  The investigation of molecular motions and chemical reactions occurring in a
constricted space has been impeded by difficulty in detection and lack of
reliable model systems. In this work, we report geometric and chemical
manipulation of O2 diffusion and ensuing O2-mediated charge transfer (CT) that
occur in the 2D space between single-layer transition metal dichalcogenides
(TMDs) and dielectric substrates. As a sensitive real-time wide-field imaging
signal, charge-density-dependent photoluminescence (PL) from TMDs was used. The
two sequential processes inducing spatiotemporal PL change could be drastically
accelerated by increasing the interfacial gap size or introducing artificial
defects serving as CT reaction centers. We also show that widely varying CT
kinetics of four TMDs are rate-determined by the degree of hydration required
for the reactions.
