Showing papers on "Electronic structure published in 2000"
TL;DR: In this paper, the authors introduce the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies, and demonstrate in situ TEM for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
Abstract: The physical and chemical properties of nanophase materials rely on their crystal and surface structures. Transmission electron microscopy (TEM) is a powerful and unique technique for structure characterization. The most important application of TEM is the atomic-resolution real-space imaging of nanoparticles. This article introduces the fundamentals of TEM and its applications in structural determination of shape-controlled nanocrystals and their assemblies. By forming a nanometer size electron probe, TEM is unique in identifying and quantifying the chemical and electronic structure of individual nanocrystals. Electron energy-loss spectroscopy analysis of the solid-state effects and mapping the valence states are even more attractive. In situ TEM is demonstrated for characterizing and measuring the thermodynamic, electric, and mechanical properties of individual nanostructures, from which the structure−property relationship can be registered with a specific nanoparticle/structure.
1,980 citations
TL;DR: In this article, the authors provide an overview of plane-wave pseudopotential density functional theory (DFT) methods applicable to studies of large periodic systems and present a number of algorithmic implementations, including ultrasoft pseudopotentials, efficient iterative schemes for solving the one-electron DFT equations, and computationally efficient codes for massively parallel computers.
Abstract: Recent developments in density functional theory (DFT) methods applicable to studies of large periodic systems are outlined. During the past three decades, DFT has become an essential part of computational materials science, addressing problems in materials design and processing. The theory allows us to interpret experimental data and to generate property data (such as binding energies of molecules on surfaces) for known materials, and also serves as an aid in the search for and design of novel materials and processes. A number of algorithmic implementations are currently being used, including ultrasoft pseudopotentials, efficient iterative schemes for solving the one-electron DFT equations, and computationally efficient codes for massively parallel computers. The first part of this article provides an overview of plane-wave pseudopotential DFT methods. Their capabilities are subsequently illustrated by examples including the prediction of crystal structures, the study of the compressibility of minerals, and applications to pressure-induced phase transitions. Future theoretical and computational developments are expected to lead to improved accuracy and to treatment of larger systems with a higher computational efficiency. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 895-910, 2000
1,514 citations
TL;DR: In this article, the electronic energy band structure, site, and angular-momentum decomposed density of states (DOS) and charge-density contours of perovskite in the paraelectric phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with the atomic-sphere approximation using density-functional theory in its local density approximation.
Abstract: The electronic-energy band structure, site, and angular-momentum decomposed density of states (DOS) and charge-density contours of perovskite ${\mathrm{BaTiO}}_{3}$ in the paraelectric phase are calculated by the first-principles tight-binding linear muffin-tin orbitals method with the atomic-sphere approximation using density-functional theory in its local-density approximation. The calculated band structure shows a direct band gap of 1.2 eV at the \ensuremath{\Gamma} point in the Brillouin zone. The total DOS is compared to the experimental x-ray photoemission spectrum. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ba and ${\mathrm{TiO}}_{3}$ is mainly ionic and that the ${\mathrm{TiO}}_{3}$ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of ${\mathrm{BaTiO}}_{3}.$ The real and imaginary parts of the dielectric function and hence the optical constants (such as the reflectivity, refractive index, extinction coefficient, absorption coefficient, and the electron energy-loss spectrum) are calculated. The calculated spectra are compared with the experimental results for ${\mathrm{BaTiO}}_{3}$ at room temperature in the ferroelectric phase and are found to be in good agreement with the experimental data in the low-energy regions. The role of band-structure calculation as regards the optical properties of ${\mathrm{BaTiO}}_{3}$ is discussed.
966 citations
TL;DR: The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously as mentioned in this paper. But it does not consider the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states.
Abstract: The ab initio multiple spawning (AIMS) method is a time-dependent formulation of quantum chemistry, whereby the nuclear dynamics and electronic structure problems are solved simultaneously. Quantum mechanical effects in the nuclear dynamics are included, especially the nonadiabatic effects which are crucial in modeling dynamics on multiple electronic states. The AIMS method makes it possible to describe photochemistry from first principles molecular dynamics, with no empirical parameters. We describe the method and present the application to two molecules of interest in organic photochemistryethylene and cyclobutene. We show that the photodynamics of ethylene involves both covalent and ionic electronic excited states and the return to the ground state proceeds through a pyramidalized geometry. For the photoinduced ring opening of cyclobutene, we show that the disrotatory motion predicted by the Woodward−Hoffmann rules is established within the first 50 fs after optical excitation.
724 citations
IBM1
TL;DR: The projection of the electronic structure surrounding a magnetic Co atom to a remote location on the surface of a Cu crystal is reported; electron partial waves scattered from the real Co atom are coherently refocused to form a spectral image or ‘quantum mirage’.
Abstract: Image projection relies on classical wave mechanics and the use of natural or engineered structures such as lenses or resonant cavities. Well-known examples include the bending of light to create mirages in the atmosphere, and the focusing of sound by whispering galleries. However, the observation of analogous phenomena in condensed matter systems is a more recent development, facilitated by advances in nanofabrication. Here we report the projection of the electronic structure surrounding a magnetic Co atom to a remote location on the surface of a Cu crystal; electron partial waves scattered from the real Co atom are coherently refocused to form a spectral image or 'quantum mirage'. The focusing device is an elliptical quantum corral, assembled on the Cu surface. The corral acts as a quantum mechanical resonator, while the two-dimensional Cu surface-state electrons form the projection medium. When placed on the surface, Co atoms display a distinctive spectroscopic signature, known as the many-particle Kondo resonance, which arises from their magnetic moment. By positioning a Co atom at one focus of the ellipse, we detect a strong Kondo signature not only at the atom, but also at the empty focus. This behaviour contrasts with the usual spatially-decreasing response of an electron gas to a localized perturbation.
713 citations
TL;DR: In this article, the full-potential linearized augmented plane-wave method has been used to investigate detailed electronic and optical properties of anatase in the structure, and the fully optimized structure obtained by minimizing the total energy and atomic forces are in good agreement with experiment.
Abstract: First-principles calculations using the full-potential linearized augmented plane-wave method have been performed to investigate detailed electronic and optical properties of ${\mathrm{TiO}}_{2}$ in the anatase structure. The fully optimized structure, obtained by minimizing the total energy and atomic forces, are in good agreement with experiment. Stabilization of the structure by the trade off between a favorable coordination in the ${\mathrm{sp}}^{2}$ hybridization and the Coulomb repulsion among oxygen atoms is also demonstrated. We calculate band structure, densities of states and charge densities, and interpret their features in terms of the bonding structure in the molecular orbital picture. The optical properties, calculated within the dipole approximation, are found to agree with recent experiments on single crystals of anatase ${\mathrm{TiO}}_{2}.$ Near the absorption edge, the results show a significant optical anisotropy in the components parallel and perpendicular to the c axis. We demonstrate that this large dichroism results from the existence of nonbonding ${d}_{\mathrm{xy}}$ orbitals located at the bottom of the conduction bands, which allows direct dipole transitions dominantly for the perpendicular component.
634 citations
590 citations
TL;DR: The theory unriddles and unifies previous band gap studies and predicts the shifting, merging, and splitting of Van Hove singularities in the density of state, and the zigzag pattern of band gap change with strains.
Abstract: Electronic structure of deformed carbon nanotubes varies widely depending on their chirality and deformation mode. We present a framework to analyze these variations by quantifying the dispersion relation and density of states. The theory is based on the H\"uckel tight-binding model and confirmed by four orbital tight-binding simulations of nanotubes under stretching, compression, torsion, and bending. It unriddles and unifies previous band gap studies and predicts the shifting, merging, and splitting of Van Hove singularities in the density of state, and the zigzag pattern of band gap change with strains. Possible applications to nanotube devices and spectroscopy research are also presented.
561 citations
TL;DR: Scanning tunnelling microscopy is used to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu 2O8+δ and reveals the long-sought four-fold symmetric quasiparticle ‘cloud’ aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials.
Abstract: Although the crystal structures of the copper oxide high-temperature superconductors are complex and diverse, they all contain some crystal planes consisting of only copper and oxygen atoms in a square lattice: superconductivity is believed to originate from strongly interacting electrons in these CuO2 planes. Substituting a single impurity atom for a copper atom strongly perturbs the surrounding electronic environment and can therefore be used to probe high-temperature superconductivity at the atomic scale. This has provided the motivation for several experimental and theoretical studies. Scanning tunnelling microscopy (STM) is an ideal technique for the study of such effects at the atomic scale, as it has been used very successfully to probe individual impurity atoms in several other systems. Here we use STM to investigate the effects of individual zinc impurity atoms in the high-temperature superconductor Bi2Sr2CaCu2O8+delta. We find intense quasiparticle scattering resonances at the Zn sites, coincident with strong suppression of superconductivity within approximately 15 A of the scattering sites. Imaging of the spatial dependence of the quasiparticle density of states in the vicinity of the impurity atoms reveals the long-sought four-fold symmetric quasiparticle 'cloud' aligned with the nodes of the d-wave superconducting gap which is believed to characterize superconductivity in these materials.
540 citations
TL;DR: In this article, an accurate method for solving the electronic Schrodinger equation that is applicable to a broad range of moleculesthe CCSD(T) method and families of basis sets that systematically converge to the complete basis set limitthe correlation consistent basis sets is presented.
Abstract: During the past decade dramatic progress has been made in calculating the binding energies of molecules. This is the result of two advances reported in 1989: an accurate method for solving the electronic Schrodinger equation that is applicable to a broad range of moleculesthe CCSD(T) methodand families of basis sets that systematically converge to the complete basis set limitthe correlation consistent basis sets. The former provides unprecedented accuracy for the prediction of a broad range of molecular properties, including molecular binding energies. The latter provides a means to systematically approach the complete basis set limit, i.e., the exact solutions of approximations to the Schrodinger equation. These two advances combined with a thorough analysis of the errors involved in electronic structure calculations lead to clear guidelines for ab initio calculations of binding energies, ranging from the strong bonds derived from chemical interactions to the extremely weak binding due to dispersion int...
495 citations
TL;DR: In this article, the C2H5• + O2 reaction has been examined in detail via highly sophisticated electronic structure methods, including the geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products.
Abstract: The C2H5• + O2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical (X 2A‘) with O2 (X 3 , a 1Δg) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1−M5) involving the ground-state reactants are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, Ea(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O2 to give e...
TL;DR: The effects of impurities and local structural defects on the conductance of metallic carbon nanotubes are calculated using an ab initio pseudopotential method within the Landauer formalism and shows a much more complex behavior than the prediction from the widely used pi-electron tight-binding model.
Abstract: The effects of impurities and local structural defects on the conductance of metallic carbon nanotubes are calculated using an ab initio pseudopotential method within the Landauer formalism. Substitutionally doped boron or nitrogen produces quasibound impurity states of a definite parity and reduces the conductance by a quantum unit $({2e}^{2}/h)$ via resonant backscattering. These resonant states show strong similarity to acceptor or donor states in semiconductors. The Stone-Wales defect also produces quasibound states and exhibits quantized conductance reduction. In the case of a vacancy, the conductance shows a much more complex behavior than the prediction from the widely used $\ensuremath{\pi}$-electron tight-binding model.
TL;DR: In this article, the electronic and magnetic properties of the layered oxide have been analyzed using the general potential linearized augmented plane wave method and it is found that the thermopower and specific heat data above 10 K are well accounted for with only moderate renormalization in spite of the fact that this is a strongly correlated system characterized by strong instabilities.
Abstract: First principles density functional calculations of the electronic and magnetic properties of the layered oxide ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}$ have been performed using the general potential linearized augmented plane wave method. The electronic transport properties are discussed in terms of the calculated paramagnetic band structure. It is found that the thermopower and specific heat data above 10 K are well accounted for with only moderate renormalization in spite of the fact that this is a strongly correlated system characterized by $W\ensuremath{\ll}U.$ Weak instabilities of itinerant magnetic character are found. The low temperature properties $(T$ below 10 K) are discussed in terms of these.
TL;DR: In this paper, electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique, and the indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively.
Abstract: Electrical and optical properties of CuAlO2, a p-type conducting transparent oxide, were examined for the thin films prepared by the pulsed laser deposition technique. The indirect and direct allowed optical band gaps were evaluated to be ∼1.8 and ∼3.5 eV, respectively. The conductivity at 300 K was ∼3×10−1 S cm−1 and its temperature dependence is of the thermal-activation type (activation energy ≈0.2 eV) at temperatures >220 K but is of the variable-range hopping type (log σ∝T−1/4) at <220 K. It was inferred that an admixed state of Cu 3d and O 2p primarily constitutes the upper valence band, which controls transport of positive holes, from a combined information on ultraviolet photoemission spectrum with x-ray photoemission spectrum. An energy band calculation by full-potential linearized augmented plane wave method substantiated the experimental findings. The present results gave a solid basis for our working hypothesis [Nature (London) 389, 939 (1997)] for chemical design of p-type conducting transpar...
TL;DR: In this paper, the unusual electronic structure of 2FeMoO 6 was analyzed by combining ab initio and model Hamiltonian approaches, and the results indicated that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites.
Abstract: We have analyzed the unusual electronic structure of $Sr_2FeMoO_6$ combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength $(U_{eff})$ at the Mo site due to these renormalized interaction strengths.
TL;DR: A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, Gd5(Si2Ge2), across its Curie temperature reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the (Si, Ge)-(Si,Ge) dimers that are richer in Ge increase their distances by 0.859(3) A and lead to twinning.
Abstract: A temperature-dependent, single crystal x-ray diffraction study of the giant magnetocaloric material, ${\mathrm{Gd}}_{5}({\mathrm{Si}}_{2}{\mathrm{Ge}}_{2})$, across its Curie temperature (276 K) reveals that the simultaneous orthorhombic to monoclinic transition occurs by a shear mechanism in which the $(\mathrm{Si},\mathrm{Ge})\ensuremath{-}(\mathrm{Si},\mathrm{Ge})$ dimers that are richer in Ge increase their distances by 0.859(3) \AA{} and lead to twinning. The structural transition changes the electronic structure, and provides an atomic-level model for the change in magnetic behavior with temperature in the ${\mathrm{Gd}}_{5}({\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}{)}_{4}$.
TL;DR: Artificial electronic structure is investigated by injecting optically a controlled number of electrons and holes into an isolated single quantum dot, which forms complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms.
Abstract: Quantum dots1,2,3,4,5,6,7 or ‘artificial atoms’ are of fundamental and technological interest—for example, quantum dots8,9 may form the basis of new generations of lasers The emission in quantum-dot lasers originates from the recombination of excitonic complexes, so it is important to understand the dot's internal electronic structure (and of fundamental interest to compare this to real atomic structure) Here we investigate artificial electronic structure by injecting optically a controlled number of electrons and holes into an isolated single quantum dot The charge carriers form complexes that are artificial analogues of hydrogen, helium, lithium, beryllium, boron and carbon excitonic atoms We observe that electrons and holes occupy the confined electronic shells in characteristic numbers according to the Pauli exclusion principle In each degenerate shell, collective condensation of the electrons and holes into coherent many-exciton ground states takes place; this phenomenon results from hidden symmetries (the analogue of Hund's rules for real atoms) in the energy function that describes the multi-particle system Breaking of the hidden symmetries leads to unusual quantum interferences in emission involving excited states
TL;DR: In this article, a combination of Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) leads to a consistent description of the electronic and structural properties in comparative studies of the three compounds.
Abstract: Bulk properties of the isostructural oxides MgO, NiO, and CoO have been calculated quantum chemically with periodic models and compared with experimental data from the literature. Ab initio Hartree-Fock, gradient-corrected density-functional methods, and hybrid approaches have been used for the calculation of the lattice constants, heats of atomization, and electronic structures. General trends of the effects of electron correlation and the treatment of exchange on the calculated properties are observed. None of the standard methods considered provided results in agreement with experimental data for all properties. A combination of Becke's three-parameter exchange functional and the Lee-Yang-Parr correlation functional (B3LYP) leads to a consistent description of the electronic and structural properties in comparative studies of the three compounds. Other methods are more accurate than B3LYP if certain properties or compounds are considered. The combination of the Hartree-Fock exchange functional with the Lee-Yang-Parr density-functional correlation is the best method for the open-shell transition-metal oxides NiO and CoO in terms of relative stability and geometry and the electronic structure of the valence band. The absolute values of calculated heats of atomization, however, are generally too small. The density-functional method based on the Perdew-Wang generalized gradient approximation (PWGGA) is preferable for the calculation of thermodynamic properties for all compounds but is less reliable in the prediction of structural and electronic properties. A hybrid approach based on the PWGGA method is proposed that improves the results for bulk geometries and electronic properties while maintaining the high quality of calculated energetic results.
TL;DR: In this paper, a theory of the electronic structure of GaN/AlN quantum dots (QD's), including built-in strain and electric-field effects, is presented.
Abstract: We present a theory of the electronic structure of GaN/AlN quantum dots (QD's), including built-in strain and electric-field effects. A Green's function technique is developed to calculate the three-dimensional (3D) strain distribution in semiconductor QD structures of arbitrary shape and of wurtzite (hexagonal) crystal symmetry. We derive an analytical expression for the Fourier transform of the QD strain tensor, valid for the case when the elastic constants of the QD and matrix materials are equal. A simple iteration procedure is described, which can treat differences in the elastic constants. An analytical formula is also derived for the Fourier transform of the built-in electrostatic potential, including the strain-induced piezoelectric contribution and a term associated with spontaneous polarization. The QD carrier spectra and wave functions are calculated using a plane-wave expansion method we have developed, and a multiband $\mathbf{k}\ensuremath{\cdot}\mathbf{P}$ model. The method used is very efficient, because the strain and built-in electric fields can be included analytically through their Fourier transforms. We consider in detail the case of GaN/AlN QD's in the shape of truncated hexagonal pyramids. We present the calculated 3D strain and electrostatic potential distributions, the carrier spectra, and wave functions in the QD's. Due to the strong built-in electric field, the holes are localized in the wetting layer just below the QD bottom, while electrons are pushed up to the pyramid top. Both also experience an additional lateral confinement due to the built-in field. We examine the influence of several key factors on the calculated confined state energies. Use of a one-band, effective-mass Hamiltonian overestimates the electron confinement energies by \ensuremath{\sim}100 meV, because of conduction-band nonparabolicity effects. By contrast, a one-band valence Hamiltonian provides good agreement with the calculated multiband ground-state energy. Varying the QD shape has comparatively little effect on the calculated levels, because of the strong lateral built-in electric field. Overall, the transition energies depend most strongly on the assumed built-in electric field. The calculated variation of transition energy with quantum dot size is in good agreement with the available experimental data.
TL;DR: In this article, the authors adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens-shaped InAs quantum dots within the ''linear combination of bulk bands'' method.
Abstract: We adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens-shaped InAs quantum dots within the ``linear combination of bulk bands'' method. We present a detailed comparison with experiment, including quantites such as the single-particle electron and hole energy level spacings, the excitonic band gap, the electron-electron, hole-hole, and electron-hole Coulomb energies and the optical polarization anisotropy. We find a generally good agreement, which is improved even further for a dot composition where some Ga has diffused into the dots.
TL;DR: In this article, the authors carried out ab initio calculations using density functional theory under the generalized gradient approximation for periodic systems and found that the edge substitution model emerges as the most stable structure and provides an excellent agreement with local structures experimentally determined on real catalysts by extended X-ray absorption fine structure.
TL;DR: In this paper, the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities were calculated using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice.
TL;DR: In this paper, it was shown that Li undergoes a structural transition from a high-pressure face-centred-cubic phase, through an intermediate rhombohedral modification, to a cubic polymorph with 16 atoms per unit cell.
Abstract: Lithium is considered a 'simple' metal because, under ordinary conditions of pressure and temperature, the motion of conduction electrons is only weakly perturbed by interactions with the cubic lattice of atomic cores. It was recently predicted that at pressures below 100 GPa, dense Li may undergo several structural transitions, possibly leading to a 'paired-atom' phase with low symmetry and near-insulating properties. Here we report synchrotron X-ray diffraction measurements that confirm that Li undergoes pronounced structural changes under pressure. Near 39 GPa, the element transforms from a high-pressure face-centred-cubic phase, through an intermediate rhombohedral modification, to a cubic polymorph with 16 atoms per unit cell. This cubic phase has not been observed previously in any element; unusually, its calculated electronic density of states exhibits a pronounced semimetal-like minimum near the Fermi energy. We present total-energy calculations that provide theoretical support for the observed phase transition sequence. Our calculations indicate a large stability range of the 16-atom cubic phase relative to various other crystal structures tested here.
TL;DR: In this article, the electronic structure of Ge nanocrystals using a sp3 tight binding description is studied and analytical laws for the confinement energies, valid over the whole range of sizes, are derived.
Abstract: The electronic structure of Ge nanocrystals is studied using a sp3 tight binding description. Analytical laws for the confinement energies, valid over the whole range of sizes, are derived. We validate our results with ab initio calculations in the local density approximation for smaller clusters. Comparing to experimental data, we conclude that, similar to the case of silicon: (a) the blue-green photoluminescence (PL) of Ge nanocrystals comes from defects in the oxide and (b) the size dependent PL in the near infrared probably involves a deep trap in the gap of the nanocrystals. We predict that the radiative lifetimes remain long in spite of the small difference (0.14 eV) between direct and indirect gaps of bulk Ge.
TL;DR: This work has shown that the multiconfiguration, self-consistent field (MCSCF) approach is capable, in principle, of providing a uniform description of the evolution of the transition-metal-main-group-element bond.
Abstract: Understanding the nature of the transition-metal (TM)-main-group-element bond is important in many areas of science, such as organometallic chemistry,1,2 surface science,3 catalysis,4 high-temperature chemistry,5,6 and astrophysics.7,8 For example, the oxides are of interest to astrophysics as the constituents of cool stars and to surface science as zero-order models for the oxidation of a transition-metal surface.9 These systems are electronically complex and very difficult to treat theoretically. Indeed, while the use of quantum chemistry to obtain useful and reliable information about small organic molecules is now routine,11 a very different situation arises in the theoretical description of molecules containing a transition element. This is due to several factors, but the most important is that the extent of electron correlation required for even qualitatively correct results is significantly raised relative to molecules containing only main-group elements. The correlation between the electronic states of a molecule and the states of its constituent atoms has been an important concept in chemistry and physics for many years. For example, we know that if a molecule is composed of atoms that have large energy differences between their various electronic states, the molecule will be characterized by electronic states that are widely spaced, or granular. In the context of the preeminent orbital theory, the Hartree-Fock (HF) theory, this means that the molecular orbitals of molecules formed from these atoms will be widely spaced in energy, and the HF configuration will dominate the wave function around equilibrium. This is, of course, the reason the HF theory has achieved its unique role as both an interpretive and predictive tool in the chemistry of the firstand second-row, main-group elements. While the situation becomes somewhat flawed as one moves away from equilibrium structures, the conceptual simplicity and interpretability of the orbital picture can be preserved by constructing self-consistent wave functions in which the HF configuration is augmented by one or more configurations. This multiconfiguration, self-consistent field (MCSCF) approach is capable, in principle, of providing a uniform description of the evolution 679 Chem. Rev. 2000, 100, 679−716
TL;DR: In this article, the role of electron-hole (e-h) interactions in quantum dots has been investigated and it has been shown that electron relaxation is dominated not by phonon emission but by the e-h energy transfer.
Abstract: To evaluate the role of nonphonon energy relaxation mechanisms in quantum dots and in particular the role of electron-hole (e-h) interactions, we have studied femtosecond carrier dynamics in CdSe colloidal nanoparticles in which the e-h separation (coupling) is controlled using different types of surface ligands. In dots capped with hole accepting molecules, the e-h coupling is strongly reduced after the hole is transferred to a capping group. By re-exciting an electron within the conduction band at different stages of hole transfer and monitoring its relaxation back into the ground state, we observe a more than tenfold increase in the electron relaxation time (from 250 fs to 3 ps) after the completion of the hole transfer to the capping molecule. This strongly indicates that electron relaxation in quantum dots is dominated not by phonon emission but by the e-h energy transfer.
TL;DR: The determination of the bulk Ce 4f electronic states of these compounds resolves differences, and the power of this technique is demonstrated by applying it to the cerium compounds CeRu2Si2 and CeRu 2.
Abstract: Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref 6), it should become feasible to probe the bulk electronic states in these materials Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2 Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences
TL;DR: The hypervalent electron counting scheme developed in this paper, along with the classical Zintl-Klemm electron counting rules, gives an easy qualitative understanding of bonding in a wide variety of intermetallic compounds of heavy main group elements.
Abstract: We construct a theory for electron-rich polyanionic networks in the intermetallic compounds of heavy late main group elements, building a bonding framework that makes a connection to well-understood hypervalent bonding in small molecules such as XeF(4), XeF(2), and I(3)(-). What we do is similar in spirit to the analogy between the Zintl-Klemm treatment of classical polyanionic networks and the octet rule for molecules. We show that the optimal electron count for a linear chain of a heavy main group element is seven electrons per atom, six electrons per atom for a square lattice, and five electrons per atom for a simple cubic lattice. Suggestions that these electron counts are appropriate already exist in the literature. We also derive electron counts for more complicated topologies, including one-dimensional ladders and one dimensional strips cut from a square lattice. We also study pairing (Peierls) distortions from these ideal geometries as well as other deformations. The presence of s-p mixing (or its absence) plays a critical role in the propensity for pairing and, in general, in determining the geometrical and electronic structure of these phases. Hypervalent bonding goes along with the relative absence of significant s-p interaction; there is a continuum of such mixing, but also a significant difference between the second-row and heavier elements. We attribute the existence of undistorted metallic networks of the latter elements to diminished s-p mixing, which in turn is due to the contraction of less-screened s orbitals relative to p orbitals down the groups in the Periodic Table. The number of electrons in the polyanionic network may be varied experimentally. An important general principle emerges from our theoretical analysis: upon oxidation a hypervalent structure transforms into a classical one with the same lattice dimensionality, while upon Peierls distortion the hypervalent structures transform into classical ones with the lattice dimensionality reduced. Dozens of crystal structure types, seemingly unrelated to each other, may be understood using the unifying concept of electron-rich multicenter bonding. Antimonides, which are explored in great detail in the current work, conform particularly well to the set of electron counting rules for electron-rich nonclassical networks. Some deviation up and down from the ideal electron count is exhibited by known stannides and tellurides. We can also make sense of the bonding in substantially more complicated alloys, including La(12)Mn(2)Sb(30) and Tl(4)SnTe(3). The hypervalent electron counting scheme developed in this paper, along with the classical Zintl-Klemm electron counting rules, gives an easy qualitative understanding of bonding in a wide variety of intermetallic compounds of heavy main group elements.
TL;DR: The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10(-15) seconds) time resolution to provide information on the mechanical forces acting on cedium atoms that are "turned on" by photoexcitation.
Abstract: The dynamics of cesium atom motion above the copper(111) surface following electronic excitation with light was studied with femtosecond (10–15 seconds) time resolution. Unusual changes in the surface electronic structure within 160 femtoseconds after excitation, observed by time-resolved two-photon photoemission spectroscopy, are attributed to atomic motion in a copper–cesium bond-breaking process. Describing the change in energy of the cesium antibonding state with a simple classical model provides information on the mechanical forces acting on cesium atoms that are “turned on” by photoexcitation. Within 160 femtoseconds, the copper–cesium bond extends by 0.35 angstrom from its equilibrium value.
TL;DR: In this paper, the use of time-dependent density functional theory (TDDFT) is considered for the determination of electronic excitation energies, and the authors highlight the problems with Rydberg excitations arising from neglect of the integer discontinuity in the potential.
Abstract: The use of time-dependent density functional theory (TDDFT) is considered for the determination of electronic
excitation energies. Using beryllium and methylene as examples, we highlight (i) problems with Rydberg excitations
arising from neglect of the integer discontinuity in the potential; (ii) the absence of pure double excitations
in calculations using conventional exchange-correlation functionals; (iii) quantitative differences
between excitation energies determined using TDDFT and the ‘delta SCF’ method; (iv) non-additivity of excitation
energies calculated using TDDFT from different electronic states; (v) an apparent failure to predict single excitations
to states that are lower than the reference
states and (vi)
the difference in quality
between excitations to singlet and triplet states.