Percolation Effects in Electrolytically Gated WS2/Graphene Nano:Nano Composites

Mixed networks of conducting and nonconducting nanoparticles show promise in a range of applications where fast charge transport is important. While the dependence of network conductivity on the loading level of conductive additive is well understood, little is known about the loading 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 WS2 nanosheets. As the graphene loading is increased, we find that both conductivity and carrier density increase in line with the 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 cm2/V s (for a WS2 network) to ∼0.3 cm2/V s (for a graphene network) which we attribute to the similarity between WS2–WS2 and graphene–graphene junction resistances. In addition, we find both the transistor on- and off-currents to scale with loading according to the percolation theory, changing sharply at the percolation threshold. Through fitting, we show that only the current in the WS2 network changes significantly upon gating. As a result, the on–off ratio falls sharply at the percolation threshold from ∼104 to ∼2 at higher loading. 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.