| AVS 56th International Symposium & Exhibition | |
| Graphene Topical Conference | Wednesday Sessions |
| Session GR+AS-WeA |
| Session: | Graphene: Surface Characterization |
| Presenter: | A.L. Vazquez de Parga, UAM & IMDEA Nanociencia, Spain |
| Authors: | B. Borca, Universidad Autonoma de Madrid, Spain S. Barja, Universidad Autonoma de Madrid, Spain M. Garnica, IMDEA Nanociencia, Spain J.J. Hinarejos, Universidad Autonoma de Madrid, Spain A.L. Vazquez de Parga, UAM & IMDEA Nanociencia, Spain R. Miranda, UAM & IMDEA Nanociencia, Spain |
| Correspondent: | Click to Email |
By means of Scanning Tunneling Microscopy/Spectroscopy (STM/STS) we investigate the electronic and structural modulation of epitaxial graphene grown on Ru(0001). The difference in lattice parameter between graphene and Ru(0001) induces in the graphene overlayer a Moiré pattern with hexagonal order and a lateral periodicity of around 3nm. The bonding with the substrate occurs through the hybridization of C p-states with Ru d states. Photoelectron spectroscopy shows that the bonding between the graphene and the metallic substrate is not carbidic and the graphene is doped with electrons from the substrate [1].
The hybridization between the carbon and ruthenium atoms changes inside the unit cell [2]. Measuring dI/dV maps we observe inhomogeneities in the charge distribution, i.e., electron pockets, in some areas of the ripples. This inhomogeneity can be understood with the help of a tight-binding model which incorporates a periodic potential associated with the structural ripples that induces a shift of the electronic levels and a corresponding charge transfer from conduction to valence bands for some atoms and the opposite in the others [3].
The influence of the modulated electronic structure in the STM images is quite strong. Large differences in corrugation values were measured in the STM images taken exactly in the same spot and changing the bias voltage applied between tip and sample. A compilation of data measured with different tips and different samples show that the apparent corrugation of the Moiré superstructure is essentially constant (0.1 nm) in the interval from -3V to -1V and diminish as the voltage goes from -1V up to +2V (0.03 nm). For a bias voltage higher than +2.5V, the contrast of the Moiré is inverted. By means of STS we measured, spatially resolved, the surface unoccupied density of states. The dI/dV spectra show that the contrast inversion is due to the presence of a strong peak at 3V above the Fermi level in the lower areas of the Moiré structure.
These results demonstrate that the electronic effects in this system are strong enough to overcome the actual geometric corrugation of the graphene layer.
[1] F. J. Himpsel et al., Surf. Sci. Lett. 115, L159 (1982)
[2] A.B. Preobrajenski et al., Phys. Rev. B 78, 073401 (2008)
[3] A. L. Vázquez de Parga et al., Phys. Rev. Lett. 100, 05680 (2008)