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    FOR 2213: Subproject "Quantum mechanically based quantitative description of metal alloy surfaces as function of the environment
    The goal of this project is to predict and understand the structural properties of metal alloy surfaces containing Au as function of their environment. This includes properties as the segregation profile of the surface, the crystallographic structure and its stability. Here, "environment" may stand for temperature, bulk concentration, or the influence of adsorbates. Based on these studies, we will be able to control the influence of the individual parameter, e.g. by analyzing the electronic structure, on the surfaces properties. Starting point will be the alloy surfaces of Ag-Au and Cu-Au, because they belong to the most promising candidates for the making of nanoporous Au. They will be studied via the combination of first-principles methods based on Density Functional Theory (DFT) with methods from statistical physics, namely the so-called Cluster Expansion method (CE) and Monte-Carlo (MC) simulations.While the CE allows for scanning huge parameter spaces, MC simulations will give us access to finite temperatures. In that way, the influence of temperature on atomic ordering phenomena in the near-surface regime will be studied by implementation of the CE Hamiltonian into Monte-Carlo programs. Then, on the basis of the stabilized cluster expansion, surface phase diagrams and ordering parameters will be constructed. These diagrams will provide a look-up-table for experimental studies, because they allow us to detect at which temperature and concentration the highest enrichment/depletion of Au in the near surface layer is reached. This also includes short-range order. In parallel, the structural results will be supported by the quantitative analysis of intensity spectra received from Low Energy Electron Diffraction (LEED) measured in the project SP5. Such structure determinations may reach accuracy smaller than one percent of the atomic diameter. This will allow for a detailed understanding of the alloy surface structures.
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    Mechanical-chemical coupling at surfaces: prediction and experiment
    While the partial atomic volume of foreign atoms in solid solutions or of constituents in intermetallic compounds, and hence the coupling between the stress state and the chemical potential, have been studied already for many decades by materials scientists, up to now the analogous processes at surfaces have not received much attention. The key goal of this project is the investigation of the coupling between the mechanics of surfaces and adsorption. The basic phenomenon is that a capillary force, the surface stress, is coupled to changes of state at the materials surface. Thus, the adsorption of atoms from gas or ions from solution, or the enrichment/depletion of electronic charge at the surface prompts deformation of the material. This phenomenon has recently found application in modern biosensors, whose miniaturized cantilevers deform measurably upon adsorption of atoms or molecules. Furthermore, it has been shown that the controlled reversible adsorption/desorption of atoms or ions can prompt large strain in nanoporous solids that promise application as metallic actuators with large amplitude and strain energy density. Understanding of the microscopic mechanisms behind the coupling of changes of state at materials surfaces and the stress state is still rudimentary at best. A theory that could predict the coupling strength for a given materials surface and adsorbate is still missing. Moreover, the surface stress changes upon reversible adsorption and desorption of light elements, which are particularly relevant for applications in actuation and sensing, remain poorly studied. In this context, the present proposal aims at 1.) providing for the first time quantitative values for the coupling between capillary force and adsorption from both experiment and theory and 2.) insight into the underlying microscopic mechanims. As model processes we plan to study the adsorption of H and O on Au, Pt and Ir surfaces. The theoretical method for calculation of the coupling strength is density functional theory. For efficient and exhaustive screening of the configuration space we additionally employ cluster expansion methods. In experiment, changes of the surface stress upon adsorption from gas and electrosorption are determined from cantilever bending. Complementary measurements of the variation of the electrode potential as a function of strain during different electrosorption processes are carried out with Dynamic Electro-Chemo-Mechanical Analysis, a technique developed by one of the applicants.
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