Intercalation of sulfur in epitaxial graphene on ruthenium(0001) studied by means of scanning tunneling microscopy and spectroscopy
This thesis studies the interaction of hydrogen sulfide (H2S) with graphene epitaxially grown on the basal plane of ruthenium (Ru(0001)). Sample growth and characterization is carried out by means of scanning tunneling microscopy and spectroscopy (STM/STS) in ultra-high vacuum (UHV) conditions. Part of the characterization was done at low temperature, and complementary UHV characterization techniques were also used. Epitaxial graphene on Ru(0001) (graphene/Ru(0001)) is grown by catalytic decomposition of ethylene in UHV conditions. Graphene/Ru(0001) shows a strong interaction between graphene and its substrate that results in a topographic and electronic modulation emerging as a moiré pattern due to the lattice mismatch between both materials. The resulting graphene layer is strongly bound to Ru(0001) and highly doped (n-type), losing its semi-metallic character and its characteristic linear dispersion relation. STM images reveal that exposing graphene/Ru(0001) to H2S results in the intercalation of sulfur between graphene and its substrate. The experiments suggest that intercalation occurs via the formation of local defects on the graphene’s surface. The intercalated sulfur atoms are structured forming different reconstructions which are described and analyzed. In order to complement the geometrical description of the intercalated sulfur layer, experiments on the adsorption of sulfur on Ru(0001) have been carried out. The different reconstructions of the intercalated system alter the properties of the graphene layer on top. The surface density of sulfur atoms and their geometrical characteristics reduce the interaction between graphene and Ru(0001) in different ways, as is evident from the corrugation reduction of the moiré in STM experiments. In order to obtain a deeper insight on the influence of the geometrical configurations of sulfur on the system’s properties we performed low-temperature STS experiments. They show the emergence of a series of evenly spaced resonances close to the Fermi level after H2S exposure. The shift of these resonances towards higher energies with increasing sulfur density suggest the reduction of the doping of the intercalated graphene with respect to graphene/Ru(0001). The origin of this phenomenon is not clear, but some possible explanations are suggested. Lastly, atomic manipulation experiments were carried out using the STM tip. When the density of the intercalated layer is low enough to allow for mobility of the sulfur atoms, geometric patterns were drawn on the surface by moving the intercalated atoms in such a way that the strong graphene-Ru interaction is recovered at specific places.
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