Mechanical Behaviour of Silver Nanowires

UoM administered thesis: Phd

  • Authors:
  • Hu Zhao

Abstract

The properties of Ag nanowires (NWs) are ideally suited for applications as flexible transparent electrodes. To predict and improve the reliability of flexible electronic devices made using Ag NWs, it is necessary to study their mechanical behaviour. Ag NWs fabricated by the commercial polyol process show a pentatwinned structure. This thesis uses transmission electron microscopy (TEM) to characterize individual Ag NWs after the deformation in tension, bending and cyclic fatigue loading. Where appropriate, the behaviour of the pentatwinned Ag NWs is compared with equivalent single crystal Ag NWs. The tensile mechanical properties of pentatwinned Ag NWs are directly compared with single crystal NWs prepared by template electrodeposition over a range of diameters, d, 80 to 300 nm. The plastic flow strengths of both sets of NWs show a significant size effect, which is shown to follow the same empirical scaling law seen with other fcc structured metals. TEM investigation of the deformation structures in each case shows little significant difference between the two classes of NW studied, with relatively low dislocation densities present after deformation. The deformed pentatwinned NWs showed no evidence for any characteristic deformation structures associated with the twin boundaries running parallel to the NW axis. The deformation structures in both pentatwinned and single crystal NWs are investigated after plastic bending deformation using TEM and electron diffraction. Structural analysis is carried out using high spatial resolution scanning precession electron diffractions (PED). Machine learning methods were used to analyse the overlapping diffraction patterns obtained from the pentatwinned NWs and determine the orientations of the silver subcrystals around localized bends in the wires. While the single crystal NWs showed bending deformation accommodated by a high angle boundary plane, the pentatwinned wires were found to display simple in-plane bends with narrow deformation zones in all 5 sub-crystals of the wire, with a common boundary plane. A coincident site lattice model has been developed to explain the bending angles seen in individual NWs and the observed distribution of angles in a population of pentatwinned NWs. The geometrical constraints of the model explain the difference in this distribution when compared to that from a population of single crystal NWs. The final part of the project investigated the mechanical behaviour of Ag NW in networks under cyclic (fatigue) loading. The extent of fibre fracture was determined through quantitative determination of the NW end point density changes enabling an analysis of current models predicting the resistance of NW networks. The microstructure of pentatwinned Ag NWs changes after fatigue tests. Bamboo-like faults were observed and characterized using PED in a similar way to the bending analysis described above. It was found that crystal rotations associated with a semi-coherent twist boundary occurred in the bamboo faults. The torsion forces leading to crystal rotation suggests that the far-field tensile stresses in the film can manifest as a variety of local stresses locally in the network particularly when the NWs are constrained by junctions. The reversible behaviour of bamboo faults from the network was observed at high cycle numbers and is supported by a high-energy state argument.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date31 Dec 2021