Non-equilibrium effects in profile spreading on stepped surfaces

We study non-equilibrium effects in spreading and collective diffusion of adatoms on stepped surfaces through Monte Carlo simulations of a lattice-gas model. The spreading density profiles are analyzed by the Boltzmann-Matano method to determine the temporal behavior of the effective collective diffusion coefficients. We find that the presence of steps induces considerable non-equilibrium effects in diffusion. For spreading along the steps, we find that these deviations can be explained by the slow approach of the different adparticle concentrations on terraces and at step edges towards equilibrium. For spreading across the steps, however, we find no such dependence, indicating the breakdown of the linear response theory at early times.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

Preparation and characterization of MgO stepped surfaces

We present a study of the preparation procedure for stepped MgO surfaces which can be used as templates for the deposition of metallic nanostructures. A cleaved sample of MgO(0 0 1) was mechanically polished to reach the desired miscut angle along the [1 1 0] direction. Then a thermal annealing was performed. The effect of an intermediate chemical etching has been also studied. The surface was analyzed by means of contact AFM in air, LEED and XPS in UHV. The role of the chemical etching and the dependence of the final morphology on the annealing time and temperature were investigated. The influence of the miscut angle on the final surface topography is also briefly discussed.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

Dissociation of carbon monoxide on stepped nickel surfaces

Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorption of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450–470 K, which was accompanied by an increase of the work function of ～0.2 eV.     

Vibration spectroscopy of water on stepped gold surfaces

The vibration spectrum of H₂O (ice) adsorbed at low temperatures on Au(1 0 0), Au(1 11 1), and Au(1 1 5) is studied using electron energy loss spectroscopy. On the Au(1 0 0) surface, the spectra show the presence of the typical H-bonded network of water molecules for all coverages from the submonolayer into the multilayer range. The absence of a non-H-bonded OH-stretching mode is indicative for the “H-down bilayer”. On stepped surfaces, on the other hand, a considerable fraction of the H-atoms remains in the non-H-bonded state; surprisingly even in the multilayer range, and even after annealing. The fraction of non-H-bonded hydrogen atoms scales with the step density. Spectral features of water adsorbed at step-sites are isolated after annealing a surface exposed to small doses of H₂O. The results are discussed in the context of recent theoretical studies as well as in conceivable relation to the experimentally found reduction of the Helmholtz-capacitance on stepped Au(1 1 n) electrodes.     

Atomic-scale friction on stepped surfaces of ionic crystals

We report on high-resolution friction force microscopy on a stepped NaCl(001) surface in ultrahigh vacuum. The measurements were performed on single cleavage step edges. When blunt tips are used, friction is found to increase while scanning both up and down a step edge. With atomically sharp tips, friction still increases upwards, but it decreases and even changes sign downwards. Our observations extend previous results obtained without resolving atomic features and are associated with the competition between the Schw?bel barrier and the asymmetric potential well accompanying the step edges.     

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.     

Theoretical study of the Ni growth on Pt stepped surfaces

We have investigated the growth of Ni on Pt stepped surfaces with (1 1 1) terraces by means of potentials derived from the second moment approximation in a tight-binding model. The activation energies associated to these processes are determined. The Schwoebel barriers of Ni atoms descending steps of Pt stepped surfaces are calculated for different kinds of straight steps (A and B steps) differing by the orientation of the ledge. In addition, we study the diffusion of Ni adatoms at fcc or hcp sites in the presence of small adislands on the terraces, in the vicinity of the A and B steps. We show that a good estimate of the potential wells and diffusion barriers could be given by introducing a lateral effective pair interaction model, the interactions extending up to the next nearest neighbors. Finally, we have carried out Kinetic Monte-Carlo simulations to investigate the Ni wire formation at Pt step edges and the influence of the exchange processes in the alloy formation.     

On the local densities of states on flat and stepped Pt surfaces

The electronic structure of the d band of both flat and stepped Pt surfaces is investigated within the tight-binding approximation, using a moment method. A sharp surface virtual bound state peak is found in the local density of states at the protruding edge of the stepped surfaces and the symmetry of states near the Fermi level are found to be rather dependent on the geometry of the surface. Possible connections with experiments are briefly discussed.     

First-principles study of the water structure on flat and stepped gold surfaces

The geometric structure and electronic properties of flat and stepped gold–water interfaces have been addressed by periodic density functional theory (DFT) calculations. This work was motivated by a recent electron energy loss spectroscopy study [H. Ibach, Surf. Sci. 604 (2010) 377] indicating that the structure of a water layer on stepped Au(511) differs significantly from the one on Au(100). Based on ab initio molecular dynamics simulations, the measured spectra have been reproduced and linked to the geometric arrangement of the water molecules. Furthermore, we find a strong polarization of the water layers which contributes to the water-induced work function change of the substrate.     

Structure and magnetism of Fe/MgO/Fe multilayered nanosystems

The dependences of the structural and magnetic properties of a nanoscale Fe/MgO/Fe planar system on the thickness of the dielectric MgO layer are reported. X-ray crystallographic analysis reveals a high-quality layered structure with abrupt interlayer boundaries and a continuous MgO-insulator layer. Fourth-order magnetocrystalline anisotropy is found in the synthesized structures. A new way to provide antiferromagnetic ordering in the nanostructure is proposed by applying a magnetic field to the investigated structure at an angle of 22° with respect to the easy magnetization axis. In this case, the antiferromagnetic ordering of magnetic moments is established in the field range of 20–50 Oe.     

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.     

Microcalorimetry of O2 and NO on flat and stepped platinum surfaces

Using the single-crystal adsorption calorimeter (SCAC), coverage-dependent heats of adsorption and sticking probabilities are reported for O₂ and NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} at 300 K. At low coverage, oxygen adsorption is dissociative for all Pt surfaces. The highest initial heat of adsorption is found on Pt{2 1 1}, with a value of 370 kJ/mol, followed by those on Pt{4 1 1} (310 kJ/mol) and Pt{1 1 1} (300 kJ/mol). We attribute this relatively large difference in the dissociative heat of adsorption at low coverage to the step character of the {2 1 1} surface. Initial sticking probabilities, s_o, are similar for the three surfaces, 0.22 on Pt{1 1 1}, 0.17 on Pt{2 1 1} and 0.18 on Pt{4 1 1}, rapidly decreasing as the oxygen coverage increases. For nitric oxide, the initial heats of adsorption are very similar and consistent with either dissociative or molecular adsorption, with values of 182 kJ/mol on Pt{1 1 1}, 192 kJ/mol on Pt{2 1 1} and 217 kJ/mol on Pt{4 1 1}. The s_o value is virtually identical for all three systems, with values ranging from 0.82 to 0.85, suggesting that the initial sticking probability is insensitive to the surface structure and adsorption is intrinsically precursor mediated. SCAC data are also used to evaluate pre-exponential factors, ν, for first-order desorption at high coverage where adsorption is non-dissociative. Values of 3 × 10¹⁸, 6 × 10¹⁸ and 2 × 10¹⁸ s^?1 for O₂, and 4 × 10¹⁹, 6 × 10¹⁷ and 2 × 10²⁰ s^?1 for NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1}, respectively, are found. These unexpectedly high values are rationalised in terms of conventional transition state theory entropy changes.     

Phase transitions on stepped and disordered surfaces: Xe adsorbed on Cu and NaCl single crystal surfaces

The influence of imperfections of single crystal surfaces on two-dimensional phase transitions is studied by LEED and AES. On a smooth, well-ordered (100)Cu or NaCl face, the first adsorbed layer of xenon undergoes a 2D gas-incommensurable solid first order transition. This transition broadens and becomes continuous on the disordered (100)Cu, the stepped (610)Cu and the disordered (610)NaCl surfaces. The corresponding small variation (±3%) of the chemical potential along the surface can be explained by a defect induced variation of adsorption energy on the substrate for the Cu surfaces or by a limitation of the size of 2D crystals by surface imperfections for the (610)NaCl substrate. Heterogeneities do not modify the lateral structure of 2D solid xenon which remains hexagonal close-packed, but they reduce the size of 2D crystallites in the case of the disordered (610)NaCl substrate. Other spectacular effects are observed on the (610)Cu surface having periodic monoatomic steps: (i) There is a 45° rotation of the 2D crystal with respect to its position on the smooth (100)Cu surface. Hence, the orientational ordering of the xenon overlayer changes drastically, (ii) The LEED pattern at T = 84 K is interpreted as due to a pseudo hexagonal close-packed xenon overlayer with a (2 × 6) coincidence mesh, orientated along the step direction. Moreover, the step edge roughness and the xenon atom size induce a static transverse distortion wave whose wavelength varies with temperature.     

Size effect on magnetism of Fe thin films in Fe/Ir superlattices

In ferromagnetic thin films, the Curie temperature variation with the thickness is always considered as continuous when the thickness is varied from n to n+1 atomic planes. We show that it is not the case for Fe in Fe/Ir superlattices. For an integer number of atomic planes, a unique magnetic transition is observed by susceptibility measurements, whereas two magnetic transitions are observed for fractional numbers of planes. This behavior is attributed to successive transitions of areas with n and n+1 atomic planes, for which the T(c)'s are not the same. Indeed, the magnetic correlation length is presumably shorter than the average size of the terraces. Monte Carlo simulations are performed to support this explanation.