Tuning the Electronic Structure of Graphene by Molecular Dopants: Impact of the Substrate

A combination of ultraviolet and X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and first principle calculations was used to study the electronic structure at the interface between the strong molecular acceptor 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F<inf>6</inf>TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge redistribution mechanisms in the two ternary systems, as a consequence of the insulating versus metallic character of the substrates. While electron transfer occurs exclusively from graphene to F<inf>6</inf>TCNNQ on the quartz support (p-doping of graphene), the Cu substrate electron reservoir induces an additional electron density flow to graphene decorated with the acceptor monolayer. Remarkably, graphene on Cu is n-doped and remains n-doped upon F<inf>6</inf>TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F<inf>6</inf>TCNNQ monolayer atop, the effect being more pronounced (~1.3 eV) on Cu compared to quartz (~1.0 eV) because of the larger electrostatic potential drop associated with the long-distance graphene-mediated Cu-F<inf>6</inf>TCNNQ electron transfer. We thus provide a means to realize high work function surfaces for both p- and n-type doped graphene. (Graph Presented).