No abstract

Great efforts have been made to synthesize ZnO nanowires (NWs) as building blocks for a broad range of applications because of their unique mechanical and mechanoelectrical properties. However, little attention has been paid to the correlation between the NWs synthesis condition and these properties. Here we demonstrate that by slightly adjusting the NW growth conditions, the cross-sectional shape of the NWs can be tuned from hexagonal to circular. Room temperature photoluminescence spectra suggested that NWs with cylindrical geometry have a higher density of point defects. In situ transmission electron microscopy (TEM) uniaxial tensile-electrical coupling tests revealed that for similar diameter, the Young's modulus and electrical resistivity of hexagonal NWs is always larger than that of cylindrical NWs, whereas the piezoresistive coefficient of cylindrical NWs is generally higher. With decreasing diameter, the Young's modulus and the resistivity of NWs increase, whereas their piezoresistive coefficient decreases, regardless of the sample geometry. Our findings shed new light on understanding and advancing the performance of ZnO-NW-based devices through optimizing the synthesis conditions of the NWs.

Quaternary Cr-V-C-N films were deposited on Si wafers through a hybrid system of arc ion plating and sputtering techniques in an Ar/N 2 /CH 4 gaseous mixture. In this work, the effects of vanadium on the microstructural evolution, mechanical properties and friction mechanism of Cr-V-C-N films were investigated. The results showed that quaternary Cr-V-C-N films consisted of nanosized crystallites of (Cr,V)(C,N) and amorphous VCN phases. The Cr-10?4 at-%V-C-N film possessed the higher hardness value of 34 GPa, compared to the 27 GPa of a Cr(C,N) film. Additionally, the friction coefficients of the Cr-V-C-N films were reduced from 0?38 for the Cr-C-N film to 0?27 for the Cr-10?4 at-%V-C-N film. Atomic force microscopy (AFM) and Auger electron spectroscopy (AES) analyses also revealed that the amorphous phase VCN phases played a role in reducing the friction coefficients of the films. The a-VCN phase (vanadium rich) was believed to cause a tribochemical reaction with ambient air during the wear process.

In situ transmission-electron-microscopy (TEM) tensile testing has been a powerful tool for revealing the underlying physical mechanism when materials are subjected to a stress [1][2][3][4][5]. With this technique, the dynamics microstructure evolution of the materials can be recorded in a nano or even atomic scale. However, all the commercial in situ TEM tensile holders available so far suffer from the absence of quantitative ability and the complexity in sample preparation. Consequently, the potential exploration ability of in situ TEM tensile holders has been hindered substantially. Supported by Department of Energy (DOE) Small Business Innovation Research (SBIR) program, recently we have developed a new tensile device for operation inside a TEM which not only yields quantitative load-displacement data concomitant with real time images of the microstructural behavior, but also simplifies the sample preparation procedure essentially.In this work, we report the current progress in the application of this quantitative in situ TEM tensile device for measuring the mechanical properties of 1D nanostructures (silicon nanowires etc). Fig. 1 is the TEM images taken after the fracture of silicon nanowire. It was surprised to see that apparent plasticity has occurred even for nano wire with a diameter of 250 nm. As shown in Fig. 1a, three zones can be identified along the residual nano wire based on the contrast. Zone I corresponds to area without plastic deformation. Zone III represents area that has experienced heavy plastic deformation. Zone II, bounded by red and green dash line, indicates gradually decrease of plastic deformation from Zone 3 to Zone 1. The insets at the upper left corner and lower right corner are corresponding selected area different patterns from zone III and zone I, respectively. Besides the slight relative change of the density of second diffraction spots (Marked by white arrow in the insets of Fig. 1 a), there are no indication for either amorphous or polycrystallization. Fig. 1 b is the magnified dark field image of zone II and III shown in Fig. 23a. Dislocation like contrast can be identified clearly. The findings indicate new deformation mechanism for silicon nano wires which has been seen as classic brittle material in bulk form.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.