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The aim of this work is to study the influence of molecular and atomic adsorbates on graphene/Ni(111) and graphene/Ir(111), towards a chemical functionalization of graphene. Functionalization means here to access and modify properties of graphene, to introduce new properties into graphene by adding new chemical components into the graphene lattice, or to vary the properties of graphene by modifying its substrate. These three pathways of functionalization are studied in the context of water and ammonia adsorption, the photolysis and subsequent oxidation from photolysis of adsorbed molecules, and by introducing a single atomic layer of manganese between graphene and its substrate. The observations and results presented below are observed by means of core level and valence band photoemission spectroscopy, near edge x-ray absorption fine structure spectroscopy, and scanning tunneling microscopy.
The adsorption of water and ammonia can be detected due to the unique electronic structure of graphene, not only on the adsorbate states but also from the substrate level modification. Due to the weak and reversible adsorption, new adsorbate-induced unoccupied electronic states are observed in graphene, using element-specific x-ray absorption spectroscopy. This weak chemical bond between the adsorbate and graphene is only detectable due to the distinct electronic structure of the single atomic graphene layer. The spectral manifestation of these induced electronic states depends on the interaction strength of the adsorbate to the graphene/metal system and permits to estimate its strength.
The incorporation of novel functional groups is achieved by adsorption and photolysis of NO2 and SO2 on graphene/Ir(111). The photolysis of these adsorbates leads to the formation of oxygen atoms on the surface, able to selectively oxidize graphene towards the epoxidic oxygen phase, locally restricted to the adsorption site and under the formation of only a small number of defects. The resulting graphene oxide layer is hence of high quality and is thermally completely reversible towards a high quality graphene layer.
By means of intercalation of manganese between graphene and the Ir(111) substrate, graphene can be functionalized not only locally, such as by oxidation, but by affecting the entire graphene layer. The intercalation process of manganese is observed at room temperature and the intercalation of manganese leads to the formation of a strong anisotropy in the electronic structure of graphene and to an enhanced surface roughness. However, many unique properties, such as the hybridization and the linear dispersion of the graphene π-band, stay intact.
