Showing papers on "Electronic structure published in 1997"

Surface electronic structure and reactivity of transition and noble metals

Abstract: We present self-consistent density functional calculations using the LMTO-ASA method of the variations in the surface electronic structure for pseudomorfic overlayers and impurities of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au on the other metals. Knowledge of these variations is of importance in understanding trends in the reactivity of metal surfaces. A simple model is presented which gives a description of the overall trends in the self-consistently calculated results.

Size, Shape, and Low Energy Electronic Structure of Carbon Nanotubes

Abstract: A theory of the long-wavelength low-energy electronic structure of graphite-derived nanotubules is presented. The propagating $\ensuremath{\pi}$ electrons are described by wrapping a massless two dimensional Dirac Hamiltonian onto a curved surface. The effects of the tubule size, shape, and symmetry are included through an effective vector potential which we derive for this model. The rich gap structure for all straight single wall cylindrical tubes is obtained analytically in this theory, and the effects of inhomogeneous shape deformations on nominally metallic armchair tubes are analyzed.

Electronic structure and optical properties of PbS and PbSe quantum dots

Abstract: The electronic structure of spherical PbS and PbSe quantum dots is calculated with a four-band envelope-function formalism. This calculation accounts for both exciton energies and wave functions with the correct symmetry of the materials. The selection rules and the strength of the dipole transitions of lead-salt quantum dots are derived accounting for the symmetry of the band-edge Bloch functions of the lead salts. The calculated energies of the optically allowed exciton states are found to be in good agreement with experimental data. The effects of many-body perturbations, such as Coulomb interactions and intervalley scattering, are also discussed.

Molecular dynamics study of water clusters, liquid, and liquid-vapor interface of water with many-body potentials

Abstract: The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structuralproperties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of vaporization, as well as the diffusion coefficient, are in excellent agreement with experimental values. Structuralproperties of liquidwater, such as the radial distribution functions, neutron, and x-ray scattering intensities, were calculated and critically evaluated against the experimental measurements. In all cases, we found the agreement between the observed data and the computed properties to be quite reasonable. The computed density profile of the water’s liquid/vapor interface shows that the interface is not sharp at a microscopic level and has a thickness of 3.2 A at 298 K. These results are consistent with those reported in earlier work on the same systems. The calculated surface tension at room temperature is in reasonable agreement with the corresponding experimental data. As expected, the computed average dipole moments of water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to their bulk values.

Isolation of Smaller Nanocrystal Au Molecules: Robust Quantum Effects in Optical Spectra

Abstract: Five massive gold-cluster molecules have been isolated in high yield and have undergone separate structural characterization, and their electronic structure has been deduced by optical absorption spectroscopy. These new molecules are distinguished by a crystalline (or quasicrystalline) core of densely packed Au atoms, ranging in size from ∼1.1 nm (∼40 atoms) to ∼1.9 nm (∼200 atoms), surrounded by a compact monolayer of various thio (RS) adsorbates. They are obtained as the thermally and environmentally stable products of the reductive decomposition of nonmetallic (−AuS(R)−) polymer in solution, are separated according to size by fractional crystallization or column chromatography, as monitored by high-mass spectrometry, and are characterized structurally by methods including X-ray diffraction (small and large angle), high-resolution electron microscopy, and scanning tunneling microscopy. The optical absorption spectra of dilute solutions of these molecules show size-dependent steplike structure with an on...

First-principles calculations of the electronic structure and spectra of strongly correlated systems: Dynamical mean-field theory

Abstract: A recently developed dynamical mean-field theory, in the iterated perturbation theory approximation, was used as a basis for the construction of a `first-principles' calculation scheme for investigating the electronic structure of strongly correlated electron systems. This scheme is based on the local density approximation (LDA) within the framework of the linearized muffin-tin orbitals (LMTO) method. The classical example of the doped Mott insulator was studied by the new method, and the results showed qualitative improvement when compared with experimental photoemission spectra.

Calculation of electronic coupling matrix elements for ground and excited state electron transfer reactions: Comparison of the generalized Mulliken-Hush and block diagonalization methods

Abstract: Two independent methods are presented for the nonperturbative calculation of the electronic coupling matrix element (Hab) for electron transfer reactions using ab initio electronic structure theory. The first is based on the generalized Mulliken–Hush (GMH) model, a multistate generalization of the Mulliken Hush formalism for the electronic coupling. The second is based on the block diagonalization (BD) approach of Cederbaum, Domcke, and co-workers. Detailed quantitative comparisons of the two methods are carried out based on results for (a) several states of the system Zn2OH2+ and (b) the low-lying states of the benzene–Cl atom complex and its contact ion pair. Generally good agreement between the two methods is obtained over a range of geometries. Either method can be applied at an arbitrary nuclear geometry and, as a result, may be used to test the validity of the Condon approximation. Examples of nonmonotonic behavior of the electronic coupling as a function of nuclear coordinates are observed for Zn2OH2+. Both methods also yield a natural definition of the effective distance (rDA) between donor (D) and acceptor (A) sites, in contrast to earlier approaches which required independent estimates of rDA, generally based on molecular structure data.

Probing the Local Effects of Magnetic Impurities on Superconductivity

Abstract: The local effects of isolated magnetic adatoms on the electronic properties of the surface of a superconductor were studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained near magnetic adsorbates reveal the presence of excitations within the superconductor's energy gap that can be detected over a few atomic diameters around the impurity at the surface. These excitations are locally asymmetric with respect to tunneling of electrons and holes. A model calculation based on the Bogoliubov-de Gennes equations can be used to understand the details of the local tunneling spectra.

Excitation Spectra of Circular, Few-Electron Quantum Dots

Abstract: Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund’s rule. These impurity-free quantum dots allow “atomic physics” experiments to be performed in magnetic field regimes not accessible for atoms.

Equilibrium Geometries and Electronic Structure of Iron-Porphyrin Complexes: A Density Functional Study

Abstract: We have performed density functional theory (DFT) calculations of iron−porphyrin (FeP) and its complexes with O2, CO, NO, and imidazole (Im). Our fully optimized structures agree well with the available experimental data for synthetic heme models. Comparison with crystallographic data for proteins highlights interesting features of carbon monoxymyoglobin. The diatomic molecule induces a 0.3−0.4 A displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron−diatomic bond increases in the order Fe−O2 (9 kcal/mol) < Fe−CO (26 kcal/mol) < Fe−NO (35 kcal/mol). The ground state of FeP(O2) is an open shell singlet. The bent Fe−O2 bond can be formally described as FeIII−O2-, and it is characterized by the anti-ferromagnetic coupling between one of the d electrons of Fe and one of the π* electrons of O2. FeP(CO) is a closed shell singlet, with a linear Fe−C−O bond. The complex with NO has a doublet ground state and a Fe−NO geometry intermed...

Molecular Mechanism of HCl Acid Ionization in Water: Ab Initio Potential Energy Surfaces and Monte Carlo Simulations

Abstract: The acid ionization of HCl in water is examined via a combination of electronic structure calculations with ab initio molecular orbital methods and Monte Carlo computer simulations. The following key features are taken into account in the modeling: the polarization of the electronic structure of the solute reaction system by the solvent, the quantum character of the proton nuclear motion, the solvent fluctuation and reorganization along with the solvent polarization effects on the proton potential, and a Grotthuss mechanism of the aqueous proton transfer. The mechanism is found to involve the following: first, a nearly activationless motion in a solvent coordinate, which is adiabatically followed by the quantum proton rather than tunneling, to produce a contact ion pair Cl-−H3O+, which is stabilized by ∼7 kcal/mol; second, motion in the solvent with a small activation barrier, as a second adiabatic proton transfer produces a solvent-separated ion pair from the contact ion pair in a nearly thermoneutral ...

Effect of Mott-Hubbard correlations on the electronic structure and structural stability of uranium dioxide

Abstract: The influence of Mott-Hubbard electron-electron correlations on the electronic structure and structural stability of uranium dioxide (UO2) has been analysed using the local spin-density approximation (LSDA) + U approach. We have found that the inclusion of a term describing the Hubbard on-site repulsion between 5f electrons results in a dramatic improvement in the description of the equilibrium electronic and magnetic structure of UO2 for which conventional LSDA calculations incorrectly predict a non-magnetic metallic ground state. We have found that the presence of electron-electron correlations in the 5f band modifies the character of chemical bonding in the material, leading to a Heitler-London type of hybridization between the 5f orbitals and giving rise to a larger value of the equilibrium lattice constant in better agreement with experimental observations.

Coupled Quantum Dots Fabricated by Cleaved Edge Overgrowth: From Artificial Atoms to Molecules

Abstract: Atomically precise quantum dots of mesoscopic size have been fabricated in the gallium arsenide-aluminum gallium arsenide material system by cleaved edge overgrowth, with a high degree of control over shape, composition, and position. The formation of bonding and antibonding states between two such "artificial atoms" was studied as a function of quantum dot separation by microscopic photoluminescence (PL) spectroscopy. The coupling strength within these "artificial molecules" is characterized by a systematic dependence of the separation of the bonding and antibonding levels, and of the PL linewidth, on the "interatomic" distance. This model system opens new insights into the physics of coupled quantum objects.

Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime

Abstract: Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude lar...

InP quantum dots: Electronic structure, surface effects, and the redshifted emission

Abstract: We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the ${\ensuremath{\Gamma}}_{1c}$-derived direct state rather than the ${X}_{1c}$-derived indirect state and (ii) unlike the prediction of $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ models, the highest occupied state in InP dots has a $1sd$-type envelope function rather than a (dipole-forbidden) $1pf$ envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated $\ensuremath{\sim}300\ensuremath{-}\mathrm{meV}$ redshift and the $\ensuremath{\sim}18$ times longer radiative lifetime relative to the dot-interior transition for the 26-\AA{} dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in selective excitation for HF-etched InP dots. (vii) The latter type of (``resonant'') redshift is compatible with the calculated screened singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state.

Interactions of hydrogen with native defects in gan

Abstract: The atomic and electronic structure of hydrogen-vacancy complexes in GaN is investigated with pseudopotential-density-functional calculations. Calculated formation energies provide information about the likelihood of incorporation of these complexes in $n$-type and $p$-type material, and binding energies provide a measure for the dissociation energy. Vibrational frequencies yield a signature of the complex that should facilitate experimental identification. The behavior of hydrogenated nitrogen vacancies during annealing of acceptor-doped GaN is discussed, and a correlation with the frequently observed luminescence band around 420 nm is proposed.

Synthesis and characterization of PbSe quantum dots in phosphate glass

Abstract: The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

Toward a Molecular Orbital Derived Empirical Potential for Liquid Simulations

Abstract: A new method for the treatment of many-body polarization effects in fluid systems is presented, making use of hybrid QM/MM techniques and the semiempirical wave function. In this approach, the electronic structure of each solvent molecule is represented by an antisymmetric determinant wave function, which can be determined by fully converged variational QM calculations at each step of the classical trajectory or can be treated as dynamic variables along with the nuclear coordinates in molecular dynamics simulations. In determining the molecular wave functions, a hybrid QM/MM method is used. This molecular orbital derived empirical potential for liquid simulations (MODEL) is analogous to the fluctuating charge model introduced by Berne and co-workers; however, the MODEL potential is a quantum chemical model as opposed to a purely empirical charge equilibration scheme employed in the FC model. The method may also be extended for the treatment of large molecular systems. The capability of the MODEL potential...

Locally self-consistent Green's function approach to the electronic structure problem

Abstract: The locally self-consistent Green’s function ~LSGF! method is an order-N method for calculation of the electronic structure of systems with an arbitrary distribution of atoms of different kinds on an underlying crystal lattice. For each atom Dyson’s equation is used to solve the electronic multiple scattering problem in a local interaction zone ~LIZ! embedded in an effective medium judiciously chosen to minimize the size of the LIZ. The excellent real-space convergence of the LSGF calculations and the reliability of its results are demonstrated for a broad spectrum of metallic alloys with different degree of order. The relation of the convergence of our method to fundamental properties of the system, that is, the effective cluster interactions, is discussed. @S0163-1829~97!06740-4#

First-principles spin-polarized calculations on the reduced and reconstructed tio2 (110) surface

Abstract: We have performed plane-wave pseudopotential density-functional theory calculations on the stoichiometric and reduced TiO2 ~110! surface, the 231 and 132 reconstructions of the surface formed by the removal of bridging-oxygen atoms, and on the oxygen vacancy in the bulk. The effect of including spin polarization is investigated, and it is found to give a qualitatively different electronic structure compared with a spin-paired description. In the spin-polarized solutions, the excess electrons generated by oxygen reduction occupy localized band-gap states formed from Ti (3d) orbitals, in agreement with experimental findings. In addition, the inclusion of spin polarization substantially lowers the energy of all the systems studied, when compared with spin-paired solutions. However, spin-polarization does not change the relative stability of the two reconstructions, which remain energetically equivalent. @S0163-1829~97!02724-0#

Electronic Structure of Amorphous Silicon Nanoclusters

Abstract: The electronic structure of amorphous silicon nanoclusters is calculated within the empirical tight-binding approximation. The electronic states are classified into three groups: extended and weakly and strongly localized. The last category practically disappears in hydrogenated amorphous silicon clusters for which the blueshift is comparable to what is predicted for crystallites. The radiative recombination rates are comparable for small clusters $(\ensuremath{\sim}1\mathrm{nm})$ but 2 orders of magnitude higher for larger clusters $(\ensuremath{\sim}2\mathrm{nm})$ of the amorphous phase due to disorder induced breaking of selection rules.

The electronic structure of PdO found by photoemission (UPS and XPS) and inverse photoemission (BIS)

Abstract: The electronic structure of the 4d transition-metal oxide PdO is investigated by photoemission (UPS and XPS), inverse photoemission (BIS; ), and electron energy loss spectroscopy in reflection geometry (REELS; primary energy, ). The valence band spectra are compared to recent theoretical ab initio band-structure calculations. Good agreement between theory and experiment is found in the occupied part of the band structure down to 8 eV below as well as in the unoccupied part up to 6 eV above . This confirms the common view that the electronic structure of the 4d transition-metal oxides, e.g. PdO, can be explained in terms of a single-electron picture. Nevertheless correlation effects among the Pd 4d electrons are clearly visible in the spectra, as e.g. satellites of the Pd core level spectra. In order to explain the origin of these satellites we performed simple cluster model calculations and as a result we can explain one satellite in a screening picture by means of a charge transfer process. In addition radiation damage effects in PdO during the electron bombardment in the BIS experiments are reported. This is explained by the formation of the Pd -like states connected with oxygen loss due to the electron bombardment.

Electronic structure of strained I n P / G a 0.51 In 0.49 P quantum dots

Abstract: We calculate the electronic structure of nm scale InP islands embedded in ${\mathrm{Ga}}_{0.51}{\mathrm{In}}_{0.49}\mathrm{P}$. The calculations are done in the envelope approximation and include the effects of strain, piezoelectric polarization, and mixing among 6 valence bands. The electrons are confined within the entire island, while the holes are confined to strain induced pockets. One pocket forms a ring at the bottom of the island near the substrate interface, while the other is above the island in the GaInP. The two sets of hole states are decoupled. Polarization dependent dipole matrix elements are calculated for both types of hole states.

Electronic structure of strainedInP/Ga0.51In0.49Pquantum dots

Abstract: We calculate the electronic structure of nm scale InP islands embedded in ${\mathrm{Ga}}_{0.51}{\mathrm{In}}_{0.49}\mathrm{P}$. The calculations are done in the envelope approximation and include the effects of strain, piezoelectric polarization, and mixing among 6 valence bands. The electrons are confined within the entire island, while the holes are confined to strain induced pockets. One pocket forms a ring at the bottom of the island near the substrate interface, while the other is above the island in the GaInP. The two sets of hole states are decoupled. Polarization dependent dipole matrix elements are calculated for both types of hole states.

Electronic, structural, and optical properties of crystalline yttria

Abstract: The electronic structure of crystalline Y2O3 is investigated by first-principles calculations within the local-density approximation (LDA) of the density-functional theory. Results are presented for the band structure, the total density of states (DOS), the atom-and orbital-resolved partial DOS. effective charges, bond order, and charge-density distributions. Partial covalent character in the Y-O bonding is shown, and the nonequivalency of the two Y sites is demonstrated. The calculated electronic structure is compared with a variety of available experimental data. The total energy of the crystal is calculated as a function of crystal volume. A bulk modulus B of 183 Gpa and a pressure coefficient B' of 4.01 are obtained, which are in good agreement with compression data. An LDA band gap of 4.54 eV at Gamma is obtained which increases with pressure at a rate of dE(g)/dP = 0.012 eV/Gpa at the equilibrium volume. Also investigated are the optical properties of Y2O3 up to a photon energy of 20 eV. The calculated complex dielectric function and electron-energy-loss function are in good agreement with experimental data. A static dielectric constant of epsilon(O)= 3.20 is obtained. It is also found that the bottom of the conduction band consists of a single band, and direct optical transition at Gamma between the top of the valence band and the bottom of the conduction band may be symmetry forbidden.

A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy− (x=1–2, y=1–5)

Abstract: The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and B 2Σg+ excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermore, the ground state vibrational frequencies of these three molecules are also similar. These observations lead us to suggest that these molecules all have a rhombuslike structure, similar to Al2O2, with the oxygen atoms sequentially attaching to the terminal aluminum atoms. The spectra are consistent with an ionic bonding view of these clusters and the vibrational frequencies are in good agreement with the theoretical results. Significant information about the structure and bonding of these small aluminum oxide clusters is obtained and discussed.

Catalytic Mechanism of the Enzyme Papain: Predictions with a Hybrid Quantum Mechanical/Molecular Mechanical Potential

Abstract: A hybrid quantum mechanical/molecular mechanical approach is used to elucidate structural and energetic features of amide hydrolysis by the enzyme papain. The role of the enzyme in stabilizing the thiolate−imidazolium ion pair is examined and the potential energy pathway for the subsequent attack of the cysteine anion and proton transfer from the imidazolium cation is determined. The reaction is found to be concerted rather than stepwise, and the transition state for the reaction is located. The effect of residue mutations both on the ion pair stability and on the barrier to amide hydrolysis is explored and found to be in agreement with experiment. In this work both high-level electronic structure and semiempirical MO methods are used, with location and characterization of stationary structures. Rearrangement of the enzyme in response to the changing electronic structure of the active site is also considered.