By combining ion beam experiments and atomistic simulations we study the production of defects in graphene on Ir(111) under grazing incidence of low energy Xe ions. We demonstrate that the ions are channeled in between graphene and the substrate, giving rise to chains of vacancy clusters with their edges bending down toward the substrate. These clusters self-organize to a graphene nanomesh via thermally activated diffusion as their formation energy varies within the graphene moiré supercell.

Free electron like image potential states are observed in scanning tunneling spectroscopy on graphene quantum dots on Ir(111) acting as potential wells. The spectrum strongly depends on the size of the nanostructure as well as on the spatial position on top, indicating lateral confinement. Analysis of the substructure of the first state by spatial mapping of constant energy local density of states reveals characteristic patterns of confined states. The most pronounced state is not the ground state, but an excited state with a favorable combination of local density of states and parallel momentum transfer in the tunneling process. Chemical gating tunes the confining potential by changing the local workfunction. Our experimental determination of this workfunction allows to deduce the associated shift of the Dirac point.PACS numbers: 73.63.Hs, 73.22.Pr, Confinement of electrons in nanostructures leads to quantum size effects as a size-dependent electronic structure and atom-like states (characterized by a set of quantum numbers). Recently, first experiments regarding the confinement of image potential states (IPSs) using the spatial resolution of the scanning tunneling microscope (STM) have been performed [1-6], transcending pioneering studies based on two photon photoemission (2PPE) [7,8]. IPSs are unoccupied states in an attractive image charge Coulomb potential between the Fermi level E F and the vacuum level E F + Φ given by the local work function Φ. Perpendicular to the surface they feature a hydrogen-like spectrum (characterized by a quantum number n) which converges to E F + Φ [9], parallel to it a two dimensional electron gas (2DEG) forms with a continuous distribution of parallel momentum k for the case of extended systems. The resulting states can be labeled Ψ (n) (k) with energies E (n) (k). In STM, IPSs appear as peaks in the local density of states (LDOS) measured while retracting the tip from the surface. As they are Stark-shifted due to the electric field between tip and sample [10] they are often referred to as field emission resonances (FERs).Confinement effects for IPSs can be induced by nanostructures fulfilling four conditions: (i) the corresponding potential well must have a sufficient depth of ∆Φ = Φ out − Φ in [2], a large ∆Φ provides strong confinement; (ii) a well-defined shape; (iii) an established preparation that allows to adjust the size in a wide range; (iv) stability under the high STM bias voltage U . Whereas previous work provides fascinating first insights into quantum size effects for IPSs, no study yet matches all four conditions: In [3] a first hint at a size dependence of the energies of IPSs confined to Co islands on Au(111) is visible, however here the size variation was less than an order of magnitude. Atom-like patterns have been observed above stacking-fault tetrahedra on Ag(111) [5], still ∆Φ is so small that the resulting weak confinement only acts on the IPSs lowest in energy. The system NaCl on metal is promising as it shows a large ∆Φ. However, up to now there is no...

We observe spatial confinement of Dirac states on epitaxial graphene quantum dots with low-temperature scanning tunneling microscopy after using oxygen as an intercalant to suppress the surface state of Ir(111) and to effectively decouple graphene from its metal substrate. We analyze the confined electronic states with a relativistic particle-in-a-box model and find a linear dispersion relation. The oxygen-intercalated graphene is p doped [E D = (0.64 ± 0.07) eV] and has a Fermi velocity close to the one of free-standing graphene [v F = (0.96 ± 0.07) × 10 6 m/s].

Using scanning tunneling microscopy, the oxygen adsorbate superstructures on bare Ir(111) are identified and compared to the ones formed by intercalation in between graphene and the Ir(111) substrate. For bare Ir(111) we observe O-(2 × 2) and O-(2 × 1) structures, thereby clarifying a persistent uncertainty about the existence of these structures and the role of defects for their stability. For the case of graphene-covered Ir(111), oxygen intercalation superstructures can be imaged through the graphene monolayer by choosing proper tunneling conditions. Depending on the pressure, temperature and duration of O2 exposure as well as on the graphene morphology, O-(2 × 2), O-(√3×√3)-R30°, O-(2 × 1) and O-(2√3 × 2√3)-R30° superstructures with respect to Ir(111) are observed under the graphene cover. Two of these structures, the O-(√3 × √3)-R30° and the (2√3 × 2√3)-R30° structure are only observed when the graphene layer is on top. Phase coexistence and formation conditions of the intercalation structures between graphene and Ir(111) are analyzed. The experimental results are compared to density functional theory calculations including dispersive forces. The existence of these phases under graphene and their absence on bare Ir(111) are discussed in terms of possible changes in the adsorbate-substrate interaction due to the presence of the graphene cover.

We present direct experimental evidence of broken chirality in graphene by analyzing electron scattering processes at energies ranging from the linear (Dirac-like) to the strongly trigonally warped region. Furthermore, we are able to measure the energy of the van Hove singularity at the M point of the conduction band. Our data show a very good agreement with theoretical calculations for free-standing graphene. We identify a new intravalley scattering channel activated in case of a strongly trigonally warped constant energy contour, which is not suppressed by chirality. Finally, we compare our experimental findings with T-matrix simulations with and without the presence of a pseudomagnetic field and suggest that higher order electron hopping effects are a key factor in breaking the chirality near to the van Hove singularity.