Environment-dependent coarsening of supported metallic nanoclusters

Iowa State University, Ames IA

Investigators

Abstract

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.