  Transition metal dichalcogenide (TMDC) monolayer has recently emerged as an
important two-dimensional semiconductor with promising potentials for
electronic and optoelectronic devices. Unlike semi-metallic graphene, layered
TMDC has a sizable band gap. More interestingly, when thinned down to a
monolayer, TMDC transforms from an indirect bandgap to a direct bandgap
semiconductor, exhibiting a number of intriguing optical phenomena such as
valley selective circular dichroism, doping dependent charged excitons, and
strong photocurrent responses. However, the fundamental mechanism underlying
such a strong light-matter interaction is still under intensive investigation.
The observed optical resonance was initially considered to be band-to-band
transitions. In contrast, first-principle calculations predicted a much larger
quasiparticle band gap size and an optical response that is dominated by
excitonic effects. Here, we report experimental evidence of the exciton
dominance mechanism by discovering a series of excitonic dark states in
single-layer WS2 using two-photon excitation spectroscopy. In combination with
GW-BSE theory, we find the excitons are Wannier excitons in nature but possess
extraordinarily large binding energy (~0.7 eV), leading to a quasiparticle band
gap of 2.7 eV. These strongly bound exciton states are observed stable even at
room temperature. We reveal an exciton series in significant deviation from
hydrogen models, with a novel inverse energy dependence on the orbital angular
momentum. These excitonic energy levels are experimentally found robust against
environmental perturbations. The discovery of excitonic dark states and
exceptionally large binding energy not only sheds light on the importance of
many-electron effects in this two-dimensional gapped system, but also holds
exciting potentials for the device application of TMDC monolayers and their
heterostructures.
