  Molecular motions and chemical reactions occurring in constrained space play
key roles in many catalysis and energy storage applications. However, its
understanding 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. The reported findings will be instrumental in designing
novel functional nanostructures and devices.
