Controlled electrochemical functionalization of epitaxial graphene*

Chemical functionalization is a promising means of modifying graphene for applications ranging from nanoelectronics to transparent electrodes. Various schemes have been demonstrated, but control over functionalization density with well-specified molecules is still a challenge. We report on the controlled electrochemical functionalization of epitaxial graphene with trifluoromethylphenylene (CF$_3$Ph), where the functionalization density was controlled by the electron injection rate. CF$_3$Ph peaks were observed in x-ray photoemission spectroscopy, along with binding energy shifts consistent with bonding between CF$_3$Ph and graphene. A maximum functionalization density of one molecule per six graphene carbons was inferred from the peak intensities. Spectroscopic Raman mapping revealed increasing graphene D:G peak intensity ratios that scaled with increasing functionalization-induced localized defects. While a minimal shift in the $\pi$ orbital structure and the emergence of CF$_3$Ph related features were observed in ultraviolet photoemission spectroscopy, a work function increase by $0.5$ eV in CF$_3$Ph-graphene suggests a shift of the electron distribution towards the CF$_3$ moieties on the surface. This work has positive implications for transparent electrode applications.