Wolfgang Schmickler

The investigation of redox reactions with a scanning tunneling microscope: Experimental and theoretical aspects

Abstract The tunneling current between the tip and a sample in a scanning tunneling microscope (STM) can be enhanced by the presence of an electroactive adsorbate on the sample. We have used the transfer Hamiltonian mechanism to determine the dependence of this current on various system parameters like the bias voltage, the energy of reorganization, and electronic coupling of the redox centre to the tip and the substrate. We estimate that a fast redox couple would rise give to currents in the pA to nA range; furthermore, the adsorbate density of states will be reflected in d j /d V . We also discuss experimental considerations in examining electron transfer via a redox adsorbate using an STM.

A model for the adsorption of a commensurate layer of metal ions on a single crystal substrate

Abstract A simple model is presented for the adsorption of a dense commensurate layer of metal atoms adsorbed on a single crystal substrate. Both the adsorbate and the substrate are modeled as jellium with an appropriate lattice of Pseudopotentials. Model calculations give a reasonable order of magnitude for the Gibbs energies of adsorption and for the surface dipole potentials. In agreement with experimental findings absorption is found to be stronger on the more open adsorbate surfaces.

Second harmonic generation at single crystal surfaces of metals in the vacuum and in a solution

Abstract The surface electronic properties of a number of metals are calculated using jellium with pseudopotentials as a model; particular attention is paid to the optical second harmonic response due to currents induced perpendicular to the metal surface. The structure of the surface is shown to have a pronounced effect on the properties investigated. The presence of a solution lowers the absolute magnitude of the second harmonic response.

Changes in surface stress caused by the adsorption of an epitaxial metal monolayer

Abstract The adsorption of a metal monolayer on a foreign substrate generates a change in the surface stress. We calculate this change for a number of substrate/adsorbate systems using the embedded-atom method. The results are compared with those obtained from a continuum model. A cycle, in which the stretching of a substrate/adsorbate system is decomposed into several steps, helps in understanding the numerical results.

Model calculations for copper clusters on Au(111) surfaces

Abstract Using the semi-empirical embedded-atom method, the structure of small copper clusters on Au(111) surfaces has been investigated both by static and dynamic calculations. By varying the size of roughly circular clusters, the edge energy per atom is obtained; it agrees quite well with estimates based on experimental results. Small three-dimensional clusters tend to have the shape of a pyramid, whose sides are oriented in the directions of small surface energy. The presence of a cluster is found to distort the underlying lattice of adsorbed copper atoms.

A model for the coadsorption of ions

Abstract The Anderson—Newns model for adsorption is extended to the case of electrosorption of two kinds of ions with opposite charges. A set of self-consistent equations for the adatom charges is derived. The magnitude of the long-range Coulomb interaction of the adsorbates is estimated. Model calculations are performed for typical system parameters. The charges on the adsorbates and the adsorption energies are shown to be strongly affected by adatom—adatom interactions.

Second harmonic generation and the microscopic structure of the metal-electrolyte interface

Abstract A model for second harmonic generation at the metal-solution interface is presented and treated by electron density functional calculations. The coefficient a , characterizing the non-linear optical response perpendicular to the surface, is calculated both for bare metal electrodes and for metals covered with an overlayer of a different metal. The analysis for bare metals is performed with a lattice of pseudopotentials, thus introducing crystal structure into the model.

Adsorption at the Metal / Solution Interface

While we would like to use similar concepts and models for the double layer in the presence and in the absence of specific adsorption, it is obvious that the adsorption of ions on the metal surface introduces complications such as the chemical interaction of the ions with the metal, their interaction with the sol vent, and adsorbate - adsorbate interactions. Much work is still to be done in this area, but some progress has been made in two relatively simple cases: the adsorption of simple ions at low coverage, and the deposition of a monolayer of metal ions on a foreign metal, which is also known as underpotential deposition (upd). We shall first consider simple ions like the alkali and halide ions, and then upd.

A simple method for calculating the corrugation of metal surfaces

Abstract We present a simple method to introduce corrugation into jellium-type models for metal surfaces. As our reference we take a one-dimensional system that is homogeneous parallel to the surface, and expand the density functional in the three-dimensional correction. By choosing a suitable family of trial functions we calculate the corrugation up to second order in the perturbation. Our results compare well with calculations based on a full three-dimensional treatment. An application to charged surfaces suggests that the electronic corrugation is larger for a positively than for a negatively charged surface. Our method should be particularly useful in the modeling of complex interfaces.

The Interface Between a Metal and a Solution in the Absence of Specific Adsorption

The theory of the double layer at the interface between a metal and an electrolyte solution dates back to the work of Helmholtz [1] in 1881. He pointed out that a difference in the electrostatic potential between the metal and the solution implies a charge separation at the interface, and that the resulting charge configuration resembles that of a parallel plate condenser. Gouy [2] in 1910 and Chapman [3] in 1913 developed the theory of the diffuse double layer, based on a model in which the solution consists of point ions in a dielectric continuum (the solvent), and the metal is a perfect conductor. In 1924, stern [4] introduced a charge-free layer, named the Stern layer or the inner layer, into the model of Gouy and Chapman, and the resulting Gouy-Chapman-Stern theory dominated double layer theory for more than five decades.