Sliding-friction-dependent stress at the graphene/LiNbO3 interface around the critical temperature of the stress-free state

The graphene/LiNbO3 structure exists in an interfacial stress-free state at the temperature at which the graphene was transferred onto the LiNbO3 substrate surface. Coupling of a surface acoustic wave with this structure revealed drastic changes in the properties of the propagating elastic wave around the critical temperature of the stress-free state. Three states, namely, tensile stress, stress-free, and compressive stress, were successively observed at the surface of the LiNbO3 substrate as the temperature was increased through the critical point. The interfacial stress increased prior to the occurrence of sliding friction and approached a constant value when the frictional force exceeded the van der Waals interaction between the graphene and LiNbO3. Consequently, the interfacial stress exhibited a step-like temperature dependence around the critical temperature of the stress-free state. The results obtained in this study indicate that the temperature used to prepare graphene layers on a substrate is a crucial parameter owing to the instability of the electrical and mechanical properties of the graphene/substrate in the vicinity of this temperature. Therefore, in the fabrication of graphene-based electronic devices, room temperature should be avoided during the preparation of the graphene layers on the substrate.