  Direct, tunable coupling between individually assembled graphene layers is a
next step towards designer two-dimensional (2D) crystal systems, with relevance
for fundamental studies and technological applications. Here we describe the
fabrication and characterization of large-area (> cm^2), coupled bilayer
graphene on SiO2/Si substrates. Stacking two graphene films leads to direct
electronic interactions between layers, where the resulting film properties are
determined by the local twist angle. Polycrystalline bilayer films have a
"stained-glass window" appearance explained by the emergence of a narrow
absorption band in the visible spectrum that depends on twist angle. Direct
measurement of layer orientation via electron diffraction, together with Raman
and optical spectroscopy, confirms the persistence of clean interfaces over
large areas. Finally, we demonstrate that interlayer coupling can be reversibly
turned off through chemical modification, enabling optical-based chemical
detection schemes. Together, these results suggest that individual 2D crystals
can be individually assembled to form electronically coupled systems suitable
for large-scale applications.
