- Email: [email protected]
Photoemission and electron microscopy of small supported palladium clusters
A138 150
Surface Science 245 (1991) 150-172 North-Holland
Growth kinetics simulation of the A1-Ga self-organization on GaAs(100) stepped surfaces Yan-Ten Lu and Horia Metiu Department of Chemistry and Physics, University of California, Santa Barbara, CA 93106, USA Received 1 August 1990; accepted for publication 29 October 1990 We present results of a stochastic kinetic simulation of AI and Ga segregation during the mobility enhanced epitaxial deposition of an AI & Ga monolayer on the As face of a stepped GaAs(100) surface. The simulation deposits the atoms on the surface and moves them from site to site so that the frequency of each event is proportional to its rate. Since the rate constants needed in the simulation have not been measured we use guesses based on the available information and analogies with similar systems. The simulations generate exactly the growth pattern corresponding to a given set of rates. We have found a set of rate constants which generates AI & Ga growth patterns having all the properties observed experimentally. The predicted aggregation patterns are stable with respect to reasonable changes in the numerical values of the kinetic parameters. By varying the corresponding rates we find how the elementary kinetic steps affect segregation. One of the more interesting results is that to obtain good segregation with smooth A I - G a borders it is essential that the AI-AI interactions are anisotropic and are stronger than the G a - G a interactions.
Surface Science 245 (1991) 173-179 North-Holland
173
Photoemission and electron microscopy of small supported palladium clusters Ch. Kuhrt and M. Harsdorff Institut fiJr Angewandte Physik, Universitiit Hamburg, Jungiusstrasse 11, W-2000 Hamburg 36, Germat~v Received 14 August 1990; accepted for publication 26 October 1990 Small palladium clusters, produced by vapour deposition onto amorphous carbon substrates, were analysed in situ by photoelectron spectroscopy (XPS, UPS) and characterized ex situ by transmission electron microscopy. These combined measurements provide the actual cluster size dependence of the two well-known effects in the photoelectron spectra of small supported clusters - the valence band narrowing and the core-electron binding energy shift - suggesting the following explanations: the valence band narrowing is due to an initial state effect, while the core-level shift is dominated by the effect of the changed final state, as it is predicted by Wertheim et al. 180
Surface Science 245 (1991) 180-190 North-Holland
Nuclear spin relaxation as a tool to study surface diffusion * Manfred Riehl-Chudoba 1, U. M e m m e r t 2 and D. Fick Fackbereich Physik and Wissenschaftliches Zentrum fftr Materialwissenschaften, Philipps-Universitiit, W-3550 Marburg, Germany Received 20 September 1990; accepted for publication 8 November 1990 The experimental investigation of nuclear spin relaxation for single alkali-metal atoms adsorbed on hot metal surfaces (1000 _< T_< 1300 K) enables to study the diffusion of adatoms. The magnetic field dependence of the relaxation rate probes in particular the characteristic spectral density of the diffusion process. The theoretical background is developed and applied to data measured previously for 7Li, adsorbed on oxygen-covered W(ll0). The magnetic field dependence as well as the temperature dependence are well described by a spectral density, which exhibits in the low frequency (high temperature) limit a logarithmic divergence. It is determined by the correlation function for two-dimensional diffusion. The analysis of the data allows for the determination of the activation energy and the pre-exponential factor for diffusion in the limit of very small coverage (0 < 0.001).
Surface Science 245 (1991) 150-172 North-Holland
Growth kinetics simulation of the A1-Ga self-organization on GaAs(100) stepped surfaces Yan-Ten Lu and Horia Metiu Department of Chemistry and Physics, University of California, Santa Barbara, CA 93106, USA Received 1 August 1990; accepted for publication 29 October 1990 We present results of a stochastic kinetic simulation of AI and Ga segregation during the mobility enhanced epitaxial deposition of an AI & Ga monolayer on the As face of a stepped GaAs(100) surface. The simulation deposits the atoms on the surface and moves them from site to site so that the frequency of each event is proportional to its rate. Since the rate constants needed in the simulation have not been measured we use guesses based on the available information and analogies with similar systems. The simulations generate exactly the growth pattern corresponding to a given set of rates. We have found a set of rate constants which generates AI & Ga growth patterns having all the properties observed experimentally. The predicted aggregation patterns are stable with respect to reasonable changes in the numerical values of the kinetic parameters. By varying the corresponding rates we find how the elementary kinetic steps affect segregation. One of the more interesting results is that to obtain good segregation with smooth A I - G a borders it is essential that the AI-AI interactions are anisotropic and are stronger than the G a - G a interactions.
Surface Science 245 (1991) 173-179 North-Holland
173
Photoemission and electron microscopy of small supported palladium clusters Ch. Kuhrt and M. Harsdorff Institut fiJr Angewandte Physik, Universitiit Hamburg, Jungiusstrasse 11, W-2000 Hamburg 36, Germat~v Received 14 August 1990; accepted for publication 26 October 1990 Small palladium clusters, produced by vapour deposition onto amorphous carbon substrates, were analysed in situ by photoelectron spectroscopy (XPS, UPS) and characterized ex situ by transmission electron microscopy. These combined measurements provide the actual cluster size dependence of the two well-known effects in the photoelectron spectra of small supported clusters - the valence band narrowing and the core-electron binding energy shift - suggesting the following explanations: the valence band narrowing is due to an initial state effect, while the core-level shift is dominated by the effect of the changed final state, as it is predicted by Wertheim et al. 180
Surface Science 245 (1991) 180-190 North-Holland
Nuclear spin relaxation as a tool to study surface diffusion * Manfred Riehl-Chudoba 1, U. M e m m e r t 2 and D. Fick Fackbereich Physik and Wissenschaftliches Zentrum fftr Materialwissenschaften, Philipps-Universitiit, W-3550 Marburg, Germany Received 20 September 1990; accepted for publication 8 November 1990 The experimental investigation of nuclear spin relaxation for single alkali-metal atoms adsorbed on hot metal surfaces (1000 _< T_< 1300 K) enables to study the diffusion of adatoms. The magnetic field dependence of the relaxation rate probes in particular the characteristic spectral density of the diffusion process. The theoretical background is developed and applied to data measured previously for 7Li, adsorbed on oxygen-covered W(ll0). The magnetic field dependence as well as the temperature dependence are well described by a spectral density, which exhibits in the low frequency (high temperature) limit a logarithmic divergence. It is determined by the correlation function for two-dimensional diffusion. The analysis of the data allows for the determination of the activation energy and the pre-exponential factor for diffusion in the limit of very small coverage (0 < 0.001).