Bending manipulation and measurements of fracture strength of silicon and oxidized silicon nanowires by atomic force mic
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Sergiy Krylyuk Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899; and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742
Albert V. Davydov Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Robert F. Cook Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 28 June 2011; accepted 8 September 2011)
In this work, the ultimate bending strengths of as-grown Si and fully oxidized Si nanowires (NWs) were investigated by using a new atomic force microscopy (AFM) bending method. NWs dispersed on Si substrates were bent into hook and loop configurations by AFM manipulation. The adhesion between NWs and the substrate provided sufficient restraint to retain NWs in imposed bent states and allowed subsequent AFM imaging. The stress and friction force distributions along the bent NWs were calculated based on the in-plane configurations of the NWs in the AFM images. As revealed from the last-achieved bending state, before fracture, fracture strengths close to the ideal strength of materials were attained in these measurements: 17.3 GPa for Si NWs and 6.2 GPa for fully oxidized Si NWs.
I. INTRODUCTION
Nanoscale mechanical properties’ measurements are a critical part of the development of new nanostructures and nanoscale devices with applications in electronics,1–3 optics,2,4 electromechanical devices,5,6 biomedicine,7 and energy conversion.8,9 Mechanical tests of elastic and plastic deformation, fracture, toughness, creep, and fatigue provide key information in selecting appropriate materials and structures for reliable and high-performance functionality of such emerging nanoscale devices. Due to its extensive use in electronics, the mechanical properties of “bulk” Si have received long-term attention; challenging requirements arising from the current and ongoing shrinking of features in nanoscale components require new tests and procedures to measure the “nanoscale” mechanical properties of Si. Readily fabricated in top-down10,11 and bottomup12,13 systems, Si nanowires (NWs) are one-dimensional Si single crystals that provide excellent vehicles for testing the mechanical properties of Si at the nanoscale.5,14–17 Due to the large surface area-to-volume ratio of nanostructures, some mechanical properties that are intrinsic material characteristics at the macroscale become strongly size dependent at the nanoscale. Theoretical and experimental a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.354 562
J. Mater. Res., Vol. 27, No. 3, Feb 14, 2012
studies have shown that the elastic moduli of various types of NWs change significantly compared with their bulk counterparts due to surface contributions. In the case of Si NWs, a size-dependent behavior for the elastic modulus (decreased with respect to the bulk elastic modulus) was found from atomistic computations18,19 for NWs with
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