▎ 摘　　要

Graphene-based plasmonic devices have recently drawn great attention. However, practical limitations in fabrication and device architectures prevent studies from being carried out on the intrinsic properties of graphene and their change by plasmonic structures. The influence of a quasi-infinite object (i.e., the substrate) on graphene, being a single sheet of carbon atoms, and the plasmonic device is overwhelming. To address this and put the intrinsic properties of the graphene-plasmonic nanostructures in focus, we fabricate large-area, freestanding, single-layer graphene-gold (LEG-Au) sandwich structures and Au nanoparticle decorated graphene (formed via thermal treatment) hybrid plasmonic nanostructures. We observed two distinct plasmonic enhancement routes of graphene unique to each structure via surface-enhanced Raman spectroscopy. The localized electronic structure variation in the LEG due to graphene-Au interaction at the nanoscale is mapped using scanning transmission X-ray microscopy. The measurements show an optical density of similar to 0.007, which is the smallest experimentally determined for single-layer graphene thus far. Our results on freestanding graphene Au plasmonic structures provide great insight for the rational design and future fabrication of graphene plasmonic hybrid nanostructures.