Adsorption of silicon atoms on the surface of the Au/W(110)

The possibility of formation of an ordered silicene-like structure on Au/W(110) surface has been considered using angle-resolved photoelectron spectroscopy and X-ray photoelectron spectroscopy. It is shown that the addition of silicon atoms results in a considerable distortion of the electron structure of the initial substrate, and the resulting electron structure cannot be attributed to silicene. The configuration of reflections in the low energy electron diffraction pattern indicates the formation of two-dimensional ordered silicon structures with a large number of multidirectional domains.

     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

Enhanced activity for supported Au clusters: Methanol oxidation on Au/TiO2(110)

Gold clusters supported on TiO₂(110) exhibit unusual activity for the oxidation of methanol to formaldehyde. Temperature programmed desorption studies of methanol on Au clusters show that both Au and titania sites are necessary for methanol reaction. Isotopic labeling experiments with CD₃OH demonstrate that reaction occurs via OH bond scission to form a methoxy intermediate. When the TiO₂ surface is oxidized with ¹⁸O₂ before or after Au deposition, methanol reaction produces H₂¹⁸O below 300 K, indicating that oxygen from titania promotes OH bond scission and is incorporated into desorbing products. XPS experiments provide additional evidence that during methanol reaction on the Au/TiO₂ surface, methanol adsorption occurs on TiO₂, given that the titania support becomes slightly oxidized after exposure to methanol in the presence of Au clusters. While the role of TiO₂ is to dissociate the OH bond and form the reactive methoxy intermediate, the role of the Au sites is to remove hydrogen from the surface as H₂, thus preventing the recombination of methoxy and hydrogen to methanol. The decrease in formaldehyde yield with increasing Au coverage above 0.25 ML suggests that reaction occurs at Au–titania interfacial sites; scanning tunneling microscopy images of various Au coverages confirm that the number of interfacial sites at the perimeter of the Au clusters decreases as the Au coverage is increased between 0.25 and 5 ML.     

Isothermal desorption of hydrogen molecules from a W(110) surface at temperature ∼5 K

Isothermal desorption of hydrogen molecules from a W(110) surface atT _s ～ 5 K upon rapid shuttering of the molecular beam is observed in a “black chamber” type of ultrahigh-vacuum apparatus. Desorption was detected from three different states, identified as a multilayer condensation state and physisorbed states in the form of a two-dimensional gas and a two-dimensional condensate. The distribution of the physisorbed molecules between these states depends on the intensity of the flux of molecules on the surface; this appears to be responsible for the anomalous decrease of the number of isothermally desorbed molecules as the flux increases.     

Spin-asymmetry in elastic scattering of low-energy electrons from ultrathin Au films on W(110)

We present joint experimental and theoretical results on the elastic scattering of spin-polarized electrons from an epitaxial Au film on a W(110) substrate in the energy range from 8 eV to 27 eV. A time-of-flight technique with a position-sensitive detector is applied to measure secondary emission spectra for spin-up and spin-down primary electrons in a specular geometry. The spin-asymmetry of coherently scattered electrons is obtained by selecting the diffraction spot on the detector. Regions of large asymmetries – with a maximum of about ?60 % – are identified for electron energies of about 14 eV. Relativistic multiple-scattering calculations produce spin-orbit-induced asymmetries which are in agreement with their experimental counterparts. They further reveal that large asymmetries are associated with high intensities. This offers the possibility of an efficient new spin polarimeter with a figure of merit of about 1.5 · 10^?2.     

Surface crystallography of W(110) and W(110) p (2×1)-O: The position of W atoms

The position of W atoms in the surface layers of clean W (110) and W (110) p (2 × 1)-O is studied using Constant-Momentum-Transfer Averaging of LEED intensities. It is shown that the clean surface is not relaxed to an uncertainty of < 0.06 Å. Analysis of superstructure beam intensity averages from W(110) p (2 × 1)-O indicates that oxygen does not reconstruct W(110) at these coverages and below 1000 ° K. An upper limit of 0.05 Å can be put on the out-of-plane displacement of W atoms by the oxygen. Substrate beam averages from W (110) p (2 × 1)-O verify the non-reconstruction. The use of CMTA for adsorbed-layer crystallography in general is briefly discussed.     

Angle-resolved investigation of Auger electrons from Cu and Au adsorbed on W(110)

The angular distribution of Cu M_2,3VV and Au N_6,7VV Auger electrons from Cu and Au mono- and double layers on W(110) is measured with the goal of obtaining information on the contribution of the backscattered wave on the angular distribution of Auger electrons from adsorbed atoms.     

Stress induced stripe formation in Pd/W(110)

A stress-induced stripe phase of submonolayer Pd on W(110) is observed by low-energy electron microscopy. The temperature dependence of the pattern is explained by the change both in the boundary free energy and elastic relaxation energy due to the increasing boundary width. The stripes are shown to disorder when the correlation length of the condensed phase becomes comparable to its period, while the condensate to lattice-gas transition takes place at a higher temperature, as revealed by low-energy electron diffraction.     

The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Kinetics of facile bilayer island formation at low temperature: Ag/NiAl(110)

Facile nucleation and growth of bilayer Ag(110) islands on NiAl(110) is observed by STM for Ag deposition at temperatures as low as 127 K. Density functional theory analysis for supported Ag films determines adatom adsorption energies (which favor bilayer islands), interaction energies, and diffusion barriers. Analysis of an atomistic lattice-gas model incorporating these energies elucidates the role of strongly anisotropic interactions in enabling the upward mass transport needed for bilayer island formation.     

Long jump rates in surface diffusion: W on W(110)

We have reexamined the diffusion of W adatoms on W(110) using the field ion microscope. The diffusivity is in good agreement with previous results and reveals no unusual features, but from observations of the distribution of displacements, corrected for diffusion during temperature transients, we have for the first time been able to measure the temperature dependence of rates for nearest-neighbor and double jumps, as well as for vertical and horizontal transitions. Activation energies and frequency factors for all the long jumps are significantly larger than for single jumps, and a simple model is proposed to account for our observations.     

Adsorption of oxygen on W(110): II. The high coverage range

The structure and composition of the interaction layer between oxygen and a W(110) surface for oxygen coverages θ above 0.5 monolayers is studied with LEED, AES, thermal desorption and work function change measurements. Oxygen is adsorbed by depositing WO₂ followed by annealing. The results are interpreted in terms of a topmost layer consisting only of oxygen atoms followed by the formation of isolated three-dimensional WO₃ crystals after saturation of the two-dimensional oxidation layer at 15 × 10¹⁴ O atoms cm^?2. All available experimental evidence is compatible with this interpretation.     

Thermal desorption spectroscopy of Ni,Cu, Ag and Au from W(110)

Ni, Cu, Ag and Au on W(110) in the submonolayer range are studied by thermal desorption spectroscopy with the goal of obtaining information on lateral interactions and on the phase state of the adsorption layer in the temperature range in which desorption occurs. Ni, Cu and Ag are found to desorb over a wide coverage range from the two-phase (vapor-condensate) region while Au desorbs only from the single-phase vapor region. Segments of the coexistence curve are determined. The desorption energies have the following limiting values: 4.35–4.95. 3.2–3.85, 2.8–3.55 and 3.3–4.1 eV for Ni, Cu, Ag and Au, respectively.     

Streuung spinpolarisierter Elektronen an Au(110)

Using the the spin modulated polarized electron gun based on photoemission from positive affinity GaAs the intensityI(E, , ) and asymmetry (E, , ) are measured in LEED from Au(110). The asymmetry is compared with the polarizationS(E, , ) which N. Müller obtained with a Mott detector after scattering an unpolarized electron beam from the same crystal. There is excellent agreement for the (00) beams, if the scattering plane is a mirror symmetry plane of the crystal (=0°, =90°). From the differences for the (00) and (11) beams at =35° and the (01/2) beam at =90° conditions for possible models for the reconstructed Au(110)–(1×2) surface may be derived.

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Die vorliegende Arbeit ist ein Auszug aus [1]

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Neue Addresse ab 1. Oktober 1980: B. Reihl, IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, USA     

Observation of a registered phase in Cs/W(110)

RHEED and AES studies at room temperature have revealed a new metastable state of Cs ad-atoms on W(110) which are in registry at least along [1&#x0304;10] W. The ad-atom density in this state is 0.81 ± 0.03 ML, where 1 ML corresponds to the final density of Cs atoms in the saturated close packed hexagonal structure. This new structure has been observed as the final structure in dynamic deposition conditions, i.e. under the constant pressure of the Cs beam. The previously observed 1 ML structure arises when deposition and AES observations are alternated. The possibility that impurities provide the means of overcoming a nucleation barrier to this final structure is discussed.