Probing Excitonic Dark States in Single-layer Tungsten Disulfide
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.