XPS and DFT Investigation of the Au-S Interface

Abstract

Self-assembled monolayers (SAMs) are semi-crystalline 2D-structures that can form spontaneously when organic molecules are adsorbed on solid or liquid substrates. The molecules that form SAMs are characterized by an “anchoring” functional group, with specific affinity for the substrate, by a carbon chain that is responsible for the long-range ordering of the monolayer, thanks to van der Waals interactions, and by a tail group, which is exposed to the external environment. The relevance of SAMs for technological and industrial applications resides in their ease of preparation combined with the possibility they offer to tailor the properties of a surface in a controlled manner. After SAM formation, indeed, the surface properties will depend on the SAM structure and composition. For example, the surface wettability, adhesion, and protein-affinity will only depend on the chemistry of the functional group of the adsorbed molecules that is exposed to the environment. Electrical properties will depend on the molecular structure and on the way they are bound to the substrate. In the last decades, one of the most studied SAMs systems has been that formed by alkanethiols on gold, due to its ease of preparation and stability. Despite the great knowledge acquired for this system about its formation mechanism, its crystalline structure, and most of its physical-chemical properties, there is still a lack of knowledge about the geometry of its gold/sulfur interface. Being able to determine the interface geometry in a specific system, or to predict such geometry as a function of the employed molecules, would be useful for various applications. However, experimental studies have not yet been able to identify this geometry, due to the interface being not directly observable, nor have theoretical calculations managed to simulate the SAM formation so as to determine the energetically favored structures, due to the system complexity. One objective of this thesis was to develop a method for studying the gold/sulfur interface geometry, by employing density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) measurements. XPS is able to detect small shifts in the energy of core-level electrons of the elements present in the top few nanometers of a sample, while DFT calculations can predict such shifts. In this thesis, it was predicted by DFT that the electrons of gold atoms at the gold/sulfur interface would have slightly different binding energies than bulk atoms, and that such binding-energy shifts would be different for different geometries. This might allow the geometry identification based on the experimentally measured shifts. Another objective was the optimization of the experimental conditions for accurately measuring the binding-energy shifts. To do so, bare gold films of different thickness and surface roughness were prepared and analyzed by XPS, and different measurement parameters were tested. The results were evaluated based on the relative intensity of surface components in the main gold signal. Measurements were also performed on SAM-functionalized gold, finding a dependence of the relative intensity of interface signals on the gold thickness. A second part of the thesis aimed at the design and optimization of flat bimetallic patterned surfaces, to be tested for mixed-SAM functionalization and for application in the field of instrumental calibration. The latter application was supported with the organization of a VAMAS interlaboratory study, in which participants were asked to measure different kinds of spatial resolution using the samples obtained in this work, especially the effective analyzed area, which is an important property that is often not known about instruments. The method for determining the analyzed area could be successfully applied to various X-ray photoelectron spectrometer designs as well as to time-of-flight secondary ion mass spectrometry and the latest generation of Auger spectrometers, which allow chemical information to be obtained with nanometer spatial resolution. This work demonstrated that with a single fabrication method it is possible to obtain many different useful characteristics on the same sample, so that it can be exploited for a variety of applications for which traditionally different kinds of samples had to be employed. The fabrication method is also very versatile, so as to allow the realization of different designs in terms of surface composition, topography, and shape of the patterns.