Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Perpendicular magnetization in Fe/Ni bilayers on GaAs(0 0 1)

Following the concept of spin-injection into a semiconductor-based device, a ferromagnetic element (like a GMR multilayer structure) can be used as a spin filter. A high spin-polarization of the electrons can be realized by the preparation of a monocrystalline multilayer structure consisting of ultrathin films of a high magnetic polarization. In the case of ultrathin films, the manipulation of the easy-axis of magnetization is possible, by changing the anisotropy terms contributing to the effective anisotropy of the structure. We report on the structural and magnetic properties of Ni/Fe and Fe/Ni bilayers epitaxially grown on GaAs(0 0 1). By a proper choice of Fe and Ni sequences (Fe/Ni/GaAs) and their thickness (up to 3 ML of Fe on the top of Ni), the rotation of magnetization from the in-plane to the out-of-plane direction was achieved.     

Cu dimer diffusion on strained Cu(0 0 1)

Cu dimer diffusion energy barrier on strained Cu(0 0 1) surfaces has been studied with nudged elastic band method (NEB) and embedded atom method (EAM). Dimer exchange and hopping mechanisms are chosen as the initial diffusion paths in the NEB method. It is shown here that the dimer exchange is dominant on tensile surfaces and the dimer hopping is dominant on compressive surfaces. For most strain conditions Cu dimer diffusion energy barrier is lower than Cu monomer diffusion barrier. The concerted movement of the remaining adatom toward the hopping adatom lowers the dimer hopping barrier. The adsorption induced relaxation makes the dimer exchange barriers lower than the monomer exchange barriers on tensile surfaces. Transition state theory is used to calculate the diffusion frequencies as a function of temperature. No surface crowdion is observed on the shear strained surfaces for the dimer diffusion.     

Structural and electronic characterization of the MgO/Mo(0 0 1) interface using STM

The nucleation and growth of ultrathin MgO films on Mo(0 0 1) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.     

Adsorption of ZnO, (ZnO)2 and (ZnO)3 on MgO(0 0 1)

We have investigated the adsorption of ZnO, atomic Zn, atomic O and (ZnO)_n on the flat MgO(0 0 1) surface by means of density functional calculations. A single ZnO molecule prefers to bind in a position parallel to the surface with the zinc atom above a surface oxygen atom and the oxygen above a magnesium atom. Adsorption of a pair of ZnO molecules leads to the formation of (ZnO)₂ on the surface with geometric parameters slightly distorted from those of the free molecule, providing an indication of surface interaction with the adsorbate. Finally, the trimer forms on the surface with the same general shape as the gas phase trimer, but with angular distortions influenced by surface-molecule interactions. The effects of increasing surface coverage are also explored.     

Electronic surface states of Cu(1 1 0) surface

Surface states are a unique and important class of quantum states that shave an important effect on the electronic properties of Cu(1 1 0) surface. The Cu(1 1 0) surface has been studied using ultraviolet photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), and reflection anisotropy spectroscopy (RAS), and shows a resonance in the RAS spectra at 2.1 eV due to a transition between occupied and unoccupied surface states. The unoccupied surface state involved in the RAS transition at an energy of 1.7 eV at the point of the surface Brillouin zone has been investigated using IPES and the occupied surface state is seen in PES spectra at 0.45 eV below the Fermi level. The energy difference of the surface states, 2.15 eV, is a good match to the transition energy found in the RAS experiments.     

A high (0 0 1)-oriented CoPt/Ag film deposited on glass substrate

The magnetic properties and structures of CoPt_15 nm/Ag_0−100 nm films deposited by DC magnetron sputtering on glass substrates have been studied. The (0 0 1) texturing was improved by introducing an Ag underlayer. As the Ag underlayer thickness was 100 nm, a nearly perfect (0 0 1)-textured CoPt film was obtained.     

A PdO(1 0 1) thin film grown on Pd(1 1 1) in ultrahigh vacuum

We present experimental results demonstrating that a high quality PdO(1 0 1) thin film can be grown on Pd(1 1 1) in ultrahigh vacuum by oxidizing the metal at 500 K using an oxygen atom beam, followed by annealing to 675 K. Low energy electron diffraction (LEED) images show that the [0 1 0] direction of the PdO(1 0 1) thin film aligns with the [−1 1 0] direction of the Pd(1 1 1) substrate, and that the PdO film grows in three degenerate domains, rotated 120° relative to one another. Based on excellent agreement between the experimental and simulated LEED patterns, we conclude that the surface structure of the PdO thin film deviates minimally from bulk-terminated PdO(1 0 1). Recent temperature programmed desorption (TPD) experiments also provide evidence that the PdO(1 0 1) thin film on Pd(1 1 1) is terminated by the stoichiometric surface in which half of the Pd atoms are coordinatively unsaturated (cus), corresponding to a cus-Pd atom density equal to about 35% of the surface density of Pd(1 1 1). The ability to generate a well-defined PdO(1 0 1) surface in ultrahigh vacuum should provide new opportunities for conducting model surface science studies of PdO, particularly studies aimed at elucidating the reactivity of PdO(1 0 1) toward species important in commercial applications of Pd catalysis.     

para-Sexiphenyl thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O surfaces

We present a reflectance difference spectroscopy (RDS) study of para-sexiphenyl (p-6P) thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O substrates. The RDS spectra show pronounced anisotropies for p-6P films formed on both substrates at room temperature, demonstrating that the molecules are uniaxially aligned within the films. Based on the RD spectra and the evolution of the optical transitions with p-6P coverage the growth mode on both substrates could be identified. From the dominating RDS feature, assigned to the lowest energy HOMO-LUMO transition, the orientation of the molecular chain can be determined. On Cu(1 1 0), the p-6P molecular chains align in the direction, i.e., along the Cu atomic rows, whereas on the Cu(1 1 0)-(2 × 1)O surface, the molecules are oriented in the orthogonal [0 0 1] direction, i.e., along the “added” Cu-O rows of the Cu(1 1 0)-(2 × 1)O surface. The energetic position and line shape of the main RDS feature differs for the two substrates and varies with p-6P coverage. This fine structure is discussed in terms of different molecular conformations, adlayer structure and vibronic replicas.     

Surface relaxation and near-surface atomic displacements in the N/Cu(1 0 0) self-ordered system

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift ⁴He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 Å, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.     

Surface phonons of GaP(1 1 0) and InAs(1 1 0)

The dispersions of low energy surface phonon modes of GaP(1 1 0) and InAs(1 1 0) measured with inelastic He-atom scattering along the and 0 0 1 directions are presented. Aside from the Rayleigh mode, additional distinct acoustic modes are observed as well as indications of optical modes. Contrary to results for GaAs(1 1 0), a rocking mode was not observed. The experimentally determined phonon dispersions are in excellent agreement with recent ab initio calculations by C. Eckl, et al. [1].     

Carbon-doped high-mobility hole gases on (0 0 1) and (1 1 0) GaAs

Since Stormer and Tsang have introduced the first two-dimensional hole gas (2DHG) in the GaAs/AlGaAs heterosystem, the choice of suitable dopants was limited to beryllium and silicon over the last 20 years. Both acceptor atoms have significant disadvantages, i.e. either high-diffusion rates or a limitation to specific growth directions. Utilizing a carbon filament-doping source, we prepared high-quality 2DHGs in the (0 0 1) and the nonpolar (1 1 0) crystal plane with carrier mobilies beyond 10⁶ cm²/Vs in quantum well and single interface structures. Low-temperature magnetoresistance measurements recover a large number of fractional QHE states and show a pronounced beating pattern from which the Rashba induced spin-splitting has been determined. In addition, 2DHGs have been grown on cleaved edges of (1 1 0) and (0 0 1) wafers with transport features in qualitative agreement to our findings on (1 1 0) substrates.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

Tailoring exchange bias by oxidizing Co film across a Cu wedge in Cu(wedge)/CoO/Co/Cu(0 0 1)

A new method was developed to control Co film oxidation in an epitaxially grown Cu(wedge)/Co/Cu(0 0 1) film. By annealing the film at 200 °C within 10⁻⁶ Torr oxygen environment, we find that the top Cu wedge controls the Co underlayer oxidation continuously as a function of the Cu film thickness. Magneto-Optic Kerr Effect measurement shows that the exchange bias of the resulting CoO/Co film exhibits a systematic variation with the Cu thickness, thus offering a new method of tailoring the exchange bias of CoO/Co films.     

Comparative dynamical study of the HD and isotopomers interacting with Pd(1 1 1) and Cu(1 1 0) surfaces

We study the dynamics of HD and H₂ molecules interacting with Pd(1 1 1) and Cu(1 1 0) using the classical trajectory method based on potential energy surfaces obtained from Density Functional Theory calculations. Our results predict a negligible isotopic effect on the dissociative adsorption probability on Pd(1 1 1) whereas on Cu(1 1 0), the adsorption probability for HD(ν_i=0) is slightly lower than for H₂(ν_i=0), mainly due to its lower initial vibrational zero point energy. The final rotational energy distribution of scattered HD and H₂ molecules are very similar. This shows that the asymmetric mass distribution of HD, barely affects the fraction of initial translational energy transferred to rotation during the scattering process. Our calculations point to the larger number of open rotational excitation channels for HD, as the main cause of rotational excitation probabilities larger than for H₂. The theoretical apparent rotational temperature, T_rot, of HD molecules scattered from Pd(1 1 1) at impact energy , is in good agreement with the experimental value. In contrast, for Cu(1 1 0) the theoretical T_rot is much lower than the value measured for Cu(1 0 0). Possible reasons for such a discrepancy between theory and experiments are discussed.     

Adsorption and NiO(1 0 0) formation by atomic and molecular oxygen on Ni(1 1 0)

The structure of the ordered p(2 × 1) and p(3 × 1) phases of adsorbed oxygen as well as the formation of ultrathin NiO(1 0 0) layers on a Ni(1 1 0) single crystal are investigated by grazing scattering of fast hydrogen atoms. Via ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of oxygen adsorbates and ultrathin nickel oxide layers. For oxidation using atomic instead of molecular oxygen, the gas exposure can be reduced by almost two orders of magnitude. We compare the experimental results with computer simulations based on classical projectile trajectories for grazing scattering of fast hydrogen atoms and test structure models for oxygen adsorbed on Ni(1 1 0) and NiO(1 0 0).     

Adsorption of NO on Au atoms and dimers supported on MgO(1 0 0): DFT studies

The adsorption of NO on single gold atoms and Au₂ dimers deposited on regular O²⁻ sites and neutral oxygen vacancies (F_s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au₁/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F_s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F_s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au₂/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au₂. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm⁻¹ to lower frequencies is observed for Au₁/MgO in comparison with free NO.     

Near-surface stresses in silicon(0 0 1)

We report here on a detailed atomistic stress analysis of the near-surface stresses on the 2 × 1 surface reconstruction on Si(0 0 1). We find that the surface stresses are spatially periodic with fairly large amplitude. We additionally investigate the stresses in the neighborhood of an S_A-type step.