Use of Optical Contrast To Estimate the Degree of Reduction of Graphene Oxide

We report an optical contrast study of graphene oxide on 72 nm Al2O3/Si(100) and 300 nm SiO2/Si(100) as a function of its reduction degree. The reduction has been performed by means of ultrahigh vacuum thermal annealing from 25 degrees C (pristine graphene oxide) to 670 degrees C. In parallel to the optical contrast investigation, performed with optical microscopy, the graphene oxide films have been characterized with core level X-ray photoemission spectroscopy and micro-Raman spectroscopy. The optical contrast of graphene oxide (normalized to the one measured for pure graphene) on both substrates ranges from similar to 0.4 to 1.0 for pristine and 670 degrees C annealed graphene oxide, respectively. Optical microscopy and X-ray photoemission spectroscopy data have been cross-correlated, leading to calibration graphs that demonstrate that just by simply measuring the optical contrast of graphene oxide one can determine with very good approximation the fraction of sp(2) hybridized carbon.