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 degrees C. Furthermore, graphene noncovalent functionalization with F4-TCNQ can be implemented not only via evaporation in ultrahigh vacuum but also by wet chemistry.