EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Bridging the gap between fundamental nanomechanical research and device engineering requires the application of innovative testing techniques under realistic loading conditions. One of the most prominent examples for nanomaterials based devices are highly flexible transparent electrodes consisting of a percolated nanowire network on a polymer substrate. To optimize the overall performance and analyze the failure mechanisms of such nanowire electrodes, the mechanical behavior has to be studied on different length scales. While stretching the polymer substrate (PDMS, PET, LDPE) in one direction, compressive strain builds up in the perpendicular direction (elastic/plastic Poisson effect). Nanowires aligned along the compressive direction show a characteristic buckling behavior which enables partial relaxation of the compressive strain. While the resulting nanowire kinks can be studied in SEM and TEM the structure and mechanical properties of the polymer can be strongly affected by the electron beam. Therefore, in situ light microscopy has been used to study the deformation behavior of aligned nanowire networks on polymer substrates upon applying uniaxial and cyclic loading. For the sample preparation, a stamping transfer of previously doctor-bladed and aligned silver nanowires has been developed. In that manner, highly directional nanowire arrays could be achieved, which enables alignment of the nanowires parallel or perpendicular to the load axis. While for small strains in compressive direction (perpendicular alignment) the nanowires show an elastic and reversible sinusoidal deformation, increasing the strain finally results in a characteristic plastic deformation into discrete nanowire kinks followed by breakage and complete failure associated with loss of conductivity. In combination with TEM analysis and further correlative in situ SEM testing, the complex interplay of defect formation and buckling behavior of silver nanowires on stretched polymer substrates could be unraveled. The microscopic insights are used to propose new coating strategies for producing transparent nanowire electrodes with enhanced flexibility and durability.