  We propose a composite layered structure for tunable, low-loss plasmon
resonances, which con- sists of a noble-metal thin film coated in graphene and
supported on a hexagonal boron nitride (hBN) substrate. We calculate electron
energy loss spectra (EELS) for these structures, and nu- merically demonstrate
that bulk plasmon losses in noble-metal films can be significantly reduced, and
surface coupling enhanced, through the addition of a graphene coating and the
wide-bandgap hBN substrate. Silver films with a trilayer graphene coating and
hBN substrate demonstrated sur- face plasmon-dominant spectral profiles for
metallic layers as thick as 34 nm. A continued-fraction expression for the
effective dielectric function, based on a specular reflection model which
includes boundary interactions, is used to systematically demonstrate plasmon
peak tunability for a variety of configurations. Variations include substrate,
plasmonic metal, and individual layer thickness for each material. Mesoscale
calculation of EELS is performed with individual layer dielectric functions as
input to the effective dielectric function calculation, from which the loss
spectra are directly determined.
