Correlative in situ light and electron microscopy studies on the deformation behavior of silver nanowires on polymer substrates

Abstract text (incl. figure legends and references)

Ongoing developments in the field of flexible electronics led to a need for transparent flexible electrodes. In this area, percolation-based electrodes have shown promising characteristics. Especially, networks based on silver nanowires (AgNWs) inhabit a very appealing combination of properties for the application case. Since, flexibility is one of the most important characteristics of these electrodes it is necessary to understand the wire behavior under different loading scenarios revealing the influence of mechanical loading on the device performance.

Within the network, the mechanical response can be governed by complex interaction of the wires with one another. Therefore, for our testing purposes a scaled back approach was chosen, investigating isolated wires placed on varying polymer substrates (PDMS, PET, LDPE). As a first step, an alignment setup up utilizing doctor blading was developed, to investigate tensile and compressive behavior independently. In this context, the compressive loading is achieved via the constriction of the polymer substrates on AgNWs directed perpendicular to the initial straining direction (plastic Poisson"s effect). To cover all facets of the nanowire deformation mechanism, a correlative workflow combining complementary microscopy techniques was selected. Stopped in situ straining tests were performed under the light microscope (LM) since visible light has in contrast to electrons no deteriorative interaction with the AgNWs. For a more detailed post characterization, ex situ scanning electron microcopy (SEM) and transmission electron microcopy (TEM) were used (see Fig.1). Here, the AgNWs are transferred to the TEM by a workflow incorporating a sacrificial PEDOT:PSS layer.

We observed that the wires on PET compensate an applied compressive load with the formation of sharp angled kink structures. Subsequent TEM studies revealed the kinks to be locations for newly formed grain boundaries (GBs). The formation of GBs in combination with a folding mechanisms enables the wire to withstand high amount of compression without failing. Changing the substrate material from PET to the elastomer PDMS opened up the ability to perform fatigue tests on the AgNWs. In contrast to the wires on PET, the wires on PDMS show a sinusoidal elastic deformation behavior before at a critical strain amount they switch to an acute plastic kink. Furthermore, additional fatigue tests uncovered the elastic deformation to be completely reversible. Whereas, the introduced GBs in the wire act as a fracture location when repeatedly strained. Overall, we demonstrate that the combination of LM and EM gives fundamental insight into the underlying deformation mechanisms of AgNWs on different substrates as well as loading conditions. We believe, that those findings are highly relevant to further improve the mechanical properties of AgNW electrodes in application of flexible devices.