We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states. DOI: 10.1103/PhysRevLett.105.246803 PACS numbers: 73.20.Àr, 73.21.Cd, 73.22.Pr, 79.60.Ài Graphitic materials have attracted strong scientific interest because they exhibit exotic phenomena such as superconductivity or the anomalous quantum Hall effect [1]. Graphene (gr) is the building block of these materials; it is wrapped up into fullerenes, rolled up into carbon nanotubes, or stacked into 3D graphite. It presents a model system to investigate the influence of many-body interactions on the electron dynamics in these materials. In addition, the exceptional electronic mobility makes graphene a candidate material for next generation electronic devices [2]. Freestanding graphene is a zero-gap semiconductor. Because most electronic applications require a gap between valence and conduction bands, considerable effort has been spent to induce and control the opening of such a band gap [3][4][5][6].A related, and for applications equally relevant, issue is the ability to tailor the band dispersion. The speed with which information can be transmitted along a graphene layer depends on the charge carrier group velocity. Close to the K points, the bands of freestanding graphene have a linear dispersion well described by the relativistic Dirac equation for massless neutrinos. The resulting Dirac cones are trigonally warped due to the chiral nature of graphene charge carriers in the equivalent A and B carbon sublattices [7]. The ability to increase and tailor this anisotropy would open a manifold of new applications. Several theoretical studies suggest that this goal can be reached by applying an external periodic potential with nanometer period giving rise to highly anisotropic Dirac cones [8][9][10][11].Epitaxial graphene layers grown on lattice mismatched close-packed metal substrates, such as Pt (111) Here we demonstrate with angle-resolved photoemission spectroscopy (ARPES) that an Ir cluster superlattice grown on the gr=Irð111Þ-(9:32 AE 0:15 Â 9:32 AE 0:15) moiré structure [13,15] gives rise to significant group velocity and Dirac cone asymmetries. We attribute this to a much stronger periodic potential caused by the cluster superlattice than by the moiré itself. H decorated gr=Irð111Þ has been reported to give rise to band gap opening but not to the Dirac cone asymmetries reported here [6]. We thereby confirm theoretical predictions and present a new way to tailor the directionality of carrier mobilit...

We investigate the effects of Na adsorption on the electronic structure of bare and Ir cluster superlattice covered epitaxial graphene on Ir(111) using angle-resolved photoemission spectroscopy and scanning tunneling microscopy. At Na saturation coverage a massive charge migration from sodium atoms to graphene raises the graphene Fermi level by about 1.4 eV relative to its neutrality point. We find that Na is adsorbed on top of the graphene layer and when coadsorbed onto an Ir cluster superlattice it results in the opening of a large bandgap of Δ Na/Ir/G = 740 meV comparable to the one of Ge and with preserved high group velocity of the charge carriers.

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Using low-energy electron microscopy, we study Co intercalation under graphene grown on Ir(111). Depending on the rotational domain of graphene on which it is deposited, Co is found intercalated at different locations. While intercalated Co is observed preferentially at the substrate step edges below certain rotational domains, it is mostly found close to wrinkles below other domains. These results indicate that curved regions (near substrate atomic steps and wrinkles) of the graphene sheet facilitate Co intercalation and suggest that the strength of the graphene/Ir interaction determines which pathway is energetically more favorable.In view of potential technological appications, the ability to modify and control the properties of a graphene layer has been a central issue since its discovery [1]. The intercalation of foreign atoms or molecules between a graphene sheet and its substrate often affects the electronic and magnetic properties of the considered interface. For example, the intercalation of noble metals [2,3] or hydrogen atoms [4] can be used to reduce the interaction between graphene and its substrate, and even restore the electronic properties of free-standing graphene, while the intercalation of alkali metals is an efficient mean to control the doping level of graphene [5]. Intercalation of a ferromagnetic transition metal can also enhance the net magnetic moment induced in carbon atoms when graphene is in contact with a magnetic surface [6], and is a promising route to fabricate graphene/ferromagnetic metal hybrid structures with perpendicular magnetic anisotropy [7,8].Understanding where and how a foreign species intercalates below graphene is a challenging task, and different scenarios have been proposed. While oxygen intercalates at the free edges of graphene grown on Ru(0001) [9, 10] and on Ir(111) [11], alkali metals instead may intercalate at the substrate step edges or at boundaries between different rotational domains in graphene/Ni(111) [5] and in graphite [12]. Regarding transition metals, the intercalation mechanism remains elusive. While it has been demonstrated that pre-existing defects in graphene, such as vacancies or pentagon-heptagon pairs, reduce the required energy to trigger intercalation [13,14], several recent experimental works have shown that other mechanisms could be at work. In particular, the formation of atomic defects, not pre-existing in the graphene layer but induced by the contact with a transition metal cluster, with subsequent restoring of the carbon-carbon bonds, has been suggested as a possible way for metal intercalation [14][15][16]. In this work, low-energy electron microscopy [17,18] (LEEM) is used to study cobalt intercalation at moderate annealing temperature (about 125 • C) underneath graphene grown on an iridium (111) surface. Depending on the rotational orientation of the graphene domain, we find Co intercalated at differ-