Local Electronic Density Of States Research Articles

Variations of the local density of electron states and short-range order in amorphous hydrated silicon films

Ultrasoft x-ray spectroscopy methods have been used to observed a change in the energy distribution of the silicon valence states after annealing a-Si:H films at 500 °C. This change appears as three distinct maxima in the density of states 3.5, 7.2, and 10.2 eV above the top of the valence band, which indicates ordering of the a-Si:H structural network. The energy distance between the latter two maxima (E−Ev=7.2 and 10.2 eV) supports electron-diffraction data indicating a decrease in the silicon-silicon interatomic distance by 0.2 A in comparison with the crystal. The presence of a third maximum (E−Ev=3.5 eV) is connected with the change in the hybridization of the s-p-functions of silicon with decrease of the coordination number.

First-principles real-space tight-binding LMTO calculation of electronic structuresfor atomic clusters

A real-space scheme has been developed for a first-principles calculation of electronic structures and total energies of atomic clusters. The scheme is based on the combination of the tight-binding linear muffin-tin orbital (TBLMTO) method and the method of real-space Green{close_quote}s function. With this approach, the local electronic density of states can be conveniently determined from the real-space Green{close_quote}s function. Furthermore, the full electron density of a cluster can be directly calculated in real space. The scheme has been shown to be very efficient due to the incorporation of the method of real-space Green{close_quote}s function and Delley{close_quote}s method of evaluating multicenter integrals. {copyright} {ital 1996} {ital The American Physical Society}

Single crystal surfaces probed by polarized nuclei

Nuclear magnetic resonance has developed into a very powerful technique to study the structure and dynamics of atomic and molecular systems, both in liquid and solid phase. However the investigation of single crystal surfaces with “conventional” NMR methods is essentially impossible due to the small sample size of less than 1015 sites on a cm2. To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin polarized atomic beam of 6Li in the one case and 8Li in the other. The latter isotope is radioactive and undergoes a \beta‐decay with a halflife of 0.84 s. Li adsorbed on the close packed Ru(001) surface is investigated. The T{in1} relaxation rate is the main observable and is used to deduce the local electronic density of states (LDOS(EF,r=0)) and the Li diffusion barriers at low and high adsorbate coverage. The second experiment uses 6Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier‐Transform NMR technique is demonstrated, by observing the time dependent flux of circularly polarized light emitted behind the foil after a 90\circ‐pulse has been employed at the surface. Development and prospects of the latter technique are presented.

Probing the Superconducting Proximity Effect in NbSe2 by Scanning Tunneling Microscopy.

Cryogenic scanning tunneling microscopy has been used as a local probe of the superconducting proximity effect across a normal metal--superconductor interface of a short coherence length superconductor. Both the topography and the local electronic density of states were measured on a superconducting NbS${\mathrm{e}}_{2}$ crystal decorated with nanometer-size Au islands. The presence of a quasiparticle bound state could be inferred even when the probe was located directly on the bare NbS${\mathrm{e}}_{2}$ surface near an Au island, indicating a severe depression of the pair potential inside the superconductor due to the proximity effect.

First-principles study of the atomistic and electronic structure of the niobium--alumina (0001) interface

We report first-principles self-consistent calculations of the atomistic structure and local electronic densities of states of the system Nb on α-Al 2 O 3 (sapphire). We find that a monolayer of Nb strongly prefers those surface sites on Al 2 O 3 (0001) that are occupied by Al in the bulk, and the bonding is very ionic. When eight layers of Nb are simulated in contact with O-terminated Al 2 O 3 (0001), the metal shows appreciable interlayer relaxation near the interface. When the Al 2 O 3 is terminated instead by Al, the adjacent Nb lattice is much less distorted. We see significant differences in the local electronic structure between these cases.

Electronic structure of Pd clusters in the tight-binding approximation: influence of spd-hybridization

We show that including spd-hybridization in tight-binding model allows an accurate description of the local electronic density of states of Pd clusters in a large range of size and gives reliable site energies. We put in evidence a narrowing of the band with decreasing coordination number accompanied by a significant variation of the local density of states near the Fermi level according to the type of site (vertex, edge, facets or core). From the energetical point of view, we find that a quasi-linear dependence of the attractive part of the potential as a function of the coordination number is better suited than the usual square root dependence. Moreover, we point out the influence of cluster symmetry by comparing the cuboctahedral structure (fcc type) with the icosahedral one (five-fold symmetry) which is adopted by the small fcc clusters.

Core attraction v.s. electron-electron repulsion in the lopsided energy sharing between a photoelectron and an auger electron

A theory on the dynamics of lopsided energy sharing between a photoelectron and an Auger electron in solids is formulated and evaluated numerically. The formulation is based on an optical response theory and incorporates both the core attraction around core holes and the electron-electron (e-e) repulsion working between those two electrons. In order to perform specific calculations, two different models are adopted, which are a s-wave model and a cubic-lattice one. The main conclusions are that (1) their effects are competing with each other, and (2) the energy distribution spectra for electrons show a considerable variety according to the local electronic density of states.

The adsorption of H2O on TiO2 and SnO2(110) studied by first-principles calculations

First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H$_2$O adsorption on the (110) surface of TiO$_2$ and SnO$_2$. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and the cases of full and half coverage are studied. Both molecular and dissociative (H$_2$O $\rightarrow$ OH$^-$ + H$^+$) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrical configurations. It is found that for both TiO$_2$ and SnO$_2$ an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO$_2$ and SnO$_2$ are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented and their relation with experimental UPS spectra is discussed.

FULLERENES ADSORPTION ON Cu(111) AND Ag(111) SURFACES

Initial stage adsorption and film growth of C 60, C 70, C 60(x) C 70(1−x), and Sc 2@ C 84 fullerene molecules on Cu (111) and Ag (111) surfaces have been investigated by field-ion scanning tunneling microscopy. Fullerene molecules are mobile on the terrace and initially segregate to the step edges. A well-ordered monolayer films form with a close-packed arrangement upon annealing the fullerence-covered surfaces. The (4×4) phases form in the case of the C 60, C 70, and C 60(x) C 70(1−x) adsorption on the Cu (111)-(1×1) surface, indicating a strong interaction between the Cu substrate and fullerenes. Intramolecular structures of the C 60 and C 70 molecules are observed and interpreted as the mappings of the local electron density of states. On the Ag (111) surface, C 60 and Sc 2@ C 84 monolayer films show several phases that have almost identical nearest neighbor distance but are rotated from each other, indicating weaker interaction between the Ag substrate and fullerenes compared with the case of Cu .

195Pt NMR of Polymer-Protected Pt/Pd Bimetallic Catalysts

Colloids of Pt, Pt0.8Pd0.2, and Pt0.2Pd0.8 particles of approximately 2.5 nm diameter with poly (N-vinyl-2-pyrrolidone) as protecting agent were dried, and the 195Pt NMR spectrum and nuclear spin−lattice relaxation rates were measured. The spectra indicate that the particles are indeed bimetallic and that the composition of their interior corresponds well to the nominal value. A strong variation of catalytic activity of these systems with composition around Pt0.2Pd0.8 has been reported in the literature. We suggest that this effect is related to corresponding variations in the local densities of electronic states at the Fermi energy on the surface sites of the catalytic metal.

THE ELECTRONIC STRUCTURE AND STRUCTURE STABILITY OF Ａｇ（１１１） AND Ａｇ（00１） SURFACES

By using the Linearized Muffin-Tin Orbital Method, the surface electronic structure of several "stable Ａｇ（１１１） surfaces" obtained by different methods were systematically studied and contrasted, the different surface electronic character of the different relaxation surfaces were presented and discussed. According to the surface energy, the most stable Ａｇ（１１１） surface was single layer inward relaxed surface, and the local electronic densities of states of this king of structure were presented and discussed. The local electronic dendities of states of ideal clean Ａｇ（00１） surface were also studied, and the results that the Ａｇ（00１） surface were also presented. the Ａｇ（００１） surface is more stable than Ａｇ（１１１）.

Deep defect states in a-Si:(H,C) alloys

The cluster-Bethe-lattice method is used to study the electronic properties of the a-Si:(H,C) alloys. The sp 3s * tight-binding-parameters are used in the calculations. The electronic local density of states for several clusters containing various local bonding configurations are calculated. There are no defect states in the gap induced by the ideal bonding of H and C in the cluster whereas there are defect states in the gap due to the Si-and C-dangling bonds. The energy level for the neutral Si dangling bond is at + 0.4 eV. For the cluster containing a Si-C bond, we found that the defect energy level associated with the C dangling bond lies at −0.8 eV, and that associated with the Si dangling bond lies at + 0.8 eV. The defect states distribution in the gap is wider with the inclusion of C in a-Si:H films.

Amorphous clusters: the electronic structure of Ge clusters with B and Al impurities

Amorphous clusters of the type XGe 20H 28 with X = B and Al have been studied using the pseudopotential self-consistent field Hartree-Fock and the HONDO program. The local electronic density of states and the charge density were obtained for structures determined by energy optimization of the position of the central impurity, and of the position of the central impurity and the four nearest neighbors, using the AM1 method implemented in the MOPAC Program. The structures obtained are such that the impurities are not located at the center of the clusters; instead they are displaced towards three of the four nearest neighbors, which indicates that there is a tendency for the impurity to remain with the same chemical valence in the cluster as when isolated. The gap energy states due to the impurities are near the valence band and are p-like; the state due to aluminum is closer to the valence band than the one introduced by boron. These results are compared with recent findings, both experimental and theoretical.

Significance of non-central forces in atomistic studies of grain boundaries in bcc transition metals

Abstract The effects of non-central forces in atomistic studies of grain boundaries in molybdenum and tungsten, the transition metals with half-filled d-band, are investigated. For this purpose we have used two different types of potential which include different number of moments of the local density of electronic states when evaluating the total energy: the central-force Finnis-Sinclair potentials which include the scalar second moment and the potentials constructed by Carlsson which include the fourth and the matrix second moments. The energy terms associated with these two moments represent non-central interactions and assure that the bcc-fcc structural energy difference is reproduced with good accuracy. For the three boundaries studied, the non-central forces have been found to be very important in determining the lowest energy structures. In particular, the energy differences between multiple structures depend on specific orientations and geometries of the atomic clusters at and near the interface. ...

Chemisorption of Li on jellium: Local density of states and nuclear-spin relaxation.

Nuclear-spin relaxation of atoms chemisorbed on metal surfaces is investigated theoretically. The underlying interaction is the Fermi contact interaction between the magnetic moments of the nucleus and the surrounding electrons. In a resulting rate equation for the nuclear polarization, the rate depends on the local density of electron states (LDOS) at the nuclear site. This LDOS is calculated in the framework of density-functional theory via solving Dyson's equation using the jellium model for the metal substrate. The model results are consistent with the data.

Dynamics of chemical bonds and local density of electron states at heterogeneous surfaces

Dependencies of the local density of electron states, effective atom charges and bond lengths on time for thermal vibrations, adsorption complex formation and surface reactions were studied by the dynamic reaction coordinate method within the scope of a cluster approach by the semiempirical AM1, MNDO/H and NDDO methods. Maximum changes of these characteristics are observed in the transition states of chemical reactions both without charge separation and without H + transfer.

NMR study of the lithium adsorption

β-NMR experiments with 8Li are well suited to probe the alkali metal adsorption with respect to electronic properties. The extremely high sensitivity of this method allows to investigate alkali metal coverages down to 10−5 ML. Since the mutual interaction between the alkali metal atoms can be neglected in this coverage regime, isolated adsorbate atoms are probed. The information which can be drawn from the experiments contains an absolute value for the local density of electronic states at the Fermi energy and the electric field gradient at the position of the nucleus, which is a fingerprint of the adsorption site. We present examples on a clean, a gold and oxygen covered Ru(001) surface.

Scanning tunneling microscope/scanning tunneling spectroscopy investigation of the structural modulation on the surface of cleaved Bi2Sr2CaCu2Oy, single crystal

The surface of cleaved Bi2Sr2 CaCu2O3 (Bi2212) single crystals has been studied by means of scanning tunneling microscope (STM) and scanning tunneling spectroscopy (STS) at room temperature in ultrahigh vacuum. We obtain atomic images of the BiO surface using logarithmic current mode and conventional mode. It is demonstrated that the Bi atoms in the BiO plane are not missing. Some Bi atoms are depressed down below the BiO surface. STS obtained at different places of the surface shows more or less the same feature, indicating that local electronic density of states does not change much due to the depression or the well-known structural modulation. The possible origins of the variation in the period of the structural modulation in the BiO plane of cleaved Bi2212 single crystals extracted from STM images are also studied.

Amorphous clusters. III. The electronic structure of Si clusters with B and Al impurities

Amorphous clusters of the type XSi 20H 28 with X  B and Al have been studied using the pseudopotential SCF Hartree—Fock and the HONDO program. The local electronic density of states and the charge density were obtained for structures determined by energy optimization of the position of the central impurity, and of the position of the central impurity and the four nearest neighbors, using the AM1 method implemented in the MOPAC program. The structures obtained in this manner are such that the impurities are not located at the center of the clusters; instead both the boron and the aluminum are displaced towards three of the four nearest neighbors, a fact that indicates that there is a tendency for the impurity to remain with the same chemical valence in the cluster as when isolated. The gap energy states introduced by both B and Al are near the valence band and are p-like; the state due to aluminum is closer to the valence band than the one introduced by boron. These results are compared with recent experimental findings.

Effect of particle size upon catalytic and electronic properties of supported Ag catalysts: combined catalytic, 109Ag NMR and quantum chemistry studies

The rate of ethylene epoxidation over Ag Al 2O 3 supported catalysts demonstrates the drastic increase at a mean size of Ag particles about 400–500 Å. The integral intensity of the 109Ag NMR line from Ag metal particles of the same catalysts increases in the same range of particle sizes. This is due to the large line broadening of the signal from the Ag particles of which the sizes are less than 400–500 Å. The self-consistent calculations of the electronic structure of semi-infinite Ag(111) crystal show the large deviation of the local density of electronic states and 109Ag NMR Knight shifts for the first surface layers of Ag compared with Ag bulk.