Graphene today is one of the most intensively studied materials due to unique massless behaviour of its charge carriers, high Fermi velocity, strengh and stability of carbon honeycomb lattice.
However, there are several poorly studied issues and challenges for graphene-based technology. Some of them are: possibility of band-gap creation and thus turning graphene into semiconductor retaining its peculiar electronic structure is complicated; the interaction between twisted graphene layers were arguably sustained to be absent using angle resolved photoemission spectroscopy, which is controversial to what is observed by STM; the use of graphene for spintronics or pseudospintronics due to its internal inversion symmetry is also an open question; and most obvious issue is the growth, production and reactive manipulation of graphene interaction with growth substrate and therefore graphene electronic properties such as, for example, doping.
We address the above issues using micro-ARPES end-station at SpectroMicroscopy beamline at Elettra Synchrotron which allows acquiring angle-resolved spectra from particularly small areas thanks to focused VUV beam (submicron beam diameter).
Our first part of the work concerns the electronic properties of the twisted multilayer graphene on SiC. We performed set of measurements for differently stacked few-layer graphene and observed influence of the twist angle on the interlayer coupling. Our measurements confirmed the presence of van Howe singularities (vHs) and dependence of their position on the twist angle as well as velocity renormalization previously seen by STS. We also observed presence of locally flat bands in the vicinity of the interacting π-bands which could be used in future for simple realization of 1D conductivity.
Further we carried out lithium intercalation leading to the shift of the π-bands to lower kinetic energies, but with preservation of vHs’s. Moreover, we observed the splitting of the Dirac cone of the middle layer in case of doping of trilayer graphene. The π-band of the middle layer became split in two coaxial cones, which can be explained by the influence of the electrostatic field from the intercalated atoms on the chirally behaving charge carriers in graphene leading to Rashba type pseudo-spin splitting.
Second part of the work was the synthesis of graphene on Ru(0001) using thermal cracking of ethylene and further oxygen intercalation. It was observed that relatively high amount of ethylene in the chamber leads to the formation of graphene layer on the surface and some excess of carbon atoms on it, that cause n-doping of graphene. Formation of the carbon layer by segregation of the carbon from bulk to the surface during anneal creates strongly coupled carbon layer that even shows no Dirac cones. After oxygen treatment such layer partly decouples and some parts of the same flake demonstrate p-doping while others are n-doped.