ISBN 9783843936927

978-3-8439-3692-7, Reihe Physik

Anja Michl
Coupling between electronic, mechanical and vibrational properties of clean and adsorbate-covered metal surfaces - a first-principles study

211 Seiten, Dissertation Technische Universität Hamburg-Harburg (2018), Softcover, A5

Zusammenfassung / Abstract

Coupling between mechanical stress or strain of a surface and its electronic properties is relevant for diverse applications. Changes of state of a metal surface - e.g. charging or adsorption - affect the surface stress. For materials with a high surface-to-volume ratio this entails a macroscopic deformation, which represents the working principle of porous metal actuators. Adsorbate-induced variations can be exploited for designing surface stress based sensing devices. The key quantity for characterizing these phenomena is the electrocapillary coupling parameter. Owing to Maxwell relations, it can be determined using apparently unrelated quantities, which are, however, considerably easier to calculate with high accuracy. For clean metal surfaces, the response of the work function to in-plane strain is evaluated as a measure for the charge-induced surface stress change. For adsorbate-covered surfaces, the correlation between adsorption energies and strain gives access to mechano-chemical coupling parameters and in addition is important for heterogeneous catalysis, where the reactivity of catalysts is sensitive to mechanical deformation. Since the interplay between different adsorbate species and coupling of their properties may be relevant in such systems as well, the impact of coadsorption on the vibrational properties of a probe molecule is considered at last.

In this thesis, a first-principles approach is used to study these coupling phenomena. Mainly, density functional theory (DFT) is employed in order to accurately determine material properties like work functions, adsorption energies or vibrational frequencies with and without applied mechanical strain. Since it is impossible to calculate every possible configuration of the adsorbate atoms on the surface with such a computationally expensive tool, a combined approach of DFT and the cluster-expansion method was chosen to tackle this problem.

Concerning the work function strain response, different facets of several sp-bonded metals are investigated. These elements are typically characterized by a rather simple electronic structure, thus making them ideal candidates for exploring which aspects of the response behavior are captured within simplified models. The coupling between mechanics and adsorption was investigated using hydrogen on Ir(111) as a model system. Finally, the interplay of different adsorbate species is addressed for coadsorption of CO and organic molecules on Ir(111).