CO vibrations on a stepped Ni surface

Electron energy loss spectra (ELS) have been measured after CO adsorption on the stepped Ni[5(111) × (11̄0)] surface as function of substrate temperature and CO exposure. At 150 K and exposures below 0.5 L, only adsorption sites close to the step edges are occupied. Pronounced rearrangements of CO molecules over terrace sites occur by increasing the temperature to 300 K. The occupation of special step sites gives rise to an exceptionally low frequency C-O stretching vibration of 1520 cm⁻¹. This frequency indicative of a weakened C-O bond is correlated with the previously found tendency for CO decomposition to occur on this surface during a flash desorption experiment.     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

Kinetics of adsorption on stepped surfaces and the determination of surface diffusion constants

Nitrogen adsorption on stepped W(110) surfaces is examined to illustrate a theory of surface kinetics. Experimental findings by Besockeet al. have shown that nitrogen chemisorbs dissociatively only at the step corner sites of a W(110) surface. Thus the rate of dissociation reveals the mobility of nitrogen and its interaction with the surface. Using continuous-time-random-walk theory, we obtain the probability that molecules reach the step corner sites as a function of time. A kinetic model of nitrogen dissociation is proposed to calculate a coverage function that is in good agreement with experiment. The surface diffusion constant of nitrogen molecules is obtained and is in accordance with previous observations that nitrogen molecules are first weakly physisorbed on the W(110) terrace. Finally, the coverage functions for different step densities are predicted.     

Extended one-dimensional supramolecular assembly on a stepped surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag110 surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.     

Thermal decomposition of CO on a stepped Ni surface

The adsorption-desorption behaviour of CO on the stepped Ni(S) [5(111) × (11̄0)] and the smooth Ni(111) plane are compared by using LEED, thermal flash desorption and AES. Above room temperature flash desorption from the Ni(111) face yields a single α peak characteristic of molecularly adsorbed CO whereas from the stepped surface in addition to the a peak α second desorption peak (β2) appears around 550°C which is assigned to associative CO desorption. If carbon and oxygen are separately chemisorbed on Ni(111) associative desorption of CO leads to a desorption peak around 350° C. It is concluded that steps lower the activation energy for CO decomposition but increase the activation energy for associative desorption.     

Nitrogen-induced reconstruction of a stepped iron surface

The dissociative adsorption of nitrogen on Fe(12, 1, 0) is found to induce an increase in the density of steps and finally cause faceting of the surface with large nitrogen exposures. The epitaxial relationship between α-Fe and Fe₄N in certain crystallographic directions can account for the observed structural changes.     

Reconstructed domains on a stepped W(100) surface

A model calculation is carried out for the scattering of low energy electron diffraction from reconstructed (√2 × √2)R45° domains on a stepped W(100) surface. The existence of monotonically increasing steps causes the integral order beams to split and the separation of the splitting oscillates with the incident electron energy. We show that due to the existence of an antiphase relationship among the randomly nucleated reconstructed domains, the half-order beams neither split nor oscillate with the incident electron energy. This nonsplitting of half order beams is in agreement with the observation by Debe and King (DK). We also show that the measured intensity profile of a half order beam is equal to the signal intensity profile from the individual finite size domains convoluted with the instrument response function. This gives a simple way to evaluate the reconstructed domain size quantitatively. From the angular distribution of the half-order beam intensity we deduce that the reconstructed domains are somewhat round in shape, instead of the “long strips” proposed by DK. Also, the long range inhibition (20 Å) of the reconstruction near the step edge suggested by DK does not necessarily follow from our analysis. As a matter of fact, there is evidence showing that the inhibition (if it exists at all) can be short range in nature. Our suggestion is in agreement with the observation of the reconstructed W(100) surface by Melmed, Tung, Graham and Smith using FIM technique.     

Vibrations of linear cobalt clusters on a stepped copper surface

The results of calculations of vibrational properties of cobalt dimers and trimers on the Cu(110) surface are reported. The local phonon density of states and their polarizations are analyzed. The calculations are performed using interatomic interaction potentials constructed in a tight-binding approximation. It was shown that the vibrational modes of the free standing dimer and trimer remain in the supported clusters but the revealed anisotropy of the local surface relaxation leads to deformation of the interatomic bonds and, as a result, to splitting and shifting of the phonon frequencies. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 67–73, January, 2009.     

Bromine atom diffusion on stepped and kinked copper surfaces

The rates of Br atom diffusion on several single crystalline Cu surfaces have been studied because of the potential impact of Br diffusion on the selectivity of alkyl bromide surface chemistry on Cu. Density functional theory (DFT) has been used to study the diffusion of isolated bromine atoms on a flat Cu surface, Cu(1 1 1), two Cu surfaces with straight steps, Cu(2 2 1) and Cu(5 3 3), and two kinked Cu surfaces, Cu(6 4 3) and Cu(5 3 1). Bromine diffusion is rapid on the flat Cu(1 1 1) surface with a barrier of ΔE_diff = 0.06 eV and a hopping frequency of ν = 4.8 × 10¹⁰ s⁻¹ at 150 K. On the stepped and kinked surfaces the effective diffusion barriers lie in the range ΔE_diff = 0.18-0.31 eV. Thus the rates of diffusion are many orders of magnitude slower on stepped and kinked Cu surfaces than on the Cu(1 1 1) surface. Nonetheless, at temperatures relevant for alkyl bromide debromination on Cu surfaces, bromine atoms remain sufficiently mobile that they can explore all available binding sites on the timescale of the debromination reaction.     

Electron confinement in surface states on a stepped gold surface revealed by angle-resolved photoemission

STM images show that vicinal Au(788) surfaces are made up of a uniform array of (111)-oriented terraces of similar width ( approximately 3.8 nm). This uniformity makes it possible to study the electronic structure of the resulting step superlattice by angle-resolved photoemission. We show that for this terrace array the surface state appears to be broken up into one-dimensional quantum-well levels, indicating total electron confinement within the terraces. The angular resolution allows the probability density of the terrace quantum well state to be mapped in reciprocal space, complementing nicely the wave function measured in real space by STM.     

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10^?5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E_ad≈ 5 kcal/mole and dipole moment of μ'_T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy ofΔE_ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ_S = 1.0 D and μ_T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E_ad. The E_ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.     

Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate-substrate system. The derived average activation energy for spreading first decreases from E_dif = 1.32 eV/atom at Θ = 0.25 ML to E_dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E_dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D₀ also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)-(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)-(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.     

Synthesis of graphite monolayer stripes on a stepped Ni(771) surface

Physics of the Solid State - Graphite stripes on a stepped Ni(771) substrate were grown in situ by cracking propylene followed by annealing at optimal temperatures between 450 and 590° C. The...     

Inward diffusion of oxygen on a silicon surface

Cluster computations with the density functional theory were performed to clarify the diffusion mechanism of an O atom adsorbed on a Si surface. The activation energy required for the oxygen diffusion into the Si substrate is estimated to be 1.6 eV, which is fairly small compared to oxygen diffusion within a Si crystal. The presence of a surface dangling bond is responsible for the lowering of the activation energy, assisting the generation of an intermediate which is involved in the rearrangement of the Si-O connections. Adsorption of another O atom on the Si surface appears to further enhance the inward oxygen diffusion.     

A search for high frequency vibrational modes at a stepped surface

In this paper we present a theory of the vibrations of atoms in the vicinity of a stepped surface on a Bravais crystal. The static relaxations in the positions of the atoms in the crystal are determined, and the atomic force constants are then calculated in the relaxed atomic configuration. The general theory is applied to a simple stepped surface, and the local phonon density of states is carried out for atoms at several points on the stepped surface by the real space continued fraction recursion method. No evidence is found for high frequency surface phonons.