= Solution
\b[Use a thin, continuous cap of <hexagonal boron nitride>], preferably assembled without exposing the cleaned MoS$_2$ surface to ambient contamination. It is chemically inert and relatively transparent in the visible range, and can isolate the film from adsorbing water, oxygen and hydrocarbons during optical measurements. A transferred sheet avoids depositing a strongly reactive precursor directly on the monolayer.
The cap must cover the illuminated region and overlap its edges; pinholes, wrinkles and unsealed edges can allow ingress. Trapped residues or bubbles can themselves change the optical response. Record the capped background and account for optical interference and dielectric-environment shifts rather than assuming that encapsulation leaves every spectral peak unchanged. A thin oxide or polymer coating may also provide protection, but deposition damage, optical background and permeability require checking. An atomically thin sheet with a perfect basal plane is not proof that the whole assembled enclosure is sealed.
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