Stacking-Order Dependence of Strain in Bilayer Graphene: Implications for High-Performance Electronics

The Cu step bunches formed during the synthesis of graphene by chemical vapor deposition (CVD) have been intensively studied to optimize the electrical and mechanical properties of graphene. For example, it has been reported that the compressive strain due to the mismatch between the thermal expansion coefficients of Cu and graphene tends to be released by forming periodic steps depending on the number of graphene layers. However, the stacking-order dependence of the step bunches in multilayer graphene has not yet been investigated. Here, we show that the twisted bilayer graphene (tBLG) with less compressive strain induces the formation of considerably smaller step bunches compared to the case of AB-stacked bilayer graphene (BLG), as evidenced by atomic force microscopy (AFM) and Raman spectroscopy. It is supposed that interlayer slipping between the weakly coupled tBLG layers weakens mechanical stiffness as well as compressive strain to deform the Cu surface. In addition, we also find that the direction of Cu step bunches depends on the lattice orientation of tBLG. Thus, our findings are expected to provide insights into understanding and improving the electrical and mechanical properties of multilayer CVD graphene for high-performance device applications.