  In monolayer transition metal dichalcogenide semiconductors, valley coherence
degrades rapidly due to a combination of fast scattering and inter-valley
exchange interaction. This leads to a sub-picosecond valley coherence time,
making coherent manipulation of exciton a highly challenging task. Using
monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate
fully valley coherent excitons by observing ~100% degree of linear polarization
in steady state photoluminescence. This is achieved in this unique design
through a combined effect of (a) suppression in exchange interaction due to
enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast
inter-layer transfer to graphene, and (c) operating in the motional narrowing
regime. We disentangle the role of the key parameters affecting valley
coherence by using a combination of calculation (solutions of Bethe-Salpeter
and Maialle-Silva-Sham equations) and a careful choice of design of experiments
using four different stacks with systematic variation of screening and exciton
lifetime. To the best of our knowledge, this is the first report in which the
excitons are found to be valley coherent in the entire lifetime in monolayer
semiconductors, allowing optical readout of valley coherence possible.
