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Adsorbate lateral interactions and islanding in surface reaction kinetics
A249 Surface Science 214 (1989) 1-16 North-Holland, Amsterdam EFFECTIVE-MEDIUM THEORY DELIMITATIONS ON ITS USE IN GaAs AND
OF CHEMICAL BINDING: AND APPLICATION TO HYDROGEN
Si
A. ZWARTKRUIS
and J.E. VAN
HIMBERGEN
Institute for Theoretical Physics, University of Utrecht, Princetonplein 5, P.O. Box 80.006, 3508 TA Utrecht, The Netherlands Received 29 July 1988; accepted for publication 9 December 1988 A derivation of the effective-medium theory (EMT) of chemical binding is given that clarifies some points in an older derivation and yields additional terms. Both new and previously recognised contributions are analysed in order to delimit the applicability of the theory. Whereas the newly found terms are relatively small for chemisorption to metals, they limit the validity of an EMT description for the adsorption on semiconductor surfaces. A proper treatment of neutral hydrogen in a crystalline semiconductor bulk is not ruled out a priori. We find a semiquantitative description of interstitial diffusion of H in Si and GaAs.
Surface Science 214 (1989) 17-43 North-Holland, Amsterdam ADSORBATE IN SURFACE
LATERAL INTERACTIONS REACTION KINETICS
M. SILVERBERG
17 AND
ISLANDING
* and A. BEN-SHAUL
Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91904, Israel Received 9 September 1988; accepted for publication 13 December 1988 The effects of lateral interactions on the kinetics of bimolecular surface reactions are studied for interactions extending to second and third nearest neighbor range. Monte Carlo simulations are performed for a model system of two adsorbates, A and B, which react to form a rapidly desorbing product. The model features both "topological" and "energetic" effects associated with lateral (A-A, A - B and B-B) interactions; the former reflect the non-random (partially ordered) lateral distribution of the adspecies while the latter correspond to the changes in their diffusion and reaction activation barriers. The general kinetic scheme of the simulations consists of three stages: (i) adsorption of A followed by diffusion and (partial) adlayer ordering; (ii) adsorption of B followed by diffusion of A and B and reorganization of the mixed overlayer; (ii) gradual temperature rise, which triggers and enhances the A + B---, AB reaction, as in temperature programmed reaction (TPR) experiments. The substrate surface is a regular two-dimensional square lattice. Adsorbate interaction potentials are chosen such that p(2 × 2) A-islands are formed at coverages below 0.25 and are gradually replaced by c(2 × 2) structures at coverages between 0.25 and 0.5. Similarly, due to repulsive A - B interactions, coadsorption of B also induces (at sufficient coverages) the compression of A's from the p(2 × 2) to the c(2 × 2) structure. The choice of the model system has been partly motivated by an experimental study of CO oxidation on Pd(100) (see Stuve et al., Surface Sci. 146 (1984) 155). Although the simulations reveal some of the important features observed experimentally, no attempt is made to reproduce or predict any specific experiment. TPR spectra are calculated for various initial conditions. The pattern of reaction is characterized by two distinct regimes, reflected by two peaks in the TPR spectra: a "contact" regime in which A and B react when forced into close contact with each other (giving rise to a low temperature peak), and a "diffusion" regime in which particles must diffuse towards each other to react. Comparison is made with combined Monte Carlo-lattice gas (Bethe-Peierls) model calculations, showing that such models can be useful for low and moderate coverages.
OF CHEMICAL BINDING: AND APPLICATION TO HYDROGEN
Si
A. ZWARTKRUIS
and J.E. VAN
HIMBERGEN
Institute for Theoretical Physics, University of Utrecht, Princetonplein 5, P.O. Box 80.006, 3508 TA Utrecht, The Netherlands Received 29 July 1988; accepted for publication 9 December 1988 A derivation of the effective-medium theory (EMT) of chemical binding is given that clarifies some points in an older derivation and yields additional terms. Both new and previously recognised contributions are analysed in order to delimit the applicability of the theory. Whereas the newly found terms are relatively small for chemisorption to metals, they limit the validity of an EMT description for the adsorption on semiconductor surfaces. A proper treatment of neutral hydrogen in a crystalline semiconductor bulk is not ruled out a priori. We find a semiquantitative description of interstitial diffusion of H in Si and GaAs.
Surface Science 214 (1989) 17-43 North-Holland, Amsterdam ADSORBATE IN SURFACE
LATERAL INTERACTIONS REACTION KINETICS
M. SILVERBERG
17 AND
ISLANDING
* and A. BEN-SHAUL
Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91904, Israel Received 9 September 1988; accepted for publication 13 December 1988 The effects of lateral interactions on the kinetics of bimolecular surface reactions are studied for interactions extending to second and third nearest neighbor range. Monte Carlo simulations are performed for a model system of two adsorbates, A and B, which react to form a rapidly desorbing product. The model features both "topological" and "energetic" effects associated with lateral (A-A, A - B and B-B) interactions; the former reflect the non-random (partially ordered) lateral distribution of the adspecies while the latter correspond to the changes in their diffusion and reaction activation barriers. The general kinetic scheme of the simulations consists of three stages: (i) adsorption of A followed by diffusion and (partial) adlayer ordering; (ii) adsorption of B followed by diffusion of A and B and reorganization of the mixed overlayer; (ii) gradual temperature rise, which triggers and enhances the A + B---, AB reaction, as in temperature programmed reaction (TPR) experiments. The substrate surface is a regular two-dimensional square lattice. Adsorbate interaction potentials are chosen such that p(2 × 2) A-islands are formed at coverages below 0.25 and are gradually replaced by c(2 × 2) structures at coverages between 0.25 and 0.5. Similarly, due to repulsive A - B interactions, coadsorption of B also induces (at sufficient coverages) the compression of A's from the p(2 × 2) to the c(2 × 2) structure. The choice of the model system has been partly motivated by an experimental study of CO oxidation on Pd(100) (see Stuve et al., Surface Sci. 146 (1984) 155). Although the simulations reveal some of the important features observed experimentally, no attempt is made to reproduce or predict any specific experiment. TPR spectra are calculated for various initial conditions. The pattern of reaction is characterized by two distinct regimes, reflected by two peaks in the TPR spectra: a "contact" regime in which A and B react when forced into close contact with each other (giving rise to a low temperature peak), and a "diffusion" regime in which particles must diffuse towards each other to react. Comparison is made with combined Monte Carlo-lattice gas (Bethe-Peierls) model calculations, showing that such models can be useful for low and moderate coverages.