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。纳米团簇的一个有吸引力的特征是通过尺寸选择来调节物理性质的可能性。例如，现在已经确定，金在纳米级范围内成为良好的催化剂，尽管它在本体形式下是化学惰性的。要探索这些从根本上有趣的和技术上重要的尺寸依赖性的现象，这是至关重要的，一个应该获得充分的知识，这些集群的原子结构，透射电子显微镜一直是一个受欢迎的探针获得原子分辨的三维结构信息支持的纳米团簇。然而，我们面临的一个共同的问题是集群的快速运动的支持，可能是由于其固有的结构不稳定性，以及他们的弱相互作用与衬底和强相互作用与入射电子束。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扫描透射电子显微镜（STEM）中的高角度环形暗场（HAADF）成像，再加上成像模拟，可以用来获得具有原子分辨率的大小选择的Au 309纳米团簇的大小，形状和取向的快照。这项工作的成功建立在近年来在伯明翰的纳米物理研究实验室开发的尺寸选择集群技术以及自2005年以来通过EPSRC通过第一笔赠款计划向PI提供支持而在集群生产，表征，结构建模和电子显微镜方面形成的强有力的合作基础上。这一进展为我们研究与纳米团簇相关的新奇和复杂性提供了丰富的机会。在目前的建议中，我们利用上述进展，并通过最近成功竞标区域科学城基金及时提供内部畸变校正STEM设施。我们建议着手对表面上的尺寸选择纳米团簇进行系统的研究，目的是在原子尺度上发现其3D原子结构的尺寸特定变化，并了解其潜在的物理机制。这种研究的最终目标是能够利用改进的理解，以便为特定应用量身定制设计集群。通过严格的尺寸选择对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成像物理学的理解的手段，HAADF-STEM成像是一种重要的纳米级分析技术，在材料科学中具有广泛的应用。拟议的计划允许物理学家和化学家组成的紧密团体通过采用最先进的实验测量和理论模拟相结合，专注于这一跨学科的科学问题。据设想，这些大小选择的纳米团簇获得的知识将是非常有趣的和相关的科学家从基础科学到纳米技术的广泛学科。

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.