120 research outputs found
Towards a first-principles theory of surface thermodynamics and kinetics
Understanding of the complex behavior of particles at surfaces requires
detailed knowledge of both macroscopic and microscopic processes that take
place; also certain processes depend critically on temperature and gas
pressure. To link these processes we combine state-of-the-art microscopic, and
macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001)
system and calculate thermal desorption spectra, heat of adsorption, and the
surface phase diagram. The agreement with experiment provides validity for our
approach which thus identifies the way for a predictive simulation of surface
thermodynamics and kinetics.Comment: 4 pages including 3 figures. Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Fluctuations of an Atomic Ledge Bordering a Crystalline Facet
When a high symmetry facet joins the rounded part of a crystal, the step line
density vanishes as sqrt(r) with r denoting the distance from the facet edge.
This means that the ledge bordering the facet has a lot of space to meander as
caused by thermal activation. We investigate the statistical properties of the
border ledge fluctuations. In the scaling regime they turn out to be
non-Gaussian and related to the edge statistics of GUE multi-matrix models.Comment: Version with major revisions -- RevTeX, 4 pages, 2 figure
Theoretical study of O adlayers on Ru(0001)
Recent experiments performed at high pressures indicate that ruthenium can
support unusually high concentrations of oxygen at the surface. To investigate
the structure and stability of high coverage oxygen structures, we performed
density functional theory calculations, within the generalized gradient
approximation, for O adlayers on Ru(0001) from low coverage up to a full
monolayer. We achieve quantitative agreement with previous low energy electron
diffraction intensity analyses for the (2x2) and (2x1) phases and predict that
an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on
Ru(0001), where the O adatoms occupy hcp-hollow sites.Comment: RevTeX, 6 pages, 4 figure
Automatic kelvin probe compatible with ultrahigh vacuum
This article describes a new type of in situ ultrahigh‐vacuum compatible kelvin probe based on a voice‐coil driving mechanism. This design exhibits several advantages over conventional mechanical feed‐through and (in situ) piezoelectric devices in regard to the possibility of multiple probe geometry, flexibility of probe geometry, amplitude of oscillation, and pure parallel vibration. Automatic setup and constant spacing features are achieved using a digital‐to‐analog converter (DAC) steered offset potential. The combination of very low driver noise pick‐up and data‐acquisition system (DAS) signal processing techniques results in a work function (wf ) resolution, under optimal conditions, of <0.1 meV. Due to its high surface sensitivity and compatibility with standard sample cleaning and analysis techniques this design has numerous applications in surface studies, e.g., adsorption kinetics, sample topography and homogeneity, sputter profiles, etc. For semiconductor specimens the high wf resolution makes it eminently suitable for surface photovoltage (SPV) spectroscopy
Surface structure of nickel oxide layers on a Rh(111) surface
The formation of nickel oxide nanolayers by oxidizing Ni overlayers on Rh(111) has been investigated and their structures are reported as a function of the nickel coverage and oxygen pressure. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and diffraction (XPD), and high-resolution electron energy loss spectroscopy (HREELS) have been applied to characterize the structure and stoichiometry of the nickel oxide nanolayers. Several different phases have been observed depending on the strain state of the metallic Ni overlayers. For the pseudomorphic Ni monolayer, two distinctly different oxide phases with (6 x 1)-Ni5O5 and (2 root 3 x 2)-Ni8O10 structures have been identified at oxygen-poor (p = 5 x 10(-8) mbar) and oxygen-rich (p >= 1 x 10(-6) mbar) conditions, respectively. Above one monolayer, where the Ni layers are relaxed, bulk-like NiO(100) films form at the O-rich conditions, whereas chemisorbed-type p(2 x 2)O-Ni(111) layers develop in the O-poor regime. X-ray photoelectron diffraction analysis has provided additional insight into the relaxation mechanism and the detailed atomic structure of the Ni-oxide nanolayers. (C) 2013 Elsevier B.V. All rights reserved
Growth and properties of strained VOx thin films with controlled stoichiometry
We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001)
substrates. The oxygen content as a function of oxygen flux was determined
using 18O2-RBS and the vanadium valence using XAS. The upper and lower
stoichiometry limits found are similar to the ones known for bulk material
(0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for
both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These
numbers are, surprisingly, very similar to those for bulk material and
consequently quite strain-insensitive. XAS measurements reveal that the
vacancies give rise to strong low symmetry ligand fields to be present. The
electrical conductivity of the films is much lower than the conductivity of
bulk samples which we attribute to a decrease in the direct overlap between t2g
orbitals in the coherently strained layers. The temperature dependence of the
conductivity is consistent with a variable range hopping mechanism.Comment: 12 pages, 16 figures included, revised versio
Low-energy electron diffraction analysis of the structure of a Cs-O/Ru(0001) coadsorbate phase
The structure of the (√3 × √3 )R30° overlayer formed by coadsorption of Cs and O atoms (θCs=θ0=0.33) on a Ru(0001) surface was determined by low-energy electron diffraction. Both adsorbates occupy hcp-type hollow sites. If compared with the structures of the respective pure adsorbate phases, the bond lengths are modified in a way consistent with an effective transfer of electronic charge from Cs to O
Simulation of Near Edge X-ray Absorption Fine Structure (NEXAFS) Measurements of CO on Supported Pd Nanoparticles
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