MAE PhD Defense – Tzu-Hsuan Chang

Title: In-situ Scanning Electron Microscopy Mechanical Characterization of Crystalline Nanowires using MEMS Devices (Advisor:  Dr. Young Zhu)

Date: August 15, 2016

Time & Location: 1:00 PM EB3 – 3235

ABSTRACT

The nanostructures such as nanowires (NWs) are important building blocks for a broad spectrum of nanotechnology applications. Therefore, it is of important to accurately characterize their mechanical response. Microelectromechanical systems (MEMS) are the reliable platform while performing the in-situ characterization of nanostructure. In this thesis, we conduct the MEMS tensile stage to study the mechanical behaviors of two important NWs: Si NWs and Ag NWs.

For the Si NWs, we focus on the study of temperature effect on the mechanical properties. Ambiguous results on either brittle or ductile failure of Si NWs are found in literatures. A novel temperature controllable MEMS tensile platform for NWs characterization is therefore designed and carefully calibrated. Multiphysics finite element analysis was carried out to optimize the structure design and minimize undesired thermal displacement during heating. The simulation results of temperature distribution on MEMS device are confirmed by the in-situ Raman spectroscopy.

Using this device, we thoroughly study the thermomechancical properties of Si NWs. Both Young’s modulus and fracture strength of the Si NWs decrease with the increasing of the temperature. A new approach in order to assess the brittle-to-ductile transition of Si NWs is proposed in this report. The activation volumes of dislocation in crystalline Si NWs acquired from in-situ SEM tensile testing show a clear deformation mechanism transition.

For the penta-twinned Ag NWs, we report an experimental effort to address the discrepancy about the size-dependent Young’s modulus in this thesis. Two independent experiments on the same NW, in-situ resonance test and tensile test in a scanning electron microscope, were used to measure the Young’s moduli. The cross-sectional shape of the Ag NWs was found to transit from pentagon to circle with decreasing NW diameter, which can modify the Young’s modulus as much as 19% for tensile test. This work confirmed that the Young’s modulus of penta-twinned Ag NWs increases with decreasing NW diameter.

BIOGRAPHY

Tzu-Hsuan was born in Taipei, Taiwan. He received his B.S. and M.S. degree in Civil Engineering in 2007 and 2009, respectively, from National Taiwan University. He started his PhD program at NC State under the advisory of Dr. Yong Zhu in August 2012. His PhD research focuses on the mechanical properties characterization of crystalline nanowires.