Percolation Effects in Electrolytically Gated WS 2 /Graphene Nano:Nano Composites.

Mixed networks of conducting and non-conducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the conductive mass fraction (M) is well understood, little is known about the M-dependence of mobility and carrier density. This is particularly important as the addition of graphene might lead to increases in the mobility of semiconducting nanosheet-network transistors. Here, we use electrolytic gating to investigate the transport properties of spray-coated composite networks of graphene and WS nanosheets. As the graphene M is increased, we find both conductivity and carrier density to increase in line with percolation theory with percolation thresholds (~8 vol%) and exponents (~2.5) consistent with previous reporting. Perhaps surprisingly, we find the mobility increases modestly from ~0.1 cm/Vs (for a WS network) to ~0.3 cm/Vs (for a graphene network) which we attribute to the similarity between WS-WS and graphene-graphene junction resistances. In addition, we find both the transistor on- and off-currents to scale with M according to percolation theory, changing sharply at the percolation threshold. Through fitting, we show that only the current in the WS network changes significantly upon gating. As a result, the on-off ratio falls sharply at the percolation threshold from ~10 to ~2 at higher M. Reflecting on these results, we conclude that the addition of graphene to a semiconducting network is not a viable strategy to improve transistor performance as it reduces the on:off ratio far more than it improves the mobility.