Departmental Doctoral Oral Examination (Thesis Title: “Atomic modification of graphene on silicon carbide: adsorption and intercalation”)

Abstract:
Graphene, the first truly 2D material to be isolated, is host to a wealth of remarkable properties. It can be modified in a variety of ways—strained, twisted, stacked, placed on a substrate, decorated with adatoms, etc.—to further enhance these properties or introduce new ones. In this thesis, we use a number of complementary surface characterization techniques to study two methods of modifying epitaxial graphene on a silicon carbide (SiC) substrate via the addition of other atoms.

In the first method, we induce the Kekulé distortion—a periodic distortion of the carbon-carbon bonds in graphene—using a small number of lithium atoms adsorbed on the graphene surface. Mediated by the graphene, the adatoms interact over large distances, leading to symmetry breaking between graphene unit cells and a (√3×√3) R30° superstructure. Using angle-resolved photoemission spectroscopy (ARPES), we observe the formation of the superstructure in the appearance of a new Dirac cone in the centre of the Brillouin zone due to band folding. The same superstructure was confirmed by the appearance of new spots in low-energy electron diffraction (LEED) experiments. ARPES data also reveals a gap opening 2Δ = (238±3) meV at the Dirac point. Using a Monte Carlo toy model, we study the importance of deposition parameters in the formation of the Kekulé phase. Finally, we show that this phase is generic to other graphene systems, regardless of charge carrier type or density.

In the second method, we intercalate a layer of copper atoms between the graphene and the SiC substrate by contacting it with copper paste and annealing in vacuum. Using scanning tunnelling microscopy (STM), we observe the formation of 2.2 Å tall islands under the graphene, which exhibit a modification of the observed graphene/SiC superstructure from SiC (6×6) to SiC (6√3×6√3) R30°. The same periodicity is observed in LEED, and the intercalation of copper is further confirmed by the appearance of additional bands in ARPES. This presents a simple method of producing metal-intercalated graphene, without the need for deposition by evaporation.