Experimental and Computational Study of Pore Morphology Evolution Mechanisms in Nanoporous Metal Thin Films Under Thermal/Electrical/Mechanical Stress Fields

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Non-technical Summary:Nanostructured metals have had a tremendous impact on a variety of applications including battery electrodes, biomedical implant coatings, and biosensors. Materials in these conditions are generally subject to various external factors (e.g., temperature, electric, and mechanical stress fields) that can gradually change the material's properties and performance. The purpose of this project is to develop a fundamental understanding of these processes using nanoporous gold (np-Au) as a model system. The np-Au belongs to the emerging class of nanoporous metals that have attracted significant interest for its catalytic, optical, mechanical, and biomedical features. The expected outcome is the generation of scientific knowledge that allows for precise control of shape and structure changes in nanoporous metals; this will improve predictions of how material properties evolve and in turn enhance the performance of nanostructured metals in applications. The instrumentation and simulation techniques developed as part of the project will be widely applicable to other material systems as well, including other nanoporous metals and metallic nanowires. The broader impacts of this project include societal benefits and educational opportunities, such as undergraduate research opportunities with the principal investigators and interactive educational tools that teach key concepts of micro-/nano-fabrication and atomistic simulations via interactive computer games.Technical Summary:The morphology of nanoporous gold (np-Au), typically produced by selectively dissolving a less noble component of an alloy to create a bicontinuous porous structure, is conventionally modulated via thermal annealing where the enhanced surface diffusion of gold atoms leads to ligament coarsening. However, both the extent of coarsening and the resulting morphologies can also be significantly affected by the application of electrical currents and mechanical stresses at low temperatures, though these effects have not yet been studied. The project hypothesizes that morphological change in np-Au can be more finely modulated, and novel structural features be developed, by applying heating, electrical current, and mechanical stresses in concert. Initially subjecting np-Au to varying temperature, electric, and mechanical stress fields will allow the effects of the individual fields to be characterized. Subsequent experiments and simulations will investigate synergistic or emergent effects from applying the fields simultaneously. Key scientific outcomes of the project will include characterization of the decoupled effects of heating, electrical current, and mechanical stresses on pore morphology evolution, identification of the underlying mechanisms and the corresponding kinetics, and a model supported by experiments and simulations to predict the morphological evolution from the combined influences of these fields. Precise control of morphology evolution in nanoporous metals will expand the set of practically accessible nanostructures and corresponding material properties that can be implemented as enhanced biomedical device coatings, catalytic fuel cells, or as plasmonic materials for sensors. In tandem, the project will employ classroom assignments to develop a library of interactive simulations of experimental procedures that emphasize micro-/nano-fabrication with a focus on nanostructured metals and visualizations of the relevant atomic processes. In addition, through a scheme where undergraduate students write a short research proposal to compete for a summer internship, the students will be employed on customized projects under mentorship of the principal investigators.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.