Nanoscience of Quasicrystals and Related Complex Metallic Alloys

当一个图案有一个基本的重复元素，就像瓷砖浴室里的方形瓷砖，我们说这个图案是周期性的。周期性是一个简单的概念，但它的影响是深远的。周期系统具有长程序；换句话说，如果我们知道图案中第一个贴图的位置，那么图案中所有其他贴图的位置都可以通过按规则的增量移动这个贴图来找到。这只适用于非常有限数量的简单形状，其中包括正方形，三角形和六边形。试着用五边形覆盖一个表面。用这种方法制作的瓷砖总会有缝隙。直到20年前，人们还认为材料要么像上面描述的那样以周期性的方式有序（在这种情况下，它们被称为晶体），要么完全无序（在这种情况下，它们被称为无定形）。人们发现，几乎所有材料的物理性质都取决于材料是结晶的还是非晶态的。1984年，人们发现了一类结构介于晶体和非晶态材料之间的材料。它们被称为准晶体。它们具有长程秩序，因为它们的结构可以由一些简单的规则建立起来。它们的结构实际上是基于至少两块具有非常特定形状的瓷砖。当这些瓷砖被放置在一起时，这些瓷砖的边缘应该匹配一些规则，然后建立一个准周期模式。这被称为彭罗斯瓷砖。准晶体可以被认为是彭罗斯瓷砖，原子群附着在瓷砖的角上，赋予它们原子结构。在过去的20年里，对准晶体性质的研究使科学家们能够理解这种新秩序在真实物理系统中的影响。特别是，我们已经学到了很多关于电子的行为，电子是材料中的导电体，以及在新的背景下的磁性。在过去的十年里，这些研究与科学的巨大发展同时发生，这些发展主要集中在极小尺度的原子排列，即纳米结构，以及这些纳米结构的潜在应用，即纳米技术领域。这个建议是将准晶体和纳米科学这两个领域结合在一起，通过许多不同的方法，包括：(i)通过在准晶体表面上喷洒少量原子并让它们自己组织成覆盖层或薄膜，来研究在准晶体表面生长的纳米级结构；（ii）使用上述步骤(i)中创建的系统作为有序分子吸附的模板；（iii）研究准晶体表面在导电液体环境中发生的变化，在这种环境中，表面的条件和状态可能受到严格控制——这被称为电化学；（iv）与准晶本身密切相关的周期性结晶材料的研究——我们称之为复杂金属合金。用于进行这些研究的主要工具是一种被称为扫描隧道显微镜的仪器。这是一种显微镜，可以让我们以一种相对快速和可控的方式对表面上的原子进行成像。我们希望这项提议的结果能够大大增加对具有准晶体性质的纳米级系统中发生的事情的理解。我们应该更多地了解这些系统的电、热、磁和结构特性，并对这些材料的哪些应用可能是可行的有一些想法。

When a pattern has a basic repeating element, like a square tile in a tiled bathroom, we say the pattern is periodic. The concept of periodicity is a simple one, but its consequences are far reaching. Periodic systems possess long-range order; in other words if we know the position of the first tile in the pattern, then the positions of all the other tiles in the pattern can be found just by shifting this tile along by regular increments. This only works for a very limited number of simple shapes, among them squares, triangle and hexagons. Try covering a surface with pentagons. There will always be gaps in a tiling made in this way. Until twenty years ago, it was thought that materials were either ordered in a periodic way as described above (in which case they were called crystals), or completely disordered (in which case they were said to be amorphous). Nearly all the physical properties of materials were found to depend on whether the material was crystalline or amorphous.In 1984 a class of materials were discovered which are in between crystals and amorphous materials in terms of their structure. They are known as quasicrystals. They have long-range order, in the sense that their structure can be built up from some simple rules. Their structure is in fact based on a minimum of two tiles which have very specific shapes. When the tiles are put together with some rules as to which edges of the tiles should match, then a quasiperiodic pattern is built up. This is known as a Penrose tiling. Quasicrystals can be thought of as Penrose tilings with groups of atoms attached to the corners of the tiles to give them atomic structure.The study of the properties of quasicrystals in the past 20 years has allowed scientists to understand the effects of this new kind of order in a real physical system. In particular we have learned much about the behaviour of electrons, which are the conductors of electricity in materials, and about magnetism in a new context. These studies have occurred in parallel with huge developments in science in the past 10 years which have focussed on extremely small scale atomic arrangements called nanostructures, and the potential applications of these nanostructures, a field called nanotechnology. This proposal is to join these two areas together, that is quasicrystals and nanoscience, through a number of different approaches which include:(i) the study of nanoscale structures which are grown on quasicrystal surfaces by sprinkling tiny amounts of atoms on the surfaces and letting them organize themselves into overlayers or thin films;(ii) The use of systems created in step (i) above as templates for ordered molecular adsorption;(iii) the study of what happens the surfaces of quasicrystals when they are placed in a conducting liquid environment where the conditions and hence the state of the surface may be strictly controlled - this is known as electrochemistry;(iv) the study of periodic crystalline materials which are closely related to quasicrystals themselves - we call these complex metallic alloys.The main tool which will be used to perform these studies is an instrument known as a scanning tunnelling microscope. This is a microscope which allows us make images of atoms on surfaces in a relatively quick and controlled way.We intend the outcome of the proposal to be a substantially increased understanding of what happens in nanoscale systems which have a quasicrystalline nature. We should understand much more about the electrical, thermal, magnetic and structural properties of these systems, and have some ideas about which applications of these materials might be feasible.