The electrical conductivity of solution-processed nanosheet networks

Solution-processed networks of 2D nanosheets are promising for a range of applications in the field of printed electronics. However, the electrical performance of these networks — represented, for example, by the mobility — is almost always inferior to that of the individual nanosheets. In this Review, we highlight the central role that the inter-sheet junctions play in determining the electrical characteristics of such networks. After briefly reviewing ink formulation and printing methods, we use a selection of electronic applications as examples to demonstrate the dependence of network conductivity on network morphology. We show the network morphology to be heavily influenced by the deposition method, the post-treatment regime and the nanosheet properties. In turn, the morphology of the network fundamentally determines the properties of the inter-sheet junctions, which, ultimately, control the electrical performance of the network. We use reported electrical data to show that three main conduction regimes exist: the network conductivity can be limited by the junctions, by a combination of junction and material properties or, very rarely, by the material properties. Using a meta-analysis of published data, we propose simple models relating network conductivity and mobility to the junction resistance. Solution-processed networks of 2D materials are promising for applications in printed electronics. This Review examines how these networks are often electrically limited by the junctions between nanosheets, a phenomenon rarely reported and poorly understood, and surveys the macroscopic electrical properties of printed 2D networks, with a focus on inter-sheet junctions.