Electronic structure of epitaxial single-layer MoS2.

The electronic structure of epitaxial single-layer MoS2 on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a (1.39±0.05) eV direct band gap at K[over ¯] with the valence band top at Γ[over ¯] having a significantly higher binding energy than at K[over ¯]. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at K[over ¯] is found to be lifted by the spin-orbit interaction, leading to a splitting of (145±4) meV. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the potassium doping does not only give rise to a rigid shift of the band structure but also to a distortion, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

Graphene as an anti-corrosion coating layer.

Graphene, a single layer of carbon atoms arranged in an aromatic hexagonal lattice, has recently drawn attention as a potential coating material due to its impermeability, thermodynamic stability, transparency and flexibility. Here, the effectiveness of a model system, a graphene covered Pt(100) surface, for studying the anti-corrosion properties of graphene, has been evaluated. Chemical vapour deposition techniques were used to cover the single crystal surface with a complete layer of high-quality graphene and the surface was characterised after exposure to corrosive environments with scanning tunnelling microscopy (STM) and Raman spectroscopy. Graphene covered Pt samples were exposed to: (i) ambient atmosphere for 6 months at room temperature and 60 °C for 75 min, (ii) Milli-Q water for 14 hours at room temperature and 60 °C for 75 min, and (iii) saltwater (0.513 M NaCl) for 75 min at room temperature and 60 °C. STM provides atomic resolution images, which show that the graphene layer and the underlying surface reconstruction on the Pt(100) surface remain intact over the majority of the surface under all conditions, except exposure to saltwater when the sample is kept at 60 °C. Raman spectroscopy shows a broadening of all graphene related peaks due to hybridisation between the surface Pt d-orbitals and the graphene π-bands. This hybridisation also survives exposure to all environments except saltwater on the hot surface, with the latter leading to peaks more representative of a quasi free-standing graphene layer. A mechanism explaining the corrosive effect of hot saltwater is suggested. Based on these experiments, graphene is proposed to offer protection against corrosion in all tested environments, except saltwater on a hot surface, and Raman spectroscopy is proposed as a useful method for indirectly assessing the chemical state of the Pt surface.

Hydrogen interaction with graphene on Ir(1 1 1): a combined intercalation and functionalization study.

We demonstrate a procedure for obtaining a H-intercalated graphene layer that is found to be chemically decoupled from the underlying metal substrate. Using high-resolution x-ray photoelectron spectroscopy and scanning tunneling microscopy techniques, we reveal that the hydrogen intercalated graphene is p-doped by about 0.28 eV, but also identify structures of interfacial hydrogen. Furthermore, we investigate the reactivity of the decoupled layer towards atomic hydrogen and vibrationally excited molecular hydrogen and compare these results to the case of non-intercalated graphene. We find distinct differences between the two. Finally, we discuss the possibility to form graphane clusters on an iridium substrate by combined intercalation and H atom exposure experiments.

Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene.

Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one distinct region of the moiré supercell. Scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements show that a highly periodic hydrogen functionalized graphene sheet can thus be prepared by controlling the sample temperature (Ts) during hydrogen functionalization. At deposition temperatures of Ts = 645 K and above, hydrogen adsorbs exclusively on the HCP regions of the graphene/Ir(111) moiré structure. This finding is rationalized in terms of a slight preference for hydrogen clusters in the HCP regions over the FCC regions, as found by density functional theory calculations. Angle-resolved photoemission spectroscopy measurements demonstrate that the preferential functionalization of just one region of the moiré supercell results in a band gap opening with very limited associated band broadening. Thus, hydrogenation at elevated sample temperatures provides a pathway to efficient band gap engineering in graphene via the selective functionalization of specific regions of the moiré structure.

Ultrafast Band Structure Control of a Two-Dimensional Heterostructure.

The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure.