Size Effects in Mechanical Deformation and Fracture of Cantilevered Silicon Nanowires

Elastic modulus and fracture strength of vertically aligned Si [111] nanowires (empty set = 100-700 nm) in an as-grown state have been measured using a new, multipoint bending protocol in an atomic force microscope. All wires showed linear elastic behavior, spring constants which scale with (length)(3), and brittle failure at the wire-substrate junction. The "effective" Young's modulus increased slightly (100 -> 160-180 GPa) as wire diameter decreased, but fracture strength increased by 2-3 orders of magnitude (MPa -> GPa). These results indicate that vapor-liquid-solid grown wires are relatively free of extended volume defects and that fracture strength is likely controlled by twinning and interfacial effects at the wire foot. Small wires (100 nm) grown with a colloidal catalyst were the best performers with high modulus (similar to 180 GPa) and fracture stress >1 GPa.