  We studied the interlayer coupling and decoupling of bilayer graphene (BLG)
by using spatially resolved electron energy loss spectroscopy (EELS) with a
monochromated electron source. We correlated the twist-angle-dependent energy
band hybridization with Moire superlattices and the corresponding optical
absorption peaks. The optical absorption peak originates from the excitonic
transition between the hybridized van Hove singularities (vHSs), which shifts
systematically with the twist angle. We then proved that the BLG decouples when
a monolayer of metal chloride is intercalated in its van der Waals (vdW) gap,
and results in the elimination of the vHS peak.
