Two-dimensional polyaniline crystal with metallic out-of-plane conductivity

Linear conducting polymers show ballistic transport, imposed by mobile carriers moving along the polymer chains, whereas conductance in the extended dimension, that is, between polymer strands or layers, remains weak due to the lack of intermolecular ordering and electronic coupling. Here we report a multilayer-stacked two-dimensional polyaniline (2DPANI) crystal, which shows metallic out-of-plane charge transport with high electrical conductivity. The material comprises columnar π arrays with an interlayer distance of 3.59 Å and periodic rhombohedral lattices formed by interwoven polyaniline chains. Electron spin resonance spectroscopy reveals significant electron delocalization in the 2DPANI lattices. First-principles calculations indicate the in-plane 2D conjugation and strong interlayer electronic coupling in 2DPANI facilitated by the Cl-bridged layer stacking. To assess the local optical conductivity, we used terahertz and infrared nanospectroscopy to unravel a Drude-type conductivity with an infrared plasma frequency and an extrapolated local d.c. conductivity of around 200 S cm−1. Conductive scanning probe microscopy showed an unusually high out-of-plane conductivity of roughly 15 S cm−1. Transport measurements through vertical and lateral micro-devices revealed comparable high out-of-plane (roughly 7 S cm−1) and in-plane conductivity (roughly 16 S cm−1). The vertical micro-devices further showed increasing conductivity with decreasing temperature, demonstrating unique out-of-plane metallic transport behaviour. By using this multilayer-stacked 2D conducting polymer design, we predict the achievement of strong electronic coupling beyond in-plane interactions, potentially reaching three-dimensional metallic conductivity.