Graphene has received great attention owing to its superior physical properties, making graphene suitable for multiple applications. Numerous graphene growth techniques have been developed in the past decade to provide scalable high quality graphene. Among these techniques, chemical vapor deposition (CVD) on catalytic metal films holds great promises for a large-scale graphene growth. Even though extensive efforts have been devoted to synthesize high quality graphene, formation of defects. In particular, grain boundaries (GBs) have a dominant effect on properties, motivating extensive efforts to tune the CVD growth process to minimize GB. Rapid imaging of GBs will significantly aid in studies of CVD graphene grain structure. Here we report a straightforward technique to optically observe GBs in CVD-grown graphene via optical microscopy, allowing rapid assessment of graphene quality as well as the number of layers. The local oxidation of copper through the damaged GBs induces an optically discernable color change in the underlying copper due to different extend of oxidation between the two copper regions under grains and GBs. Our observation technique for GBs of graphene paves a path for understanding fundamental mechanisms of graphene growth and efficient quality evaluation of large-scale graphene sheet for mass production.
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