Charge neutrality and band-gap tuning of epitaxial graphene on SiC by molecular doping

Epitaxial graphene on SiC(0001) suffers from strong intrinsic $n$-type doping. We demonstrate that the excess negative charge can be fully compensated by noncovalently functionalizing graphene with the strong electron-acceptor tetrafluorotetracyanoquinodimethane (F4-TCNQ). Charge neutrality can be reached in monolayer graphene as shown in electron-dispersion spectra from angular-resolved photoemission spectroscopy. In bilayer graphene the band-gap that originates from the SiC/graphene interface dipole increases with increasing F4-TCNQ deposition and, as a consequence of the molecular doping, the Fermi level is shifted into the band-gap. The reduction in the charge-carrier density upon molecular deposition is quantified using electronic Fermi surfaces and Raman spectroscopy. The structural and electronic characteristics of the graphene/F4-TCNQ charge-transfer complex are investigated by x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The doping effect on graphene is preserved in air and is temperature resistant up to $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry.