| dc.description.abstract | Graphene, as the first two-dimensional material successfully produced, its excellent properties, from extremely high specific surface to electron mobility, has attracted the attention of researchers in the various field. However, the lack of band gap limits applicability in graphene device. Graphene oxide (GO) engineers band gap through introduction of sp3 bonding. Tuning the electric properties by controlling the degree of oxidation and reduction is one of the most popular method. So far, there are many methods evolving to reduce GO, e.g., chemical, thermal, and optical methods. The degree of reduction affects many electronic properties much, such as electron mobility. Therefore, it is important to effectively remove the oxidized functional groups on graphene. Hence, understanding the mechanism of reduction of GO is necessary for both the application of graphene and interaction of functional groups on a 2D material.
In our previous work, we have shown the reduction dynamics of micron-scaled defective oxidized graphene patterns done on CVD-grown graphene on SiO_2 by scanning probe lithography (SPL), which provides localized functionalization of graphene. These patterns were subsequently reduced by the irradiation of photoelectrons induced by a focused beam of soft x-ray. By in-situ monitoring the chemical configuration of the irradiated defects during the reduction process, the evolution of each oxygen functional group is resolved by scanning photoelectron microscope (SPEM) and x-ray photoelectron spectra (XPS). From the coupled reduction behavior of each bond, we proposed a model that describes the dynamics of reduction with a step-by-step process. That is C=O→C-OH→C-sp^3→C-sp^2. By the result of least-squares fitting, we find the rate constant of each step of reaction and we consider that C-sp^3→C-sp^2, the recovery of sp^2 is the limiting step of the whole process. We can also speculate that this step requires more energy to achieve.
In this study, we want to know the contribution of photoelectrons excited by X-rays from the substrate for the reduction process. Therefore, we select calcium fluoride (CaF_2) which has the slightly larger binding energy than graphene and similar optical properties of SiO_2. Using the appropriate energy of X-ray, it is found that the reduction still happens even without the photoelectrons excited from the substrate. The photoelectrons of graphene itself trigger the reduction, but the approach of reduction changes. We speculate that the kinetic energy of photoelectrons is not enough to achieve repairing sp^2 by sp^3, but directly repaired by oxygen-related bonds. So that, we increase the X-ray energy to excite the photoelectron from CaF_2 with the similar kinetic energy of the photoelectron from the SiO_2 in our original experiment, and investigate the reduction pathway and dynamics of oxidized graphene on different substrates by the photoelectron of different kinetic energy owing to the different X-ray energy.
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