  The discovery of electric field induced bandgap opening in bilayer graphene
opens new door for making semiconducting graphene without aggressive size
scaling or using expensive substrates. However, bilayer graphene samples have
been limited to um size scale thus far, and synthesis of wafer scale bilayer
graphene posts tremendous challenge. Here we report homogeneous bilayer
graphene films over at least 2 inch x 2 inch area, synthesized by chemical
vapor deposition on copper foil and subsequently transferred to arbitrary
substrates. The bilayer nature of graphene film is verified by Raman
spectroscopy, atomic force microscopy (AFM), and transmission electron
microscopy (TEM). Importantly, spatially resolved Raman spectroscopy confirms a
bilayer coverage of over 99%. The homogeneity of the film is further supported
by electrical transport measurements on dual-gate bilayer graphene transistors,
in which bandgap opening is observed in 98% of the devices.
