initio electronic structure calculations.

Spin waves and electronic interactions in La2CuO4

Abstract: The magnetic excitations of the square-lattice spin-1/2 antiferromagnet and high- T(c) parent compound La2CuO4 are determined using high-resolution inelastic neutron scattering. Sharp spin waves with absolute intensities in agreement with theory including quantum corrections are found throughout the Brillouin zone. The observed dispersion relation shows evidence for substantial interactions beyond the nearest-neighbor Heisenberg term which can be understood in terms of a cyclic or ring exchange due to the strong hybridization path around the Cu4O4 square plaquettes.

Highly luminescent silicon nanocrystals with discrete optical transitions.

Abstract: A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 °C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25−40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely...

Stabilization of Ferromagnetic States by Electron Doping in Fe-, Co- or Ni-Doped ZnO

Abstract: Detailed guidelines for controlling magnetic states in ZnO-based diluted magnetic semiconductors are given based on ab initio electronic structure calculations within the local spin density approximation using the Korringa-Kohn-Rostoker method. Effects of disorder were taken into account by the coherent potential approximation. It was found that the ferromagnetic state was stabilized by electron doping in the case of Fe-, Co- or Ni-doped ZnO. From the view point of practical applications, it is possible to realize a high-Curie-temperature ferromagnet, because n-type ZnO is easily available.

A Three-Dimensional Synthetic Metallic Crystal Composed of Single-Component Molecules

Abstract: Molecular metals normally require charge transfer between two different chemical species. We prepared crystals of [Ni(tmdt) 2 ] (tmdt, trimethylenetetrathiafulvalenedithiolate) and carried out crystal structure analyses and resistivity measurements. The analyses and measurements revealed that these single-component molecular crystals are metallic from room temperature down to 0.6 kelvin. Ab initio molecular orbital calculations suggested that π molecular orbitals form conduction bands. The compact molecular arrangement, intermolecular overlap integrals of the highest occupied and lowest unoccupied molecular orbitals, and tight-binding electronic band structure calculation revealed that [Ni(tmdt) 2 ] is a three-dimensional synthetic metal composed of planar molecules.

Compensation mechanism for N acceptors in ZnO

Abstract: We present a mechanism for the compensation of N acceptors in ZnO through real-space multigrid electronic structure calculations within the local-density-functional approximation. We find that at low N doping levels using a normal ${\mathrm{N}}_{2}$ source, O vacancies are the main compensating donors for N acceptors, while N acceptors are compensated via the formation of defect complexes with Zn antisites at high doping levels. When an active plasma ${\mathrm{N}}_{2}$ gas is used to increase the N solubility, N acceptors are still greatly compensated by ${\mathrm{N}}_{2}$ molecules at oxygen sites and N-acceptor--${\mathrm{N}}_{2}$ complexes, explaining the difficulty in achieving low-resistivity p-type ZnO.

First-principles elastic constants and electronic structure of α − Pt 2 Si and PtSi

Abstract: We have carried out a first-principles study of the elastic properties and electronic structure for two room-temperature stable Pt silicide phases, tetragonal $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Pt}}_{2}\mathrm{Si},$ and orthorhombic PtSi. We have calculated all of the equilibrium structural parameters for both phases: the a and c lattice constants for $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Pt}}_{2}\mathrm{Si}$ and the a, b, and c lattice constants and four internal structural parameters for PtSi. These results agree closely with experimental data. We have also calculated the zero-pressure elastic constants, confirming prior results for pure Pt and Si and predicting values for the six (nine) independent, nonzero elastic constants of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Pt}}_{2}\mathrm{Si}$ (PtSi). These calculations include a full treatment of all relevant internal displacements induced by the elastic strains, including an explicit determination of the dimensionless internal displacement parameters for the three strains in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Pt}}_{2}\mathrm{Si}$ for which they are nonzero. We have analyzed the trends in the calculated elastic constants, both within each material as well as among the two silicides and the pure Pt and Si phases. The calculated electronic structure confirms that the two silicides are poor metals with a low density of states at the Fermi level, and consequently we expect that the Drude component of the optical absorption will be much smaller than in good metals such as pure Pt. This observation, combined with the topology found in the first-principles spin-orbit split band structure, suggests that it may be important to include the interband contribution to the optical absorption, even in the infrared region.

Energetics of native defects in ZnO

Abstract: We have investigated the formation energies and electronic structure of native defects in ZnO by a first-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type conductivity of undoped ZnO.

Finite-Temperature Magnetism of Transition Metals: An ab initio Dynamical Mean-Field Theory

Abstract: We present an ab initio quantum theory of the finite-temperature magnetism of iron and nickel. A recently developed technique which combines dynamical mean-field theory with realistic electronic structure methods successfully describes the many-body features of the one electron spectra and the observed magnetic moments below and above the Curie temperature.

Energetics and electronic structures of encapsulated C60 in a carbon nanotube.

Abstract: We report total-energy electronic structure calculations that provide energetics of encapsulation of ${\mathrm{C}}_{60}$ in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the $(10,10)$ nanotube, whereas the processes are endothermic for the $(8,8)$ and $(9,9)$ nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 \AA{}. We also find that the ${\mathrm{C}}_{60}@(10,10)$ is a metal with multicarriers each of which distributes either along the nanotube or on the ${\mathrm{C}}_{60}$ chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.

Bonding and XPS chemical shifts in ZrSiO 4 versus SiO 2 and ZrO 2 : Charge transfer and electrostatic effects

Abstract: The degree of ionic/covalent character in oxides has a great influence on the electronic structure and the material's properties. A simple phenomenological rule is currently used to predict the evolution of covalence/ionicity in mixed oxides compared to the parent ones, and is also widely used to interpret the x-ray photoelectron spectroscopy (XPS) binding-energy shifts of the cations in terms of charge transfer. We test the validity of this simple rule and its application to XPS of mixed oxides with a prototypical system: zircon ${\mathrm{ZrSiO}}_{4}$ and parent oxides ${\mathrm{ZrO}}_{2}$ and ${\mathrm{SiO}}_{2}.$ The ionic charges on Si, Zr, and O were extracted from the density functional theory in the local density approximation calculations in the plane-wave formalism. In agreement with the predictions of the phenomenological rule, the most ionic cation (Zr) becomes more ionic in ${\mathrm{ZrSiO}}_{4}$ than in ${\mathrm{ZrO}}_{2},$ while the more covalent one (Si) experiences a corresponding increase in covalence with respect to ${\mathrm{SiO}}_{2}.$ The XPS chemical shifts of the O $1s,$ Si $2p,$ and Zr ${3d}_{5/2}$ photoelectron lines in the three oxides were measured and the respective contributions of charge transfer and electrostatic effects (initial state), as well as extra-atomic relaxation effects (final state) evaluated. The validity of the phenomenological rule of mixed oxides used in x-ray electron spectroscopy as well as the opportunity to use the $\mathrm{O}1s$ binding-energy shifts to derive a scale of covalence in silicates is discussed.

Structurally Tailored Organic−Inorganic Perovskites: Optical Properties and Solution-Processed Channel Materials for Thin-Film Transistors

Abstract: The structures, optical properties, and field-effect mobilities of three semiconducting m-fluorophenethylammonium-based (C6H4FC2H4NH3)2SnI4 perovskites (m = 2, 3, or 4) are reported and compared with the analogous measurements for the nonfluorosubstituted phenethylammonium system, (C6H5C2H4NH3)2SnI4. The (4-fluorophenethylammonium)2SnI4 system adopts a fully ordered monoclinic (P21/c) cell with the lattice parameters a = 16.653(2) A, b = 8.6049(8) A, c = 8.7551(8) A, β = 98.644(2)°, and Z = 2. Both (3-fluorophenethylammonium)2SnI4 and (2-fluorophenethylammonium)2SnI4 are refined in a monoclinic (C2/c) subcell with the lattice parameters a = 34.593(4) A, b = 6.0990(8) A, c = 12.254(2) A, β = 103.917(2)°, and Z = 4 and a = 35.070(3) A, b = 6.1165(5) A, c = 12.280(1) A, β = 108.175(1)°, and Z = 4, respectively. Each hybrid structure consists of sheets of corner-sharing distorted SnI6 octahedra separated by bilayers of fluorophenethylammonium cations. The dominant low energy feature in the optical absorption ...

Size-Dependent Optical Spectroscopy of a Homologous Series of CdSe Cluster Molecules

Abstract: The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and ph...

Electronic, bonding, and optical properties of CeO2 and Ce2O3 from first principles - art. no. 115108

Abstract: First-principles electronic structure calculations of cerium oxide in two forms, CeO2 and Ce2O3, are Presented. The 4f state of Ce is treated as a part of the inner core in Ce2O3 and as a valence-band-like state in CeO2,. The calculated ground-state and m

Absolute Hydration Free Energy of the Proton from First-Principles Electronic Structure Calculations

Abstract: The absolute hydration free energy of the proton, ΔGhyd298(H+), is one of the fundamental quantities for the thermodynamics of aqueous systems. Its exact value remains unknown despite extensive experimental and computational efforts. We report a first-principles determination of ΔGhyd298(H+) by using the latest developments in electronic structure theory including solvation effects. High level ab initio calculations have been performed with a supermolecule-continuum approach based on a recently developed self-consistent reaction field model known as surface and volume polarization for electrostatic interaction (SVPE) or fully polarizable continuum model (FPCM). In the supermolecule-continuum approach, part of the solvent surrounding the solute is treated quantum mechanically and the remaining bulk solvent is approximated by a dielectric continuum medium. With this approach, the calculated results can systematically be improved by increasing the number of quantum mechanically treated solvent molecules. ΔGh...

Interplay of magnetism and high-Tc superconductivity at individual Ni impurity atoms in Bi2Sr2CaCu2O8+delta.

Abstract: Magnetic interactions and magnetic impurities are destructive to superconductivity in conventional superconductors1. By contrast, in some unconventional macroscopic quantum systems (such as superfluid 3He and superconducting UGe2), the superconductivity (or superfluidity) is actually mediated by magnetic interactions. A magnetic mechanism has also been proposed for high-temperature superconductivity2,3,4,5,6. Within this context, the fact that magnetic Ni impurity atoms have a weaker effect on superconductivity than non-magnetic Zn atoms in the high-Tc superconductors has been put forward as evidence supporting a magnetic mechanism5,6. Here we use scanning tunnelling microscopy to determine directly the influence of individual Ni atoms on the local electronic structure of Bi2Sr2CaCu2O8+δ. At each Ni site we observe two d-wave impurity states7,8 of apparently opposite spin polarization, whose existence indicates that Ni retains a magnetic moment in the superconducting state. However, analysis of the impurity-state energies shows that quasiparticle scattering at Ni is predominantly non-magnetic. Furthermore, we show that the superconducting energy gap and correlations are unimpaired at Ni. This is in strong contrast to the effects of non-magnetic Zn impurities, which locally destroy superconductivity9. These results are consistent with predictions for impurity atom phenomena5,6 derived from a magnetic mechanism.

Evolution of III-V nitride alloy electronic structure: the localized to delocalized transition.

Abstract: Addition of nitrogen to III-V semiconductor alloys radically changes their electronic properties. We report large-scale electronic structure calculations of GaAsN and GaPN using an approach that allows arbitrary states to emerge, couple, and evolve with composition. We find a novel mechanism of alloy formation where localized cluster states within the gap are gradually overtaken by a downwards moving conduction band edge, composed of both localized and delocalized states. This localized to delocalized transition explains many of the hitherto puzzling experimentally observed anomalies in III-V nitride alloys.

Strong coupling between local moments and superconducting 'heavy' electrons in UPd2Al3.

Abstract: The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.

Interplay of magnetism and high-Tc superconductivity at individual Ni impurity atoms in Bi2Sr2CaCu2O8+d

Abstract: In conventional superconductors, magnetic interactions and magnetic impurity atoms are destructive to superconductivity. By contrast, in some unconventional systems, e.g. superfluid 3He and superconducting UGe2, superconductivity or superfluidity is actually mediated by magnetic interactions. A magnetic mechanism has also been proposed for high temperature superconductivity (HTSC) in which an electron magnetically polarizes its environment resulting in an attractive pairing-interaction for oppositely polarized spins. Since a magnetic impurity atom would apparently not disrupt such a pairing-interaction, it has also been proposed that the weaker influences on HTSC of magnetic Ni impurity atoms compared to those of non-magnetic Zn are evidence for a magnetic mechanism. Here we use scanning tunneling microscopy (STM) to determine directly the influence of individual Ni atoms on the electronic structure of Bi2Sr2CaCu2O8+d. Two local d-wave impurity-states are observed at each Ni. Analysis of their energies surprisingly reveals that the primary quasiparticle scattering effects of Ni atoms are due to non-magnetic interactions. Nonetheless, we also demonstrate that a magnetic moment coexists with unimpaired superconductivity at each Ni site. We discuss the implications of these phenomena, and those at Zn, for the pairing-mechanism.

Playing with Polarity

Abstract: We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga- and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed two-dimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices.

Electronic structure and properties of transition metal-benzene complexes.

Abstract: A comprehensive theoretical study of the geometries, energetics, and electronic structure of neutral and charged 3d transition metal atoms (M) interacting with benzene molecules (Bz) is carried out using density functional theory and generalized gradient approximation for the exchange-correlation potential. The variation of the metal-benzene distances, dissociation energies, ionization potentials, electron affinities, and spin multiplicities across the 3d series in MBz complexes differs qualitatively from those in M(Bz) 2. For example, the stability of Cr(Bz)2 is enhanced over that of CrBz by almost a factor of 30. On the other hand, the magnetic moment of Cr(Bz)2 is completely quenched although CrBz has the highest magnetic moment, namely 6IB ,i n the 3d metal-benzene series. In multidecker complexes involving V2(Bz)3 and Fe2(Bz)3, the metal atoms are found to couple antiferromagnetically. In addition, their dissociation energies and ionization potentials are reduced from those in corresponding M(Bz)2 complexes. All of these results agree well with available experimental data and demonstrate the important role the organic support can play on the properties of metal atoms/clusters.

On the Aromaticity of Square Planar Ga42- and In42- in Gaseous NaGa4- and NaIn4- Clusters

Abstract: We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa4- and NaIn4- Photoelectron spectra of the anions were obtained and compared with ab initio calculations We found that the ground state of the two anions contains a square planar dianion interacting with a Na+ cation The Ga42- and In42- dianions both possess two delocalized π electrons and are considered to be aromatic, similar to that recently found in Al42- Using calculations for a model compound, we showed that a recently synthesized Ga4−organometallic compound also contains an aromatic −Ga42-− unit, analogous to the gaseous clusters

Ground state electronic structures and spectra of zinc complexes of porphyrin, tetraazaporphyrin, tetrabenzoporphyrin, and phthalocyanine: A density functional theory study

Abstract: Density functional theory (DFT) electronic structure calculations were carried out to predict the structures and ground-state spectra for zinc complexes of porphyrin (ZnP), tetraazaporphyrin (ZnTAP), tetrabenzoporphyrin (ZnTBP), and phthalocyanine (ZnPc). All four porphyrins are found to have stable D4h structures. Structurally, meso-tetraaza substitutions significantly reduce the central hole in ZnTAP and ZnPc compared to ZnP. The excitation energies and oscillator strengths, computed by time-dependent DFT, provide a good account of the observed spectra of all four compounds. The TDDFT spectrum of ZnPc has a number of bands in the Soret region, in agreement with the experimental spectra that have been determined through spectral deconvolution. The low energy n→π* transition (Q′) reported for ZnPc, however, was not found in the computed spectrum. The effects of meso-tetraaza substitutions and tetrabenzo annulations on the spectrum of ZnP are discussed.

Surface electronic structure of plasma-treated indium tin oxides

Abstract: X-ray photoelectron spectroscopy (XPS) has been used to study the electronic structures of indium tin oxide (ITO) surfaces treated by O+, Ar+, and NHx+ plasmas. The XPS data show that there is a significant change in core level energies (In 3d5/2 O 1s, and Sn 3d5/2), in donor concentration (Sn4+), in valence band maximums (VBM), and in work functions on ITO surfaces being treated by O+ and NHx+ plasmas, compared with that of virgin and Ar+ plasma treated surfaces. Based on these experimental data, a surface band-bending theory is proposed. The theory explains that when Fermi energy of the plasma-treated surface is shifted towards the middle of the band gap: core levels will shift their energies to lower binding energies, VBM will bend upward, and work function will increase, as observed.

Bilayer Splitting in the Electronic Structure of Heavily Overdoped Bi 2 Sr 2 CaCu 2 O 8 + δ

Abstract: The electronic structure of heavily overdoped Bi2Sr2CaCu2O8+delta is investigated by angle-resolved photoemission spectroscopy. The long-sought bilayer band splitting in this two-plane system is observed in both normal and superconducting states, which qu