Optical reflection spectroscopy for characterizing defects in high-quality CVD graphene on copper

Graphene is known to be an excellent diffusion barrier for practically all gas atoms and molecules including oxygen. Therefore, a defect free graphene grown on copper prevents the oxidation of copper in air even for months. On the other hand, some defects in CVD graphene allow oxygen to penetrate and oxidize copper beneath the graphene. Copper oxide thus formed reduces the reflectivity of visible light especially in the short wavelength range and tarnishes the graphene covered copper. The wavelength dependent reduction on light reflectivity is sensitive to the types, density, and distribution of defects in CVD graphene. In comparison, micro-Raman spectroscopy usually measures Raman scattering at small spots of laser illumination. As long as the laser illuminated area is defect free, the presence of defects in other parts of the graphene is not detected. Since these defects affect adversely the overall electronic properties, an effective and simple means of detecting defects and domain boundaries is desirable. Instead of expensive fabrication of devices for measuring the electron mobility and the Raman scattering characterization, we demonstrate a simple and effective means based on optical refection spectroscopy to effectively differentiate high quality graphene from poor quality graphene by measuring the wavelength dependent reflectivity of graphene on copper at the ambient temperature or at an elevated temperature to accelerate the penetration of oxygen through defects for the oxidation of copper heating the graphene.