Structures and Stabilities of Nanoscale Bimetallic Clusters

纳米技术通常指的是原子和分子层面的探索性工程，或者是自下而上而不是自上而下地建造东西。这种自下而上的方法类似于使用乐高积木来构建一个具有我们想象力的模型，这是每个孩子都喜欢的游戏。在纳米世界中，这些块可以小到元素原子本身，如氢、碳或金。建造的模型不仅比它们的大模型更小、更轻，而且还展示了许多迷人的现象。例如，由数百个金原子组成的团簇--一个微小的乐高积木--呈现出微红的颜色，而不是黄色。事实上，如果我们能够制备出不同大小的团簇，就可以得到全谱的颜色。在这个项目中，我们希望改变乐高积木的物理性质，而不是通过改变它们的物理尺寸，而是它们的原子组成和空间排列。我们将集中讨论由两种不同原子物种组成的金属乐高积木--它们被称为双金属纳米团簇。科学家们已经找到了许多巧妙的方法来制备这些具有独特性质的纳米簇。例如，人们可以用银取代块中的一些金原子，得到非常不同的颜色。然而，人们对性质和内部原子排列之间的关系知之甚少。事实上，在大多数情况下，人们不能确定是否已经产生了所需的结构。这是因为人们在研究块状材料时积累的智慧可能不适用于纳米结构。因此，研究的关键部分是表征双金属纳米团簇的内部结构，作为科学理解其物理性质的基础。众所周知，现代电子显微镜能够对纳米团簇进行原子尺度的成像。但这项传统技术无法告诉我们星团中的原子是金还是银。我们建议使用扫描透射式电子显微镜(STEM)，在该显微镜中，电子束聚焦到一个小探针上。当探针在样品上扫描时，从样品上散射的电子被收集起来形成图像。它可以形成原子的图像，因为原子中心的散射最强。它还可以区分金和银，因为较重的金可以比较轻的银散射更多的电子。虽然这种方法已经有一段时间了，但探测器的大小还不够细，不足以研究小星团。电子显微镜中的探头尺寸受电子光学质量的限制。最近在像差校正方面的技术进步使探测器尺寸显著减小到0.1 nm或更小(小于原子半径)。我们最近已经证明，使用如此精细的探针的STEM成像能够揭示超小金/银团簇的内部核-壳结构。我们想利用这一强大的‘化学’成像工具来研究双金属纳米团簇的结构/性质关系。我们希望找出双金属纳米团簇可以采用什么样的内部结构以及能持续多久。根据结合方式的不同，团簇中的两种金属物种可以像油和水一样相分离，也可以像糖和水一样相互溶解。我们想知道，在纳米尺度上，前一种情况下是否可以形成混合结构，或者在后一种情况下，溶解度是否与尺寸有关。我们还希望找出哪些因素，无论是在生长阶段还是在使用阶段，对结构的形成或转变有很大的影响，以及它们是否与那些更大尺寸的材料有任何不同。我们相信，这样的研究对于制造具有特定物理性质的设计者的纳米团簇至关重要。在这方面，我们将集中在光学性质上。最终的目标是能够为特定的应用设计和实现新型的功能纳米结构。

Nanotechnology often refers to exploratory engineering at atomic and molecular level, or to building things from the bottom up instead of from top down. This bottom up approach is akin to using Lego building blocks to construct a model with our imagination, a game every kid loves. In the nano-world, the blocks can be as small as elemental atoms themselves, like hydrogen, carbon, or gold. Not only models constructed are smaller and lighter than their larger counterparts, but also display many fascinating phenomena. For example, clusters - a tiny Lego block - comprising a few hundreds gold atoms display reddish colour instead of yellow. Indeed a full spectrum of colours may be obtained if we can prepare these clusters with different sizes. In this project, we wish to turn physical properties of Lego blocks not by changing their physical size, but their atomic composition and spatial arrangement. We will concentrate on metallic Lego blocks made of two different atomic species - they are called bimetallic nanoclusters. Scientists have found many ingenious ways of preparing these nanoclusters displaying unique properties. For example, one can replace some of gold atoms in the block by silver and get very different colour. However, very little is known about the relationship between properties and the internal atomic arrangement. In fact, in most cases, one cannot be sure if a desired structure has ever been produced. This is because the wisdom one accumulated in studying bulk materials might not apply for nanostructures. Thus the key part of the research is to characterize the internal structure of bimetallic nanoclusters as a basis for scientific understanding of the physical properties. It is well known that a modern electron microscope is capable of atomic scale imaging of nanoclusters. But this conventional technique cannot tell us if atoms within the clusters are gold or silver. We propose to use a scanning transmission electron microscope (STEM) in which the electron beam is focused to a small probe. As the probe is scanned across the sample, electrons scattered off the sample are collected to form an image. It can form an image of an atom because the scattering is strongest at the centre of the atom. It can also distinguish between gold and silver because the heavier gold can scatter more electrons than the lighter silver. Although this method has been available for sometime, the probe size was not fine enough to study small clusters. The probe size in an electron microscope is limited by the quality of electron optic. Recent technical advancements in aberration correction have enabled a significant reduction of the probe size down to 0.1 nm or less (smaller than atomic radius). We have shown recently that STEM imaging using such a fine probe is capable of revealing internal core-shell structures of ultrasmall gold/silver clusters. We want to take advantages of this powerful 'chemical' imaging tool to study the structure/property relationship of bimetallic nanoclusters. We hope to find out what kind of internal structures a bimetallic nanocluster can adopt and for how long. Depending on combination, two metallic species in clusters may be phase separated like oil and water, or dissolved each other like sugar and water. We want to know, at the nanometre scale, if a mixed structure can be formed in the former case, or if solubility is size dependent in the latter case. We also hope to find out what factors, either in the growth stage or in the service stage, have strong influence on the structure formed or transformed and if they are any different from those materials with larger dimensions. We believe such a study is crucial in fabricating designer's nanoclusters with specific physical properties. In this respect, we will concentrate on optical properties. The ultimate goal is to enable novel functional nanostructures to be designed and implemented for specific applications.