  Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit novel
electrical and optical properties and are emerging as a new platform for
exploring 2D semiconductor physics. Reduced screening in 2D results in
dramatically enhanced electron-electron interactions, which have been predicted
to generate giant bandgap renormalization and excitonic effects. Currently,
however, there is little direct experimental confirmation of such many-body
effects in these materials. Here we present an experimental observation of
extraordinarily large exciton binding energy in a 2D semiconducting TMD. We
accomplished this by determining the single-particle electronic bandgap of
single-layer MoSe2 via scanning tunneling spectroscopy (STS), as well as the
two-particle exciton transition energy via photoluminescence spectroscopy (PL).
These quantities yield an exciton binding energy of 0.55 eV for monolayer
MoSe2, a value that is orders of magnitude larger than what is seen in
conventional 3D semiconductors. This finding is corroborated by our ab initio
GW and Bethe Salpeter equation calculations, which include electron correlation
effects. The renormalized bandgap and large exciton binding observed here will
have a profound impact on electronic and optoelectronic device technologies
based on single-layer semiconducting TMDs.
