Mechanical Characterization of Nanowires Based on Optical Diffraction Images of the Bent Shape

A mechanical characterization technique for nanowires (NWs) longer than approximately 10 Am is proposed, based on optical microscopic observations under bending test. Low flexural rigidity of NWs often results in large deflection, which rules out the use of linear beam theory; however, the largely bent shape is optically visible as a diffraction image under transmitted illumination. The NW standing on a rod-like substrate was deflected by means of a micro-cantilever, where interactive forces, such as van der Waals forces, provide sufficient adhesion for fixing the free end of the NW. The reactive force was measured from the cantilever deflection and detected by a laser interferometer. The luminance profile of the diffraction image provided a good measure of the NW diameter. Inverse analysis using geometrically nonlinear mechanics for the bent shape enabled successful evaluation of the Young's modulus. In addition, a fracture test was conducted by manipulating the cantilever for intense deformation of the NW, such as buckling. The maximum curvature was observed at the freely suspended part of the bent NW where fracture was assured. The bending strength was determined from observation of the curvature at the fracture. Examples for CuO NWs of 40 nm to 190 nm in diameter indicated dependence of the Young's modulus and strength on the NW diameter.