Bilayer Graphenes Superlattices
Two dimensional superlattices with tunable periodicity can be created by stacking two single layer graphene with relative rotation. Such superlattices have attracted much theoretical investigation, but their experimental evidences remain elusive. Using chemical vapor deposition, we synthesized twisted bilayer graphene islands with well defined edge termination such that the relative rotation can be obtained from the relative edge misalignment. The band structure of superlattices is revealed by new phonon mode and enhanced 2D line in Raman scattering. The folded phonons arises from the reduced Brillouin zone of the superlattices, while the enhanced 2D Raman intensity is due to the constructive quantum interference between two Raman paths as a result of degenerate Dirac cone. The spectra of the folded phonon and 2D line depend strongly on rotation angle and laser excitation energy. The observation of two dimensional superlattices opens up further basic study and novel applications of graphene.