Surface Studies of Oxide Quasicrystals

1982年，Dan Shechtman发现了准晶体，导致了晶体学“传统”观点的重大转变。虽然最初晶体被认为是原子的周期性排列，形成有序的固体，但谢赫特曼发现了一种被禁止的五重对称铝锰合金，似乎推翻了这一观点。虽然最初受到科学界的嘲笑，但他的发现最终导致了2011年的诺贝尔化学奖[1]。准晶现在是物理学，化学和材料科学的主要研究领域。一般来说，准晶可以定义为表现出长程有序但没有周期性的材料，通常导致常规晶体中限制的旋转对称性（例如五倍、十倍、十二倍等）。在各种金属间、胶体和超分子体系中已经观察到这种结构的实例。与周期性晶体相比，准晶的独特结构导致不同的电子，机械和热性能。这反过来又导致宏观材料中的有趣特性，包括例如观察到摩擦减小和不粘特性的表面。准晶表面在催化方面也具有广泛的意义，在各种工业应用中具有重要意义[2]。对准晶表面的许多研究都集中在外延吸附层上。这种研究已经通过使用两种主要技术进行;在准晶表面上沉积单一元素adlayers和在周期性衬底表面上沉积准晶薄膜。从这个意义上说，这两种技术都为准晶表面及其与周期性材料的界面提供了有价值的信息[3]。除了这些研究，最近发现的氧化物准晶为准晶结构的研究提供了另一种途径。非周期性氧化物层在周期性单元素衬底的界面处的自发生长形成了二维氧化物准晶的基础。第一个例子是在3重对称的Pt（111）金属衬底上沉积钙钛矿型钛酸钡BaTiO 3，从而形成12重对称的BaTiO 3衍生准晶薄膜。这种膜的形成是通过在分子束外延工艺中从Ti棒中蒸发Ti和从BaTiO 3陶瓷中蒸发BaO来实现的，1以Pt（111）作为衬底。由于匹配的晶格条件，BaTiO 3在氧气中退火工艺之后在衬底上形成周期性BaTiO 3（111）膜。在UHV中进一步加热样品可以形成准晶层[3]。对于SrTiO 3-Pt（111）系统中的SrTiO 3衍生的准晶薄膜也报道了类似的结果[4]。这些发现证明了在异质外延生长时，在其他周期性材料之间的界面处的挫折如何导致二维准晶层的形成。与其他准晶相相比，这些结构的维数较低，可以使它们在准晶形成的进一步研究中发挥关键作用。此外，观察到的二维准晶结构和周期性结构之间的可逆性（由氧化学势控制）提供了与逆模型催化剂的相似性，潜在地允许研究催化反应过程中的基本步骤[5]。本研究的目的是进一步研究氧化物准晶，寻找可以产生二维准晶结构的类似系统。在初步模拟上述一些重要结果后，将进行其他研究，包括分子和元素外延，以及催化性能的研究。各种表面科学技术将被采用。

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