Nanoporous Silicon: Probing Dimensionally Constrained Deformation in Non-Metals

The objective of the proposed research is to understand the combined effects of small deformation volume and large amounts of surface area on the mechanical behavior and stability of nanoporous silicon (np-Si), as a model for nanostructured non-metal systems. This joint computational-experimental effort leverages an ongoing collaboration between the University of Kentucky and the Karlsruhe Institute of Technology and will strengthen and augment previous success in experimental testing, characterization, and materials synthesis. Furthermore, the cooperative aspects of this project and the complementary makeup of the three research groups involved will allow for synergistic effects, increasing the scientific revenue and providing a more challenging and diverse learning environment for the students involved in this project. The motivation for this study arises from increasing interest in understanding and controlling the mechanical properties of nanoscale structures. In addition, the technological importance and wide application of Si in various electrical, optical and mechanical systems favors the choice of np-Si as a model non-metal system. Further, nanoporous materials represent a fertile test bed for comprehensive experimental and computational study intended to explore the role of various mechanisms for deformation in nanostructures. The proposed research combines material synthesis, complementary mechanical test techniques, in situ observation of sample behavior during deformation, and computational modeling of deformation based on experimental characterization of real systems. This integrated approach is required to build a functioning understanding of the deformation mechanisms and is needed if nanostructured materials are to transition to even broader and more impactful technological application. The intellectual merit of this proposal lies in its aim to uncover the fundamental mechanisms governing the mechanical behavior and stability of nanostructured non-metals, with np-Si as a model material. Recently, a number of research groups have demonstrated that nanoscale Si systems exhibit ductile deformation below a critical size. Motivated by these results, we propose to study the mechanical behavior of nanoporous Si networks, where the Si ligament structure is equivalent to an interconnected network of fine Si nanowires. Previous computational and experimental studies have identified a range of mechanisms potentially underlying the plasticity of nanoscale Si, but no consensus has emerged. The relative importance of Si amorphization and/or recrystallization, stress-induced surface diffusion, plastic flow, dislocation nucleation and motion, and pressure-induced phase transformations are not yet clear. In order to best interpret and model the deformation behavior of nanoscale Si systems—and therefore enable transformative new technologies based on such materials—we must understand the actual mechanisms that control both deformation behavior and transitions in mechanical properties. Differentiating among possible mechanisms and understanding the mechanical behavior that enables practical control of nanoscale systems in engineered devices requires the joint application of experimental characterization and computational methods. Nanoporous materials offer the opportunity to explore the deformation behavior of highly confined volumes and to understand the mechanisms that govern mechanical behavior at the nanoscale, both computationally and experimentally. Examples of broader impacts include the use of nanoporous Si in small-scale structures, for example photovoltaics, flexible electronics, or batteries. Additional broader impacts of this proposal include both graduate and undergraduate education and research, connecting experimental and computational technique, and enhancing an ongoing exchange of students between Kentucky and Germany. This experience will provide UK doctoral students with international exposure and help them learn how to live and work in a global society. Undergraduate involvement in this research is a continuing focus of the PI, who will recruit UK undergraduate students to work with graduate students and with visiting German students in the laboratory. Ideally, this will spark an interest in undergraduates to participate in the German Engineering Exchange program that was established between Kentucky and Karlsruhe in January 2007. The results of this project will be presented at conferences and disseminated in the scientific literature, with joint authorship by the collaborative team of student researchers from the three collaborating research groups.