Mechanical properties of metal nanowires and applications in flexible electronics

Abstract

Noble metal nanowires have recently attracted considerable attention due to their interesting optical, catalytic, and electronic characteristics. Furthermore, the application of the metal nanowire networks as a flexible transparent electrode is of considerable interest in the flexible display industry owing to its higher conductivity compared to those of major competing materials, such as graphene and carbon nanotubes. Before the metal nanowire network technology can be commercially used for flexible transparent electrodes, however, it is critical to understand the deformation behaviors of individual metal nanowires as well as of collective nanowire networks. In the first part of this study, the deformation behavior of defect-free [110] Au nanowires was studied by using in-situ SEM/TEM tensile tests with push-to-pull (PTP) devices. PTP devices are the specifically designed MEMS devices for performing nanoscale tensile tests on 1-D nanomaterials within a SEM or TEM chamber. Since the manipulation of nanowire was performed with a sharp tungsten tips while viewing through optical lenses, any damages induced by focused ion beam (FIB) on nanowires are able to be avoided. In addition, surface oxidation can also be ignored for the Au nanowires due to their high resistance to oxidation. Real-time observation in SEM together post-deformation TEM analysis revealed that the Au nanowires having diameter less than ~170 nm showed deformation twinning while perfect dislocation mediated plasticity was observed in the nanowire with diameter larger than ~170 nm. Therefore, the critical dimension where the deformation mechanism switches from deformation twinning to ordinary dislocation plasticity was experimentally determined to be ~170 nm. Nanoribbons with fixed thickness but varying width-to-thickness ratios (up to 9:1) were also studied to show that an increase in the surface energy due to the crystal re-orientation suppresses the deformation twinning. Molecular dynamics simulations confirmed that the transition from partial dislocation mediated plasticity to perfect dislocation plasticity with increase in the width-to-thickness ratio is due to the effect of the surface energy. In the second part of the dissertation, the mechanical behavior of collective nanowire networks were evaluated. In order to gain understanding of deformation behavior of collective nanowire networks, bending fagiue tests were performed. Such bending fatigues tests are of tenchological importance since the metal nanowire based transparent electrodes are to be applied in many of the emminent flexible devices. The bending fatigue tests indicate that Ag nanowire networks electrodes are highly reliable that showed only a 1.5% increase in 500,000 cycles at 1.0% strain. Resistance increase of Ag nanowire networks was determined to be due to the failure at the thermally locked-in junctions, followed by propagation of nearby junction failures. The progression of the junction failures in a nanowire network geometry is more difficult than the crack formation in a thin film, and, therefore, is the reason for the enhanced fatigue behavior. In addition, careful examination showed that the resistance decrease occurred in the early stage of bending due to mechanical welding upon application of bending strain. Based on the observations from this study, a new methodology by using mechanical welding effect was also proposed for highly reliable Ag nanowire network with no prior thermal annealing. In the final chapter, the role of the reduced graphene oxide (RGO) in chemical stability and its effect on the mechanical reliability were studied for Ag nanowire/RGO hybrid transparent electrodes. Although Ag nanowire network is highly reliable as explained above, the major drawback of in application to transparent electrodes is that the Ag nanowires are easily oxidized when exposed to moist air, which results in the decrease in conductivity of the Ag nanowire networks. Therefore, a suitable protective layer is needed on the Ag nanowire electrode to suppress the oxidation of the Ag nanowires. Here, reduced graphene oxide (RGO) was used as the protective layer owing to its low permeability to gas or $H_2O$ molecules and excellent optical transmittance. The RGO layer was formed on the Ag nanowire networks, and the mechanical reliability of the fabricated Ag nanowire/RGO hybrid electrodes were examined by using a bending fatigue tester. Thin layer of RGO with optimized thickness of ~0.8 nm deposited on the Ag nanowire networks sustained excellent reliability of the Ag nanowire networks, where the fractional resistance increase was a 2.7% after 800,000 cycles. Furthermore, adopting the RGO layer significantly lowered oxidation of Ag nanowires, and the bending fatigue properties after exposure to ambient air for 132 h at $70^\circ C$ indicated remarkable enhancement due to the prevention from forming oxide particles on Ag nanowires. Lastly, highly reliable Ag nanowire/RGO hybrid electrode was fabricated using mechanical welding by subjecting it to bending strain to form localized junction without any post annealing process. Overall, this thesis addresses the deformation behavior of 1-D individual Au nanowire as well as the fatigue behavior of network of 1-D Ag nanowires. The size-dependent twin deformation of Au nanowire was investigated, which revealed the critical factors that govern the deformation mechanism of Au nanowires. As for the nanowire network, the fatigue behavior of nanowire network was systematically studied as well as novel method to enhance the mechanical reliablility of nanowire networks under fatigue was suggested. In addition, the enhancement of chemical stability of Ag nanowire networks was achieved by adopting RGO layers on nanowire networks. As mentioned above, the clear understanding of the deformation behavior of an individual nanowire and network of the individual nanowire is repuired to enhance the reliability of electronic devices using metal nanowires. Therefore, the finding of this thesis will be practical guidance for the fabrication of highly reliable electronic devices using metal nanowires.