  Semiconducting transition metal dichalcogenides (TMDs) are desired as active
materials in optoelectronic devices due to their strong excitonic effects. They
can be exfoliated from their parent layered materials with low-cost and for
mass production via a liquid exfoliation method. However, the device
application of TMDs prepared by liquid phase exfoliation is limited by their
poor photoluminescence quantum efficiencies (PLQE). It is crucial to understand
the reason to low PLQE for their practical device development. Here we evaluate
the quality of monolayer-enriched liquid phase exfoliated (LPE) WS$_2$
dispersions by systematically investigating their optical and photophysical
properties and contrasting with mechanically exfoliated (ME) WS2 monolayers. An
in-depth understanding of the exciton dynamics is gained with ultrafast
pump-probe measurements. We reveal that the energy transfer between monolayer
and few-layers in LPE WS$_2$ dispersions is a substantial reason for their
quenched PL. In addition, we show that LPE WS$_2$ is promising to build high
performance optoelectronic devices with excellent optical quality.
