Raman spectroscopy for the study of reduction mechanisms and optimization of conductivity in graphene oxide thin films

Highly reduced few-layer graphene oxide films with conductivities of up to 500 S cm^-1 are obtained. The thin films with an optimized compromise between sheet resistance (3.1 kΩ sq^-1) and transparency (around 80% to 90%) are suitable for touch screens and transparent electrodes in OLEDs. We discuss the effects of low temperature annealing and chemical reduction on the properties of the films and present an optimized reduction process that allows the original 2D/G Raman intensity ratio of few-layer graphene to be recovered. The Raman spectrum of graphene oxide is found to be only related to oxygen-free graphene-like regions with Raman bands at 1130 and 3155 cm ^-1 that probably involve C-H vibrations of rings and edges, while a band at 1700 cm^-1 is assigned to irregular rings such as Stone Wales defects. All the bands involve resonant Raman processes and disappear in highly reduced samples. Clear correlations of the D band width with the sp² content in thin films and resistivity have been observed, indicating that this is a good Raman parameter for evaluating the quality of the samples. The structural defects produced by the release of embedded water and some of the oxygen functional groups during annealing are detrimental for intra-grain conductivity but greatly enhance inter-grain connectivity.