  Monolayer transition metal dichalcogenides (1L-TMDs) have tremendous
potential as atomically thin, direct bandgap semiconductors that can be used as
convenient building blocks for quantum photonic devices. However, the short
exciton lifetime due to the defect traps and the strong exciton-exciton
interaction in TMDs has significantly limited the efficiency of exciton
emission from this class of materials. Here, we show that exciton-exciton
dipolar field interaction in 1L-WS2 can be effectively screened using an
ultra-flat Au film substrate separated by multilayers of hexagonal boron
nitride. Under this geometry, dipolar exciton-exciton interaction becomes
quadrupole-quadrupole interaction because of effective image dipoles formed
inside the metal. The suppressed exciton-exciton interaction leads to a
significantly improved quantum yield by an order of magnitude, which is also
accompanied by a reduction in the exciton-exciton annihilation (EEA) rate, as
confirmed by time-resolved optical measurements. A semiclassical model
accounting for the screening of the dipole-dipole interaction qualitatively
captures the dependence of EEA on exciton densities. Our results suggest that
fundamental EEA processes in the TMD can be engineered through proximal
metallic screening, which represents a practical approach towards
high-efficiency 2D light emitters.
