  The lubricating ability of graphite largely depends on the environmental
humidity, essentially the amount of water in between its layers. In general,
intercalated molecules in layered materials modify their extraordinary
properties by interacting with the layers. To understand the interaction of
intercalated water molecules with graphene layers, we performed Raman
measurements on bi-layer graphene at various humidity levels and observed an
additional peak close to that of the low-frequency layer breathing mode between
two graphene layers. The additional peak is attributed to the vibration between
an intercalated water layer and the graphene layers. We further propose that
the monolayer coverage of water increases between bilayer graphene with
increasing environmental humidity while the interaction between the water layer
and graphene layers remains approximately unchanged, until too much water is
intercalated to keep the monolayer structure, at just over 50\% relative
humidity. Notably, the results suggest that unexpectedly humidity could be an
important factor affecting the properties of layered materials, as it
significantly modifies the interlayer interaction.
