In situ transmission electron microscopy studies of mechanical properties of one-dimensional nanostructures

The unique high-aspect ratio and low mass properties of intrinsic one-dimensional (1-D) nanostructures offer an opportunity for the development of ultra-sensitive nanoresonator-based devices. This dissertation studies the mechanical properties of various 1-D nanostructures, including single-walled carbon nanotubes (SWNTs), SWNT bundles, C60-filled SWNT bundles, and gallium nitride (GaN) nanowires, through observation of their mechanical resonances in a transmission electron microscope. A special specimen holder was custom designed and built to actuate these materials to their resonance through an applied tunable ac signal. The Euler-Bernoulli beam theory is employed to relate the measured resonant frequency to the sample's elastic modulus. These studies illustrate the increased importance of surface and interfacial interactions in materials at these length-scales to the mechanical properties of these structures. For SWNT bundles, the bending modulus E decreases with increasing bundle diameters, and is found to be in the range of 70 +/- 6 to 442 +/- 36 GPa for bundle diameters between 11-68 nm. Inter-tube shear is believed to be responsible for these results. The onset of nonlinear frequency response is observed in these bundles upon large excitation amplitudes. We also studied the effects of structural modifications on the mechanical properties of these bundles through the creation of cross links between nanotubes in the bundled structures, and through the introduction of C60 molecules to the hollow inner area of the nanotube structure. Significant enhancement in the stiffness is observed after the bundle is irradiated with low to moderate electron doses, since inter-tube shear is prohibited. In the C60 SWNT structures, we observe a similar diameter-dependent bending modulus; yet, the modulus is significantly higher than the unfilled system. We also attempt to measure the mechanical properties of isolated individual SWNTs. With the combined uses of focused ion beam nano-patterning and in situ TEM characterization, we successfully measure the resonant frequency of individual SWNTs. E for a SWNT diameter of 5 nm is 1.34 +/- 0.06 TPa. Finally, we extend the use of our in situ resonance detection technique to measure the mechanical properties of GaN nanowires. E is close to the theoretical bulk value (300 GPa) for a large diameter nanowire (84 nm diameter) but is significantly reduced for smaller diameters. The quality factor of GaN nanowires is also size-dependent, and decreases with increasing surface-to-volume ratio.