Theoretical studies of the bonding of sulfur to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for a S atom bonded to Ni clusters consisting of 1 to 4 atoms designed to model bonding to the Ni(100) surface. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. Modeling the (100) surface with four Ni atoms, we find the optimum S position to be 1.33 Å above the surface, in good agreement with the value (1.30 ± 0.10 Å) from dynamic LEED intensity calculations. The bonding is qualitatively like that in H₂S with two covalent bonds to one diagonal pair of Ni atoms. There is a S pπ pair overlapping the other diagonal pair of Ni atoms. [Deleting this pair the S moves in to a position 1.04 Å from the surface.] There are two equivalent such structures, the resonance leading to equivalent S atoms and a c(2 × 2) structure for the S overlayer. The Ni in the layer beneath the surface seems to have little effect (~0.03 Å) on the calculated geometry. Bonding the S directly above a single Ni atom leads to a much weaker bond (D_e = 3.32 eV) than does bonding in a bridge position (D_e = 5.37 eV).     

The H atom in CaTiO 3 : Structure and electronic properties

In the present work we explore the effects that an H impurity produces upon the geometry and electronic structure of the CaTiO 3 crystal considering the cubic and orthorhombic crystallographic lattices of the material. A quantum-chemical method based on the Hartree-Fock formalism and the periodic large-unit-cell model is used throughout the work. The analysis of the results shows that the interstitial H impurity binds to one of the O atoms forming the so-called O-H group. At equilibrium, the O-H distances are found to be equal to 0.89 and 1.04 Å for cubic and orthorhombic lattices respectively. Atomic displacements and relaxation energies are analysed comparing the obtained results in the cubic lattice with those in the orthorhombic lattice. In the cubic phase the computed relaxation energy of vicinity of the O-H group is found to be equal to 1.1 eV and the atomic displacements generally obey the Coulomb law. So, the negatively charged O atoms move outwards from the defective region by about 0.09 Å while the positively charged Ti and Ca atoms move towards the defective region by about 0.05 and 0.01 Å respectively. A similar effect is observed in the orthorhombic lattice of CaTiO 3 doped with an H atom. It is necessary to mention that different O positions in the orthorhombic structure are considered for the O-H bond creation. The computed relaxation energies of the atomic displacements in this structure are found to be equal to 2.3 and 2.1 eV depending on the crystallographic type of the bonding O atom.     

Theoretical studies of the geometries of O and S overlayers on the (100) surface of nickel

Geometries for O and S overlayers on the (100) surface of Ni have been calculated using $\text{a b initio}$ wavefunctions for O and S bonded to small clusters of Ni atoms (1 to 5 Ni atoms). The calculated distance of the adatom from the surface is 0.96 Å and 1.33 Å for O and S, respectively, in excellent agreement with the results of dynamic LEED intensity calculations, 0.9 ± 0.1 Å and 1.3 ± 0.1 Å, respectively. This indicates that accurate geometries of chemisorbed atoms may be obtained from calculations using clusters.     

Bond length inequalities and relaxations at the Rh(110)-c(2 × 2)-S surface structure studied by LEED crystallography

A tensor LEED analysis has been made for the Rh(110)-c(2 × 2)-S surface structure using intensity-versus-energy curves measured for twelve independent beams at normal incidence. Each S atom chemisorbs on a centre site of the Rh(110) surface. It bonds to the second layer Rh atom directly below, with a bond distance equal to about 2.27 Å, and to four neighbouring first layer Rh atoms at close to 2.47 Å. A significant feature of this structure is that the second metal layer is buckled; those Rh atoms directly below the S atoms relax down by about 0.11 Å compared with the other second layer Rh atoms. This buckling is apparently driven by the need to reduce the difference that would otherwise occur between these two types of S-Rh bond lengths. A component in the observed difference between the S-Rh distances appears to be dependent on the metallic coordination number for the Rh atoms; in this regard, a comparison is made with the structural details for O chemisorbed on reconstructed Ni(110).     

LCGTO-Xα model cluster study for the chemisorption of CO on twofold sites of Ni surfaces

The cluster Ni₂CO is studied as a simplified model for the chemisorption of CO on twofold bridging sites of transition metal surfaces. Using the LCGTO-Xα method we have calculated the potential energy surface for the totally symmetric stretching motion keeping the NiNi distance fixed at the bulk value. The minimum energy is found at a NiC distance of 1.72 Å and a CO bond length of 1.19 Å. The vibrational frequency for the CO bond (1850 cm^?1) shows reasonable agreement with EELS data (1810, 1870 cm^?1), whereas the (Ni₂)C frequency of 495 cm^?1 is remarkably higher than the experimental values (380, 400 cm^?1) indicating an overestimation of the chemisorption bond strength in this simple cluster model. The bonding between CO and Ni is analyzed using orbital correlations, ionization energies and Mulliken population analysis. Important bonding contributions from π backdonation are identified while the a₁orbital manifold exhibits strong antibonding effects.     

Molecular cluster calculations for the interpretation of CO chemisorption on a Ni(100) surface

Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of carbon monoxide on a Ni(100) surface are presented. In earlier calculations of this type the CO molecule has been assumed to be chemisorbed in a hollow position of C_4v symmetry. A recent EELS experiment shows however that in the most stable configuration CO is linearly bonded to the Ni atoms, i.e. a top position of the CO-molecule. This experiment indicates also that there exists an additional bridge bonding of the CO molecule to the two nearest neighbour Ni atoms. The variation of the energy levels, binding energies and charge distribution with the height of the CO molecule above the nickel surface is calculated for the top position using the NiCO and Ni₅CO clusters and for the bridge bonding configuration using the Ni₂CO cluster. The CO 1π level is found to be split by about 0.8 eV in bridge bonding geometry. For both hollow and top positions the 1π and 5σ levels are separated by about 0.5 eV. The energy separation to the 4σ level is about 3 eV, which is in good agreement with experimental data. Theoretical ionization energies relative to the Fermi energy for top position geometry at a bond distance of 3.5 au between the carbon atom and nickel surface were found to be 25.7, 11.7, 8.7 and 8.2 eV for the 3σ, 4σ, 5σ and 1π levels which should be compared with the experimental data of 29.0, 10.8, 8.4 and 7.8 eV, respectively. The corresponding ionization energies for a bond angle of 99° in bridge bonding were 23.7, 12.1, 7.3, 7.0 and 7.9 eV. The two last values represent the 1π level which is split into two levels in this geometry. The variation of the C-O stretch vibrational frequencies with the height of the CO molecule above the surface for the top-position geometry is estimated from the 5σ and 2π gross orbital populations and from the CO σ and π overlap populations.     

The structure and bonding of furan on Pd(111)

The structure and bonding of molecular furan, C₄H₄O, on Pd(111) has been investigated using density functional theory (DFT) calculations and the results compared with those of a recent experimental investigation using scanned-energy mode photoelectron diffraction (PhD). The DFT results confirm the orientation of the molecular plane to be essentially parallel to the surface and show a clear energetic preference for one of the two possible structures identified in the PhD study, namely that with the molecule centred over the hollow sites of the surface. Two slightly different geometries at the hollow sites are found to be essentially energetically equivalent; in both cases, one Pd surface atom bonds to two C atoms, while two other Pd atoms each bond to one C atom. These structures differ in that in one case the pair of C atoms bonding to a single Pd atom are both β-C (C atoms not bonded to O in the furan molecule), whereas in the second case this pair of C atoms comprises one β-C and one α-C (adjacent to the O atom in furan). In both structures the C–Pd bonding is accompanied by displacements of the H and O atoms away from the surface and out of the molecular plane and local C–Pd coordination consistent with a rehybridisation of the C bonding to sp³ character.     

A consistent interpretation of the atomic and electronic structure of the Si(111)−7×7 surface

A simple mechanism leading to the 7×7 reconstruction of the Si(111) surface is proposed. In this model a charge-density wave with the 7×7 pattern acts as the driving force which precipitates the dehybridisation of the surface atoms. The atoms at the surface form terraces of height ～0.3 Å. This small corrugation of the surface gives rise to a much larger (～3 Å) corrugation of the charge in agreement with recent He atom scattering experiments. The model is also in qualitative agreement with photoemission, optical absorption and ion scattering experiments.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

Molecular dynamics simulation studies of cryoprotective agent solutions: the relation between melting temperature and the ratio of hydrogen bonding acceptor to donor number

Molecular dynamics simulations have been carried out for glycerol–water–sodium chloride ternary solution due to its important role in cryopreservation engineering. The radial distribution functions for atom pairs potentially related to C–H ··· O and O–H ··· O hydrogen bonds were calculated. The radial distribution functions for the H (connected to C)–O atom pair do not exhibit peaks between 2 and 3 Å, whereas the radial distribution functions for the C–O atom pair exhibit distinct peaks between 3 and 4 Å. The reason for this is because most C–H ··· O geometries are bent and deviate from linearity. The ratios of acceptor to donor numbers for water and glycerol molecules decrease as the solute concentration increases. A characteristic concentration has been found that divides solutions with different mechanisms. Below the characteristic concentration, the melting temperature is linearly related to the ratio of acceptor to donor number for water molecules, whereas above the characteristic concentration, the melting temperature is linearly related to the ratio of acceptor to donor number for glycerol molecules. Further studies indicate that the relations are independent of hydrogen bonding criteria and temperature.     

Structure and phonon spectrum of a submonolayer Ni film on the surface of Cu(100)

The equilibrium atomic structure and the phonon spectra of a submonolayer (θ = 0.5 monolayer) Ni film deposited on the surface of Cu(100) are calculated using the potentials obtained by the embedded atom method. We consider atomic relaxation, the vibrational state density distribution on Ni and substrate atoms, and polarization of vibrational modes. Variation of the phonon spectrum upon segregation of Cu atoms on the film surface is considered. It is shown that mixing of vibrations of Ni adatoms with vibrations of substrate atoms occurs in the entire frequency range, leading to a frequency shift of the vibrational modes of the substrate and to the occurrence of new vibrational states atypical of a clean surface. The Cu(100)–c(2 × 2)–Ni structure is dynamically stabler when placed in the subsurface layer of the substrate.     

不同退火方式对Ni/SiC接触界面性质的影响

采用快速热退火(rapid thermal annealing, RTA)法和脉冲激光辐照退火(laser spark annealing, LSA)法, 在n型4H-SiC的Si面制备出Ni电极欧姆接触. 经传输线法测得RTA样品与LSA样品的比接触电阻分别为5.2×10^-4 Ω·cm², 1.8× 10^-4 Ω·cm². 使用扫描电子显微镜、原子力显微镜、透射电子显微镜、拉曼光谱等表征手段, 比较了两种退火方式对电极表面形貌、电极/衬底截面形貌和元素成分分布、SiC衬底近表层碳团簇微结构的影响. 结果表明, 相比于RTA, LSA法制备出的欧姆接触在电极表面形貌、界面形貌、电极层组分均匀性等方面都具有明显优势, 有望使LSA成为一种非常有潜力的制备欧姆接触的退火处理方法.     

Angle resolved auger surface spectroscopy

The angular distribution of electrons ejected in core-valence-valence Auger transitions of atoms chemisorbed on metal surfaces is considered theoretically. Since the valence electrons participating in the Auger transition are also involved in chemical bonding to the surface, these initial states contain information pertaining to the chemisorption bonding geometry. The role of the initial state symmetry in determining the angle resolved Auger surface spectrum (ARASS) is investigated through model calculations and is found to be small. Thus the ARASS is expected to be a smoothly varying function of angle with ? ± 15% modulations due to diffraction effects, in agreement with recent experimental results for S adsorbed on Ni(100).     

Geometry,vibrational frequencies,and ionization potentials for CO/Ni(100); explanation of the disappearance of the 5σ peak in pes

Using a cluster of 14 Ni atoms to model a Ni(100) surface, we used ab initio methods [generalized valence bond (GVB)] to study CO chemisorbed at the on-top site. Reported are the optimum geometry, vibrational frequencies, and ionization potentials. We propose a new explanation for the two lowest levels of free CO (5σ and 1π) reducing to one level of chemisorbed CO.     

Radiation damage in single-crystal ice studied by 100 keV proton channeling

A detailed study has been undertaken of the Ni{100} (2 × 2)C structure formed by cracking ethylene on a clean Ni{100} surface. The LEED pattern shows characteristic missing spots which can be attributed to the presence of glide lines and indicate a space group symmetry of p4g. We show that this can be readily interpreted in terms of a distortion of the top nickel layer both parallel and perpendicular to the surface, which accompanies the carbon adsorption. Detailed comparisons of LEED intensity data with dynamical calculations indicate that the top layer nickel atoms are displaced 0.35 ± 0.05 Å parallel to the surface, 0.20 ± 0.05 Å outwards from the surface, and that the carbon atoms are in 4-fold hollows (now distorted) at a spacing of 0.1 ± 0.1 Å from the surface. These conclusions lead to a nickel-carbon nearest neighbour spacing of 1.803 ± 0.015 Å.     

Structure determination of Ni(110)-(2 × 3)-N by use of SEXAFS measurements: on-surface and sub-surface sites on a pseudo-(100) reconstructed surface

Surface-extended X-ray-absorption fine-structure measurements have been performed on the Ni(110)-(2 × 3)-N system. The data are consistent with a model in which nitrogen chemisorbs on a pseudo-(100) reconstructed and largely corrugated surface with a nearest-neighbour N-Ni bond length of 1.86 ± 0.03 Å. The corrugation results in two raised and two lowered [11̄0] Ni rows which are not uniformly distributed in the [001] direction. Nitrogen chemisorbs in four-fold hollow sites slightly (0.19Å) above the lowered Ni rows and in pseudo-four-fold hollow sites slightly (0.38 Å) below the plane defined by a raised and a lowered Ni row. In both sites N is equidistantly bonded to Ni atoms in the second layer. Structural models with long-bridge adsorption sites can be safely excluded.     

Adsorption behavior and mechanism of multiple Mg atoms on the surface of AlNi compound at Mg alloy/steel interface

The interface between Mg alloy and the intermetallic compounds (IMCs) is the weak point during the welding of Mg alloy and steel. The models of AlNi surface under different number of absorbed magnesium atoms were established. The absorption energy and electronic properties of the Nth (N represents the number of Mg atoms, which may be 1, 2, 3, and 4 in this paper) Mg atom on the AlNi surface were calculated using first-principles calculations. The results show that the previously absorbed Mg atoms will promote the absorption of the latter Mg atom when the distance between them is equal or less than 2.89 Å. The absorption energy of the Mg atoms on the AlNi surface is related to the electronic exchange between different atoms. The results will provide a new perspective for understanding the atomic scale mechanism of the bonding between the Mg atom and AlNi IMCs.     

The local structure of SO2 and SO3 on Ni(1 1 1): A scanned-energy mode photoelectron diffraction study

O 1s and S 2p scanned-energy mode photoelectron diffraction (PhD) data, combined with multiple-scattering simulations, have been used to determine the local adsorption geometry of the SO₂ and SO₃ species on a Ni(1 1 1) surface. For SO₂, the application of reasonable constraints on the molecular conformation used in the simulations leads to the conclusion that the molecule is centred over hollow sites on the surface, with the molecular plane essentially parallel to the surface, and with both S and O atoms offset from atop sites by almost the same distance of 0.65 Å. For SO₃, the results are consistent with earlier work which concluded that surface bonding is through the O atoms, with the S atom higher above the surface and the molecular symmetry axis almost perpendicular to the surface. Based on the O 1s PhD data alone, three local adsorption geometries are comparably acceptable, but only one of these is consistent with the results of an earlier normal-incidence X-ray standing wave (NIXSW) study. This optimised structural model differs somewhat from that originally proposed in the NIXSW investigation.     

Effects of metal-support interactions on the electronic structures of metal atoms adsorbed on the perfect and defective MgO(1 0 0) surfaces

The electronic structures of Ni, Pd, Pt, Cu, and Zn atoms adsorbed on the perfect MgO(1 0 0) surface and on a surface oxygen vacancy have been studied at the DFT/B3LYP level of theory using both the bare cluster and embedded cluster models. Ni, Pd, Pt, and Cu atoms can form stable adsorption complexes on the regular O site of the perfect MgO(1 0 0) surface with the binding energies of 19.0, 25.2, 46.7, and 17.3 kcal/mol, respectively, despite very little electron transfer between the surface and the metal atoms. On the other hand, adsorptions of Ni, Pd, Pt, and Cu atoms show strong interaction with an oxygen vacancy on the MgO(1 0 0) surface by transferring a significant number of electron charges from the vacancy to the adsorbed metal atoms and thus forming ionic bonds with the vacancy site. These interactions on the vacancy site for Ni, Pd, Pt, and Cu atoms increase the binding energies by 25.8, 59.7, 85.2, and 19.1 kcal/mol, respectively, compared to those on the perfect surface. Zn atom interacts very weakly with the perfect surface as well as the surface oxygen vacancy. We observed that the interaction increases from Ni to Pt in the same group and decreases from Ni to Zn in the same transition metal period in both perfect and vacancy systems. These relationships correlate well with the degrees of electron transfer from the surface to the adsorbed metal atom. The changes in the ionization potentials of the surface also correlate with the adsorption energies or degrees of electron transfers. Madelung potential is found to have significant effects on the electronic properties of metal atom adsorptions on the MgO(1 0 0) surface as well as on an oxygen vacancy, though it is more so for the latter. Furthermore, the Madelung potential facilitates electron transfer from the surface to the adsorbed metal atoms but not in the other direction.     

An investigation of the kinetics of structural changes during the early oxidation stages of a Ni(100) surface using low energy ion scattering (LEIS)

Low energy ion scattering has been used to investigate the early stages of the oxidation of a Ni(100) surface. This technique allows simultaneous study of the oxygen uptake in the surface and the development of surface structures. Bombardment induced surface damages was minimised by performing the experiments with low ion doses, while keeping the target at 200–300°C. The measured kinetics of the oxygen uptake are in good agreement with recent work, using different techniques. It is concluded that during the early chemisorption, a two stage process takes place: an initial oxygen adsorption during which the O atoms probably reside within the fourfold surface hollows, followed by a reconstruction process, caused by the combined action of at least two nearest neighbour O atoms, trapping mobile Ni adatoms, after which the O atoms stabilise at a site in or close to the reconstructed 〈001̄〉 row. Observed structural changes at higher exposures are compatible with a transition into a (3 × 1) structure and subsequently NiO, but cannot, as yet be positively identified.