Daniel F. Förster

Is keV ion-induced pattern formation on Si(001) caused by metal impurities?

We present ion beam erosion experiments performed in ultrahigh vacuum using a differentially pumped ion source and taking care that the ion beam hits the Si(001) sample only. Under these conditions no ion beam patterns form on Si for angles theta < or = 45 degrees with respect to the global surface normal using 2 keV Kr+ and fluences of approximately 2 x 10(22) ions m(-2). In fact, the ion beam induces a smoothening of preformed patterns. Simultaneous sputter deposition of stainless steel in this angular range creates a variety of patterns, similar to those previously ascribed to clean ion-beam-induced destabilization of the surface profile. Only for grazing incidence with 60 degrees < or = theta < or = 83 degrees do pronounced ion beam patterns form. It appears that the angular-dependent stability of Si(001) against pattern formation under clean ion beam erosion conditions is related to the angular dependence of the sputtering yield, and not primarily to a curvature-dependent yield as invoked frequently in continuum theory models.

The Backside of Graphene: Manipulating Adsorption by Intercalation

The ease by which graphene is affected through contact with other materials is one of its unique features and defines an integral part of its potential for applications. Here, it will be demonstrated that intercalation, the insertion of atomic layers in between the backside of graphene and the supporting substrate, is an efficient tool to change its interaction with the environment on the frontside. By partial intercalation of graphene on Ir(111) with Eu or Cs we induce strongly n-doped graphene patches through the contact with these intercalants. They coexist with nonintercalated, slightly p-doped graphene patches. We employ these backside doping patterns to directly visualize doping induced binding energy differences of ionic adsorbates to graphene through low-temperature scanning tunneling microscopy. Density functional theory confirms these binding energy differences and shows that they are related to the graphene doping level.

Sheet plasmons in modulated graphene on Ir(111)

The sheet plasmon of graphene on Ir(111) was investigated in this paper by means of high-resolution electron energy loss spectroscopy. The perfect lateral coordination of sp2-hybridized C atoms on a large scale is manifested by brilliant moire diffraction images. However, the modulation of the graphene films caused by hybridization at the interface limits the lifetimes of the collective excitation modes. This modulation within the films can be lowered owing to intercalation of Na. Linear dispersion was found, but surprisingly the overall slope of the dispersion is not dependent on the chemical potential within the graphene films. The dispersion measured for graphene on Ir(111) is almost identical to that measured on SiC(0001), although the carrier densities differ by two orders of magnitude. This contradicts the model that the relevant carrier density for a two-dimensional plasmon is given by (2π)− 1kF2.

Phase coexistence of clusters and islands: europium on graphene

The adsorption and equilibrium surface phases of europium (Eu) on graphene on Ir(111) are investigated in the temperature range from 35 to 400?K and for coverages ranging from a small fraction of a saturated monolayer to the second layer by scanning tunnelling microscopy. Using density functional theory including 4f-shell Coulomb interactions and modelling of electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion and work function is obtained. Most remarkably, at 300?K in an intermediate coverage range a phase of uniformly distributed Eu clusters (size 10?20?atoms) coexists in two-dimensional equilibrium with large Eu-islands in a structure. We argue that the formation of the cluster phase is driven by the interplay of three effects. Firstly, the metallic Eu?Eu binding leads to the local stability of structures. Secondly, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Thirdly, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters.

Europium underneath graphene on Ir(111): Intercalation mechanism, magnetism, and band structure

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a (2×2) or a (3×3)R30∘ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n doping. The hybridization of the Ir surface state S1 with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially, as found by x-ray magnetic circular dichroism. The (2×2) Eu structure displays plain paramagnetic behavior, whereas for the (3×3)R30∘ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.

Mechanical exfoliation of epitaxial graphene on Ir(111) enabled by Br2 intercalation

We show here that Br(2) intercalation is an efficient method to enable exfoliation of epitaxial graphene on metals by adhesive tape. We exemplify this method for high-quality graphene of macroscopic extension on Ir(111). The sample quality and the transfer process are monitored using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), scanning electron microscopy (SEM) and Raman spectroscopy. The developed process provides an opportunity for preparing graphene of strictly monatomic thickness and well-defined orientation including the transfer to poly(ethylene terephthalate) (PET) foil.

Structure and magnetic properties of ultra thin textured EuO films on graphene

We present a straightforward and reproducible method to grow stoichiometric and single phase (100) textured EuO thin films on epitaxial graphene. Depending on coverage, either separated EuO grains or fully closed layers can be prepared. Room temperature preparation followed by annealing in Eu vapor leads to a random distribution of the in-plane orientation, whereas growth under distillation conditions at 720 K induces a fixed orientation with respect to the substrate. Magneto-optical Kerr effect (MOKE) shows that the films are ferromagnetic with an enhanced Curie temperature.