George Comsa

STM investigation of single layer graphite structures produced on Pt(111) by hydrocarbon decomposition

STM has provided new insight into the nucleation, growth and nature of the graphite layer formed on Pt(111) by hydrocarbon decomposition. Annealing an ethylene covered surface to 800 K results in the formation of small (about 20–30 A in diameter) graphite islands which are initially uniformly distributed over the surface. With further annealing above 1000 K, the graphite is observed to accumulate, forming a layer at the lower step edges and also forming large, regularly shaped islands on the terraces. It has been determined that hydrocarbon decomposition at elevated temperatures results in formation of a single layer of graphite on the Pt surface. It is interesting to note that in the STM images of this single layer of graphite only three of the six carbon atoms in the graphite lattice are visible. This result cannot be accounted for by the usual explanation given in terms of inequivalent carbons atoms for the bulk graphite surface. Superstructures with periodicities varying up to 22 A are evident on the graphite areas and are due to a higher order commensurability of the graphite and Pt lattices at different relative rotations.

Dynamical parameters of desorbing molecules

Abstract The aim to uncover fundamental details of the desorption process as well as the needs of more applied research, like fusion, led recently to a growing interest in the study of the parameters of desorbing molecules. The main experimental methods and the most significant experimental results obtained so far are reviewed and discussed. The investigation of the parameters of desorbing molecules being one of the oldest fields of surface science, a number of pre- and misconceptions still hamper the mutual understanding and hereby impede progress. Accordingly, a relatively large pace is dedicated to the historical development, to definitions, and even to semantics.

The purely “fast” distribution of H2 and D2 molecules desorbing from Cu(100) and Cu(111) surfaces

Abstract Velocity distributions of H2 and D2 molecules desorbing from Cu(100) and (111) surfaces were measured as a function of desorption angle ϑ and sulphur coverage by the time-of-flight (TOF) cross-correlation method. The H and D atoms were supplied to the surface by permeation through the bulk of the Cu crystal. The TOF spectra are very narrow and correspond to mean energies of about 8kTs. i.e. four times larger than the energy corresponding to the surface temperature Ts = 1000 K. The velocity distributions of H2 and D2 from Cu appear to be the first known case in which all desorbing molecules belong to the group of “fast” molecules. This provides an opportunity to study the desorption of “fast” molecules in their pure form. The present data on Cu as well as former ones on Ni and Pd may be tentatively explained by assuming that hydrogen atoms permeating through the bulk to the sample surface have a much higher probability to equilibrate in some subsurface potential well, e.g. in the recently proposed subsurface state of H, than in the chemisorption well. The excess kinetic energy of the desorbing molecules resulting from recombination of atoms in this state originates from the potential barrier separating the subsurface state from the chemisorption well. The height of this barrier appears to correlate with the activation barrier for the bulk permeation of hydrogen.

Determination of surface reconstruction with impact-collision alkali ion scattering

Abstract 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.

The homoepitaxial growth of Pt on Pt(111) studied with STM

The homoepitaxial growth of Pt on Pt(111) has been investigated by STM and the results have been compared to recent thermal He scattering (TEAS) data obtained on the same system. Additional information on the growth modes is obtained and the real space aspect of the growing surface, which results in TEAS and RHEED oscillations is evidenced. The three different growth modes, including the reentrant layer-by-layer growth at low temperatures, are confirmed. The limited diffusion along the adatom island edges, which causes their fractal aspect with dendritic structures, appears to play a significant role in the appearance of the low temperature layer-by-layer growth.

Temperature dependence of the sputtering morphology of Pt(111)

Abstract The morphology of the Pt(111) surface after sputtering with 600 eV Ar + ions as a function of the annealing temperature, during and after sputtering, has been investigated by scanning tunneling microscopy. The data confirm the transition from pit formation to layer-by-layer removal regime and emphasize the role of thermally created adatoms and/or vacancies in the mass transport between atomic layers. The measurements show further that the shape of the vacancy islands upon annealing above 700 K is equilibrium-like. The application of the Wulff construction to the two-dimensional vacancy islands allows the determination of the ratio of the free energies of the two types of step edges which bound the islands.

The interaction of hydrogen with platinum(s)−9(111) × (111) studied with helium beam diffraction

Abstract The H 2 Pt(S) −9(111) × (111) interaction system has been studied using helium beam diffraction in the temperature range 250–370 K. Information on the H coverage is obtained from the reduction of coherent scattering upon the presence of disordered adsorbates. The features of this recently proposed new technique are discussed in detail. The incident He beam of thermal energy is essentially non-perturbing and non-penetrating. The diffraction experiment yields surface sensitive information on the interaction of adsorbates (well-suited for H2 too!) with ideal regions of the surface. The method enables discrimination between step edge and terrace properties in a direct geometrical way. So, for instance, isosteric heats of adsorption of 22 and 19 kcal mole have been obtained for the step edges and the terraces, respectively. The obtained preexponentials for desorption have “normal” values for a second order process. The measured isotherms can be fitted well with Langmuir isotherms. The internal consistency of the model description is checked by comparing adsorption and desorption measurements with equilibrium results.

Magnetically suspended cross‐correlation chopper in molecular beam‐surface experiments

The advantages of the cross‐correlation time of flight (TOF) method when applied to surface scattering experiments under ultrahigh vacuum conditions are demonstrated in comparison with the conventional TOF method. The principle of the cross‐correlation TOF technique including the deconvolution procedure and statistical accuracy given by this method is discussed. The main parts of the spectrometer: chopper blade, magnetic rotor suspension and drive, and data acquisition system are described followed by the test of the TOF spectrometer, and by an illustration of the efficiency of this technique by means of TOF distributions measured for hydrogen molecules desorbing from metal surfaces.

The angular dependence of flux, mean energy and speed ratio for D2 molecules desorbing from a Ni(111) surface

Abstract Experimental data are presented for the angular dependence of the relative flux, the mean energy and the speed ratio of deuterium molecules desorbing from a Ni(111) crystal surface at a surface temperature of Ts = 1143 K and at sulphur coverages ranging between 30% and less than 2% of a monolayer.The angular flux distribution is sharply peaked in the forward direction (cosdθwith 3 ⩽ d ⩽ 5) and the mean energy 〈E〉 of the desorbate depends strongly on the desorption angle θ. For normal desorption (θ = 0°) 〈E〉 2k is about 700 K higher than Ts and for glancing angles (θ = 80°) it decreases to about 400 K below Ts The results obtained on sulphur free and sulphur covered Ni(111) surfaces are compared with our former data on polycrystalline nickel. The main differences in the kinetic features can be ascribed to the surface roughness. Accordingly, the angular distributions of flux, mean energy, and speed ratio, which deviate strongly from the Knudson and Maxwellian law, do not seem to depend considerably on sulphur coverage and surface structure. A qualitative explanation for these deviations is presented using the principle of detailed balancing.

Direct observation of surface reactions by scanning tunneling microscopy : ethylene→ethylidyne→carbon particles→graphite on Pt(111)

We have used variable temperature, ultrahigh vacuum scanning tunneling microscopy (STM), in both static and time‐dependent experiments, to study the chemistry of the ethylene/Pt(111) system. Images of ethylene which exhibit long‐range order have been obtained at a sample temperature of 160 K. The conversion of ethylene to ethylidyne has been observed directly in STM images. This conversion reaction is observed to occur in a ‘‘patchy’’ manner across the surface at saturation coverage. As the reaction proceeds, well‐defined islands of unreacted ethylene continue to be clearly observed. Further dehydrogenation of the ethylidyne formed from ethylene leads to carbon containing particles dispersed randomly across the sample. After annealing the ethylidyne covered sample to 500 K, the surface is uniformly covered with carbon containing particles which exhibit a bimodal distribution of heights (one and two atomic layers) consisting of an average of ten and twenty carbon atoms, respectively. Further annealing to 7...