Environment-dependent coarsening of supported metallic nanoclusters

“大自然讨厌真空”是一句古老的格言，但它的现代和同样有效的对应句可能是“大自然讨厌纳米颗粒”。热力学中体现的自然力表明，裸露的纳米颗粒本质上是不稳定的；它们将不可避免地合并（变粗），在正常情况下变得更大（更少）。最终，它们将失去小的固有属性。这可能是有害的，因为纳米粒子的特性在许多技术中被利用。Drs。蒂尔和埃文斯正在研究粗化过程中一个主要的、很大程度上尚未被探索的方面，即在纳米颗粒之间来回穿梭物质、使它们能够改变大小的物种的本质。这是一项具有挑战性的任务，但泰尔博士通过研究支撑在平面上的纳米颗粒模型系统，获得了深刻而令人惊讶的新见解。她的搭档詹姆斯·埃文斯（James Evans）对这些系统进行了计算建模，并帮助解释蒂尔博士的实验成果。专注于铜、银和金的纳米颗粒，它们具有宝贵的光学和催化性能，Thiel博士可以直接“看到”与粗化有关的化学计量表面复合物。这些复合物可以被认为是可移动的分子，其中包含来自纳米粒子的一些原子和来自环境的一些原子。例如，她发现了一种含有两个铜原子和三个硫原子的心形“分子”，它可以使铜原子在表面上穿梭。本研究涉及的仪器和实验/理论方法是复杂的。正因为如此，再加上她的支持性指导，蒂尔博士的学生作为优秀的实验家和科学家享有广泛的声誉。例如，它们在微电子工业和其他高科技环境中很有价值，它们构成了国家人类技术基础设施的一部分。Evan博士的学生也因其出色的计算建模技能而受到高度要求。在这个研究项目中，蒂尔博士和爱荷华州立大学的埃文斯博士得到了大分子、超分子和纳米化学（MSN）项目的支持，研究化学添加剂存在下表面金属纳米团簇粗化的机制和动力学。纳米科技的成功取决于这些合成的功能纳米结构的鲁棒性，特别是在操作环境中的鲁棒性。他们的分析利用了在受控超高真空环境中的实验研究和预测系统特定理论/建模研究的紧密结合。原位扫描隧道显微镜（STM）研究提供了粗化机制和动力学的关键信息（例如，小簇消失的Ostwald成熟与簇扩散和聚结的Smoluchowski成熟）。质量传递是由添加剂-金属配合物介导的，其中金属添加剂与表面的相互作用有可能引起结构调整，这些研究中特别感兴趣的是这种配合物的形成。理论研究利用密度泛函理论来评估控制上述行为的金属-金属和金属-加性能量学。这些能量为描述STM研究中观察到的表面结构和动力学的统计力学模型提供了信息。该项目为初级科学家提供了一系列复杂的实验和通用建模技术的培训，这些技术使化学材料和纳米技术对美国具有重要意义。

Environment-dependent coarsening of supported metallic nanoclusters "Nature abhors a vacuum" is an old adage, but its modern-day and equally-valid counterpart could be "Nature abhors a nanoparticle". The forces of nature embodied in thermodynamics dictate that bare nanoparticles are intrinsically unstable; they will inevitably merge (coarsen), becoming larger (and fewer) under normal conditions. Eventually they will lose the properties inherent to smallness. And that can be detrimental, since the properties of nanoparticles are exploited in numerous technologies. Drs. Thiel and Evans are investigating a major, and largely unexplored, aspect of the coarsening process, namely, the nature of the species that shuttle material back and forth between nanoparticles, allowing them to change size. This is a challenging task, but Dr. Thiel is gaining deep and surprising new insights by studying model systems of nanoparticles supported on flat surfaces. Her co-PI, James Evans, models these systems computationally, and helps interpret Dr. Thiel's experimental efforts. Focusing on nanoparticles of Cu, Ag, and Au, which have valuable optical and catalytic properties, Dr. Thiel can directly "see" stoichiometric surface complexes that are involved in coarsening. These complexes can be thought of as mobile molecules containing a few atoms from the nanoparticles and a few atoms from the environment. She has discovered, for example, a heart-shaped "molecule" containing two copper atoms and three sulfur atoms, which can shuttle copper atoms across a surface. The instrumentation and experimental/theoretical approach involved in this research are complex. Because of this, as well as her supportive mentoring, Dr. Thiel's students have a wide reputation as excellent experimentalists and scientists. They are valued, for instance, in the microelectronics industry and other high-tech environments, where they constitute a part of the human technological infrastructure of the nation. Dr. Evan's students are also in high demand for their excellent computational modeling skills.In this research program, Dr. Thiel and Dr. Evans of Iowa State University are supported by the Macromolecular, Supramolecular and Nanochemistry (MSN) Program to study the mechanisms and kinetics of coarsening of metallic nanoclusters on surfaces in the presence of chemical additives. The success of nanoscale science and technology is dependent on robustness of such synthesized functional nanostructures, especially in operating environments. Their analysis exploits a closely integrated combination of experimental studies in a controlled ultra-high vacuum environment and predictive system-specific theory/modeling studies. In-situ Scanning Tunneling Microscopy (STM) studies provide key information on the coarsening mechanism (e.g., Ostwald ripening where small clusters disappear, versus Smoluchowski ripening where cluster diffuse and coalesce) and kinetics. mass transport is mediated by additive-metal complexes, where the interaction of metal additives with the surface has the potential to induce restructuring, and the formation of such complexes is of particular interest in these studies. Theoretical studies utilize Density Functional Theory to assess the metal-metal and metal-additive energetics which control the above described behavior. These energies inform statistical mechanical models describing the surface structures and dynamics observed in the STM studies. This project provides training for junior scientists in a range of sophisticated experimental and versatile modeling techniques which enable chemical materials and nanotechnologies of importance to the US.