Showing papers by "Jens K. Nørskov published in 1995"
TL;DR: In this paper, a simple surface reaction, the dissociation of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table, was studied.
Abstract: THE unique role that gold plays in society is to a large extent related to the fact that it is the most noble of all metals: it is the least reactive metal towards atoms or molecules at the interface with a gas or a liquid. The inertness of gold does not reflect a general inability to form chemical bonds, however—gold forms very stable alloys with many other metals. To understand the nobleness of gold, we have studied a simple surface reaction, the dissociation of H2 on the surface of gold and of three other metals (copper, nickel and platinum) that lie close to it in the periodic table. We present self-consistent density-functional calculations of the activation barriers and chemisorption energies which clearly illustrate that nobleness is related to two factors: the degree of filling of the antibonding states on adsorption, and the degree of orbital overlap with the adsorbate. These two factors, which determine both the strength of the adsorbate-metal interaction and the energy barrier for dissociation, operate together to the maxima] detriment of adsorbate binding and subsequent reactivity on gold.
2,721 citations
TL;DR: In this article, the authors present a consistent picture of some key physical properties determining the reactivity of metal and alloy surfaces, and suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present).
2,008 citations
TL;DR: A kinetic model of methanol synthesis is presented in this paper, where all parameters in the model are estimated from gas-phase thermodynamics and surface science studies, and the rate-limiting step in the kinetic model was determined from Cu(100) single-crystal experiments.
249 citations
TL;DR: One of the main conclusions of the theoretical analysis is that, due to the plastic deformation of the nanowires formed by the STM, the typical length scale of the variations in the shape of the boundary is not an atomic radius but rather five times that value.
Abstract: We present experimental and theoretical results for the conductance and mechanical properties of atom-sized wires between two metals. The experimental part is based on measurements with a scannng tunneling microscope (STM) where a point contact is created by indenting the tip into a gold surface. When the tip is retracted, a 10--20 \AA{} long nanowire is formed. Our measurements of the conductance of nanowires show clear signs of a quantization in units of 2${\mathit{e}}^{2}$/h. The scatter around the integer values increases considerably with the number of quanta, and typically it is not possible to observe more than up to four quanta in these experiments. A detailed discussion is given of the statistical methods used in the analysis of the experimental data. The theoretical part of the paper addresses some questions posed by the experiment: Why can conductance quantization be observed, what is the origin of the scatter in the experimental data, and what is the origin of the scaling of the scattering with the number of conductance quanta? The theoretical discussion is based on a free-electron-like model where scattering from the boundary of the nanowire is included. The configurations of the nanowires are deduced from molecular dynamics simulations, which also give information about the mechanical properties of the system. We show that such a model can account semiquantitatively for several of the observed effects. One of the main conclusions of the theoretical analysis is that, due to the plastic deformation of the nanowires formed by the STM, the typical length scale of the variations in the shape of the boundary is not an atomic radius but rather five times that value. This is the reason why scattering is sufficiently small to make conductance quantization observable by STM.
233 citations
TL;DR: It is shown that in general isotropic two-dimensional strain as well as its relief via dislocations have a drastic effect on surface diffusion and nucleation in heteroepitaxy and are thus of significance for the film morphology in the kinetic growth regime.
Abstract: The influence of strain on diffusion and nucleation has been studied by means of scanning tunneling microscopy and effective-medium theory for Ag self-diffusion on strained and unstrained (111) surfaces. Experimentally, the diffusion barrier is observed to be substantially lower on a pseudomorphic Ag monolayer on Pt(111), 60 meV, compared to that on Ag(111), 97 meV. The calculations show that this strong effect is due to the 4.2% compressive strain of the Ag monolayer on Pt. It is shown that in general isotropic two-dimensional strain as well as its relief via dislocations have a drastic effect on surface diffusion and nucleation in heteroepitaxy and are thus of significance for the film morphology in the kinetic growth regime.
232 citations
143 citations
TL;DR: The growth of one monolayer of Au on Ni(111) is shown to lead to an ordered array of misfit dislocation loops in the underlying Ni( 111) surface, and atomistic simulations are used to relate the observed surface structure to that of the buried interface.
Abstract: The growth of one monolayer of Au on Ni(111) is shown to lead to an ordered array of misfit dislocation loops in the underlying Ni(111) surface. The signature of these loops is observed by scanning tunneling microscopy, and atomistic simulations are used to relate the observed surface structure to that of the buried interface. The new interface structure is different from normal misfit dislocation structures in three respects: (i) it forms already during growth of a single Au monolayer, (ii) it forms in the substrate and not in the overlayer, and (iii) it is controlled by the interface energy rather than by the strain in the two phases.
134 citations
TL;DR: In this article, a set of density-functional calculations for clean and O-covered Al(111) are presented and the potential energy surface (PES) of chemisorbed O is investigated.
Abstract: A set of density-functional calculations for clean and O-covered Al(111) are presented. At low O coverages the potential energy surface (PES) of chemisorbed O is investigated. The PES indicates large barriers (0.8 eV) against O diffusion and a large corrugation of the equilibrium O height over the Al(111) while only a moderate energy gain (5 eV per atom) is found upon ${\mathrm{O}}_{2}$ dissociation over the surface. The possible existence of ``hot'' O adatoms after ${\mathrm{O}}_{2}$ dissociation is discussed on the basis of the presented PES and existing dynamical simulations on model potentials. At high O coverages an attractive O-O interaction is identified together with an enhancement in the dipole moment induced per O atom. Finally, Tersoff-Hamann-type scanning tunneling microscopy (STM) topographs are derived based on the calculated one-electron wave functions and spectra. For the clean Al(111) a theoretical STM height corrugation compatible with the experimentally observed one is obtained if the tunneling conductance is assumed dominated by contributions from orbitals of atomic p character centered on the tip. For the O-covered Al(111) the theoretical topographs agree well with the observed ones.
118 citations
TL;DR: In this paper, the size dependence of phase separation in small bimetallic solid clusters is investigated using Monte Carlo simulations, and the interatomic interactions are described approximately using the effective medium theory.
Abstract: Aspects of the size dependence of phase separation in small bimetallic solid clusters are investigated using Monte Carlo simulations. The interatomic interactions are described approximately using the effective-medium theory. For the Ag-Cu system, which in the bulk shows phase separation for a broad range of mixing ratios up to the melting temperature, we find a strong size dependence of the maximum temperature where phase separation is possible. We also find that for clusters smaller than a critical cluster size N0 of about 270 atoms there is no phase separation at all. The results are discussed in terms of a simple continuum model, which is found to be adequate for describing the main characteristics of the problem. Using the simple model we propose a description of N0 as a function of the interface and mixing energies of an alloy, which can be used to estimate N0 for other bimetallic alloys.
88 citations
71 citations
TL;DR: A direct desorption pathway from the mono-hydride phase is found which is compatible with experimental activation energies and the importance of substrate relaxation for this process is demonstrated.
Abstract: The energetics of H$_2$ interacting with the Si(100) surface is studied by means of {\em ab initio} total energy calculations within the framework of density functional theory. We find a direct desorption pathway from the mono-hydride phase which is compatible with experimental activation energies and demonstrate the importance of substrate relaxation for this process. Both the transition state configuration and barrier height depend crucially on the degree of buckling of the Si dimers on the Si(100) surface. The adsorption barrier height on the clean surface is governed by the buckling via its influence on the surface electronic structure. We discuss the consequences of this coupling for adsorption experiments and the relation between adsorption and desorption.
TL;DR: In this article, the authors used artificial atoms with modified size to probe the stress and elasticity of individual layers of low-index fcc surfaces using effective medium theory potentials.
Abstract: Surface stress and surface elasticity of low-index fcc surfaces have been studied using effective-medium theory potentials. In addition to total-energy calculations giving stress components and elastic data for the surface as a whole, the use of artificial atoms with modified size allows us to probe the stress and elasticity of individual layers. This method of artificial atoms provides a direct way to study the contribution of atomic size to segregation in alloys as well as the driving force of reconstructions driven by surface stress. As an example, we give a qualitative explanation of the face-dependent segregation of Pt-Ni alloys. We also compare results of these atomic-scale calculations with continuum elasticity.
TL;DR: In this paper, Nucleation, aggregation, self-assembly, and epitaxial growth were investigated using the Web of Science Record created on 2009-04-14, modified on 2017-05-12.
Abstract: Keywords: Nucleation ; Aggregation ; Self-Assembly ; Epitaxial Growth Reference LNS-ARTICLE-1996-005View record in Web of Science Record created on 2009-04-14, modified on 2017-05-12