Structures and Stabilities of Nanoscale Bimetallic Clusters

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.