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Electronic structure of a Cu75Pt25 disordered alloy
A268 248
Surface Science 249 (1991) 248 ~264 North-Holland
The effect of phase transitions, surface diffusion, and defects on heterogeneous reactions" multiplicities and fluctuations * D.G. Vlachos, L.D. Schmidt * and R. Aris Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Received 27 September 1990; accepted for publication 17 December 1990 The influence of attractive absorbate-absorbate interactions and surface diffusion on phase transitions in unimolecular surfac¢: reactions is examined by Monte Carlo simulations. Detailed studies in parameter space are performed assuming various forms of desorption and reaction rate expressions. Introduction of rate anomalies can lead either to a kinetic or to an equilibrium phase transition, depending on the reaction kinetics and the rate-determining processes. It is demonstrated that migration affects the reaction rate, nucleation, and phase transitions, and, therefore, detailed study of them at a molecular level is necessary to describe heterogeneous reactions. Hysteresis in reactant coverage and reaction rate are studied and large fluctuations are found to dominate the system dynamics near cusp points. The influence of defects which act as nucleation centers on metastability is also investigated. The Monte Carlo data are compared with the predictions of mean-field theory, and limitations of the latter are discussed.
Surface Science 249 (1991) 265-280 North-Holland
265
Electronic structure, magnetism and growth of ultrathin films of transition metals F. Gautier and D. Stoeffler IPCMS Gemme (UMR 46 CNRS), Universit~ Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France Received 23 November 1990; accepted for publication 2 January 1991 The conditions which determine the growth mode (Frank-van der Merwe, Stranski-Krastanov, Volmer-Weber) on a perfect surface are obtained in terms of the electronic structure of ultrathin films and interfaces. The various contributions (elastic, structural, chemical, magnetic, etc.) to the spreading energy and to the interface energy are obtained at zero Kelvin for ovedayers of a transition metal A on a transition metal B. We use a tight-binding model and the recursion method to get general trends for all transition metal pairs (A, B) but the electronic structure derived from this simple model agrees when available with the ab-initio (LSDA) calculations. The spreading energy results from a delicate balance between its contributions but the wetting of the substrate by a monolayer (Yl < 0) is generally obtained when the thermodynamicat criterion ~'A < "/a is satisfied. Finally the stability of an overlayer with respect to the formation of two-dimensional ordered compounds is investigated.
Surface Science 249 (1991) 281-288 North-Holland
281
Electronic structure of a Cu 75Pt25 disordered alloy E. Arola a C.J. Barnes a, R.S. Rao a,b, A. Bansil c and M. Pessa a Physics Department, Tampere University of Technology, 33101 Tampere, Finland b National Physical Laboratory, Theory Group, New Delhi, 110012, India c Department of Physics, Northeastern University, Boston, MA 02115, USA Received 16 October 1990; accepted for publication 4 January 1991 We present angle-resolved photoemission spectra from the (100) surface of a Cu75Pt25 substitutionally disordered alloy single crystal for He I, Ne I, He II, and Ne II radiation and compare the results with the corresponding spectra from Cu(100). The results are interpreted in terms of fully relativistic K o r r i n g a - K o h n - R o s t o k e r coherent-potential approximation (KKR-CPA) calculations of the electronic densities of states, Bloch spectral densities, and complex energy bands in the alloy. Our theoretical results provide a reasonable overall understanding of the experimental results. In particular, we find that in Cu-rich CuPt alloys, a nearly dispersionless Pt-induced band centered at 1.4 eV binding energy appears above the Cu d-band edge, with an additional weak Pt-induced emission around 5 eV binding energy near the bottom of the Cu d-band complex. The C u d bands remain quite distinct in the alloy, but move to a lower binding energy of 0.6 eV compared to Cu. We compare and contrast the electronic spectra of Cu-rich CuPt and CuPd alloys, and find that Pt induces a substantially larger density of states at the Fermi energy in Cu compared to Pd, suggesting that an electronic factor may possibly be involved in explaining the better catalytic properties of CuPt over CuPd alloys.
Surface Science 249 (1991) 248 ~264 North-Holland
The effect of phase transitions, surface diffusion, and defects on heterogeneous reactions" multiplicities and fluctuations * D.G. Vlachos, L.D. Schmidt * and R. Aris Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Received 27 September 1990; accepted for publication 17 December 1990 The influence of attractive absorbate-absorbate interactions and surface diffusion on phase transitions in unimolecular surfac¢: reactions is examined by Monte Carlo simulations. Detailed studies in parameter space are performed assuming various forms of desorption and reaction rate expressions. Introduction of rate anomalies can lead either to a kinetic or to an equilibrium phase transition, depending on the reaction kinetics and the rate-determining processes. It is demonstrated that migration affects the reaction rate, nucleation, and phase transitions, and, therefore, detailed study of them at a molecular level is necessary to describe heterogeneous reactions. Hysteresis in reactant coverage and reaction rate are studied and large fluctuations are found to dominate the system dynamics near cusp points. The influence of defects which act as nucleation centers on metastability is also investigated. The Monte Carlo data are compared with the predictions of mean-field theory, and limitations of the latter are discussed.
Surface Science 249 (1991) 265-280 North-Holland
265
Electronic structure, magnetism and growth of ultrathin films of transition metals F. Gautier and D. Stoeffler IPCMS Gemme (UMR 46 CNRS), Universit~ Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France Received 23 November 1990; accepted for publication 2 January 1991 The conditions which determine the growth mode (Frank-van der Merwe, Stranski-Krastanov, Volmer-Weber) on a perfect surface are obtained in terms of the electronic structure of ultrathin films and interfaces. The various contributions (elastic, structural, chemical, magnetic, etc.) to the spreading energy and to the interface energy are obtained at zero Kelvin for ovedayers of a transition metal A on a transition metal B. We use a tight-binding model and the recursion method to get general trends for all transition metal pairs (A, B) but the electronic structure derived from this simple model agrees when available with the ab-initio (LSDA) calculations. The spreading energy results from a delicate balance between its contributions but the wetting of the substrate by a monolayer (Yl < 0) is generally obtained when the thermodynamicat criterion ~'A < "/a is satisfied. Finally the stability of an overlayer with respect to the formation of two-dimensional ordered compounds is investigated.
Surface Science 249 (1991) 281-288 North-Holland
281
Electronic structure of a Cu 75Pt25 disordered alloy E. Arola a C.J. Barnes a, R.S. Rao a,b, A. Bansil c and M. Pessa a Physics Department, Tampere University of Technology, 33101 Tampere, Finland b National Physical Laboratory, Theory Group, New Delhi, 110012, India c Department of Physics, Northeastern University, Boston, MA 02115, USA Received 16 October 1990; accepted for publication 4 January 1991 We present angle-resolved photoemission spectra from the (100) surface of a Cu75Pt25 substitutionally disordered alloy single crystal for He I, Ne I, He II, and Ne II radiation and compare the results with the corresponding spectra from Cu(100). The results are interpreted in terms of fully relativistic K o r r i n g a - K o h n - R o s t o k e r coherent-potential approximation (KKR-CPA) calculations of the electronic densities of states, Bloch spectral densities, and complex energy bands in the alloy. Our theoretical results provide a reasonable overall understanding of the experimental results. In particular, we find that in Cu-rich CuPt alloys, a nearly dispersionless Pt-induced band centered at 1.4 eV binding energy appears above the Cu d-band edge, with an additional weak Pt-induced emission around 5 eV binding energy near the bottom of the Cu d-band complex. The C u d bands remain quite distinct in the alloy, but move to a lower binding energy of 0.6 eV compared to Cu. We compare and contrast the electronic spectra of Cu-rich CuPt and CuPd alloys, and find that Pt induces a substantially larger density of states at the Fermi energy in Cu compared to Pd, suggesting that an electronic factor may possibly be involved in explaining the better catalytic properties of CuPt over CuPd alloys.