Roll-to-roll synthesis and patterning of graphene and 2D materials

Graphene has been intensively studied due to their outstanding electrical, mechanical and optical properties, such as high electrical conductivity, mechanical flexibility, and optical transmittance. The key challenges to make commercially viable graphene-based electronic devices are enabling large-area production of high quality graphene and subsequently patterning graphene into desirable structures. With the recent advances in chemical vapor deposition (CVD), large-area growth of graphene by CVD on Cu substrates was successfully demonstrated for industrial applications. However, reliable methods are still required to transfer the large-area graphene sheet to the application substrate and pattern for the desired applications without damaging or leaving undesired residues on the graphene surface. As for the graphene transfer, the wet transfer method using a support layer such as poly(methyl methacrylate) (PMMA) is typically used but generally difficult to scale up, and the surface tension experienced by the floating graphene at the air-water interface causes rippling, rolling, and break of the films during transfer. The complete removal of PMMA residues is also problematic and most of flexible substrates are either dissolved in acetone or cannot withstand the annealing temperature; thus, graphene can only be transferred to a limited number of flexible substrates. On the other hands, the transfer method using a thermal release tape (TRT) is easy to transfer large-area graphene onto flexible or rigid substrates, but inevitably contaminates the transferred graphene with the adhesive from the TRT film. The adhesion of polymer supports (i.e., PMMA and TRT) to the graphene mainly depends on the chemical adhesion of the polymer film. The residual PMMA or adhesive left on the graphene surface is inevitable. Unlike the adhesive-based transfer mechanism such as PMMA or TRT, a dispersive adhesion-based transfer methods have been demonstrated including the work by Allen, M. J. et al. using a soft PDMS stamp. The transfer mechanism there was based on the difference in dispersive adhesion at the PDMS-graphene and graphene-substrate interfaces. For most materials, the PDMS interface is weaker than the substrate interface, due to the extremely low surface energy of PDMS.