Twisted bilayer graphene on SiC(0001̅) grown via molecular beam epitaxy: scanning tunneling microscopy/spectroscopy studies and local environment tight-binding calculations insights

Abstract

Twisted bilayer graphene (TBG) grown on the 4H-SiC(0001̅) surface via molecular beam epitaxy was characterized using scanning tunneling microscopy and spectroscopy (STM/STS), complemented by local environment tight-binding (LETB) calculations. STM imaging showed the preservation of the original step-terrace structure of the substrate surface, indicating a graphene growth of ≤2 ML. Atomically resolved images of the flat regions revealed the honeycomb lattice of the topmost graphene layer and a Moiré superlattice corresponding to a twist angle of θ = 4.4°. STS measurements of the local density of states (LDOS) were compared with LETB-calculated DOS, revealing partial agreement, specifically, the presence of a van Hove singularity (vHS) peak above the Fermi level. However, the absence of a lower energy vHS peak and a significant peak broadening are features that do not correspond with the LETB calculations. These discrepancies are likely caused by STM tip instability, which also restricted the study to investigating only one Moiré superlattice on the surface. These findings highlight the need for further STM tip treatment to probe additional areas on the surface and thus, improve the quantity and quality of STM/STS data.