Reduced graphene oxide is used in sensors, biosensors and energy storage systems. The material is obtained by subjecting graphene oxide to a reduction process which precisely follows a pre-determined pattern. However until very recently the machinery of this process was not well understood. One hypothesis suggests that the material is heated as it absorbs light which results in removal of oxygen groups. According to an alternative view, light directly and selectively breaks carbon-oxygen bonds. There is also a third hypothesis which combines the photothermal and photochemical arguments.
In order to find which of the three hypotheses is correct, the Tomsk Polytech team implemented a joint research project with colleagues from the Mountain University of Leoben (Austria), the Northwestern Polytechnical University (China) and the Szechuan University (China). The experimenters used glass platters covered with ultrathin graphene oxide film, and lasers of different power and wavelength (blue, green and red). In the course of laser treatment the team was monitoring oxide temperature.
They found that after exposure to laser irradiation graphene oxide emits red light for some time, but afterwards its luminescence quickly diminishes with no correlation to the intensity of heating. In particular, when placed under red laser light the material would accumulate an exponentially greater amount of heat compared to blue light, while the intensity of luminescence accompanying reduction was similar in both samples. The researchers interpret it as an argument in favor of the photochemical hypothesis, which says that light selectively breaks carbon-oxygen bonds causing graphene oxide reduction.
The research findings will make it possible to control the properties of graphene oxide without significant heating. In particular, this will enable photolithographic processing of the material for subsequent application in microelectronics.
“The beauty of our strategy to control graphene oxide fluorescence is in its precision – by fine tuning laser power rate, on the graphene oxide film we can write data which are not visible under an optical microscope. But even more important is fundamental understanding of the mechanisms that shape the process. This knowledge will help predict behavior of the material under a variety of conditions to inform technological innovation,” says Yevgeny Sheremet, PhD, Professor at the Research School of Chemistry & Applied Biomedical Sciences, Tomsk Polytechnic University, as cited by the Russian Science Foundation.
