Dynamics of Epitaxial Graphene growth and adsorptions of Cobalt

Abstract

This thesis begins with revisiting the precursor phase of graphene i.e. 6H-SiC(0001)-(6sqrt3x6sqrt3)R30° (hereafter 6sqrt3 for short), followed by probing the graphitisation mechanism adopted by 6H-SiC(0001) and adsorption studies of Co at both room and elevated temperatures. Scanning tunneling microscopy (STM) and photoelectron spectroscopy were used as main characterisation tools. Beginning with the 6sqrt3 phase, considerable amount of Si are found to exist on this surface. Their presence may be the reason the carbon-rich 6sqrt3 is refrained from directly converting to graphene. The graphene growth is found to begin from step edges with simultaneous collapse of the 6sqrt3 surface and three silicon carbide (SiC) bilayers underneath where the later disintegrated into a fresh 6sqrt3-like layer at the interface. This interface acts as a buffer to graphene from direct interaction with bulk SiC. For multilayer graphene formation, the same mechanism repeats again for each monolayer formed. In addition, kinetic analysis reveals an activation energy of 3.0 ± 0.4 eV for the surface to be completely graphitised. This value is close to binding energy of a Si-C bond. At room temperature, Co is physisorbed and form 3-dimensional dome-shaped clusters on graphite, 6sqrt3 and graphene. The presence of 6sqrt3-like layer at the interface is found to influence the nucleation of Co on graphene where scaling analyses reveal a critical nucleus size of i*= 3 for Co/graphene but i*= 0 for Co/graphite. For Co/6sqrt3, smaller but higher density of Co clusters are formed due to impedance of Co diffusion by the corrugation of 6sqrt3. i*= 1 is found for this system. At elevated temperatures, growth of Co on graphene transforms into multilayer flat top islands with diameter as wide as 50 nm. Along with these flat islands, two types of atomic clusters with very contrasting physical appearance are also observed. They are labelled as dim clusters (thickness less than 1 Å) and bright clusters (thickness between 2 to 3 Å). The dim clusters are often found cluttering around the edges of the flat island but not the bright clusters. Post-growth annealing experiments reveal that the flat islands formation is mediated by the dim clusters. Scaling analysis of flat islands shows a single dim cluster can transforms into a flat island i.e. i*= 0. Bright clusters may also aid the formation of flat islands by releasing Co adatoms via dissolution. However its contribution is very limited during growth and is likely to occur significantly during annealing. More interestingly, STM and X-ray photoelectron spectroscopy reveal that the flat islands are sandwiched between the 6sqrt3 interface and graphene surface. These flat islands, as a consequence, stable against ambient oxidation i.e. they remained metallic. Formation of buried Co islands is most likely driven by huge difference in surface energy between Co and graphene. Penetration of Co, however, is kinetically limited at room temperature.