Twinkling graphene on polycrystalline Cu substrate: A scanning electron microscopy study

An unexpected "twinkling" appearance of partially covered chemical vapor deposition-grown graphene on the Cu substrate, showing strong grain orientation-dependent contrast of graphene - brighter, darker, or even invisible against the oxidized substrate - was observed with a scanning electron microscope. The intricate interplay between the electron channeling contrast, oxidation layer, and the imaging parameters was discussed. Imaging conditions affording stronger mass-thickness contrast was proposed to lessen the twinkling effect and thus enable unambiguous discrimination of graphene from the substrate. The findings reported here will have important implications for the reliable characterization of graphene and other 2D materials as well as the growth mechanism study of graphene.