Thiel Research Goup

Formation of adlayers and thin films during deposition involves a competition between the effect of deposition driving the system out-of-equilibrium, and equilibrating surface diffusion processes.

A periodic structure in higher-dimensional space (the two-dimensional grid of blue dots) is used to generate a nonperiodic structure in lower-dimensional space (the segmented green line). Each dot defines the location of a bar, and the line is divided into segments by the points at which the bars cut the line. The segments are labeled L and S. If the line has an appropriate slope, ƒ¿, then the sequence of segments is ordered but nonperiodic. In this figure, ƒ¿ is chosen such that the sequence is part of a Fibonacci chain, often observed in real quasicrystals.

Research

STM and LEED Images

Metal Thin Films

We are exploring many aspects of thin metal film growth on metallic substrates. These aspects include reconstruction in the film, place-exchange between atoms in the film and substrate, diffusion and rearrangement of metallic clusters, nucleation and growth of islands during deposition, roughness of the film during growth, and Ostwald ripening of islands and vacancies. In this project, we enjoy a collaboration with a group whose expertise is in the area of non-equilibrium statistical mechanics.

The following article explains epitaxial thin films in depth: J. W. Evans, P. A. Thiel and M. C. Bartelt, Surf. Sci. Rep., 2006, 61, 1-128. doi:10.1016/j.surfrep.2005.08.004

Quasicrystals

Quasicrystals are well-ordered structures which fall outside the realm of conventional crystallography. Essentially, the materials were regarded as useless curiosities following their discovery by Schechtman in 1984--although the fact of their existence was highly controversial. While practical uses for the bulk materials remain elusive today, recent work has shown that coatings of quasicrystals can have spectacular properties. The properties which are especially exciting include low coefficients of friction, good wear-resistance, and good oxidation-resistance. Furthermore, quasicrystalline films can be prepared in an environmentally-benign manner, which makes them an attractive alternative to conventional, chemically-based platings for machine parts. Applications of these materials are not purely in the realm of the potential; they exist already in the realm of the marketplace. Today, non-stick cookware can be purchased which contains a quasicrystal coating, rather than a Teflon coating, and which is impervious to scratching by metal utensils.

The properties of interest--friction, adhesion, corrosion- and wear-resistance--are all determined by phenomena at surface or interfacial regions. However, very little is known about these underlying phenomena. Elucidating them is a focus of our work. Thus, our work will provide the scientific underpinnings for a set of important physical properties which are just beginning to be exploited, but which are not being understood.

More specifically, our research revolves around three main questions. First, what is the atomic-scale structure and composition of the clean surface? Second, what is the surface and interface chemistry involved in corrosion resistance, friction, fracture, and adhesion? And third, how are the surface and interface properties of quasicrystalline materials distinct from the properties of metallic alloys with similar composition? In other words, which of the interesting properties discovered so far are due to quasicrystallinity, and which are due simply to chemical composition? We are currently well underway to obtaining answers to all of these questions.

This research project is built largely upon collaborations with several experts in quasicrystals here in Ames, in fields ranging from condensed matter physics to metallurgy. Experts in these odd materials are rare enough, but a cluster of them such as we have here simply does not exist elsewhere in the United States. We therefore enjoy a very unique and exciting position from which to undertake our work.

The following article is a great introduction to quasicrystals: P. A. Thiel, Annu. Rev. Phys. Chem., 2008, 59, 129-152. doi:10.1146/annurev.physchem.59.032607.093736