Obtaining Ferromagnetic Graphene By Nitrogen Doping

Graphene is a two-dimensional network of sp2 hybridized carbon atoms packed into hexagonal structure. Since long-range π-conjugation in graphene yields extraordinary mechanical, thermal, optical, and electrical properties, an enormous effort has been devoted to exploration of its many applications in nanoelectronics, materials science, and condensed-matter physics. Notably, magnetism of graphene and its derivatives is of particular interest since the light weight magnets could open up new ways to design adaptable and flexible information storage systems. Especially, the greatly potential application of graphene-based magnets in spintronics is promising, since graphene has extraordinary carrier mobility and may provide an easy way to integrate spin and molecular electronics. The long spin diffusion lengths and coherent times arising from the weak spin-orbit and hyperfine interactions in graphene can provide ideal conditions for coherent spin manipulation which can act as the next-generation spintronic devices. However, graphene is usually intrinsic non-magnetic and lacks of localized magnetic moments due to a delocalized π bonding network, which limits its applications in spintronic devices. Therefore, synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance.One of the most feasible methods to control the semiconducting properties of graphene is by doping nitrogen, which is used to tailor the optical and electrical properties of graphene. Theoretically, N-doping can induce magnetic moments. In short, a combination of vacancy defects and N atoms may provide a unique way for enhancing the magnetic moment. Moreover, N atoms can effectively increase the localized magnetic moments at the edges. Furthermore, the N atom may also act as a stable attractor to attract other atoms, forming a magnetic defect complex, which can contribute magnetic moment. Thanks to the strong electron affinity, N atoms may act as bound magnetic polarons (BMPs) and trap free electrons, and the electrons trapped in these BMPs act as the mediate for the magnetic coupling of localized magnetic moments, resulting in ferromagnetism. However, experimentally, the magnetic property of NG is almost completely unknown. In this paper, we doped graphene with nitrogen and realized ferromagnetic graphene.Despite the magnetic properties, effort has been devoted to exploration of its electronic and optical properties, one of the effective method is to tune the band gap. In the quest to open and tune an energy gap in graphene, various approaches have been developed to improve the semiconducting properties, exemplified by forming confined geometries of quantum dots and doping graphene. One of the most feasible methods to control the semiconducting properties of graphene is by doping N or F, which is used to tailor the optical and electrical properties of graphene. It is clear that tuning the energy gap will not only open an area for semiconducting materials to be used for spintronics, they also can lead to the development of new multifunctional devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies, applications in nanoelectronics, materials science, and condensed matter physics. The main results are as follows:1. N-doped graphene (NG) has been prepared by annealing reduced graphene oxide (RGO) in ammonia. The magnetic properties of RGO and NG have been studied. The results showed after doping, the samples keep paramagnetic with an increasing magnetization, and the highest saturated magnetization is 0.514 emu/g,1.67 times higher than reported before. Moreover, it is found that doping RGO with N at a relatively low temperature (≤600℃) can increase its magnetization, and which can be increased by 64.1% at the annealing temperature of 500℃. However, it still shows typically paramagnetic property.2. Synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance.In this study, we demonstrate that doping graphene oxide (GO) with N can obtain a very high magnetization (1.66 emu/g) and a significant increase of magnetization of GO (ca. 1509.1%). Most importantly, it makes the magnetism of GO from purely spin-half paramagnetism to ferromagnetism with Curie temperature (TC) of ca.100.2 K. Clearly, our findings can offer the easy realization of ferromagnetic GO with high magnetization, therefore, push the way for potential applications in spintronic devices.3. In order to increase the Curie temperature of ferromagnetic grapheme, we obtained high-content N doping by F-assisted annealing. Subsequently, we show that NG with very high N content (29.82 at.%), and high contents of N-6 (18.48 at.%) and N-5 (9.54 at.%) can be obtained by thermal treatment of FG powder in ammonia. Also we Moreover, we have performed magnetic measurements on NG with high N content. It reveals that the high-content N-doping in graphene results in the appearance of ferromagnetism with a Curie-temperature of 250.1 K and a very high magnetization of ca.2.3 emu/g. This work may push the way for potential applications in spintronics further.4. The single nanodevice-based ultrathin N-doped helical carbon nanotube (N-doped HCNT) was fabricated for systematical examinations of photoconductivity. A 515 nm focused beam of about 300μm in diameter was applied for irradiation. The study showed that the N-doped HCNT has high generation of photocarriers and high bimolecular recombination rate. The PL properties of the N-doped HCNTs were examined systematically. The results indicated that the photoresponse can be improved further by selecting an appropriate wavelength excitation.