Epoxy to Carbonyl Group Conversion in Graphene Oxide Thin Films: Effect on Structural and Luminescent Characteristics

The conversion of epoxy to carbonyl group in graphene oxide (GO) thin films has been carried out via oxygen plasma treatment, and the effects of this conversion on structural and optical properties were investigated. Hydrophilicity of the prepared GO solution allows it to be uniformly deposited onto substrates in the form of thin films. High-resolution transmission electron microscopy and electron diffraction analysis confirmed 4-5 layers of the graphene oxide layers which are polycrystalline in structure, and the oxygen plasma treatment results in short-range order crystallization in graphene oxide films with an increase in interplanar spacing which can be attributed to the presence of oxygen functional groups on the graphene oxide layers. Electron energy loss spectroscopy (EELS) and Raman spectroscopy confirm the presence of the sp(2) and sp(3) hybridization due to the disordered crystal structures of the carbon atoms results from oxidation, and XPS analysis shows that epoxy pairs convert to more stable C=O and O-C=O groups with oxygen plasma treatment. The broad energy level distribution resulting from the broad size distribution of the sp(2) clusters produces excitation-dependent photoluminescent (PL) emission in a broad wavelength range from 400 to 650 nm. Our results suggest that as oxygen pressure increases, there is a change from epoxide to carbonyl linkages which also resulted in variation in PL emission.