  Monolayer transition metal dichalcogenide semiconductors host strongly bound
two-dimensional excitonic complexes, and form an excellent platform for probing
many-body physics through manipulation of Coulomb interaction. Quantum confined
Stark effect is one of the routes to dynamically tune the emission line of
these excitonic complexes. In this work, using a high quality
graphene/hBN/WS$_2$/hBN/Au vertical heterojunction, we demonstrate for the
first time, 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 non-monotonic Stark shift with electric
field arises from a competition between the conventional quantum confined Stark
effect driven red shift and a suppressed binding energy driven anomalous blue
shift 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
Bethe-Salpeter Equation that uses seven-band per spin tight binding
Hamiltonian. The findings have important implications in probing many-body
interaction in the two dimension as well as in developing layered semiconductor
based tunable optoelectronic devices.
