Mechanical and Electromechanical Properties of Semiconducting and Metallic Nanowires.

摘要

Nanowires are envisioned as the building blocks of future electronics, sensing and actuation devices, nanostructured materials, among many applications. This technological potential arises because the properties of nanowires tend to be superior to those of bulk structures. However, unambiguous characterization of these properties has not been yet achieved, due to the challenging nature of nanoscale experimentation. In this thesis, we aimed at advancing the unambiguous characterization of mechanical and electromechanical properties of nanowires, by employing and improving MEMS-based (Microelectromechanical Systems) characterization technologies, which allow in-situ electron microscopy testing. Furthermore, we coupled the experimental results with atomistic simulations in order to attain fundamental understanding, and allow the determination of structure-property relations. This synergy between experiments and simulations also provides guidelines for improvements in both the experimental and computational techniques.;In the context of semiconducting specimens, we characterized the elastic modulus of GaN nanowires. We find that below 20 nm in diameter, the nanowires display enhanced elastic moduli. Above this size, nanowires show bulk behavior. The measured trends are consistent both in experiments and simulations. The modulus enhancement is caused by local contraction of the atomic bonds near the surface of the nanowires, which leads to a locally higher modulus at the surface.;For metallic specimens, we characterized the mechanical behavior of fivefold-twinned silver nanowires below 120 nm in diameter. To better match the loading condition between experiments and simulations, we implement a MEMS device for displacement-controlled testing, and subsequently employ it to characterize the cyclic plastic behavior of the nanowires. Experimentally, Bauschinger effect and partial recovery of the plastic deformation are observed. In-situ TEM experiments and atomistic simulations reveal that the twinned structure promotes reversibility in certain partial dislocations, leading to the observed plastic recovery.;Finally, leveraging the experience acquired on mechanical testing, we implement a MEMS device for four-point electromechanical characterization of nanowires. We validate the methodology using the fivefold twinned silver specimens, and then characterize the piezoresistance of n-doped silicon nanowires, which are found to display piezoresistive behavior of the same order of magnitude as bulk. Furthermore, we discover that contact resistance varies as a function of applied strain.