We investigate the adsorption of graphene sheets on h-BN substrates by means of first-principles calculations in the framework of adiabatic connection fluctuation-dissipation theory in the random phase approximation. We obtain adhesion energies for different crystallographic stacking configurations and show that the interlayer bonding is due to long-range van der Waals forces. The interplay of elastic and adhesion energies is shown to lead to stacking disorder and moiré structures. Band structure calculations reveal substrate induced mass terms in graphene which change their sign with the stacking configuration. The dispersion, absolute band gaps and the real space shape of the low energy electronic states in the moiré structures are discussed. We find that the absolute band gaps in the moiré structures are at least an order of magnitude smaller than the maximum local values of the mass term. Our results are in agreement with recent STM experiments.

We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

We present a joint theoretical and experimental investigation of charge doping and electronic potential landscapes in hybrid structures composed of graphene and semiconducting single layer MoS2. From first-principles simulations we find electron doping of graphene due to the presence of rhenium impurities in MoS2. Furthermore, we show that MoS2 edges give rise to charge reordering and a potential shift in graphene, which can be controlled through external gate voltages. The interplay of edge and impurity effects allows the use of the graphene-MoS2 hybrid as a photodetector. Spatially resolved photocurrent signals can be used to resolve potential gradients and local doping levels in the sample.Being a truly two-dimensional material [1], graphene can be integrated into hybrid structures with other 2D crystals such as boron nitride (BN), tungsten disulfide (WS 2 ) or molybdenum disulfide (MoS 2 ) [2-6]. The ability to build 'on demand' complex heterostructures via layer-by-layer integration establishes a whole family of new materials with widely varying characteristics and exciting possibilities for novel 2D nanodevices [7][8][9]. A prerequisite for future electronic applications lies in the understanding of interface effects when different building blocks come together. In particular, the electronic properties of realistic interfaces of graphene and two-dimensional materials present an open problem.In this work, we investigate heterostructures made of graphene and the semiconducting transition metal dichalcogenide MoS 2 , a system that has already been utilized for vertical field-effect transistors [3]. We study how different charge transfer mechanisms control relative Fermi level positions, built-in electric fields and charge reordering at realistic graphene-MoS2 interfaces. The simulated charge and potential landscapes are compared to photovoltaic measurements.In order to investigate the graphene-MoS 2 hybrid structures theoretically, we performed first-principles density functional theory (DFT) simulations [10]. As the lattice constant of isolated graphene is about 23% smaller than the one of isolated MoS 2 , we constructed a supercell consisting of a 5x5 layer graphene (50 C atoms) coated with a 4x4 layer MoS 2 (16 Mo atoms and 32 S atoms) with a stacking as shown in Fig. 1b to account for this lattice mismatch. The remaining lattice mismatch of about 3-4% is reasonably small and compensated by a slight strain of graphene to the MoS 2 lattice constant.The graphene-MoS 2 structure was then fully relaxed, which leads to an equilibrium graphene-MoS 2 distance of 3.35 A in good agreement with Ref.[11] and indicates a weak interlayer bonding of van der Waals-type. Based on this setup, we simulated realistic edge and impurity effects on the electronic properties of graphene-MoS 2 hybrids.We first address the role of impurity effects. To this end, we consider the fully MoS 2 -covered graphene as in Fig. 1b without impurities and compare it to the case with impurities in the MoS 2 . The band diagram of the pristine sy...

The influence of graphene islands on the electronic structure of the Ir(111) surface is investigated. Scanning tunneling spectroscopy (STS) indicates the presence of a two-dimensional electron gas with a binding energy of -160  meV and an effective mass of -0.18me underneath single-layer graphene on the Ir(111) surface. Density functional calculations reveal that the STS features are predominantly due to a holelike surface resonance of the Ir(111) substrate. Nanometer-sized graphene islands act as local gates, which shift and confine the surface resonance.

We have investigated the magnetism of the bare and graphene-covered (111) surface of a Ni single crystal employing three different magnetic imaging techniques and ab initio calculations, covering length scales from the nanometer regime up to several millimeters. With low temperature spinpolarized scanning tunneling microscopy (SP-STM) we find domain walls with widths of 60 -90 nm, which can be moved by small perpendicular magnetic fields. Spin contrast is also achieved on the graphene-covered surface, which means that the electron density in the vacuum above graphene is substantially spin-polarized. In accordance with our ab initio calculations we find an enhanced atomic corrugation with respect to the bare surface, due to the presence of the carbon pz orbitals and as a result of the quenching of Ni surface states. The latter also leads to an inversion of spinpolarization with respect to the pristine surface. Room temperature Kerr microscopy shows a stripe like domain pattern with stripe widths of 3 -6 µm. Applying in-plane-fields, domain walls start to move at about 13 mT and a single domain state is achieved at 140 mT. Via scanning electron microscopy with polarization analysis (SEMPA) a second type of modulation within the stripes is found and identified as 330 nm wide V-lines. Qualitatively, the observed surface domain pattern originates from bulk domains and their quasi-domain branching is driven by stray field reduction.