Thermal expansion near plane and stepped surfaces

A theory of the thermal expansion of the plane and (11n) stepped surfaces of fcc crystals is presented. The temperature dependent relaxations arise from cubic anharmonic terms in the crystal potential energy. We show that the thermal expansion depends on the positions of the atoms with respect to the steps and is greatest for the atom of the upper corner. The knowledge of these new atomic positions at each temperature allows us to calculate new atomic force constants and then new vibrational properties at this temperature. The application is made for a Ni crystal for which we give the corrections, due to the thermal expansion, on the mean square displacements of stepped surface atoms. The variation with temperature of the optical modes due to a light monolayer is also presented.     

Modeling of neutral-atom scattering at the surface of a crystal for the case of grazing incidence

The features of the angular distributions of accelerated neutral atoms at grazing angles of incidence on the Al(001) surface are studied using the mathematical modeling method. The interaction of accelerated atoms with crystal-lattice atoms and the electronic properties and atomic structure of the Al(001) surface are calculated using the electron-density-functional method. The angular distributions of scattered atoms are modeled by taking into account their interaction with several atomic layers in the crystal lattice and atomic displacements during thermal oscillations. The influence of crystal surface-layer relaxation on rainbow scattering, i.e., the difference between the distances of planes on the surface and in the volume, is established. The possibilities of using the effect of rainbow scattering to study the structural features of a crystal surface are discussed.     

A statistical model for the scattering of atoms from randomly stepped surfaces

The scattering of an atomic beam from a randomly stepped surface has been calculated using the hard corrugated wall model. Under the basic assumption that scattering from the step edges may be neglected the scattering equation can be solved without any further approximation. The solution displays the usual diffraction peaks each of them is broaden by a term characterizing the step configuration. If the step distribution is ergodic and stationnary in space this term is the characteristic function of the difference of level between two point of the surface considered as a random variable. Statistical models for the step repartition at the surface are proposed and the scattering intensity is derived in a closed form. The main result is that the broadening of the peaks varies from no broadening to a maximum according as the interferences from waves reemitted by the various terraces are constructive or destructive. A comparison is made with previous experimental data from which an estimation of the average step separation can be drawn. The sensitivity of the atomic beam scattering to steps is found to be more than one step every one hundred crystal atoms.     

Xe原子吸附对GaAs(110)表面重构的影响

基于第一性原理的自洽场密度泛函理论(DFT)和广义梯度近似(GGA),利用缀加平面波加局域轨道(APW+lo)近似方法，建立了五层层晶超原胞模型，模拟了GaAs(110)表面结构和单个Xe原子在其表面的吸附.利用牛顿动力学方法，对GaAs(110)表面原子构形的弛豫和Xe原子在GaAs(110)表面的吸附进行了计算.从三种不同的初始构形出发，即Xe原子分别在Ga原子的顶位，As原子的顶位以及桥位，都发现Xe原子位于桥位时体系能量最低.由此，认为Xe原子在GaAs(110)表面的吸附位置在桥位，并且发现吸附Xe原子后GaAs(110)表面有趋向于理想表面的趋势，表面重构现象趋于消失，表面原子间键长有一定的恢复,这与理论预言相符合. 关键词： 密度泛函理论 表面结构 APW 表面原子吸附     

Semiempirical theory for surface core-level shifts

Numerical results are given for the surface core-level shifts at clean single crystal surfaces, stepped and reconstructed surfaces and surfaces with chemisorbed atoms. These results demonstrate that surface core-level shifts can be used as a diagnostic tool to study various surface effects.     

Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

The adsorption of CO,O2, and H2 on Pt: I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O₂, and h₂ on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O₂ adsorption at steps or defects than on terraces.     

Manipulation of atoms and molecules with a low temperature scanning tunneling microscope

Apart from its ability to image surfaces with atomic resolution the scanning tunneling microscope has evolved as a tool to manipulate single atoms and molecules. In this paper we present several examples of atomic manipulation and the formation of nanostructures with this technique. As examples for lateral manipulation, i.e. the sliding of atoms and molecules along the surface, we show CO, C₂H₄ and Pb on a stepped copper surface, the Cu(211). As examples for vertical manipulation, i.e. the transfer of atoms and molecules between the tip and the surface, we discuss Xe and C₃H₆. The design of our low temperature scanning tunneling microscope is shortly described.     

Low energy electron diffraction studies of high index crystal surfaces of platinum

Several high Miller Index crystal surfaces of platinum have been examined by low energy electron diffraction (LEED) and have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The ordered stepped surfaces were found to be stable in ultra-high vacuum up to 1500 K, and the reasons for this remarkable stability are discussed. The presence of ordered atomic steps may be considered as a general structural property of high index surfaces regardless of the chemical bonding in the crystal. A nomenclature is suggested to identify the surface structures of stepped surfaces.     

Modeling of Atomic Displacements in the Bombardment of Copper by Ar and Xe Ions with Near-Sputtering-Threshold Energies

Using the molecular dynamics method, collision-induced cascades are modeled, which are generated in the normal bombardment of a copper crystal (100) by Ar and Xe ions with energies 25, 40, and 50 eV as well as by Xe ions at 15 eV and crystal temperature 300 K. Time dependences are derived for the formation of vacancies and interstitial atoms as a function of depth of the crystal and radiation-adsorption atoms (adatoms) on its surface during the cascade development. The adatoms are shown to contribute considerably to the change in r.m.s. atomic displacement in the thermal stage of the cascade. It is proved that Ar ions produce more adatoms on the surface vacancies while Xe ions do so in the bulk of the crystal below the first atomic layer and exhibit more effective mixing in the [100] direction.     

Migration brownienne de cristallites sur une surface et relation avec l'épitaxie: II. Partie théorique

The thermal migration of small crystal buildings on a crystalline substrate is considered from two points of views. In the first the surface self diffusion of the atoms on the crystallites is supposed to be able to move the crystallites. In the second, special atomic configurations located in the interface crystallites-substrate are rendered responsible of these motions. Both hypotheses are formulated and compared with some experiments given in the fore going paper. The second theory is in qualitative accordance with these experiments so that the formation of an epitaxy is now better understood.     

Structural, electronic, and magnetic properties of bcc iron surfaces

Trends in atomic multilayer relaxations, surface energy, electronic work function, and magnetic structure of several low-Miller-index surfaces of iron are investigated employing density functional theory total energy calculations. The calculated topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness. The multilayer relaxation sequences correlate with the reduced coordination in surface layers and can be explained in terms of a simple electrostatic picture. The surface energies scale almost linearly with the surface roughness. They agree well with the experimental surface tensions and show a small anisotropy in agreement with predictions based on measurements for other metals. The equilibrium shape of a bcc Fe crystal is determined and discussed. The work function anisotropy is calculated and rationalized in terms of changes in the valence charge distribution. Significantly increased local magnetic moments of atoms in the surface region are determined. The correlation between the anisotropy of the surface magnetic moments and atomic coordination in the outermost layers is demonstrated to follow a simple rule.     

Three-beam X-ray standing wave analysis: A two dimensional determination of atomic positions

Measurements with X-ray standing wavefields were carried out for the first time in a three-beam diffraction geometry for Ge(000)(333)(115). The experimental results and the analysis using the dynamical multi-beam theory show that two independent Fourier components of the atomic density function can be determined simultaneously. This opens the way to a two dimensional position analysis of atoms in the bulk and on the surface of a single crystal.     

Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p_3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.     

Angle resolved photoemission study of surface states on the Pt(997) vicinal surface

One-dimensional atomic chains can be synthesized on stepped surfaces and the electronic structure of the high vicinal surface plays an essential role in determining the physical properties of atomic chains grown on top of it. We have applied surface analysis techniques to study the surface of a Pt(997) single crystal. The STM image of the surface showed that the surface was uniform with a well defined distance between the terraces. Angle resolved photoemission spectroscopy (ARPES) was used to characterize the electronic states of the Pt(997) surface, and confinement of electrons with wave vector perpendicular to the step direction was observed.     

Focussed inelastic resonances in particle scattering from surfaces

Particles, such as atoms or electrons, inelastically scattered from a crystal surface in resonance with a bound state are predicted to focus around a discrete set of final angles with a defined energy for each angle. The final angle and energy of such focussed inelastic resonances (FIR) are shown to be independent of the initial state. A calculation of the FIR amplitude indicates favorable conditions for the observation of the so far elusive bound states of He on low-index metal surfaces and of image states on stepped metal surfaces.     

The physical process of melting and freezing

It is suggested that at the melting temperature the wavelength of the average thermal phonon vibration is equal to or is a harmonic of the distance separating the atomic layers in the crystal. This resonance between the phonon and lattice vibrations equals out the energy of the vibrating atoms in the surface layer. If this “uniform” energy is higher than the energy corresponding to the metastable transition state then all the surface atoms lose its position stability. In order to make the jump to the next potential well energy is required to overcome on the viscous resistance of the liquid. If this energy, latent heat of fusion, is supplied then the atomic/molecular sheet or platelets from the surface are detached and melting occurs. The proposed model is consistent with all of the characteristic features of melting and freezing. Equations calculating the average phonon wavelength and the corresponding lattice distance at the melting temperature are derived from fundamental thermodynamic relationships. The required thermodynamic parameters are determined from experiments of the nine selected highly symmetrical solids. The calculated wavelengths of the phonon vibration are equal to or is a harmonic of the $d$-spacing of the atomic/molecular layers in agreement with theory.     

The effect of surface channeling of neutral atoms

Special features of reflection of neutral atoms from crystal surfaces are considered in the case of grazing incidence. It is shown that polarization effects and the long-range part of the potential of interaction of ions with atomic chains and planes can play an important role in describing processes of reflection of atoms from the surface. The analysis in this paper shows that the polarization of accelerated neutral atoms in the case of grazing incidence on the surface can lead to the formation of bound surface states. The threshold condition for the angle of arrival of atoms at the plane at which the capture by such states is possible is obtained.     

Dynamic Potential of Interaction between Nitrogen Atoms and Aluminum Crystal Surface

Specific features of the angular distributions of accelerated neutral nitrogen atoms at the grazing angles of incidence on the Al(001) crystal surface have been investigated by the computer simulation method. The N–Al pair interaction potential is approximated by the three-parameter Morse potential with the energydependent coefficients. The angular distributions of scattered atoms have been simulated taking into account the interaction between atoms and several atomic layers in the lattice and the atomic displacement during thermal oscillations. The parameters of the pair potential of accelerated neutral nitrogen atoms in the energy range from 10 to 70 keV have been determined according to the best agreement between the calculated dependence of the rainbow scattering angle on the energy of particles incident on the crystal surface and the available experimental data.

     

The influence of sulfur adsorption on the step structure of vicinal Mo(100): a LEED and STM study

The effect of sulfur adsorption on the step structure of vicinal Mo(100) surfaces has been studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). STM was used for low densities of steps on a nominally flat (100) surface. LEED was used to study the more highly and regularly stepped (910), (911), and (28,4,1) oriented surfaces. Steps and the sulfur adsorbate were found to have a strong interaction. The presence and orientation of steps on the surface governed the formation of ordered domains of sulfur, and sulfur modified the structure of the steps on the surface. Both techniques show that monatomic steps predominate on the clean surfaces. When sulfur was adsorbed on the surfaces, however, steps coalesced to form groups of steps, double atomic height steps, and multiple height steps, depending on sulfur coverage and initial step density. The results are discussed in the framework of the theory of equilibrium crystal shape and provide information on how the Mo step-step interactions are affected by sulfur adsorption.