Controlled Ambipolar‐to‐Unipolar Conversion in Graphene Field‐Effect Transistors Through Surface Coating with Poly(ethylene imine)/Poly(ethylene glycol) Films

Abstract A controlled ambipolar‐to‐unipolar (n‐type) conversion, along with a maximum fourfold increase in the electron mobility, in graphene field‐effect transistors (FETs) is achieved by coating the surface of graphene with a layer of a mixed polymer system, poly(ethylene imine) (PEI) in poly(ethylene glycol) (PEG). The PEG serves as a physisorption adhesion agent for the PEI. Both unipolar and ambipolar n‐type doping can be realized by adjusting the thickness of PEI films atop the graphene channel. The observed phenomena are attributed to the doping/dedoping effects of the external PEI film. The study provides a guide to engineering graphene transport properties through chemical modifications.