Work function modulation of graphene with binary mixture of Cu and C60F36

Abstract

Graphene is a promising candidate for flexible, transparent electrodes in organic electronic devices. In order to achieve enhanced device performance, work function (WF) modulation of graphene for better electric contact at the electrode/active materials interface is of significant importance. Herein, we have demonstrated the effective modulation of graphene WF with binary mixture of Cu and C60F36 and the highest WF of ∼6.04 eV is obtained after in situ thermal annealing at 150 °C. The ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurement indicate that the growth of Cu on C60F36-modified graphene leads to C–F bonds breaking of C60F36 and the formation of Cu–F bonds on the basal plane of graphene, which is responsible for the large WF increase of graphene. The mechanism for the WF increase of binary mixture of Cu and C60F36 modified graphene (BMG) has been corroborated by density-functional theoretical (DFT) calculations, which shows that the C–F bond breaking of C60F36 is facilitated by Cu adatoms, and the WF increase can be attributed to the formation of surface dipole layer with the direction pointing from vacuum to graphene, which is consistent with UPS and XPS results. Moreover, our results indicate that other 3d transition metal adatoms (Ti, V, Fe, Co, and Ni) on graphene can also facilitate the C–F bond breaking of C60F36 and the formation of surface dipole layer, resulting in the WF increase of graphene.