Torsional Behavior and Nanoelectromechanical Systems of Carbon and Inorganic Nanotube

The aim of my Ph. D. has been the investigation of the torsional mechanics and electromechanics of carbon and inorganic nanotubes, envisioning their incorporation into nanoelectromechanical systems (NEMS), fibers, and nanocomposites. Amongst various deformation modes, torsion attracted our attention for two reasons. First, twisting a carbon nanotube (CNT) modifies its electrical resistance, making CNTs promising building blocks for torsional nanosensors. Second, torsion of a multiwall (MW) nanotube is a powerful tool to probe friction at the atomic scale. While CNTs possess among the highest elastic modulus and tensile strength, weak shear interactions between layers dramatically alter their mechanical properties and the sensitivity of CNT-based NEMS. Our goal was therefore to find a material that combines high stiffness and strength, high mechanical interlayer coupling, and a strong electronic response to torsion, so as to find the best possible material for these applications.;Boron nitride nanotubes (BNNTs), owing to their high Young's modulus, the polar nature of the B-N bond, and their tunable electronic properties, were an appealing candidate. We investigated the torsional mechanics of BNNTs, and showed that unlike CNTs, BNNTs exhibit a strong mechanical interlayer coupling, which makes them up to an order of magnitude stiffer and stronger than CNTs. Using both experimental and theoretical results, we attributed these properties to the faceted nature of BNNTs, and developed a model that predicts the conditions for facet formation and their stability under torsional stress. Following our failure to detect any electrical response of BNNTs to torsion, we aimed at combining BNNT outstanding mechanical properties and CNT exceptional electrical properties into a single material. The first approach we considered was the electron irradiation of multiwall CNTs, in order to induce covalent cross-linking between the walls, and thereby increase mechanical interlayer coupling. While further work is still needed to determine the optimal experimental parameters, our results point out to a moderate increase of CNT torsional stiffness. The second approach was the investigation of the torsional mechanics and electromechanics of composite boron carbonitride nanotubes (BCNNTs), whose mechanical and electromechanical properties were still unexplored so far. BCNNTs, which were produced by post-synthesis C-doping of BNNTs, retain the faceted structure of BNNTs, and were found to be several times stiffer and stronger than CNTs; BCNNT conductivity displayed dramatic variations under torsion. These remarkable mechanical and electromechanical properties make BCNNT possibly the best building blocks for NEMS and nanocomposites. Finally, torsional mechanics of grown nanowires was also investigated for the first time; GaN nanowires displayed surprising high strength and resilience, retaining their elasticity over hundreds of press cycles.