Surface Studies of Oxide Quasicrystals

The discovery of quasicrystals by Dan Shechtman in 1982 led to a major transition from the "traditional" views of crystallography. Whilst originally crystals were considered to be a periodic arrangement of atoms forming an ordered solid, Shechtman's discovery of a forbidden fivefold symmetric aluminium-manganese alloy seemingly disproved this. Although initially ridiculed by many in the scientific community, his findings eventually led to a Nobel prize in chemistry in 2011 [1].
Quasicrystals are now a major area of research across physics, chemistry and materials science. In general, a quasicrystal may be defined as a material exhibiting long-range order but no periodicity, often resulting in a rotational symmetry restricted in a conventional crystal (such as fivefold, tenfold, twelvefold etc.). Examples of such structures have been observed in various intermetallic, colloidal and supramolecular systems. The unique structure of quasicrystals when compared to periodic crystals results in differing electronic, mechanical and thermal properties. This in turn leads to interesting characteristics in the macroscopic material, including the surface where, for example, reduced friction and non-sticking properties are observed. Quasicrytalline surfaces are also of wider interest in catalysis, providing significance in various industrial applications [2].
Many investigations into the surfaces of quasicrystals have focussed on epitaxial adlayers. Such research has proceeded via the use of two main techniques; the deposition of single elemental adlayers on quasicrystalline surfaces and the deposition of quasicrystalline thin films on periodic substrate surfaces. In this sense, both such techniques have provided valuable information on quasicrystalline surfaces and how they interface with periodic materials [3].
Alongside these studies, the more recent discovery of oxide quasicrystals has provided an alternative route for the study of quasicrystalline structure. The spontaneous growth of an aperiodic oxide layer at the interface to a periodic single-element substrate forms the basis of the two-dimensional oxide quasicrystal. The first example of this involved the deposition of the perovskite Barium Titanate BaTiO3 on the 3-fold symmetric Pt(111) metal substrate, resulting in the formation of a 12-fold symmetric BaTiO3-derived quasicrystalline thin film. The formation of such a film was realised via the evaporation of Ti from a Ti rod and BaO from a BaTiO3 ceramic in a molecular beam epitaxy process, 1 with Pt(111) as the substrate. Due to matching lattice conditions, the BaTiO3 forms a periodic BaTiO3(111) film on the substrate after a process of annealing in oxygen. Further heating of the sample in UHV allows for the quasicrystalline layer to form [3]. Similar results have also been reported for a SrTiO3-derived quasicrystalline thin film in the SrTiO3-Pt(111) system [4].
These findings demonstrate how frustration at the interface between otherwise periodic materials can cause a two-dimensional quasicrystalline layer to form, when grown heteroepitaxially. The lower dimensionality of these structures when compared with other quasicrystaline phases could give them a key role in further studies of quasicrystalline formation. Furthermore, the observed reversibility between a 2D quasicrystalline structure and a periodic structure (controlled by the oxygen chemical potential) provides similarities to an inverse model catalyst potentially allowing for the study of elementary steps in catalytic reaction processes [5].
The aim of this research is to further the investigations into oxide quasicrystals, looking for similar systems which can produce a two-dimensional quasicrystalline structure. After initially emulating some of the important results described above additional studies will be performed, including molecular and elemental epitaxy, as well as the investigation of catalytic properties. Various surface science techniques will be emplo