We present a study on the growth and characterization of high-quality single-layer MoS 2 with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS 2 layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements.Spin-and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials.

The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.

VS 2 is a challenging material to prepare stoichiometrically in the bulk, and the single layer has not been successfully isolated before now. Here we report the first realization of single-layer VS 2 , which we have prepared epitaxially with high quality on Au(111) in the octahedral (1T) structure. We find that we can deplete the VS 2 lattice of S by annealing in vacuum so as to create an entirely new twodimensional compound that has no bulk analogue. The transition is reversible upon annealing in an H 2 S gas atmosphere. We report the structural properties of both the stoichiometric and S-depleted compounds on the basis of low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and scanning tunneling microscopy experiments.

Intercalation of C60 molecules at the graphene-substrate interface by annealing leads to amorphous and crystalline structures. A comparison of topography and electronic structure with wrinkles and moiré patterns confirms intercalation. The intercalated molecules imprint a local strain/deformation on the graphene layer whose magnitude is controlled by the intermolecular distance. The crystalline intercalated structure exhibits a superlattice peak in the local density of states. This work provides control of local strain in graphene.

Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB2 make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB2. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.

We present a complete characterization at the nanoscale of the growth and structure of singlelayer tungsten disulfide (WS 2 ) epitaxially grown on Au(111). Following the growth process in real time with fast x-ray photoelectron spectroscopy, we obtain a singly-oriented layer by choosing the proper W evaporation rate and substrate temperature during the growth. Information about the morphology, size and layer stacking of the WS 2 layer were achieved by employing x-ray photoelectron diffraction and low-energy electron microscopy. The strong spin splitting in the valence band of WS 2 coupled with the single-orientation character of the layer make this material the ideal candidate for the exploitation of the spin and valley degrees of freedom.

The spin structure of the valence and conduction bands at the K and K' valleys of single-layer WS2 on Au(111) is determined by spin-and angle-resolved photoemission and inverse photoemission. The bands confining the direct band gap of 1.98 eV are out-of-plane spin polarized with spin-dependent energy splittings of 417 meV in the valence band and 16 meV in the conduction band. The sequence of the spin-split bands is the same in the valence and in the conduction bands and opposite at the K and the K' high-symmetry points. The first observation explains "dark" excitons discussed in optical experiments, the latter points to coupled spin and valley physics in electron transport. The experimentally observed band dispersions are discussed along with band structure calculations for a freestanding single layer and for a single layer on Au(111).

We employ time-and angle-resolved photoemission spectroscopy to study the spin-and valleyselective photoexcitation and dynamics of free carriers at the K and K points in singly-oriented single layer WS2/Au(111). Our results reveal that in the valence band maximum an ultimate valley polarization of free holes of 84 % can be achieved upon excitation with circularly polarized light at room temperature. Notably, we observe a significantly smaller valley polarization for the photoexcited free electrons in the conduction band minimum. Clear differences in the carrier dynamics between electrons and holes imply intervalley scattering processes into dark states being responsible for the efficient depolarization of the excited electron population. arXiv:1907.10553v1 [cond-mat.mtrl-sci]