Mechanical, electromechanical, and optical properties of germanium nanowires
In order to completely assess the potential of semiconductor nanowires for multifunctional applications such as flexible electronics, nanoelectromechanical systems (NEMS), and composites, a full characterization of their properties must be obtained. While many of their physical properties have been well studied, explorations of mechanical, electromechanical, and optical properties of semiconductor nanowires remain relatively sparse in the literature. Two major hurdles to the elucidation of these properties are: (1) the development of experimental techniques which are capable of mechanical and electromechanical measurements coupled with detailed structural analysis, and (2) the synthesis of high quality nanowires with the high yields necessary to produce the quantities needed for composite fabrication. These issues are addressed in this dissertation by utilizing the supercritical fluid-liquid-solid (SFLS) synthesis method to produce germanium (Ge) nanowire specimens for mechanical and electromechanical measurements coupled with high-resolution transmission electron microscopy (HRTEM). In addition, excellent dispersibility and large quantities allow for optical measurements of dispersions and composites. Ge cantilever nanoelectromechanical resonators were fabricated and induced into resonance. From the frequency response, the Young's modulus of the nanowires was determined to be insensitive to diameter and on par with the literature values for bulk Ge. The mechanical quality factors of the resonators were found to decrease with decreasing diameter. The data indicate that energy dissipation from the oscillating cantilevers occurs predominantly via surface losses. The mechanical strengths of individual Ge nanowires were measured by in situ nanomanipulation in a scanning electron microscope (SEM). The nanowires were found to tolerate diameter-dependent flexural strains more than two orders of magnitude higher than bulk Ge. Corresponding bending strengths were in agreement with the ideal strength of a perfect Ge crystal, indicative of a reduced presence of extended defects. The nanowires also exhibited plastic deformation at room temperature, becoming amorphous at the point of maximum strain. The optical absorbance spectra of Ge nanowires were measured and found to exhibit spectra markedly different from bulk Ge. Simulations using a discrete dipole approximation (DDA) model suggest that the difference in light absorption results from light trapping within the nanowires.