Controlled thermodynamics for tunable electron doping of graphene on Ir(111)

The electronic properties and surface structures of K-doped graphene supported on Ir(111) are characterized as a function of temperature and coverage by combining low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory (DFT) calculations. Deposition of K on graphene at room temperature (RT) yields a stable (root 3 x root 3) R30 degrees surface structure having an intrinsic electron doping that shifts the graphene Dirac point by E-D = 1.30 eV below the Fermi level. Keeping the graphene substrate at 80 K during deposition generates instead a (2 x 2) phase, which is stable until full monolayer coverage. Further deposition of K followed by RT annealing develops a double-layer K-doped graphene that effectively doubles the K coverage and the related charge transfer, as well as maximizing the doping level (E-D = 1.61 eV). The measured electron doping and the surface reconstructions are rationalized by DFT calculations. These indicate a large thermodynamic driving force for K intercalation below the graphene layer. The electron doping and Dirac point shifts calculated for the different structures are in agreement with the experimental measurements. In particular, the K-4s bands are shown to be sensitive to both the K intercalation and periodicity and are therefore suggested as a fingerprint for the location and ordering of the K dopants.