New insights into complex molecule-surface interactions through local structure determination

固体通过其表面与周围环境相互作用，因此，表面性质决定了固体作为催化剂的反应性，它们对腐蚀性环境的反应，以及它们与其他固体结合制造电子设备的方式。了解这些表面的结构(原子和分子的相对位置)是大多数试图了解化学和电子性质的关键。这个项目试图使用一种特殊的方法，扫描能量模式光电子衍射(PHD)，来研究一系列特定的结构问题，这些问题被选中来洞察不同类别的表面过程。这种方法的独特之处在于，它可以提供键合在表面上的分子中特定原子的局部几何形状，区分不同元素的原子和不同局部成键环境中相同元素的原子。这意味着人们可以解决日益复杂的问题，因此与“真实世界”地表过程的相关性越来越强。例如，与蛋白质和DNA等生物大分子表面的相互作用，在医学疾病筛查和确保人体不排斥医学植入物(如人工髋关节)方面具有潜在的重要意义。这些分子太大，无法用任何可用的方法在原子尺度上探测它们的表面相互作用。然而，这些分子通过关键的分子组分与表面相互作用，虽然即使是这些‘小’分子也对表面结构的确定构成了重大挑战，但使用PHD方法应该能够确定局部的成键结构；因此，这项工作的一个目标是了解表面与这些最简单的组分分子的相互作用。其他要解决的问题主要涉及更简单的分子和原子吸附，但更复杂的表面。特别是，虽然大多数表面科学实验都是在金属和半导体表面上进行的，但具有实际重要性的表面化学性质的一类非常重要的材料是氧化物，但这些材料大多是绝缘体，很难用标准方法进行研究。特别是，关于氧化物表面的结构信息非常匮乏。最近使用PHD技术探测超薄氧化物薄膜表面的工作被证明在模型表面获得吸附结构方面非常成功，其中一些结果突显了目前对这些材料的理论理解的失败。这项工作的一个重要扩展是深入我们对分子-氧化物表面相互作用的理解，这是该研究计划的一个关键组成部分。该项目还旨在探索该技术的一个重要的新扩展，使其能够在“真实的”化学反应条件下确定结构。原子和分子与表面相互作用的方式支撑了多相催化这一极其重要的领域，但这项技术的实际应用，如净化汽车催化转化器中的汽车尾气，是在一个大气压下运行的。然而，大多数探测表面性质的方法只能在非常好的(超高)真空条件下工作；降低表面周围气体(空气)的压力会降低分子从空气中到达并污染表面的速度，为了确保这个过程足够慢，足以保持样品一小时左右的清洁，人们需要在大约十万亿分之一的大气压力下工作(与外层空间的压力相当，但略高于)。因此，找到一种方法在更接近真实反应的压力下获得局部结构信息是一个重要的目标，该提议试图通过一种能够解开这种表面的复杂性的方法来探索一种可能的方法来实现这一点。

Solids interact with their surroundings through their surfaces, so it is the surface properties which dominate the reactivity of solids as catalysts, their response to corrosive environments, and in the way they can bond to other solids to make electronic devices. Knowledge of the structure of these surfaces (the relative positions of the atoms and molecules) is the key to most attempts to understand the chemical and electronic properties. This project seeks to use one particular specialised method, scanned-energy mode photoelectron diffraction (PhD), to investigate a series of specific structural problems selected to give insight into distinct classes of surface processes. The special features of this method are that it can provide the local geometry of specific atoms within a molecule bonded to a surface, distinguishing atoms of different elements and atoms of the same element in different local bonding environments. This means that one can tackle problems of increasing complexity and thus of increasing relevance to 'real world' surface processes. For example, the interaction with surfaces of large biological molecules, such as proteins and DNA, is potentially important in medical screening for disease, and in ensuring the body does not reject medical implants (e.g. artificial hip joints).These molecules are far too large for their surface interaction to be probed at an atomic scale with any available methods. However, these molecules interact with surfaces through key molecular components, and while even these 'small' molecules present a major challenge for surface structure determination, use of the PhD method should allow the local bonding structure to be determined; one objective of this work is thus to understand the interaction of surfaces with the simplest of these component molecules. Other problems to be addressed mostly involve simpler molecular and atomic adsorbates, but more complex surfaces. In particular, while most surface science experiments have been performed on metal and semiconductor surfaces, a very important class of materials with surface chemical properties of practical importance are oxides, but these are mostly insulators and prove difficult to study by standard methods. In particular, there is a dearth of structural information on oxide surfaces. Recent work using the PhD technique to probe the surfaces of ultra-thin oxide films has proved very successful in obtaining adsorption structures on model surfaces, some of the results highlighting failures in current theoretical understanding of these materials. A significant extension of this work to make inroads into our understanding of molecule-oxide surface interactions is envisaged as a key ingredient of this research programme.The project also aims to explore an important novel extension of the technique to allow structure determination under 'real' chemical reaction conditions. The way atoms and molecules interact with surfaces underpins the hugely important area of heterogeneous catalysis, but the practical application of this technology, such as the clean-up of car exhaust gases in the car's catalytic converter, operate around one atmosphere of pressure. Most methods to probe surface properties, however, have been developed to work only in very good ('ultra-high') vacuum conditions; lowering the pressure of the gas (air) surrounding a surface lowers the rate at which molecules arrive from the air and contaminate the surface, and to ensure this process is slow enough to keep a sample clean for an hour or so, one needs to work in a pressure of about one million-billionth of an atmosphere (comparable to, but somewhat higher than, the pressure in outer space). Finding a way to obtain local structural information at pressures closer to those of 'real' reactions is thus an important goal, and this proposal seeks to explore one possible method to achieve this in by a method capable of unravelling the complexity of such a surface.