  The collective excitations of electrons in the bulk or at the surface, namely
plasmons, play an important role in the properties of materials, and have
generated the field of plasmonics. We report the observation of a highly
unusual plasmon mode on the surface of Van der Waals heterostructures of
graphene monolayer on 2D transition metal dichalcogenide substrate. Since the
exponentially decaying fields of surface plasmon wave propagating along
interface is highly sensitive to the ambient refractive index variations, such
heterostructures are useful for ultra-sensitive bio-sensing.
