MSE Colloquium: Rodrigo Bernal, Mechanical and Electromechanical Properties of Nanowires for Sensors and Electronics

Abstract

Nanowires are envisioned as components of future sensors, electronics, nanostructured materials, among many applications. This technological relevance has stimulated research in the characterization and understanding of their physical properties. In this talk, I will present results on the mechanical and electromechanical characterization of metallic and semiconducting nanowires, obtained with in-situ electron microscopy experiments. By allowing visualization of the materials at high resolution during characterization, in-situ experiments allow us to draw insights on the fundamental mechanisms controlling nanowire properties.

In the context of semiconducting specimens, we characterized the elastic modulus of gallium nitride nanowires. We find that below 20 nm in diameter, the nanowires display enhanced elastic modulus, due to reduced interatomic spacing near the surface. For metallic specimens, we characterized the tensile behavior of fivefold-twinned silver nanowires below 120 nm in diameter. We observe the Bauschinger effect and recoverable plasticity, and establish these behaviors are caused by reversible dislocation activity promoted by the twinned structure. For both semiconducting and metallic specimens, complementary atomistic simulations agree with the experimental results. Leveraging the experience acquired on mechanical testing, we implemented a MEMS (Microelectromechanical System) device for electromechanical characterization. We characterized the piezoresistance of n-doped silicon nanowires, which is found to be of the same order of magnitude as bulk.

The ability to characterize both the mechanical properties of nanomaterials, and the effect of mechanical deformation on other properties, such as electron transport, will pave the way for the discovery of novel nanomechanical multiphysics phenomena, which will underpin the next generation of sensors and electronics.

Bio

Rodrigo Bernal was born and raised in Colombia. He earned B.Sc. degrees in Mechanical and Electronics Engineering from the University of the Andes, in Bogotá. After this, he moved to the U.S to pursue a Ph.D. in Mechanical Engineering at Northwestern University, under the advisement of Prof. Horacio Espinosa, graduating in 2014. At Northwestern, he investigated the mechanical and electromechanical properties of metallic and semiconducting nanowires, employing in-situ Scanning and Transmission Electron Microscopy (SEM/TEM). Currently, he is a postdoctoral researcher in the Mechanical Engineering and Applied Mechanics Department, at the University of Pennsylvania, supervised by Prof. Robert Carpick.  He is investigating the fundamental mechanisms of wear and friction in advanced carbon-based materials. His future research interests are in the area of nanomechanical multiphysics: nanoscale phenomena where mechanical deformation can control and tune other properties such as electrical, thermal or chemical.