Electronic structure of metal clusters

Photoemission spectra of valence electrons in metal clusters, together with threshold ionization potential measurements, provide a coherent picture of the development of the electronic structure from the isolated atom to the large metallic cluster. An insulator-metal transition occurs at an intermediate cluster size, which serves to define the boundary between small and large clusters. Although the outer electrons may be delocalized over the entire cluster, a small cluster remains insulating until the density of states near the Fermi level exceeds 1/kT. In large clusters, with increasing cluster size, the band structure approaches that of the bulk metal. However, the bands remain significantly narrowed even in a 1000-atom cluster, giving an indication of the importance of long-range order. The core-electron binding-energy shifts of supported metal clusters depend on changes in the band structure in the initial state, as well as on various final-state effects, including changes in core hole screening and the coulomb energy of the final-state charge. For cluster supported on amorphous carbon, this macroscopic coulomb shift is often dominant, as evidenced by the parallel shifts of the core-electron binding energy and the Fermi edge. Auger data confirm that final-state effects dominate in cluster of Sn and some other metals. Surface atom core-level shifts provide a valuable guide to the contributions of initial-state changes in band structure to cluster core-electron binding energy shifts, especially for Au and Pt. The available data indicate that the shift observed in supported, metallic clusters arise largely from the charge left on the cluster by photoemission. As the metal-insulator transition is approached from above, metallic screening is suppressed and the shift is determined by the local environment.     

Energetics and bonding in aluminosilicate rings with alkali metal and alkaline-Earth metal charge-compensating cations

The stabilizing effect of alkali and alkaline-earth metal ions on the oxygen donors of four- and six-membered faujausite-like rings has been calculated in terms of Kohn-Sham core-level (O1s) energy shifts with respect to these same complexes without cations. The results confirm and complement earlier investigations by Vayssilov and co-workers where Na(+) and K(+) were the only complexing cations. The oxygen donor centers in six-membered rings are stabilized by -3.6 ± 0.4, -3.9 ± 0.5, -7.3 ± 0.1, and -7.6 ± 0.2 eV by K(+), Na(+), Ca(2+), and Mg(2+) adions, respectively. The energy shifts are even greater for four-membered rings where the stabilization effects attain -3.7 ± 0.1, -4.1 ± 0.1, -8.1 ± 0.1, and -9.0 ± 0.1 eV, respectively. These effects are also observed on the low-lying σ-bonding and antibonding molecular orbitals (MOs) of the oxygen framework, but in a less systematic fashion. Clear relationships with the core-level shifts are found when the effects of alkali metal complexation are evaluated through electron localization/delocalization indices, which are defined in terms of the whole wave function and not just of the individual orbitals. Complexation with cations not only involves a small but significant electron sharing of the cation with the oxygen atoms in the ring but also enhances electron exchange among oxygen atoms while reducing that between the O atoms and the Si or Al atoms bonded to them. Such changes slightly increase from Na to K and from Mg to Ca, whereas they are significantly enhanced for alkaline-earth metals relative to alkali metals. With respect to Al-free complexes, Si/Al substitution and cation charge compensation generally enhance electron delocalization among the O atoms, except between those that are linked through an Al atom, and cause either an increased or a decreased Si-O ionicity (smaller/higher electron exchange) depending on the position of O in the chain relative to the Al atom(s). The generally increased electron delocalization among O atoms in the ring is induced by significant electron transfer from the adsorbed metal to the atoms in the ring. This same transfer establishes an electric field that leads to a noticeable change in the ring-atom core-level energies. The observed shifts are larger for the oxygen atoms because, being negatively charged, they are more easily polarizable than Al and Si. The enhanced electron delocalization among O atoms upon cation complexation is also manifest in Pauling's double-bond nature of the bent σ-bonding MO between nonadjacent oxygen centers in O-based ring structures.     

FT-IR,XPS, and DFT Study of Adsorption Mechanism of Sodium Acetohydroxamate onto Goethite or Hematite

The adsorption of sodium acetohydroxamate on the goethite or hematite surface was investi-gated by Fourier transform infrared spectroscopy (FT-IR), X-ray photoemission spectroscopy and periodic plane-wave density functional theory (DFT) calculations. The core-level shifts and charge transfers of the adsorbed surface iron sites calculated by DFT with periodic in-terfacial structures were confronted to the X-ray photoemission experiments. FT-IR results reveal that the interfacial structure of sodium acetohydroxamate adsorbed on the goethite or hematite surface may be assigned to a ve-membered ring complex. In agreement with the adsorption energies determined by the DFT calculations, a ve-membered ring complex is formed via bonding of one surface iron atom of goethite (101) or (100) to both oxygen atoms of hydroxamate group, and these two oxygen atoms of the hydroxamate group correspond-ingly attach to two neighboring iron atoms of the goethite surface. But a ve-membered ring complex between two oxygen atoms of the hydroxamate group and one surface iron atom of hematite (001) is formed without any extra attachments. The calculated core-level shifts of Fe2p for the interfacial structures are correspondingly in good agreement with the experimental observed one, which con rmed the reliability of the calculated results.     

SEMIEMPIRICAL CALCULATION OF THE INNER SHELL BINDING ENERGY SHIFTS IN MOLECULES INVOLVING SULPHUR ATOM

Abstract

The inner core binding energy shifts of sulphur atom in various chemical environments were studied by the semiempirical self-consistent charge molecular orbital method. The relaxation energy was taken into account by using two distinct approaches:(a) reorganizatinoo potential method and (b) transition potential method. The changes in ESCA chemical shifts of sulphur are satisfactorily accounted for by the latter method, the s tandard deviation from the experimental data being 0.2 eV. It appears that the reorganization energy plays an important role in rationlizing inner core binding energy shifts of sulphur.     

Diffusion dynamics of the li atom on amorphous carbon: A direct molecular orbital-molecular dynamics study

Direct molecular orbital-molecular dynamics (MO-MD) calculation was applied to diffusion processes of the Li atom on a model surface of amorphous carbon and compared with the diffusion mechanism of Li+ ion. A carbon sheet composed of C96H24 was used as the model surface. The total energy and energy gradient on the full dimensional potential energy surface of the LiC96H24 system were calculated at each time step in the trajectory calculation. The optimized structure, where the Li atom is located at the center of mass of the model surface, was used as the initial structure at time zero. Simulation temperatures were chosen in the range of 200-1250 K. The dynamics calculations showed that the Li atom vibrates around the initial position below 250 K, and it moves above 300 K. At middle temperature, the Li atom translates freely on the surface. At higher temperature (1000 K), the Li atom moves from the center to edge region of the model surface and is trapped in the edge. The activation energy calculated for the Li atom is larger than that for the Li+ ion. This difference is due to the fact that the Li atom diffuses together with an unpaired electron on the carbon surface. The diffusion mechanism of the Li atom was discussed on the basis of the theoretical results.     

The effect of atomic and extra-atomic relaxation on atomic binding energies

An equivalent-cores-relaxation model is given for calculating atomic binding energies from orbital energies using only ground-state atomic properties. The agreement with experiment is excellent for the noble gases. On the basis of present knowledge of atomic relaxation, the phenomenon of “extra-atomic relaxation”, in which electronic charge is attracted toward a hole-state atom, is shown to have an important effect in lowering atomic core-level binding energies in condensed phases. This will affect the interpretation of most core-level binding energies measured to date.     

Molecular layers of ZnPc and FePc on Au(111) surface: Charge transfer and chemical interaction

We have studied zinc phthalocyanine (ZnPc) and iron phthalocyanine (FePc) thick films and monolayers on Au(111) using photoelectron spectroscopy and x-ray absorption spectroscopy. Both molecules are adsorbed flat on the surface at monolayer. ZnPc keeps this orientation in all investigated coverages, whereas FePc molecules stand up in the thick film. The stronger inter-molecular interaction of FePc molecules leads to change of orientation, as well as higher conductivity in FePc layer in comparison with ZnPc, which is reflected in thickness-dependent differences in core-level shifts. Work function changes indicate that both molecules donate charge to Au; through the π-system. However, the Fe3d derived lowest unoccupied molecular orbital receives charge from the substrate when forming an interface state at the Fermi level. Thus, the central atom plays an important role in mediating the charge, but the charge transfer as a whole is a balance between the two different charge transfer channels; π-system and the central atom.     

Poisson-Boltzmann theory of the charge-induced adsorption of semi-flexible polyelectrolytes

A model is suggested for the structure of an adsorbed layer of a highly charged semi-flexible polyelectrolyte on a weakly charged surface of opposite charge sign. The adsorbed phase is thin, owing to the effective reversal of the charge sign of the surface upon adsorption, and ordered, owing to the high surface density of polyelectrolyte strands caused by the generally strong binding between polyelectrolyte and surface. The Poisson-Boltzmann equation for the electrostatic interaction between the array of adsorbed polyelectrolytes and the charged surface is solved for a cylindrical geometry, both numerically, using a finite element method, and analytically within the weak curvature limit under the assumption of excess monovalent salt. For small separations, repulsive surface polarization and counterion osmotic pressure effects dominate over the electrostatic attraction and the resulting electrostatic interaction curve shows a minimum at nonzero separations on the Angstrom scale. The equilibrium density of the adsorbed phase is obtained by minimizing the total free energy under the condition of equality of chemical potential and osmotic pressure of the polyelectrolyte in solution and in the adsorbed phase. For a wide range of ionic conditions and charge densities of the charged surface, the interstrand separation as predicted by the Poisson-Boltzmann model and the analytical theory closely agree. For low to moderate charge densities of the adsorbing surface, the interstrand spacing decreases as a function of the charge density of the charged surface. Above about 0.1 M excess monovalent salt, it is only weakly dependent on the ionic strength. At high charge densities of the adsorbing surface, the interstrand spacing increases with increasing ionic strength, in line with the experiments by Fang and Yang [J. Phys. Chem. B 101, 441 (1997)].     

Electronic states near dislocations in transition metals: An application of quantum chemistry in technology

This paper outlines a model for calculating the localized states of a 〈 100 〈 edge dislocation in α-Fe. The model used for the calculations is based on the multiple-scattering model (SCF -X α-SW ). The purpose of this research is twofold: (1) To determine changes in electronic structure of the lattice near the core region of defects in α-Fe. (2) The variations of hydrostatic pressure about an edge dislocation produce a rearrangement of the conduction electrons. The question is what electrical interaction might be expected between a dislocation and a charged solute atom. The calculations show that the electrons tend to flow away from the compression side toward the dilated regions. The electrical contribution to the binding energy of a solute atom and a dislocation in α-Fe is of the order of 0.01 Ry/electronic unit charge of the atom.     

Probing the basicity of regular and defect sites of alkaline earth metal oxide surfaces by BF3 adsorption: a theoretical analysis.

The basicity of regular and low-coordinate (LC) sites (steps, edges and corners) at the surface of alkaline earths with NaCl structure (MgO, CaO, SrO, and BaO) has been investigated by using BF3 as a probe molecule. B-O and B-F distances; O-B-F bond angles; B-F asymmetric stretching frequencies; O, B and F 1s core-level binding energies; and the interaction energy of adsorbed BF3 were determined by means of DFT calculations on cluster models. These adsorption properties were compared with those of complexes of BF3 with molecules with various basicities (water, ammonia, phosphine, etc.). We show that many properties of adsorbed BF3, and in particular the experimentally accessible shifts in vibrational frequency, in B and F 1s core levels, and in BF3 desorption temperature, exhibit a linear correlation with the surface basicity as measured by the vertical ionization potential of the oxide anions. On the other hand, shifts of the O 1s core level binding energy do not provide a simple way to detect surface basicity. On a given oxide surface, the differing basicities of various sites result in measurable differences in adsorption properties. This suggests the potential use of BF3 as a probe molecule for titrating LC sites on the surface of ionic oxides.     

簇模型ZnO(100)面上吸附CO_2的密度泛函理论研究

采用电荷自洽方法, 以嵌入原子簇Zn4O4为模型, 使用量子化学的密度泛函理论, 研究了二氧化碳在六方ZnO非极化的(1010)面的可能吸附态。计算表明, CO2垂直底物表面吸附, 氧原子只能与Zn原子配位, 并且吸附能为很弱的1.8 kJ/mol;吸附质分子平行于底物表面时, 得到了5种平衡吸附构型, 其中采用CZn配位和η2O, O二齿配位时, 吸附很弱, 经BSSE校正后的吸附能在8.8~6.6 kJ/mol。 采用η2C, O方式分别与O和Zn配位时, 吸附能为31.1 kJ/mol; C原子与表面O配位时计算得到了唯一的一个化学吸附态, 吸附能为139.6 kJ/mol, 与实验结果一致。     

新型二维碳化物晶体储锂性能的理论研究

MXenes是一类新型的二维金属碳化物或碳氮化物.本文使用密度泛函理论,分别研究了其中Ti₂C、Nb₂C和V₂C的结构与储锂性能,以及表面氟(F)官能团对其结构和储锂性能的影响.吸附能、态密度和电荷布居的计算结果表明:该二维晶体表面可以吸附双层锂原子. Ti₂C、V₂C和Nb₂C最高储Li的理论比容量分别为995.04、941.31、541.93 mAh·g^-1.此外, F官能团可以有效提高这三种二维晶体锂化结构的稳定性与导电性.     

Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping

Density-functional calculations of the adsorption of molecular hydrogen on a planar graphene layer and on the external surface of a (4,4) carbon nanotube, undoped and doped with lithium, have been carried out. Hydrogen molecules are physisorbed on pure graphene and on the nanotube with binding energies about 80-90 meV/molecule. However, the binding energies increase to 160-180 meV/molecule for many adsorption configurations of the molecule near a Li atom in the doped systems. A charge-density analysis shows that the origin of the increase in binding energy is the electronic charge transfer from the Li atom to graphene and the nanotube. The results support and explain qualitatively the enhancement of the hydrogen storage capacity observed in some experiments of hydrogen adsorption on carbon nanotubes doped with alkali atoms.     

Crystallography Aided by Atomic Core-Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Br?nsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.     

Photoemission and near-edge X-ray absorption fine structure studies of the bacterial surface protein layer of Bacillus sphaericus NCTC 9602

The electronic structure of the regular, two-dimensional bacterial surface protein layer of Bacillus sphaericus NCTC 9602 has been examined by photoemission (PE) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both the O 1s and the N 1s core-level PE spectra show a single structure, whereas the C 1s core-level spectrum appears manifold, suggesting similar chemical states for each oxygen atom and also for each nitrogen atom, while carbon atoms exhibit a range of chemical environments in the different functional groups of the amino acids. This result is supported by the element-specific NEXAFS spectra of the unoccupied valence electronic states, which exhibit a series of characteristic NEXAFS peaks that can be assigned to particular molecular orbitals of the amino acids by applying a phenomenological building-block model. The relative contributions of the C-O, C-N, and C-C bond originating signals into the C 1s PE spectrum are in good agreement with the number ratios of the corresponding bonds calculated from the known primary structure of the bacterial surface protein. First interpretation of the PE spectrum of the occupied valence states is achieved on the basis of electronic density-of-states calculations performed for small peptides. It was found that mainly the pi clouds of the aromatic rings contribute to both the lowest unoccupied and the highest occupied molecular orbitals.     

Lithium adsorption on graphite from density functional theory calculations

The structural, energetic, and electronic properties of the Li/graphite system are studied through density functional theory (DFT) calculations using both the local spin density approximation (LSDA), and the gradient-corrected Perdew-Burke-Ernzerhof (PBE) approximation to the exchange-correlation energy. The calculations were performed using plane waves basis, and the electron-core interactions are described using pseudopotentials. We consider a disperse phase of the adsorbate comprising one Li atom for each 16 graphite surface cells, in a slab geometry. The close contact between the Li nucleus and the graphene plane results in a relatively large binding energy (larger than 1.1 eV). A detailed analysis of the electronic charge distribution, density difference distribution, and band structures indicates that one valence electron is entirely transferred from the atom to the surface, which gives rise to a strong interaction between the resulting lithium ion and the cloud of pi electrons in the substrate. We show that it is possible to explain the differences in the binding of Li, Na, and K adatoms on graphite considering the properties of the corresponding cation/aromatic complexes.     

X-ray photoelectron spectroscopy of thermally grown silicon dioxide films on silicon

At various stages of in situ thermal oxidation of Si(111) monocrystals, X-ray photoelectron spectroscopy (XPS or ESCA) reveals a shift in the silicon core-level binding energies which varies continuously from 2.4 to 4.2 eV. From the oxygen and silicon ESCA peak intensities, these films can be said to have the silicon dioxide composition with an excess in oxygen concentration. By correlating the silicon 2p or 2s binding-energy shifts with oxygen KLL Auger energy and oxygen 1s binding-energy shifts, it is shown that a Fermi level shift and differential extra-atomic relaxation energy in the interfacial region must be invoked, in addition to chemical structure considerations, to interpret these data.     

Theoretical study of mixed silicon-lithium clusters Si(n)Li(p)(+) (n=1-6, p=1-2)

Theoretical study on the structures of neutral and singly charged Si(n)Li(p)((+)) (n=1-6, p=1-2) clusters have been carried out in the framework of the density functional theory (DFT) with the B3LYP functional. The structures of the neutral Si(n)Li(p) and cationic Si(n)Li(p)(+) clusters are found to keep the frame of the corresponding Si(n), Li species being adsorbed at the surface. The localization of the lithium cation is not the same one as that of the neutral atom. The Li(+) ion is preferentially located on a Si atom, while the Li atom is preferentially attached at a bridge site. A clear parallelism between the structures of Si(n)Na(p) and those of Si(n)Li(p) appears. The population analysis show that the electronic structure of Si(n)Li(p) can be described as Si(n)(p)(-)+pLi(+) for the small sizes considered. Vertical and adiabatic ionization potentials, adsorption energies, as well as electric dipole moments and static dipolar polarizabilities, are calculated for each considered isomer of neutral species.     

Interaction of atomic hydrogen with the beta-SiC(100) 3 x 2 surface and subsurface

We investigate clean and atomic hydrogen exposed beta-SiC(100) 3 x 2 surfaces by synchrotron radiation-based Si 2p core-level photoemission spectroscopy. The clean 3 x 2 surface reconstruction exhibits three surface and subsurface components. Upon hydrogen exposures, those surface and subsurface components are shifted to lower binding energies by large values, indicating significant charge transfer to the surface and subsurface regions, in excellent agreement with the recently discovered H-induced beta-SiC(100) 3 x 2 surface metallization. In addition, the interaction of hydrogen results in a large reactive component at Si 2p supporting an asymmetric charge transfer in the third plane below the surface, in agreement with previous experimental investigations. However, the results are inconsistent with recent ab initio theoretical "frozen" calculations predicting H atom to be in a bridge-bond position.     

First-principles calculation of core-level binding energy shift in surface chemical processes

Combined with third generation synchrotron radiation light sources, X-ray photoelectron spectroscopy (XPS) with higher energy resolution, brilliance, enhanced surface sensitivity and photoemission cross section in real time found extensive applications in solid-gas interface chemistry. This paper reports the calculation of the core-level binding energy shifts (CLS) using the first-principles density functional theory. The interplay between the CLS calculations and XPS measurements to uncover the structures,...