Size Effect on the Mechanical Properties in Zinc Oxide Nanowires

Open Access
Author:
Desai, Amit
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 26, 2007
Committee Members:
  • Md Amanul Haque, Committee Chair
  • Christopher Rahn, Committee Member
  • Eric M Mockensturm, Committee Member
  • Akhlesh Lakhtakia, Committee Member
  • Melik C Demirel, Committee Member
Keywords:
  • experimental nanomechanical characterization
  • electromechanical coupling
Abstract:
As the size of material decreases to nanoscale, the fundamental material properties such as Young’s modulus become different from that at the bulk scale. New phenomena begin to emerge when the size of the material is reduced that are unique to the small scale. Nanowires are quasi one-dimensional solids, and their diameters are typically in the range of 10 nanometers to 500 nanometers. The range of nanowire diameters coincides with the critical length scales in materials, where the properties and phenomena are different from bulk. Hence, nanowires serve as excellent systems for studying material properties and behavior at the small scale. Along with their scientific significance, nanowires are also technologically important, and are expected to play an important role in the trend towards miniaturization. However, the small size of the nanowires and lack of sophisticated instrumentation pose several challenges in performing experiments on nanowires. The size effect on the mechanical properties of nanowires was investigated, specifically Young’s modulus and fracture strain. The representative material was chosen to be zinc oxide because of its technologically significant properties, such as being both, a semiconductor and piezoelectric material. The zinc oxide nanowires were synthesized using vapor-liquid-solid process. Uniaxial tensile and cantilever bending experiments were performed in-situ inside electron microscopes on single zinc oxide nanowires. Novel application of post-buckling mechanics was exploited to develop microdevices for high resolution force and displacement in sensing and actuation. For performing the mechanical characterization experiments, different techniques for nanowires specimen preparation were developed that were generic, reliable and robust. Fracture strain and Young’s modulus of zinc oxide nanowires were measured for different diameters and lengths. The fracture strains of zinc oxide nanowires were experimentally measured for the first time, and the strains were between 4 % and 14 %, which is unusually high considering zinc oxide is a brittle material (ceramic) at the bulk scale. It was also observed that the fracture strains increased with decreasing diameter of the wires. The high fracture strains and diameter dependent strains were attributed to reduced number of defects and increasing contribution of surfaces. The Young’s modulus of the zinc oxide nanowires was measured to be 26 ± 9 GPa by uniaxial tensile experiments and 18 – 44 GPa by cantilever bending experiments. The tensile experiments were used to obtain stress-strain plots for the first time for single crystal zinc oxide. The Young’s modulus values were comparable to modulus values estimated by other researchers on zinc oxide nanostructures, but significantly lower than the modulus of bulk zinc oxide (140 GPa). The significant decrease in Young’s modulus cannot be explained by existing theories such as surface effects and non-ideal boundary conditions. New mechanisms, such as strain induced charge redistribution and influence of electromechanical coupling, were explored. These mechanisms could qualitatively and to some extent, quantitatively explain the reasons for the significant decrease in modulus of zinc oxide nanowires, compared to bulk zinc oxide.