Solution-processed 2-dimensional hole-doped ionic graphene compounds
Contacts with suitable work functions are important not only for ohmic injection of carriers but also to set up the built-in potential required for various semiconductor processes including current rectification, light emission and photovoltaic generation. For two-dimensional (2D) materials, one way to shift the work function is by intercalation doping with suitable donors or acceptors. Here, using an atomic sheet transfer methodology, we report layer-by-layer assembly of centimeter-square sizes of graphene–fluorofullerene multilayers through directed stacking of chemical-vapor-deposited (CVD) graphene sheets and self-assembly of fluorofullerene acceptors, for example C60F48, to give a 1 eV increase in work function to 5.7 eV, which is unprecedented for a well-defined compound. These assemblies exhibit an unusual motif of fully-ionized large dopants in open packing with the graphene sheets. As a consequence, they show a sizeable electrostatic dipole to give ultrahigh work function at acceptor-terminated surfaces even for a moderate hole doping level of 1.6 × 1013 cm−2 per sheet. They exhibit little additional carrier scattering and a remarkable chemical stability. Hall measurements reveal unity doping efficiency with temperature-independent hole density, mobility and electrical conductivity down to 2.5 K, which are atypical of conventional graphite intercalation compounds. These materials provide the first examples of a novel domain of doped 2D assemblies where large ions are incorporated through room-temperature solution processing, which opens new opportunities beyond van der Waals semiconductor heterostructures.