Fatigue and Fracture of Stretchable Transparent Conducting Films

Flexible and stretchable transparent conducting films have potential applications for displays, photovoltaics and in wearable and body-mounted sensors. This PhD project will investigate the fundamental failure mechanisms, under cyclic loading, of a class of transparent conducting thin films that consist of a Ag nanowire network deposited onto polymeric substrates. Prior EPSRC funded work in Manchester has shown that the sheet resistance of these networks is a simple function of the surface coverage and the network resistance, which is a function of the length, aspect ratios and electrical resistivity of the wires and the resistance of interwire contacts. The sheet resistance has been shown to increase after repeated strain cycling and that the change in network resistance is independent of surface coverage but is a function of both the strain magnitude and number of strain cycles.

This project aims to help achieve our overall objective of understanding the mechanisms that lead to this degradation in the electrical properties of these transparent conducting films under cyclic mechanical loading. The following research questions will be explored:
- Is the main mechanism for the increase in electrical resistance nanowire fracture, nanowire junction failure or an increase in the intrinsic resistance of the nanowires.
- What are the mechanisms of microstructural change that occur within the nanowire during cyclic deformation.
- What is the influence of defects such as crystallographic twins, which form naturally during the growth of Ag nanowires, on the two major points of study listed above.

The student will use the facilities within the University to manufacture Ag NW networks by spray deposition and characterize the sheet resistance using established procedures. We have developed novel experimental procedures to identify specific regions of the network that can be isolated and imaged through electron microscopy after controlled strain cycling

Prior work shows that the sheet resistance of these thin films, which can be made with single-crystal or penta-twinned crystal NWs, increases with the number of strain cycles as the network becomes increasingly fatigued. This increase in resistance is also shown to be due largely to the failure of the NWs themselves (as opposed to the failure of junctions or changes in intrinsic NW properties). How this micro-scale behaviour of the NW network relates to the sheet resistance of the thin film is captured by a statistical model (for random line networks). However, the literature also shows that penta-twinned NWs perform as well as single-crystal NWs with regards to fatigue, despite the expectation for the former to fracture more easily due to the greater geometric constraints that result from penta-twinning. This implies a set of more fundamental, nanoscale deformation behaviours in penta-twinned NWs that prolong their functional lifetime - behaviours that have yet to be fully discerned.

In this project, the main focus will be on understanding such deformations of penta-twinned NWs. The project will re-evaluate previous work done by this research group, which suggests bending and twisting deformations in these NWs might be enabled by high-angle-grain-boundary twinning operations, using TEM alongside scanning nanobeam diffraction and electron beam precession. This work will then be expanded to investigate other types of deformation - compressive, tensile, shear, etc - to build up a more comprehensive picture of the ecosystem of deformations that populate a given NW network. The project will screen NW networks that have undergone cyclic loading using SEM, to obtain area statistics on the frequency of different deformation types, thus bridging the gap between the nano- and micro-scale behaviours observed. This may in turn inform of ways in which the aforementioned statistical model could be improved, given this more complete and emergent view of the thin film system.