Anomalous Stark shift of excitonic complexes in monolayer WS2

Monolayer transition-metal dichalcogenide (TMDC) semiconductors host strongly bound two-dimensional excitonic complexes, and form an excellent platform for probing many-body physics through manipulation of Coulomb interaction. The quantum confined Stark effect is one of the routes to dynamically tune the emission line of these excitonic complexes. In this paper, using a high-quality graphene/hexagonal boron nitride (hBN)/WS2/hBN/Au vertical heterojunction, we demonstrate an out-of-plane electric-field driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. Such universal nonmonotonic Stark shift with electric field arises from a competition between the conventional quantum confined Stark effect driven redshift and a suppressed binding-energy driven anomalous blueshift of the emission lines, with the latter dominating in the low-field regime. We also find that the encapsulating environment of the monolayer TMDC plays an important role in wave-function spreading, and hence in determining the magnitude of the blue Stark shift. The results for neutral and charged excitonic species are in excellent agreement with calculations from the Bethe-Salpeter equation that use a seven-band per spin tight-binding Hamiltonian. The findings have important implications in probing many-body interaction in two dimensions as well as in developing layered semiconductor based tunable optoelectronic devices.