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NIST Nanomechanical Properties Group researchers have developed a scanning probe microscopy technique that allows the stress state at buried interfaces in nanostructures to be determined. The method was applied in the investigation of the mechanical properties of as-grown and oxidized Si nanowires (NWs) as a function of wire diameter. The wires were grown and oxidized by NIST Metallurgy Division researchers. From contact-resonance atomic force microscopy (CR-AFM) measurements, the effect of the compressive stress at the Si-SiO2 interface was revealed in the diameter dependence of the elastic modulus of Si NWs oxidized at 900 °C and 1000 °C, with diameters in the range 30 nm to 90 nm. A modified core-shell model that includes the interface stress developed during oxidation captured the observed diameter dependence. The magnitude of the compressive stress, about a few gigapascals, as well as the width of the stressed transition region at the Si-SiO2 interface, about a nanometer, agree with those reported in simulations and experiments. Diameter-independent values were observed for the elastic modulus of as-grown and fully oxidized Si NWs. For these NWs, the elastic moduli determined from CR-AFM measurements are in the same range as those found in tensile tests on single crystal Si NWs and three-point bending tests on amorphous SiO2 NWs.
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