Spectroscopic Investigation And Application Of Monolayer Graphene Halide

Graphene has attracted much attention in last decade due to its unique physical and chemical properties, such as high carrier mobility, high Young’s modulus, high surface area and good optical transparency. Although graphene has many potential applications, the nature of pristine graphene with zero band gap brings some difficulties for its application in the electronic device field. Chemical modification of graphene is an effective route to open the band gap of graphene. The covalent modification of graphene is much reliable to apply in the future electronic device, in contrast with the physisorption of chemical species. Graphene halides, one of graphene derivatives including fluoride, chloride, bromide and iodide, have been recently synthesized by various methods, such as mechanical exfoliation of graphite fluoride, chemical exposure to the XeF2 vapor, F-based gas plasma treatment, and have been studied in applications in high-energy density battery and lubricant. Chlorine doped graphene, however, is mainly synthesized by post-treatment with chlorine gas, which is highly toxic and unsafe. In this thesis, the endeavor has been mainly focused on the controllable plasma-fluorinated monolayer graphene and application for fast responded ammonia gas sensor and one-step synthesis of chlorine doped graphene by plasma enhanced chemical vapor deposition.We have firstly investigated the fluorine concentration of single layer fluorinated graphene(FG). The fluorine concentration can be controlled by the plasma treatment time. Raman results suggest that defects are introduced into monolayer graphene during fluorination. XPS measurement clearly distinguishes that the fluorine content reach the maximum after 20 s treatment and the further plasma treatment decreases the fluorine concentration. Moreover, the plasma-fluorinated graphene is chemically stable in the ambient at room temperature and the fluorine lost less than 20% within a month. Interestingly, the fluorine concentration is also in accordance with the optical transparency of FG films. Although there are some reports about the chemical structure of C-F bond, its chemical environment in graphene fluoride is still ambiguous, which is very necessary for understanding the mechanism of gas sensor application. Near edge X-ray absorption spectroscopy(NEXAFS) was used to investigate the chemical bonding and unoccupied electronic structure of FG and revealed that the fluorine atoms are attached to the outside of graphene matrix to form the covalent bonds.Metal oxide is still the main material applied for gas detecting, such as SnO2 and ZnO. But most of them work under high temperature. Intrinsic graphene has been investigated for detecting NH3 and NOx, but its responsibility and response/recovery time still need to be greatly improved compared with metal oxide. In the third chapter, single layer fluorinated graphene of different fluorine concentration were used to detect 100 ppm NH3 under room temperature and showed better performance than the intrinsic graphene. Moreover, fluorinated graphene with higher fluorine concentration exhibits higher response.Chlorine doped graphene, another kind of graphene halide, was mainly synthesized by two-step method in the literatures, which need the chlorine gas as the rather dangerous reaction source for doping. In the fourth chapter, a simple approach has been developed to synthesize the chlorine doped single layer graphene(Cl-G) by plasma enhanced chemical vapor deposition. Copper foil was simply treated with hydrochloric acid and then CuCl2 formed on the surface was used as Cl doping source under the assistance of plasma treatment during growth. Compared with other two-step methods by post plasma/photochemical treatment of CVD-grown single layer graphene(SLG), one-step Cl-G synthesis approach is quite straightforward and effective.