Determination of surface reconstruction with impact-collision alkali ion scattering

The method of low energy ion scattering in the impact collision mode (ICISS) has been substantially improved by the use of alkali ions. Alkali ions extend the capabilities of ICISS towards the relative positioning of atoms lying in the first and deeper layers, which is a premise for the characterization of surface reconstruction. The self calibration procedure based on the experimental determination of an effective shadow cone is an important advantage. The small contribution of multiple scattering effects in ICISS allows a straightforward determination of the locations of surface atoms. On the basis of a large number of ICISS data the oxygen induced surface reconstruction of Cu(110) has been shown to be correctly described by the missing row model.     

STM investigation of the adsorption and temperature dependent reactions of ethylene on Pt(111)

The adsorption and reactions of ethylene adsorbed in UHV on Pt(111) have been studied as a function of temperature by STM. The STM images taken at 160K show an ordered structure of adsorbed ethylene. Annealing to 300 K produces ethylidyne (C-CH₃) irreversibly, as has been demonstrated by a wide variety of surface science techniques. The ethylidyne on Pt(111) is not visible to the STM at room temperature. Cooling the sample allows direct observation of the ethylidyne ordered structure by STM. Annealing above 430 K results in further dehydrogenation, eventually leaving only carbon on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt(111) surface. The annealed graphite islands exhibit several supersturctures with lattice parameters of up to 22 Å, which are thought to result from the higher order commensurability with the Pt(111) substrate at different relative rotations.     

Helium scattering and work function investigation of co adsorption on Pt(111) and vicinal surfaces

CO adsorption on Pt(111) and vicinal Pt(111) surfaces has been studied by means of work function variation and He scattering measurements. AES and LEED were used mainly for correlations with other work. Special attention has been paid to the low coverage regime (θ_co < 0.1) with emphasis on surface structural dependencies. The minimum of the work function versus CO exposure curve occurs at a coverage less than 11% on “kink-free” surfaces. This is much lower than the hitherto commonly accepted value of 33%, and does not relate to any observed LEED superstructure. The value of Δφ_min depends strongly on the surface structure. For an “ideal” Pt(111) surface with a step density less than 10^?3 at a temperature of 300 K, Δφ_min = ?240 meV. The scattering cross section Σ of CO adsorbed on Pt(111) for 63 meV He is typically > 250 Ų, i.e. much larger than expected from the Van der Waals radii of He and CO. For two nominal Pt(111) surfaces with step densities of 10^?2 and less than 10^?3, respectively, the measured Σ values varied by a factor of three. This can be explained by preferential CO occupation of defect sites, which are already not “seen” by thermal helium. By comparing results on a stepped (997) and a kinked (12 11 9) Pt surface with similar defect densities, the kinks are proven to play a decisive role. They probably form saddles in the recently proposed activation barrier for migration between terrace and step sites.     

Effect of size on the vibrational specific heat of ultrafine palladium particles

The specific heat of Pd particles smaller than 100 Å was measured between 1.4 and 30 K. The observed enhancement of the vibrational part in comparison with the bulk is discussed in terms of a Bose—Einstein summation of the vibrational modes as proposed by Baltes and Hilf. Satisfactory agreement between experiment and theory is found when size and temperature dependent particle sound velocities are assumed. This involves a considerable reduction of the Debye temperature of the Pd particles comparable with the results for other metals.     

Step edge diffusion and step atom detachment in surface evolution: ion erosion of Pt(111)

The temperature dependent morphological evolution of Pt(111) under 1 keV Xe+ normal incidence ion bombardment has been investigated up to 600 monolayers removed. Coarsening of the surface structures during erosion and a qualitative change in roughness evolution between 650 and 700 K are found to be caused by different atomic processes: the former by diffusion of atoms along steps, the latter by the onset of step atom detachment.     

New concepts for controlled homoepitaxy

On the basis of a kinetic growth model we discuss new methods to grow atomically flat homoepitaxial layers in a controlled way. The underlying principle of these methods is to change the growth parameters during growth of an atomic layer in such a way that nucleation on top of a growing layer is suppressed, and thus, layer-by-layer growth is achieved. Experimentally, this can be realized by changing the substrate temperature or deposition rate during monolayer growth in a well-defined way. The same can be achieved at constant temperature and deposition rate by simultaneous ion bombardment during the early stages of growth of a monolayer, or by adding suitable surfactants to the system. Model experiments on Ag(111) and on Cu(111) using thermal energy atom scattering and scanning tunneling microscopy demonstrate the success of these methods.