The Magnetic Properties Of Fluorine-doped Defective Graphene

Graphene has long spin diffusion length due to weak spin orbit interaction, which makes it very attractive for novel spintronic devices. The chemical modification of graphene by light elements, which is a possible route to introduce unpaired spins by the hybridization of carbon atoms from sp2 to sp3, can effectively engineer its electronic and magnetic properties. Fluorinated graphene (FG) is verified as wide-gap semiconductor, and have interesting optical and magnetoelectronics properties. More recently, Nair et al. revealed an inefficiency of introducing paramagnetic centres in FG, which may be due to the clustering of F atoms in the perfect pristine graphene. It is reported that graphene nanocrystals exhibit no ferromagnetism but only a weak paramagnetic contribution because of point defects, and the paramagnetism corresponds approximately to one magnetic moment per crystallite. While theoretical analysis of interaction through the indirect charge carriers between localized spins in graphene sheets leaves little doubt of magnetic ordering in graphene-based systems with high intensity of unpaired spin density.In this work, we fluorinated defective graphene-reduced graphene oxide (RGO) and investigated the magnetic properties. In the case of fluorine clustering, a magnetic contribution from the interior of a fluorine cluster is expected to be zero. The magnetic contribution can come only from cluster edges and would be determined by a particular configuration of adatoms near the edges. Hence, in order to obtain graphene with high magnetization, the efficiency of F atoms introducing magnetic moment is to be enhanced. In other words, how to get the high intensity of edge adatoms is greatly important. The innovation of our work is to fluorinate RGO materials with many vacancy defects, which is favorable for getting more small fluorine clusters and increasing the efficiency of introducing magnetic centers during fluorination process. The main results are as follows:1. The RGO with many vacancy defects was obtained by thermal reduction of graphene oxide. The results showed that fluorination of RGO could greatly enhance its magnetization and spin density. Such F-RGO can exhibit a high magnetization of 0.83 emu/g, which is 4.34 times higher than maximum value of the FG reported by Nair. The efficiency can reach high up to one magnetic moment per 100 F atoms, which is explained by that many small F clusters can be preferably formed around vacancies in RGO, and produce a lot of magnetic edge adatoms. Furthermore, it is interesting that an average local magnetic moments of the F-RGO-0.46 with the highest unpaired spin density is 2μB, which may correspond to two magnetically coupled spins. Thus, the introduction of high unpaired spin intensity is the preliminary of the existence of the magnetic ordering, obtaining graphene with high spin intensity is of both fundamental and technological importance.2. The F-RGO sample with high fluorinated degree is annealed at different temperature. Thermal reduction (>300℃) removes fluorine from the skeleton of graphene, and introducing fluorine vacancies in fluorine clusters.The magnetism of the annealed F-RGO samples is greatly enhanced, for the fluorine vacancies in fluorine clusters of graphene can results in local unpaired spins. For instance, the F-RGO-350 sample exhibits a high magnetization of 1.76 emu/g, which is almost quintupling higher than that of F-RGO sample. The efficiency can reach high up to three magnetic moments per 100 F atoms. More importantly, it is found the existence of the ferromagnetic ordering in the annealed F-RGO samples. We infer that the high localized magnetic moments or the weak localization of π electron can avail the magnetic ordering in graphene. Thus, our method offers the easy fabrication of graphene with high-density localized magnetically coupling spins, which can push the way for potential applications in magnetic graphene.3. Fluorinated graphene quantum dots (F-GQDs) with a diameter of ca.1-7 nm and 23.68% F content were successfully synthesized. Different from the ultraviolet luminescent FG accompanied by weak blue emmision, the F-GQDs possess bright blue photoluminescence(PL) with a ca.6.28% quantum yield. One can find that compared to that of GQDs, the maxium PL peak of the F-GQDs located at ca.440 nm on excitation at 320 nm shows a clear red-shift by ca.10 nm in the PL emission spectra. Moreover, it is found that the F-GQDs possess clear upconversion PL properties. The strong upconverted PL of the F-GQDs is excited by visible light, which indicates that F-GQDs may be used as an effective energy-transfer component in photocatalyst design for environmental and energy.