  The optical response of semiconducting monolayer transition-metal
dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable
even at room temperature. However, substrate-related effects such as screening
and disorder in currently available specimens mask many anticipated physical
phenomena and limit device applications of TMDCs. Here, we demonstrate that
that these undesirable effects are strongly suppressed in suspended devices.
Extremely robust (photogain >1,000) and fast (response time <1ms) photoresponse
combined with the high quality of our devices allow us to study, for the first
time, the formation, binding energies, and dissociation mechanisms of excitons
in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions
of peaks in the photocurrent and by comparing them with first-principles
calculations, we obtain binding energies, band gaps and spin-orbit splitting in
monolayer TMDCs. For monolayer MoS2, in particular, we estimate an extremely
large binding energy for band-edge excitons, Ebind > 570meV. Along with
band-edge excitons, we observe excitons associated with a van Hove singularity
of rather unique nature. The analysis of the source-drain voltage dependence of
photocurrent spectra reveals exciton dissociation and photoconversion
mechanisms in TMDCs.
