Christodoulou, C.; Giannakopoulos, A.; Ligorio, G.; Oehzelt, M.; Tirnpel, M.; Niederhausen, J.; Pasquali, L.; Giglia, A.; Parvez, K.; Müllen, M.; Beljonne, D.; Koch, N.; Nardi, M.V.: Tuning the Electronic Structure of Graphene by Molecular Dopants: Impact of the Substrate. ACS Applied Materials & Interfaces 7 (2015), p. 19134-19144
10.1021/acsami.5b04777

Abstract:
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 (F6TCNNQ) and a graphene layer supported on either a quartz or a copper substrate. We find evidence for fundamentally different charge re-distribution 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 F6TCNNQ 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 ndoped, and remains n-doped upon F6TCNNQ deposition. On both substrates, the work function of graphene increases substantially with a F6TCNNQ 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- F6TCNNQ electron transfer. We thus provide means to realize high work function surfaces for both p- and n-type doped graphene.