Collective motion of atoms in grain boundary migration of a bcc metal

Molecular dynamics simulations of grain boundary (GB) migration of a bcc metal, tungsten, have been carried out. The GB is of asymmetrical ? 110? tilt type. Detailed examinations of atomic processes in the migration, show that the GB migration consists mainly of GB dislocation glides. Furthermore, each motion of a GB dislocation involves a cooperative motion of about three atoms on each of the atomic planes perpendicular to the tilt axis, leading to their realignment from the receding grain to the advancing grain. This collective motion is not synchronized in all of the atomic planes, but appears to be in two or three adjacent planes, suggesting a kink mechanism for glides of the GB dislocations.     

Field emission through atoms chemisorbed on a metal surface

An exact general expression for the field emission current is derived under two alternative forms in the frame of a new formalism of tunnelling developed in preceding articles. This formalism is applied to study qualitatively the TED (total energy distribution) in the presence of a chemisorbed atom. It is shown in particular that the overlap between the atomic electronic state and the continuum of states of the vacuum induces, in the high-field regime, a non-negligible variation of the position and width of the resonance peak with the electric field.     

Single atom chemisorption on a bcc metal

The effect of chemisorption of a single atom on the (001) surface of a bcc metal is investigated via the Green's function and the phase shift techniques using the LCAO method and the tight-binding approximation. In particular, we obtain the change in the electronic density of states Δn due to two different binding sites, the on-site and the centered four-fold-site. For each site, the adatom energy level E_a is placed both inside and outside the band, and several adatom-substrate interaction strengths σ are considered. By varying these parameters we obtain a qualitative understanding of the effects on Δn due to either the adsorption of different atoms or a readjustment of E_a arising from a flow of charge onto or away from the adsorbed atom. We compare our results to previous studies of chemisorption on the (001) surface of an s-band simple cubic crystal and find that, although some details distinctive of the bcc metal arise, the overall qualitative features are the same.     

Localized phonons due to the adsorption of a monolayer of atoms on a crystal surface

Localized surface phonons due to a monolayer of adsorbed atoms on a free surface are studied with a simple crystal model. Surface localized modes inside a ‘surface gap’ between the bulk bands of frequencies, and inside a bulk band are reported. Resonant surface states well defined in energy are also found.     

Ionization of xenon Rydberg atoms at a metal surface

Experiments in which a thermal-energy beam of xenon Rydberg atoms is directed at near grazing incidence onto a flat Au(111) surface are described that provide new insight into charge transfer and electron tunneling during atom/surface interactions. Analysis of the data shows that for the present range of principal quantum number n, 13 < or = n < or = 20, ionization occurs at an atom/surface separation Z(i) = (4.5+/-0.9)n2a0, where n2a0 is the Bohr radius of the atom. This result is in good agreement with the value Z(i) approximately 3.8n2a0 predicted by ab initio hydrogenic theory.     

Adsorption behavior of Fe atoms on a naphthalocyanine monolayer on Ag(111) surface

Adsorption behavior of Fe atoms on a metal-free naphthalocyanine(H2Nc) monolayer on Ag(111) surface at room temperature has been investigated using scanning tunneling microscopy combined with density functional theory(DFT)based calculations. We found that the Fe atoms were adsorbed on the centers of H2 Nc molecules and formed Fe–H2Nc complexes at low coverage. DFT calculations show that Fe sited in the center of the molecule is the most stable configuration, in good agreement with the experimental observations. After an Fe–H2Nc complex monolayer was formed, the extra Fe atoms self-assembled to Fe clusters of uniform size and adsorbed dispersively at the interstitial positions of Fe–H2Nc complex monolayer. Therefore, the H2 Nc monolayer grown on Ag(111) could be a good template to grow dispersed magnetic metal atoms and clusters at room temperature for further investigation of their magnetism-related properties.     

Chemisorption of biomolecules on a metal surface and its role in anomalously intensive Raman scattering

Amino acids and their derivatives are used as an example to study the mechanism of enhancement of the Raman spectra of molecules adsorbed on silver hydrosols. It is shown that adsorption occurs as a result of unshared electron pairs of nitrogen and oxygen atoms in amino and carboxyl groups, as well as a result of the formation of-electron bonds between aromatic rings and adsorption centers on the surface of the metal. It is shown that only the groups of atoms in direct contact with the metal undergo enhancement, i.e., the enhancement mechanism is local in character. A model of the bonding of amino acids with the metal surface is presented.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 33–37, March, 1985.     

Contribution of recoil implantation to removal of monolayer impurity atoms on metal surfaces

The decay of one monolayer gold atoms at the nickel surface by bombardment with 5 keV Ar⁺ ions at room temperature has been studied by means of the RBS technique. It is found that the gold concentration decreases bi-exponentially with increasing ion fluence. It is shown that the fast decay component gives the removal cross-section which is the summation of the desorption and the recoil-implantation cross-sections. The recoil-implantation cross-section is evaluated from the difference between the removal cross-section and the desorption cross-section obtained at a previous high-temperature experiment.     

Interaction between two hydrogen atoms approaching a metal surface

Two regimes of HH interaction outside a metal surface are considered: (i) Beyond the spill-out region of the metal electrons (physisorption) (ii) Embedded protons in the electron spill-out. Using an extension of the Heitier-London model to treat (i), the H₂ binding energy for the molecule parallel to the surface is reduced, whereas for the perpendicular configuration stronger binding obtains. In (ii), the asymptotic form of the screening charge round a single proton is considered, in a linear response framework, and for an infinite barrier model of the surface. If the screened potential has no singularities in k space, then the interaction energy ΔE(X), X being the distance between the protons, falls off as X^?5 times an oscillatory function, for H₂ parallel to the planar metal surface. The effect of self-consistency on this result is then examined and it is concluded that the asymptotic interaction energy is unchanged in form, though the amplitude is altered.     

Charge transfer of Rydberg H atoms at a metal surface

The charge transfer of Rydberg hydrogen atoms at a metal surface is investigated for the first time. The surface ionization of Stark states with various electron density distributions with respect to the surface is examined. Unlike the nonhydrogenic species studied previously, genuine control over the orientation of the electronic wave function in the surface-ionization process is demonstrated. A comparison of the results for a range of collisional velocities for the most redshifted Stark state with principal quantum numbers n=20-36 with the classical over-the-barrier approach shows a good agreement for the onset of the ion signal, but the shallow rise in signal is not accounted for. An excellent fit of the experimental results can be achieved using a simple semiempirical model.     

电介质或导体表面吸附原子的共振光现象

本文借助于线性响应理论方法求得固体表面附近原子的光学布洛赫(Bloch)方程。借助于光学布洛赫方程和量子回归定理,求得了吸附原子的共振荧光谱。通过对该谱的数值分析可得出下述结论:固体表面的存在将对光谱中峰的高度以及两个边带的位置产生明显的影响。当被吸附的原子非常接近固体表面时,两个边带会向两侧移动,其中一个几乎消失,同时各个峰的高度也会大幅度降低。     

Direct observations of the behavior of single silicon atoms on a metal surface

The behavior of single silicon adatoms on the W {110} plane has been successfully studied for the first time. Single atom diffusion parameters are found to be E_d = 0.70 ± 0.07 eV, and $\text{d}{}_0\text{= 3.08 × 10}{}^{\mathrm{?4}}\text{× 10}{}^{\mathrm{\pm 1.28}}\text{solcm}{}^2\text{s}$. The field desorption behavior of Si atoms is similar to that of metal adatoms. SiSi adatom-adatom interaction shows nonmonotonic distance dependence, but the repulsive region around 3.2 Å is much weaker than those found in metal adatom interactions.     

SiO2-羟基表面上金属原子的第一性原理研究

采用第一性原理方法研究了SiO2-羟基表面上几种金属原子的吸附性质,发现In和Ga在SiO2-羟基表面上的结合很弱,而Fe,Co,Ni在该表面上与Si,O形成强的化学键.等势能面和扩散势垒计算表明In(Ga)的扩散激活能只有0.1-0.3 eV,表明这两种原子容易在表面上扩散.这些结果可以定性地解释纳米合成中的一些实验现象.     

Chemisorption of single fluorine atoms on the surface of zigzag single-walled carbon nanotubes: A model calculation

We report a model calculation of the chemisorption energies ΔE_ads of single fluorine atoms on the outer surface of zigzag single-walled carbon nanotubes (Z-SWCNTs) (p,0) with p ranging from 11 to 21. A simplified model based on an effective-mass theory is adopted to describe the electronic structure of the nanotubes. Chemisorption is treated within the Anderson–Newns approach, which takes account of Coulomb interaction between adsorbate electrons. Considering adsorption of an adatom directly on top of a surface carbon atom, we find that in the case of a fluorine atom bonded to the sidewall of the nanotubes, the absolute values of ΔE_ads are in the range for Z-SWCNTs with typical diameters of , larger |ΔE_ads| values being associated with semiconducting tubes. For the latter ones, |ΔE_ads| decreases rather significantly as the radius R of the tubes increases, tending towards the “infinite” radius graphene case, whereas for metallic tubes |ΔE_ads| slightly increases with increasing R. The localized acceptor states induced by a fluorine atom in the band gap of the semiconducting tubes are found to be responsible for such difference in the behaviour of ΔE_ads for the two above-mentioned types of tubes. The results obtained shed light on the possible mechanism of the atomic fluorine adsorption-induced hole-doping of the semiconducting tubes, which might significantly affect the transport properties of these tubes.     

Magnetic monolayer in a metal

A magnetic monolayer embedded in a metal has been investigated theoretically in the framework of the s-d interaction. An effective Hamiltonian that incorporates the conduction electron effects has been derived for a magnon. For an anisotropic s-d interaction, an energy gap appears in a certain temperature range and in contrast to an isolated monolayer, the magnetically ordered phase may remain at nonzero temperature.     

Chemisorption of atomic oxygen on silicon surface

We investigated the electronic structure of oxygen adsorbed on silicon surface in a head-on position, and showed that only the corresponding bonding structure is able to explain the features in UPS and ELS spectra that were not interpreted satisfactorily before. We also found that charge is transferred from backbonds to SiO bond and induces empty states localized between the first and second layers of the silicon surface.