  Van der Waals (vdW) heterostructures, which are produced by the precise
assemblies of varieties of two-dimensional (2D) materials, have demonstrated
many novel properties and functionalities. Here we report a nano-plasmonic
study of vdW heterostructures that were produced by depositing ordered
molecular layers of pentacene on top of graphene. We find through nano-infrared
(IR) imaging that surface plasmons formed due to the collective oscillations of
Dirac fermions in graphene are highly sensitive to the adjacent pentacene
layers. In particular, the plasmon wavelength declines systematically but
nonlinearly with increasing pentacene thickness. Further analysis and density
functional theory (DFT) calculations indicate that the observed peculiar
thickness dependence is mainly due to the tunneling-type electron transfer from
pentacene to graphene. Our work unveils a new method for tailoring graphene
plasmons and deepens our understanding of the intriguing nano-optical phenomena
due to interlayer couplings in novel vdW heterostructures.
