  Graphene, a two-dimensional honeycomb lattice of carbon atoms, is of great
interest in (opto)electronics and plasmonics and can be obtained by means of
diverse fabrication techniques, among which chemical vapor deposition (CVD) is
one of the most promising for technological applications. The electronic and
mechanical properties of CVD-grown graphene depend in large part on the
characteristics of the grain boundaries. However, the physical properties of
these grain boundaries remain challenging to characterize directly and
conveniently. Here, we show that it is possible to visualize and investigate
the grain boundaries in CVD-grown graphene using an infrared nano-imaging
technique. We harness surface plasmons that are reflected and scattered by the
graphene grain boundaries, thus causing plasmon interference. By recording and
analyzing the interference patterns, we can map grain boundaries for a large
area CVD-grown graphene film and probe the electronic properties of individual
grain boundaries. Quantitative analysis reveals that grain boundaries form
electronic barriers that obstruct both electrical transport and plasmon
propagation. The effective width of these barriers (~10-20 nm) depends on the
electronic screening and it is on the order of the Fermi wavelength of
graphene. These results uncover a microscopic mechanism that is responsible for
the low electron mobility observed in CVD-grown graphene, and suggest the
possibility of using electronic barriers to realize tunable plasmon reflectors
and phase retarders in future graphene-based plasmonic circuits.
