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 morphologyinduced 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.
Wrinkles are often formed on CVD-graphene in an uncontrollable way. By designing the surface morphology of growth substrate together with a suitable transfer technique, we are able to engineer the dimension, density, and orientation of wrinkles on transferred CVD-graphene. Such kind of wrinkle engineering is employed to fabricate highly aligned graphene nanoribbon (GNR) arrays by self-masked plasma-etching. Strictly consistent with the designed wrinkles, the density of GNR arrays varied from ∼0.5 to 5 GNRs/μm, and over 88% GNRs are less than 10 nm in width. Electrical transport measurements of these GNR-based FETs exhibit an on/off ratio of ∼30, suggesting an opened bandgap. Our wrinkle engineering approach allows very easily for a massive production of GNR arrays with bandgap-required widths, which opens a practical pathway for large-scale integrated graphene devices.
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