  Janus monolayer transition metal dichalcogenides, where one of the two
chalcogen layers is substituted with a different kind of chalcogen atoms, are
pushing the properties of two dimensional materials into new territories. Yet
only little is known about this new kind of material class, mainly due to the
challenging synthesis. In this work we propose a method to prepare high quality
Janus MoSSe monolayers from as-exfoliated MoSe2 by thermal sulfurization. With
this we aim to pave a way for more exotic Janus monolayers, which have been out
of the experimental reach thus far. The synthesized MoSSe is diligently
characterized by room- and low-temperature Raman and photoluminescence
spectroscopy, atomic force microscopy correlated with Raman mappings, and
time-correlated single-photon counting. The latter providing new information on
the lifetime of excitons in Janus MoSSe monolayers. In addition, we report an
enhanced trion formation at low temperatures and a relatively high excitonic
transition energy that is indicative of less defect states and strain, and
therefore a high sample quality.
