Mechanical Properties In Zinc Oxide Nanowires: Experimental And Theoretical Nanomechanics

Mechanical properties are amongst the most basic properties in materials, andinvestigations on the size effects of mechanical properties in nano-sized specimens notonly manifest the intrinsic structure-property relationships, but also play essential rolesin the progress of micro-and nano-electromechanical devices.In this dissertation, methodologies for in situ uniaxial tensile testing in scanningelectron microscope (SEM) are completed. Axial alignment of nanowires (NWs) aremaintained throughout testing, and stress-strain curves are accurately measured, basedon which the tensile moduli, fracture strengths, and fracture strains in [0001]-orientedsingle-crystalline zinc oxide (ZnO) NWs are quantitatively determined, and theoreticalmodels are developed for their diameter (D) dependences.Measured in ZnO NWs with D ranging from18to204nm, the tensile moduliincrease as D decreases; meanwhile, behaviors of size effect are affected by the loadingmode. Tensile moduli are lower than the previously measured bending moduli, andincrease slower than the latter with decreasing D. However, they get close rapidly to thebending moduli as D decreasing below about30nm. Based on an energy minimizationapproach including the surface-related and strain-gradient terms, the radial-distributedrelaxation function in nanowires are analytically derived, the diameter dependence ofwhich well explains our experimental findings. Moreover, physical meaning of the shellthickness in the widely-interested core-shell model is clarified for the first time.Although surface relaxation works as the basic mechanism for elasticity size effect,direct observation of the atomic structures in relaxed surfaces remains challenging. Inthis dissertation, the characteristic quantities of ZnO{1010} surface relaxation, as wellas their in-depth distributions, are directly measured with a sub-angstrom resolution,based on combining aberration-corrected transmission electron microscopy with abinitio calculations, the well-predicted surface structure in ZnO is thus verified. Thisstudy also supports the conception of relaxation function. Moreover, a novel mechanismfor surface reconstruction in ZnO{1010} is revealed for the first time.Diameter dependence of fracture strengths and fracture strains in single-crystallineZnO NWs with D ranging from18to114nm is experimentally revealed by in situ SEM.The strength properties are remarkably scattered, with their lower-bound following a modified power-form scaling law. Based on in situ cathodoluminescence measurementsand molecular dynamics simulations of the uniaxial tensile stress-strain curves, theattendance of point defects are confirmed, the diameter-dependent quantities of which,as well as their stochastic spatial configurations, dominate the NW strengths. Therefore,the Griffith's classic fracture mechanics still works well in single-crystalline NWs, aslong as the critical defect sizes are attributed to the effective quantities of point defects.Our studies provide a simple, but basic, understanding for the size effect of strengths insingle crystalline NWs.