Surface-state mediated three-adsorbate interactions: electronic nature and nanoscale consequences (original) (raw)
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Surface-state mediated three-adsorbate interaction: electronic nature and nanoscale consequences
2003
The interaction energy of a three-adsorbate cluster on a surface consists of the sum of pair interactions plus a trio contribution. The trio interaction is produced by interference of electrons which propagate around the cluster perimeter, d 123 . We investigate such interactions for a noble-metal (1 1 1) surface, where the isotropic Shockley surfacestate band produces cluster-interaction energies significant for the low-temperature adsorbate dynamics. We provide experimentally testable interaction-energy predictions, specified by the s-wave phase shift d F that characterizes the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate-pair interactions, the trio contribution exhibits a slightly weaker amplitude and a slightly faster asymptotic decay. We discuss how the trio interaction can affect the cluster geometry by introducing an additional angular dependence in the adsorbate interaction.
Applied Surface Science, 2003
The interaction energy of a three-adsorbate cluster on a surface consists of the sum of pair interactions plus a trio contribution. The trio interaction is produced by interference of electrons which propagate around the cluster perimeter, d 123 . We investigate such interactions for a noble-metal (1 1 1) surface, where the isotropic Shockley surfacestate band produces cluster-interaction energies significant for the low-temperature adsorbate dynamics. We provide experimentally testable interaction-energy predictions, specified by the s-wave phase shift d F that characterizes the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate-pair interactions, the trio contribution exhibits a slightly weaker amplitude and a slightly faster asymptotic decay. We discuss how the trio interaction can affect the cluster geometry by introducing an additional angular dependence in the adsorbate interaction.
Surface-state–mediated three-adsorbate interaction
Europhysics Letters (EPL), 2002
The interaction energy of three adsorbates on a surface consists of the sum of the three pair interactions plus a trio contribution produced by interference of electrons which propagate the entire perimeter, d123, of the three-adsorbate cluster. Here we investigate this triple-adsorbate interaction that is mediated by the isotropic Shockley surface-state band found on noble-metal (111) surfaces. Our experimentally testable result depends on the s-wave phase shift, δF = 0, characterizing the standing-wave patterns seen in scanning-tunneling microscopy (STM) images. Compared with the adsorbate-pair interactions, and in contrast to bulk-mediated interactions, the trio contribution has a slightly weaker amplitude and asymptotically decays slightly faster, ∝ d −5/2 123. It also has a distinctive oscillation period dependent on d123. We finally compare the asymptotic description with exact model calculations.
Long-ranged adsorbate-adsorbate interactions mediated by a surface-state band
Journal of Physics: Condensed Matter, 2000
The adsorbate-adsorbate interaction mediated by Shockley surface states on, for example, the (111) faces of noble metals, yields an oscillatory form modulated by a 1/d 2 envelope at asymptotic adsorbate separations d. For this interaction we obtain a non-perturbative analytical estimate, specified by experimentally accessible Shockley-state parameters and the finite Fermilevel phase shift δ F = 0, which characterize the standing-wave patterns observed in scanning tunnelling microscopy (STM) images. We provide explicit interaction results for the phase shift value δ F = −π/2 suggested by the STM measurements of sulphur adsorbates on Cu(111), and we attempt to relate our results to the corresponding observations of correlations in the adsorbate distribution function.
Dynamics of surface-localised electronic excitations studied with the scanning tunnelling microscope
Progress in Surface Science, 2007
The decay rates of electron and hole excitations at metal surfaces as determined by a scanning tunnelling microscope are presented and discussed. Surface-localised electron states as diverse as Shockley-type surface states and quantum well states confined to ultrathin alkali metal adsorption layers are covered. Recent developments in the analysis of the experimental procedures that are used to determine decay rates with the scanning tunnelling microscope, namely the analysis of line shapes and the spatial decay of standing wave patterns, are discussed.
Journal of Molecular Catalysis
A theoretical simulation of STM/STS has been performed for various surface systems, based on the first-principles local density functional (LDA) calculation. Cluster models made of lo-20 atoms are utilized for the tip, and slab models with several atomic layers are adopted for the sample surface. The tunnel current is almost concentrated on a single apex atom, if the other atoms on the top of the tip are not located on the same level. In such a case the STM image is normal, with atomic resolution. However, if the apex of the tip is formed by more than one atom, abnormal images tend to be formed. We verify this feature by numerical results for graphite, Si(loo), and Si (111) /Ag surfaces, and discuss the light emission from STM, based on realistic calculations of the electronic states of the tip/sample systems.
Tunneling spectroscopy as a probe of adsorbate-surface interactions
Journal of Electron Spectroscopy and Related Phenomena, 1983
Tunneling spectroscopy is a sensitive probe of two classes of adsorbatesurface interactions: interactions of the adsorbate with the substrate on which it is adsorbed and adsorbate interactions with the top metal electrode that is evaporated on top of it. The talk by Professor Hipps focuses on the first of these classes. This talk focuses on the second. In general, the interaction of the adsorbed molecules with the top metal electrode produces a down-shift in the vibrational mode position ranging in size from-< 0.1% to 2 10% depending on the dipole derivative of the mode and the type of top metal electrode.
Recovering hidden electronic states using energy-resolved imaging of metal clusters at surfaces
New Journal of Physics, 2007
Low-temperature scanning tunneling spectroscopy allows to probe the electronic properties of clusters at surfaces with unprecedented accuracy. By means of quantum transport theory, using realistic tunneling tips, we obtain conductance curves which considerably deviate from the cluster's density of states. Our study explains the remarkably small number of peaks in the conductance spectra observed in recent experiments. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure. 36.40.Cg The investigation of the properties of atomic clusters is of great fundamental interest since this class of systems allows to bridge the gap between single atoms on one side and bulk material on the other side [1]. This applies to both free clusters in the gas phase and clusters deposited at surfaces. Compared to the gas phase, where characteristic effects like magic numbers and size dependent bond-character changes are well studied, the understanding of supported clusters is much less developed due to the the numerous technical difficulties the problem poses to both experiment and theory. However, there is a growing interest in supported clusters, mainly for two reasons: firstly, by means of a scanning tunneling microscope (STM) one is now able to perform single-cluster studies [2], much more difficult to achieve in the gas phase because of the low beam intensity of mass-selected clusters. Additionally, scanning tunneling spectroscopy (STS) provides valuable information on the electronic properties, also allowing spatial resolution, in contrast to the non-local character of photoelectron spectroscopy . Secondly, supported clusters promise to be efficient catalysts as oxidation experiments with small gold clusters indicate . STM/STS techniques represent unique tools to analyze supported clusters and thus to shed light on the oxidation mechanism.
Atom-resolved surface chemistry studied by scanning tunneling microscopy and spectroscopy
Physical review, 1989
We have used scanning tunneling microscopy and spectroscopy to study the reaction of Si(111)-(7X 7) with NH3. We have found that by use of topographs obtained at different energies, as well as atom-resolved spectra, reacted and unreacted surface sites can be imaged selectively. Thus we have been able to probe the spatial distribution of the surface reaction on an atom-by-atom basis. We find that there are significant differences in reactivity between the various dangling-bond sites on the Si(111)-(7X 7) surface. Specifically, rest-atom sites are more reactive than adatom sites and, moreover, center-adatom sites are more reactive than corner-adatom sites. We ascribe the reduced reactivity at adatom sites to the delocalized nature of their dangling-bond state. We suggest that a bonding interaction between adatoms and the Si atoms directly below them is responsible for this behaviora suggestion supported by electronic-structure calculations. Thus, while reaction at a rest-atom site can be considered a dangling-bond saturation process, reaction at an adatom site involves the formation of a hypervalent (fivefold-coordinated) adatom. We tentatively ascribe the reactivity differences between center and corner adatoms to differences in the strain they induce upon reaction on the dimer bonds. Atom-resolved spectroscopy allows us to probe interactions and charge transfer between surface sites, and for the first time, we can directly observe how chemisorption affects the substrate electronic structure at neighboring unreacted sites.
Physical Review Letters, 2004
A nearly-free-electron (NFE) model to describe STM spectroscopy of (111) metal surfaces with Kondo impurities is presented. Surface states are found to play an important role giving a larger contribution to the conductance in the case of Cu(111) and Au(111) than Ag(111) surfaces. This difference arises from the farther extension of the Ag(111) surface state into the substrate. The different line shapes observed when Co is adsorbed on different substrates can be explained from the position of the surface band onset relative to the Fermi energy. The lateral dependence of the line shape amplitude is found to be bulk-like for R || < ∼ 4Å and surface-like at larger distances, in agreement with experimental data.