  Despite having outstanding electrical properties, graphene is unsuitable for
optical devices because of its zero band gap. Here, we report two-dimensional
excitonic photoluminescence (PL) from graphene grown on Cu(111) surface, which
shows an unexpected remarkably sharp and strong emission near 3.16 eV
(full-width at half-maximum $\leq$ 3meV) and multiple emissions around 3.18 eV.
As temperature increases, these emissions blue-shift, showing the
characteristic negative thermal coefficient of graphene. Observed PLs originate
from significantly suppressed dispersion of excited electrons in graphene
caused by hybridization of graphene $\pi$ and Cu d orbitals of the 1st and 2nd
Cu layers at a shifted saddle point 0.525(M+K) of Brillouin zone. This finding
provides a new pathway to engineering novel optoelectronic graphene devices,
whilst maintaining the outstanding electrical properties of graphene.
