In Situ Investigation On Electromechanical Properties And Service Behaviors Of Single ZnO Nanowires

ZnO is a semiconductor material with piezoelectric characteristic. One dimensional ZnO nanomaterials play an important role in building strain sensors and nanogenerators. However, the investigations are not deep enough on the electrical, mechanical, electromechanical service behaviors, and the nanodamage and nanofracture mechanism. In this thesis, ZnO nanowires were synthesized by chemical vapor deposition. The electrical, mechanical, electromechanical properties and corresponding service behaviors in the above fields were studied by nano-manipulator, TEM, C-AFM systems. The nanodamage and nanofracture mechanism of ZnO nanowires were also analysized deeply.The electrical properties and service behaviors of ZnO nanowires were investigated by nano-manipulator in SEM. The electric-induced nanodamage threshold voltages of ZnO nanowires with diameters range from103nm to807nm were distributed between15-60V, and increased linearly with the increasing of diameters. The critical current densities were at the magnitude of107Am-2, and decreased exponentially with the increasing of diameters. The thermal core-shell model was established to illustrate the electric-induced nanodamage and fracture mechanism of ZnO nanowires. The main reason leading the ZnO nanowires damage and fracture was Joule heat generated at the metal-semiconductor junction. Once the temperature raised exceeds the melting point of ZnO nanowires, the ZnO nanowires would be fusing.The mechanical properties and service behaviors of ZnO nanowires in the uniaxial tensile and compression were investigated by in-situ TEM. The strain gradient effect existed in the ZnO nanowire with diameter of99.45nm in the process of tensile and compression deformation to mechanical nanodamage and fracture. The largest strain was at the fracture plane and became smaller far away from the fracture. The fracture of the ZnO nanowire in the tensile and compression was ductile fracture. The strain gradient effect in the deformation of the ZnO nanowire was certified by the constant volume before and after the mechanical nanodamage and fracture.The investigation of fatigue properties and service behavior of ZnO wires under high-cycle strain was performed by in situ TEM electromechanical resonance method. No damage occurred in the ZnO nanowires after resonance for about108-109cycles, while the ZnO nanowires subjected to e-beam irradiation fractured after resonance for seconds. The elastic modulus of ZnO nanowires with diameters less than100nm approaches or exceeds140GPa—the elastic modulus of bulk ZnO, while the elastic modulus of ZnO wires with diameters more than100nm is far below140GPa.The mechanical properties and service behaviors of ZnO nanowires were investigated by AFM under different scanning angles. The force calibration equation about the actual external force applied on the ZnO nanowires and scanning angles was determined at the scanning rate of14.8μm/s. The threshold force equation was also obtained between the fracture threshold force and diameters of ZnO nanowires. The actual forces applied on the surface of ZnO nanowires were strengthened when scanning angles exist. The actual mechanical nanodamage threshold forces had no relationship with the scanning angles, but increased linearly with the increase of diameters of ZnO nanowires. The criterion equation for security service of ZnO nanowires was established by the combination of threshold force equation and force calibration equation. The elastic moduli of the ZnO NWs obtained in the tests were far below the values measured under the three-point tests in previous reports, but fracture strengths had good consistency.The electromechanical properties and service behaviors of ZnO nanowires were investigated by C-AFM. The electromechanical nanodamage and fracture threshold voltages of the ZnO nanowires with equal diameters decreased linearly with the increase of external forces. While the electromechanical nanodamage and fracture threshold voltages increased linearly with the increase of diameters under the constant external force. Stress concentration effect strengthened the accumulation of electrons at the interface of ZnO nanowires and AFM tip, which led more Joule heat generated and melted the ZnO nanowires at a lower threshold voltage.