Mechanical Characterization of One-Dimensional NanomaterialsBei Peng, Ph.D., Northwestern University, 2008.
Major Professor: Dr. Horacio D. Espinosa.
Novel materials and nanostructures with superior electro-mechanical properties are emerging in the development of novel devices. Engineering application of these materials requires accurate electro-mechanical characterization, which in turn requires development of novel experimental techniques. This dissertation outlines the investigation of the mechanical and electrical properties of one-dimensional nanomaterials.
One-dimensional nanomaterials such as carbon nanotubes (CNTs), ZnO and GaN nanowires were investigated using MEMS technology and in situ SEM/TEM experimentation. This nanoscale materials testing system (nMTS) allows the direct correlation of stress-strain state and defect nucleation and propagation. For CNTs, the fracture strengths of arc-discharge-grown multi-walled carbon nanotubes (MWCNTs) were measured using nMTS within a TEM. Single-shell failures were observed with a mean fracture strength in excess of 100 GPa, which exceeds prior observations by more than a factor of three. Such strengths are in excellent agreement with quantum mechanical estimates for CNTs containing only an occasional vacancy defect and are ~80% of the values expected for defect-free tubes. Electron irradiation at incident energies of 200 keV facilitated crosslinking of multiple shells yielding improvements of nearly a factor of 12 in the maximum sustainable load compared to that of non-irradiated samples of similar diameter. High-resolution imaging allowed direct determination of the number of fractured shells and the outer tube’s chirality.
Understanding the mechanical properties of nanowires (NWs) made of semiconducting materials is central to their application in piezoelectronic devices. Using the same experiment, the Young’s modulus and tensile strengths of  oriented ZnO NWs were measured with diameters ranging from 20.4 to 412.9 nm. Uniaxial tensile loading eliminates many source of experimental error and simplifies the analysis of the results. The Young’s modulus is found to be inversely related to the diameter. A value of ~140 GPa for the Young’s modulus, consistent with the Young’s modulus of bulk ZnO, is found for nanowires with diameters greater than 80 nm. Molecular dynamics simulations are carried out to model ZnO nanowires of diameters up to 20 nm. The computational results demonstrate similar size dependence, confirming the experimental findings. Computational analysis provides insight into the physics of this size dependence. The measured fracture strength ranged from 3.33 to 9.53 GPa and showed a quantized property that is well predicted by nanoscale Griffith’s and Weibull’s theory. In situ TEM experiments as well as the analysis of stress-strain response for individual ZnO NW indicated that the deformation was pure elastic with a large fracture strain of about ~5% and the fracture cleavage plane was (0001). The tensile strength was found to depend on the number of critical defects within the nanostructure. The implications of deformation and fracture strength are discussed in the context of functional devices made of ZnO NWs.
Finally, the nMTS was extended to investigate the electro-mechanical properties of GaN NWs. The Young's modulus of GaN NWs was measured to be 301.3 ± 9.1 GPa, close to the values of bulk GaN. The average fracture strain of the tested samples was 4.34%, which is much larger than the failure strain of bulk crystal (less than 1%). In addition to the mechanical properties, the piezoresistance and piezoelectric effect were both observed for the GaN NWs. We have shown that GaN NWs exhibit large longitudinal piezoresistance and piezoelectric coefficients. Systematic theoretical calculations are required to reveal the underlying mechanisms for the observations. The enhanced piezoresistance and piezoelectric effect could find applications in sensor nanotechnology, flexible electronics, as well as in NEMS for harsh environments. In particular, intrinsic strains may exist in many nanoscale materials, and strain sensitivity could be a basic issue affecting the performance of these nanostructure-based electronics.