Surface Studies of Oxide Quasicrystals

1982年，丹·谢克特曼发现准晶，使结晶学的“传统”观点发生了重大转变。虽然晶体最初被认为是形成有序固体的原子的周期性排列，但谢克特曼发现了一种被禁止的五重对称铝锰合金，这似乎证明了这一点。尽管最初受到科学界许多人的嘲笑，但他的发现最终导致了2011年的诺贝尔化学奖[1]。准晶现在是跨越物理、化学和材料科学的一个主要研究领域。一般而言，准晶可以被定义为一种具有长程有序性但没有周期性的材料，通常会导致常规晶体中受限的旋转对称性(如五倍、十倍、十二倍等)。这种结构的例子已经在各种金属间、胶体和超分子系统中被观察到。与周期晶体相比，准晶的独特结构导致了不同的电子、机械和热学性质。这反过来又导致宏观材料中的有趣特征，包括表面，例如，在那里可以观察到减少的摩擦和不粘着的特性。准晶表面在催化方面也具有广泛的兴趣，在各种工业应用中具有重要意义[2]。许多对准晶表面的研究都集中在外延层上。这种研究是通过两种主要技术进行的：在准晶表面沉积单元素附着层和在周期性衬底表面沉积准晶薄膜。从这个意义上讲，这两种技术都提供了关于准晶表面及其与周期性材料的界面的有价值的信息[3]。在这些研究中，氧化物准晶的最新发现为准晶结构的研究提供了另一条途径。在周期性单元素衬底的界面上自发生长的非周期氧化层形成了二维氧化物准晶的基础。第一个例子是在3重对称的铂(111)金属衬底上沉积了钙钛矿型钛酸钡BaTiO_3，形成了12重对称的BaTiO_3准晶薄膜。这种薄膜的形成是通过在分子束外延过程中蒸发钛棒中的钛和钛酸钡陶瓷中的BaO来实现的，1以铂(111)为衬底。由于匹配的晶格条件，BaTiO_3在氧气中热处理后在衬底上形成了周期性的BaTiO_3(111)薄膜。在超高电压下进一步加热样品可形成准晶层[3]。在钛酸锶-铂(111)系统中的准晶薄膜也有类似的结果[4]。这些结果表明，当异质外延生长时，周期性材料之间的界面受挫可以导致形成二维准晶层。与其他准晶相相比，这些结构的维度较低，这可能使它们在进一步研究准晶形成过程中发挥关键作用。此外，观察到的2D准晶结构和周期结构(由氧化学势控制)之间的可逆性提供了与反向模型催化剂的相似之处，从而有可能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，寻找能够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之后，将进行进一步的研究，包括分子和元素外延，以及催化性能的研究。将推广各种表面科学技术

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