  Monolayer transition metal dichalcogenides (TMDs) possess superior optical
properties, including the valley degree of freedom that can be accessed through
the excitation light of certain helicity. While WS2 and WSe2 are known for
their excellent valley polarization due to the strong spin-orbit coupling, the
optical bandgap is limited by the ability to choose from only these two
materials. This limitation can be overcome through the monolayer alloy
semiconductor, WS2xSe2(1-x), which promises an atomically thin semiconductor
with tunable bandgap. In this work, we show that the high-quality BN
encapsulated monolayer WS0.6Se1.4 inherits the superior optical properties of
tungsten-based TMDs, including a trion splitting of ~ 6 meV and valley
polarization as high as ~60%. In particular, we demonstrate for the first time
the emerging and gate-tunable interlayer electron-phonon coupling in the
BN/WS0.6Se1.4/BN van der Waals heterostructure, which renders the otherwise
optically silent Raman modes visible. In addition, the emerging Raman signals
can be drastically enhanced by the resonant coupling to the 2s state of the
monolayer WS0.6Se1.4 A exciton. The BN/WS2xSe2(1-x)/BN van der Waals
heterostructure with a tunable bandgap thus provides an exciting platform for
exploring the valley degree of freedom and emerging excitonic physics in
two-dimension.
