  Thermal radiation can be substantially enhanced in the near-field scenario
due to the tunneling of evanescent waves. The monolayer graphene could play a
vital role in this process owning to its strong infrared plasmonic response,
however, which still lacks an experimental verification due to the technical
challenges. Here, we manage to make a direct measurement about plasmon-mediated
thermal radiation between two macroscopic graphene sheets using a custom-made
setup. Super-Planckian radiation with efficiency 4.5 times larger than the
blackbody limit is observed at a 430-nm vacuum gap on insulating silicon
hosting substrates. The positive role of graphene plasmons is further confirmed
on conductive silicon substrates which have strong infrared loss and thermal
emittance. Based on these, a thermophotovoltaic cell made of the
graphene-silicon heterostructure is lastly discussed. The current work
validates the classic thermodynamical theory in treating graphene and also
paves a way to pursue the application of near-field thermal management.
