Infoscience

Thesis

A nano-tensile testing system for studying nanostructures inside an electron microscope: design, characterization and application

Mechanical properties of nanostructures could be remarkably different from their bulk counterparts owing to scale effects, which have attracted considerable research interest in recent years. However, nanomechanics studies are hindered by the difficulties of conducting well-instrumented mechanical testing. The objective of this thesis is to develop a novel tensile stage that can be used to probe mechanical properties of universal one-dimensional (1D) nanostructures, like nanowires and nanotubes, inside a scanning/transmission electron microscope (SEM/TEM). The main challenges of performing tensile tests at the nanoscale are: (1) specimen alignment and fixation on the tensile stage; (2) application and measurement of tensile force with nano-Newton resolution; (3) measurement of specimen elongation with nanometer resolution. Previous studies have shown that micro-electromechanical system (MEMS) technology combined with advanced microscopy (e.g. SEM and TEM) provides promising perspectives to address these challenges. Two types of nano-tensile stages, fabricated in a silicon on insulator (SOI) wafer, were developed in this thesis, which consisted of a comb-drive actuator and either a differential capacitive force sensor or a double clamped beam force sensor. The optimized comb-drive actuators could output an in-plane force of about 210 µN at a drive voltage of 120 V, and the force sensors achieved resolutions of better than 50 nN. Individual 1D nanostructures were placed on the MEMS device by in-situ nanomanipulations and fixed at their two ends via focused electron beam induced deposition (FEBID). A strategy of modifying device topography, e.g. in the form of trenches or pillars, was proposed to facilitate the specimen preparation by in-situ manipulation that could achieve a high yield of about 80%. The mechanical testing function of the developed micro devices was demonstrated by tensile tests on individual Co and Si nanowires (NWs) inside an SEM. The average apparent Young's modulus, tensile strength and fracture strain of the electrochemically deposited Co NWs were measured to be (75.3±14.6) GPa, (1.6±0.4) GPa and (2.2±0.6) %, respectively. The measured Young's modulus is significantly lower than that of Co in the bulk form (209 GPa), which is likely caused by structural defects (e.g. pores) and surface effects (e.g. surface contaminations and surface oxide layers). The phosphorous-doped SiNWs grown bottom up by the vapor-liquid-solid (VLS) technique showed an average Young's modulus of (170.0±2.4) GPa and a tensile strength larger than 8.3 GPa. This finding confirms that materials strength increases as their sizes scale down. The top down electroless chemically etched Si <100> NWs show a tensile strength of 5.4 GPa. The developed MEMS devices and experimental techniques enable an alternative way of in-situ nanomechanical characterization based on electron microscopy. The design methodology and learning presented in this thesis would be useful to develop nano-tensile stages of other configurations with more advanced functions.

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