Size effects on the stiffness of silica nanowires.

The stiffness of silica, a covalently bonded network solid, has not been found to be a function of physical dimensions for bulk specimens and microscale fibers of diameter >5 mm. However, recent indirect mechanical characterization of drawn silica nanowires suggested a substantial and unexplained decrease in the Young s elastic modulus E for diameters D below 100 nm. We present new data from direct mechanical characterization using a scanning probe microscope (SPM), and show that silica wires with diameters as small as 280 nm exhibit the stiffness of bulk silica. Further, we present results from molecular dynamics simulations that predict an increase in stiffness for diameters up to 6 nm. Together, these results suggest that the elastic weakening of silica nanowires of intermediate diameters (43 to 98 nm) cannot be explained solely by recourse to the intrinsic properties of amorphous silica as captured either by direct measurements on drawn nanowires of smaller surface-area-to-volume ratios, or by classical molecular dynamics simulations of silica nanowires of larger surface-area-tovolume ratios. Silica nanowires have promising applications in optoelectronic nanodevices, due to the optical waveguide properties of this material s structure and the capacity of such wires to exhibit small radii of curvature without fracture. Application of this material will require improved understanding of the mechanical properties of such covalently bonded nanostructures. In general, material fibers or wires of nanometer-scale diameter exhibit much higher strength than the corresponding bulk materials. However, indirect measurements of elastic moduli have suggested that amorphous silica nanowires can be much more compliant than the corresponding bulk material, 5] while fibers of micrometer-scale diameter exhibit the stiffness of bulk silica. This work aims to elucidate the transition between bulk and nanoscale behavior and investigate possible mechanisms for the size effect on stiffness in such covalent network solids. In addition to suitability for emerging technologies, silica structures of such small physical dimensions provide an opportunity to understand chemical/mechanical interactions in materials, owing to inherently high ratios of surface area to volume and nearly defect-free microstructures. An important example of such interactions is stress corrosion, in which water dramatically reduces the tensile strength of silica. Moreover, nanowires represent material structures that can bridge experimental and simulation length scales, since the estimation of the theoretical strength through molecular dynamics simulation is usually restricted to systems that are too small to investigate by conventional experimental means. A reliable mechanical characterization of these wires is essential for the interpretation of strength measurements. Mechanical properties of conventional silica fibers can be determined via conventional experiments such as twopoint bending. 9] Wires of smaller diameter, however, present significant experimental challenges, due to difficulties in simultaneously imaging, gripping, applying, and measuring the nanoscale forces and displacements. For silica wires of diameter <1 mm, only indirect measurements of elastic moduli E via resonant frequency measurements have been reported. At the nanoscale, several methods have been proposed to measure mechanical properties. Among these, direct measurement of force during controlled displacement of a compliant cantilevered probe within a scanning probe microscope (SPM) has been implemented to measure the stiffness and strength of carbon nanotubes and nanorods, as well as nanowires made of silicon carbide, gold, silver, and manganese oxide, among others. The force–displacement behavior is interpreted according to continuum beam theory in order to obtain the stiffness and strength of the material. In this work, we apply this method to determine the elastic moduli E of silica wires with uniform diameters ranging from 280 to 1950 nm. We relate these results to predictions of E as a function of nanowire diameter via classical molecular dynamics simulations for diameters from 3.7 to 6 nm. Figure 1 shows effective bending stiffness of the silica wires P/d calculated from vertical-force experiments. Points are irregularly spaced due to the manual positioning of the SPM cantilever. Figure 2 shows the effective bending stiffness of the silica wires P/d calculated from in-plane loading experiments. Note that regular spacing of acquired force– displacement responses in x was provided by the xy piezoelectric scanner (see Experimental Section). Fitting of both sets of experimental results via Eq. (2) (see below) identifies the corresponding elastic modulus E for each wire, as summarized in Table 1. [*] E. C. C. M. Silva, Prof. S. Yip, Prof. K. J. Van Vliet Department of Materials Science and Engineering Massachusetts Institute of Technology 77 Massachusetts Avenue, 8-237, Cambridge, MA 02139 (USA) Fax: (+1)617-253-8745 E-mail: krystyn@mit.edu