Structural determination of bilayer graphene on SiC(0001) using synchrotron radiation photoelectron diffraction

In recent years there has been growing interest in the electronic properties of 'few layer' graphene films. Twisted layers, different stacking and register with the substrate result in remarkable unconventional couplings. These distinctive electronic behaviours have been attributed to structural differences, even if only a few structural determinations are available. Here we report the results of a structural study of bilayer graphene on the Si-terminated SiC(0001) surface, investigated using synchrotron radiation-based photoelectron diffraction and complemented by angle-resolved photoemission mapping of the electronic valence bands. Photoelectron diffraction angular distributions of the graphene C 1s component have been measured at different kinetic energies and compared with the results of multiple scattering simulations for model structures. The results confirm that bilayer graphene on SiC(0001) has a layer spacing of 3.48 Å and an AB (Bernal) stacking, with a distance between the C buffer layer and the first graphene layer of 3.24 Å. Our work generalises the use of a versatile and precise diffraction method capable to shed light on the structure of low-dimensional materials

Band-gap expansion in the surface-localized electronic structure of MoS2(0002)

The electronic band structure of MoS2 single crystals has been investigated using angle-resolved photoelectron spectroscopy and first-principles calculations. The orbital symmetry and k dispersion of these electronic states responsible for the direct and the indirect electronic band gaps have been unambiguously determined. By experimentally probing an increase of the electronic band gap, we conclude that a MoS2 (0002) surface localized state exists just below the valence band maximum at the Gamma point. This electronic state originates from the sulfur planes within the topmost layer. Our comprehensive study addresses the surface electronic structure of MoS2 and the role of van der Waals interlayer interactions.open112625Nsciescopu

Shape-resonant superconductivity in nanofilms: from weak to strong coupling

Ultrathin superconductors of different materials are becoming a powerful platform to find mechanisms for enhancement of superconductivity, exploiting shape resonances in different superconducting properties. Here we evaluate the superconducting gap and its spatial profile, the multiple gap components, and the chemical potential, of generic superconducting nanofilms, considering the pairing attraction and its energy scale as tunable parameters, from weak to strong coupling, at fixed electron density. Superconducting properties are evaluated at mean field level as a function of the thickness of the nanofilm, in order to characterize the shape resonances in the superconducting gap. We find that the most pronounced shape resonances are generated for weakly coupled superconductors, while approaching the strong coupling regime the shape resonances are rounded by a mixing of the subbands due to the large energy gaps extending over large energy scales. Finally, we find that the spatial profile, transverse to the nanofilm, of the superconducting gap acquires a flat behavior in the shape resonance region, indicating that a robust and uniform multigap superconducting state can arise at resonance.Comment: 7 pages, 4 figures. Submitted to the Proceedings of the Superstripes 2016 conferenc

STM study of site selective hydrogen adsorption on Si(111) 7×7

Adsorption of atomic hydrogen has been studied by scanning tunneling microscopy (STM) and photoelectron spectroscopy with a focus on the different adsorption sites provided by the Si(111) 7×7 surface. At low temperature, the hydrogen atoms adsorb preferentially on adatoms while at elevated temperatures the rest atoms are the first to become hydrogen terminated. The hydrogen-terminated rest atoms are no longer visible in the STM images and the surrounding adatoms appear brighter compared to the clean 7×7 surface. This indicates that there is a local charge transfer back to the adatoms from the rest atoms. Three kinds of modified triangular subunit cells of the 7×7 reconstruction have been identified corresponding to one, two, and three hydrogen-terminated rest atoms, respectively. A detailed study of the apparent height using STM line profiles through the adatom and rest atom positions on the surface is presented. These line profiles show a characteristic and reproducible variation of the apparent heights of the adatoms for the different kinds of triangular subunit cells and the changes are interpreted in terms of charge transfer. The very local nature of the charge transfer is concluded from the fact that only the hydrogen termination of neighboring rest atoms is significantly affecting the apparent height of an adatom

High-resolution angle-resolved photoemission spectroscopy study of monolayer and bilayer graphene on the C-face of SiC

High-energy and k-space resolution angle-resolved photoemission spectroscopy experiments were achieved on nominally single and bilayer graphene grown by Si-flux assisted molecular beam epitaxy (MBE) on the C-face of SiC. This material shows the same structure as the graphene grown by standard high-temperature annealing of SiC, noticeably the rotational disorder and the very weak electronic coupling between stacked layers. The SiC substrate induces a strong doping by charge transfer, with a Dirac point located 320 meV below the Fermi level for monolayer graphene. The efficient screening by the successive graphene layers results in a reduction of this value to 190 meV for bilayer graphene. The opening of an energy band gap, whose width is inversely dependent on the thickness, is also reported. These measurements emphasize the potentialities of the Si-flux assisted MBE technique, more particularly for homogeneous low thickness graphene growth on the C-face of SiC