Inherent strains in chemical-vapor-deposited bilayer graphene on Cu

The electrical properties of bilayer graphene (BLG) are strongly affected by external forces. Here we use Raman spectroscopy to study the inherent strain states of BLG synthesized by chemical vapor deposition method on Cu, by manipulating 13C isotope atoms to substitute part of one layer in AB-stacked and twisted BLG to differentiate the Raman signals from the two layers. Results show that, both layers of twisted BLG are under compression but the compressive strain in the bottom layer is larger, which is caused by the mismatch of coefficients of thermal expansion between graphene and the substrate, whereas for AB-stacked BLG its top layer is under compression and the bottom layer is under tension due to the interlayer dislocations. These experimental data are further confirmed by molecular dynamics simulations. We believe that a clear understanding of the strain states of chemical-vapor-deposited BLG will provide the necessary route to optimize its future technologies especially in electronics.