  Because of the dominant role of the surface of molecules and their
individuality, molecules behave dis-tinctively in a confined space, which has
far-reaching implications in many physical, chemical and bio-logical systems.
Here, we demonstrate that graphene forms a unique atom-thick interstitial space
that enables the study of molecular diffusion in 2-dimensions with underlying
silica substrates. Raman spec-troscopy visualized intercalation of water from
the edge to the center underneath graphene in real time, which was dictated by
the hydrophilicity of the substrates. In addition, graphene undergoes
reversible deformation to conform to intercalating water clusters or islands.
Atomic force microscopy confirmed that the interfacial water layer is only ca.
3.5 angstroms thick, corresponding to one bilayer unit of normal ice. This
study also demonstrates that oxygen species responsible for the ubiquitous hole
dop-ing are located below graphene. In addition to serving as a transparent
confining wall, graphene and possibly other 2-dimensional materials can be used
as an optical indicator sensitive to interfacial mass transport and charge
transfer.
