The Origin of Wrinkles on Transferred Graphene

When two-dimensional graphene is exfoliated from three-dimensional highly oriented pyrolytic graphite (HOPG), ripples or corrugations always exist due to the intrinsic thermal fluctuations. Surface-grown graphenes also exhibit wrinkles, which are larger in dimension and are thought to be caused by the difference in thermal expansion coefficients between graphene and the underlying substrate in the cooling process after high temperature growth. For further characterization and applications, it is necessary to transfer the surface-grown graphenes onto dielectric substrates, and other wrinkles are generated during this process. Here, we focus on the wrinkles of transferred graphene and demonstrate that the surface morphology of the growth substrate is the origin of the new wrinkles which arise in the surface-to-surface transfer process; we call these morphology-induced wrinkles. Based on a careful statistical analysis of thousands of atomic force microscopy (AFM) topographic data, we have concluded that these wrinkles on transferred few-layer graphene (typically 1-3 layers) are determined by both the growth substrate morphology and the transfer process. Depending on the transfer medium and conditions, most of the wrinkles can be either released or preserved. Our work suggests a new route for graphene engineering involving structuring the growth substrate and tailoring the transfer process.