Deformation and Fracture of Metallic Nanostructures - In-situ TEM Experiments and Atomistic Models

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This Award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY:Novel experimental techniques are developed to investigate size-scale plasticity in one dimensional metallic nanostructures. Additionally, atomistic models of the experiments are proposed to pursue one-to-one comparison of deformation fields and fracture. The proposed research involves in-situ tensile experiments in the transmission electron microscope (TEM) on one-dimensional fcc crystal nanostructures which are characterized in terms of their atomic structure by high-resolution TEM. Atomic scale modeling of tested specimens will be used to validate existing EAM potentials. A computational framework will be formulated to identify criteria for yielding and failure of metallic fcc nanostructures under uniaxial loading given their initial atomic structure. Limitations in testing specimens of suitable size and atomic structure will be overcome by employing a previously developed nanoscale-material testing system based on micro-electro-mechanical systems sensing (MEMS) and actuation, using protocols for sample fabrication and manipulation. Dislocation nucleation and/or activation will be identified and modeled atomistically for comparison. In order to address temporal and spatial resolution issues, high-speed TEM imaging will be carried out. Likewise, in order to capture material instabilities and failure, the MEMS technology will be extended to achieve load sensing with feedback control, which will ensure testing under displacement control. Boundary conditions on the atomistic models will be imposed in the form of displacement fields obtained from TEM images taken during the experiments. The atomic rearrangement predicted will then be directly compared with the TEM observations at different strain levels. Deformation of the nanowires will then be modeled including the surface and internal defects experimentally observed. A systematic study will be carried out to develop a criterion capable of predicting nanowire strength and failure, given its size and atomic structure. Although very challenging, if successful this research will constitute a major step in the quest for connecting experiments and simulations at the atomic level. NON-TECHNICAL SUMMARY:One dimensional nanostructures are envisioned as key components in the next generation of electronics and sensors as either nanoelectromechanical systems or interconnects. The protocols developed under this project will allow the identification of mechanical and electrical properties that are essential in the design of these systems and will also impact the development of computational tools used in their design. In the long term, the combined experimental-computational approach developed under this project could be employed in the study of electro-mechanical coupling in semiconducting nanowires, field effect transistors, and other nanosystems. The strain dependence of electrical properties, surface reconstruction and interaction between molecules, all important phenomena essential in the design of nanoscale sensors and devices, could then be investigated following the same ideas. The educational and outreach component of this project will focus on providing opportunities to undergraduate and minority students, through existing programs within the NSF-NSEC at Northwestern University, to participate in 9-week summer internships. Likewise, the PI will add a lab on in-situ SEM nanomanipulation and testing of nanowires in the dual level course Experiments in Micro/Nano Science and Engineering he teaches at Northwestern University. The PI will also develop course materials on atomistic/quantum modeling using EAM type potentials and reactive force fields for his graduate level course Special Topics in Nano Engineering.