Towards an Atomic-scale Understanding of the 3D Structures of Size-selected Clusters on Surfaces

纳米团簇是尺寸在纳米范围内的有限大小的聚集体。纳米团簇的一个吸引人的特征是可以通过尺寸选择来调节物理性质。例如，现在公认的是，金在纳米尺度上是一种很好的催化剂，尽管它在化学上是惰性的。要探索这些有趣的和技术上重要的尺寸依赖现象，关键是要充分了解这些团簇的原子结构。透射式电子显微镜是获得支撑型纳米团簇的原子分辨三维结构信息的首选探针。然而，我们面临的一个共同问题是团簇在载体上的快速运动，这可能是由于它们固有的结构不稳定性以及它们与衬底的弱相互作用和与入射电子束的强相互作用。这个问题限制了可以使用的电子束密度、观察的持续时间以及可以研究的纳米团簇系统和现象。最近，我们证明了在像差校正的扫描电子显微镜(STEM)中的高角度环形暗场(HAADF)成像，结合成像模拟，可以获得原子分辨率的尺寸选择的Au309纳米团簇的大小、形状和取向的快照。这项工作的成功是建立在伯明翰纳米物理研究实验室近年来开发的尺寸选定的集群技术以及自2005年以来通过EPSRC通过对PI的第一个赠款计划支持在集群生产、表征、结构建模和电子显微镜方面形成的强大合作的基础上的。这一进展为我们提供了大量机会来研究与纳米团簇相关的新颖性和复杂性。在本提案中，我们利用了上述进展，并通过最近成功竞标区域科学城基金，及时提供了内部矫正偏差的STEM设施。我们打算开始对表面上尺寸选定的纳米团簇进行系统的研究，目的是在原子尺度上发现它们的三维原子结构中特定尺寸的变化，并了解其潜在的物理机制。这类研究的最终目标是能够利用改进后的理解，以便为具体应用量身定做集群。严格的尺寸选择对团簇结构的限制也为我们提供了一种测试和提高我们对HAADF-STEM成像的物理理解的手段，这是一种重要的纳米级分析技术，在材料科学中有着广泛的应用。拟议的方案允许一个紧密联系的物理学家和化学家小组通过采用最先进的实验测量和理论模拟相结合的方式，专注于这一跨学科的科学问题。预计，从基础科学到纳米技术等广泛学科的科学家将对所获得的关于这些尺寸选定的纳米团簇的知识非常感兴趣和相关。

Nanoclusters are finite sized aggregates with dimensions in the nanometer range. An attractive feature of nanoclusters is the possibility of tuning physical properties by size selection. For example, it is now well established that gold becomes a good catalyst in the nanoscale regime, though it is chemically inert in the bulk form. To explore these fundamentally interesting and technologically important size-dependent phenomena, it is crucial that one should gain full knowledge of the atomic structure of these clusters.Transmission electron microscopy has been a favoured probe for obtaining atomically resolved three-dimensional structural information on supported nanoclusters. However, a common problem we face is the rapid motion of clusters on the support, due probably to their intrinsic structural instability as well as their weak interaction with the substrate and strong interaction with the incident electron beam. This problem constrains the electron beam density that can be used, the duration of the observation and the nanocluster systems and phenomena that one can investigate. Recently, we demonstrated that high-angle annular dark field (HAADF) imaging in an aberration-corrected scanning transmission electron microscope (STEM), coupled with imaging simulations, can be used to obtain a snap-shot of the size, shape and orientation of size-selected Au309 nanoclusters with atomic-resolution. The success of this work builds upon the size-selected cluster technology developed in the Nanoscale Physics Research Laboratory in Birmingham over recent years and the strong collaboration formed on cluster production, characterization, structural modeling and electron microscopy since 2005 through support from EPSRC via the First Grant scheme to the PI. The advancement provides us with a wealth of opportunities for studying the novelty as well as the complexity associated with nanoclusters. In the present proposal, we take advantage of the above progress and the timely availability of an in-house aberration-corrected STEM facility through the recent successful bid to the regional Science City funds. We propose to embark on a systematic investigation of size-selected nanoclusters on surfaces, with the goal of discovering size-specific changes in their 3D atomic structures on the atomic-scale and of understanding the underlying physical mechanisms. The ultimate goal of such research is to be able to exploit the improved understanding in order to tailor-design clusters for specific applications. The constraints placed on the structures of clusters by the tight size-selection also offer us a means to test and improve our understanding of the physics of HAADF-STEM imaging, which is an important nanoscale analytical technique, with a wide range of applications in material science. The proposed programme allows a close-knit group of physicists and chemists to focus on this interdisciplinary scientific issue by employing a combination of state-of-art experimental measurements and theoretical simulations. It is envisaged that the knowledge gained about these size-selected nanoclusters will be highly interesting and relevant to scientists from a wide range of disciplines ranging from basic science to nanoscale technology.