Highly Anisotropic Dirac Cones in Epitaxial Graphene Modulated by an Island Superlattice

Abstract

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...

Combined ARPES and STM study of Pb/Au(111) Moiré structure: One overlayer, two symmetries

“…3 exhibit replicas with the periodicity of the Moiré structure. This contrasts, e.g., with recent observations for graphene on a metallic substrate, 15,16 where the Moiré structure gives rise to folded bands, with intensities proportional to the strength of the superlattice potential.…”

“…[1][2][3] Characteristic signatures of superperiodicities have been observed by ARPES in bulk materials [4][5][6] and at ordered interfaces. [7][8][9][10][11][12][13][14][15][16] Lead overlayers on both metallic and semiconducting substrates are especially interesting in this respect. It is found that, due to their high lateral stiffness, monolayers (ML) of Pb form dense Moiré superstructures.…”

Sheet plasmons in modulated graphene on Ir(111)

“…Furthermore, due to the chemical inertness of graphene itself and the low solubility of carbon in the Ir bulk, the growth process is self-limiting and always leads to one, and only one, layer of graphene [12]. The chemical interaction between the Ir substrate and graphene is rather weak, as evidenced by a nearly unperturbed band structure [13,14] and a long vertical distance between the C and Ir atoms [20], in contrast to other systems such as graphene/Ni(111) or graphene/Ru(0001), where the strong hybridization of electronic bands opens up a gap in the Dirac cone. In addition, graphene on Ir(111) is nearly undoped, whereas for example graphene on SiC(0001) shows strong n-doping as a result of charge transfer between graphene and the substrate.…”

Epitaxial Growth of Thin Films

“…For the coverage used in this experiment, the mean cluster size is 13 atoms. Apart from the interest of such clusters in catalysis and for seeding of magnetic materials, these clusters act as point scatterers for the electrons in the underlying graphene and lead to a band gap of 0.34 eV, while fully preserving the graphene electron group velocities, a necessary condition for the high charge carrier mobilities required in applications [34]. A similar effect has been obtained by H adsorption on specific sites of the moiré pattern [186].…”

“…A topical example is the CVD growth of graphene on single-crystal metal surfaces, which we only briefly discuss here, as more details are given in Chapter 25 of this volume. The carbon containing molecules are, for example, ethylene [31][32][33][34][35] and ethene [36][37][38]. At most metal surfaces, C 2 H 4 deprotonates to ethylidene (C 2 H 3 ) already at 300 K [31,39]; above 430 K, further dehydrogenation occurs, until, at 700 K, only chemisorbed C atoms remain at the surface.…”