Effects of Dielectric Encapsulation on the Optical Properties of WSe2 Monolayers

We present a systematic and comprehensive study of the optical response of mechanically exfoliated WSe2 monolayer flakes encapsulated with dielectric SiOx and AlxOy oxide layers by employing different chemical and physical deposition techniques. Conformal flakes coating is successfully demonstrated by all the techniques except for atomic layer deposition, where a cluster-like oxide coating is observed. Moreover, no significant compositional or strain state changes in the flakes are detected upon encapsulation by any of the techniques except pulsed laser deposition, where swallow implantation of Al2O3 species at the flakes during deposition is detected. A direct comparison of the photoluminescence emission of individual flakes, before and after encapsulation, unequivocally shows a strong influence of the quality of the encapsulating oxide (stoichiometry). Remarkably, their optical emission is similar to the reference uncapped flakes when the encapsulation is performed with stoichiometric oxides. Only when the encapsulation is carried out with sub-stoichiometric oxides two 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, among the investigated oxides and methods, stoichiometric oxides obtained by physical deposition techniques represent the best choice to encapsulate WSe2 monolayers with no significant modification of their optical performance.