  Two-dimensional transition-metal-dichalcogenide semiconductors have emerged
as promising candidates for optoelectronic devices with unprecedented
properties and ultra-compact performances. However atomically thin materials
are highly sensitive to surrounding dielectric media, which imposes severe
limitations to their practical applicability. Hence for their suitable
integration into devices, the development of reliable encapsulation procedures
that preserve their physical properties are required. Here, the excitonic
photoluminescence of WSe2 monolayer flakes is assessed, at room temperature and
10 K, on mechanically exfoliated flakes encapsulated with SiOx and AlxOy layers
employing chemical and physical deposition techniques. Conformal flakes coating
on untreated - non-functionalized - flakes is successfully demonstrated by all
the techniques except for atomic layer deposition, where a cluster-like oxide
coating is observed. No significant compositional or strain state changes in
the flakes are detected upon encapsulation by any of the techniques.
Remarkably, our results evidence that the flakes' optical emission is strongly
influenced by the quality of the encapsulating oxide - stoichiometry -. When
the encapsulation is carried out with slightly sub-stoichiometric oxides two
remarkable phenomena are observed. First, there is a clear electrical doping of
the monolayers that is revealed through a dominant trion - charged exciton -
room-temperature photoluminescence. Second, a strong decrease of the monolayers
optical emission is measured attributed to non-radiative recombination
processes and/or carriers transfer from the flake to the oxide. Power- and
temperature-dependent photoluminescence measurements further confirm that
stoichiometric oxides obtained by physical deposition lead to a successful
encapsulation.
