Tensile and shear interactions for graphene grown on sapphire

Abstract

Despite being at the forefront of materials research for nearly two decades, scalable, high-yield transfer of 2D materials from their growth substrates to suitable target substrates continues to pose severe limitations in their transition to industrial applications. Conventional wet transfer methods are time-expensive and highly involved processes that do not provide an effective means to be automated. Dry transfer processes relying on mechanical delamination circumvent some of the challenges of wet transfer, however most of the current efforts at dry transfer have used Mode I loading which are not easy to scale to wafer sizes. In the present study, we performed dry transfer of graphene from sapphire under a wide range of mode-mixes (combinations of tension and shear), something that had not been accomplished heretofore. Laminated beam type specimens (graphene/sapphire/epoxy/sapphire) were fabricated using CVD graphene on sapphire and tested using a previously developed dual actuator loading device. By varying the ratio of the actuator displacements several fracture mode conditions were explored. We observe a reduction in the overall fracture toughness of the graphene-sapphire interface as we go from pure Mode I (Γ_ss ∼ 2J/m² for ψ = 0⁰) to mixed-mode loading (lowest Γ_ss ∼ 0.3 J/m² for ψ = 82⁰). A beam-on elastic foundation model was used in obtaining the adhesion energies and strengths of the graphene-sapphire interface under the range of mode-mixes. The yield of the dry transfers has been characterized using Raman, SEM and AFM microscopy. All the dry transfers explored in this work have shown remarkably high yields consistently greater than 95 %. The results of this study provide key implications in exploring other loading configurations that can be scalable to wafer sizes, such as three-point bending, for carrying out dry transfers of 2D materials.