The Preparation Of Ferromagnetic Graphene

Since the pioneering work on the successful fabrication of single-layer graphene in 2004 by mechanical exfoliation from graphite,there has been immense interest devoted to unveiling its impressive electrical,mechanical,optical,and thermal properties.Graphene possesses high carrier mobility,long spin lifetime,and weak spin-orbit and hyperfine interactions,making it a very promising candidate for spintronics applications.However,in the case of pristine graphene sheets,every carbon atom is sp2 hybridized with three neighboring atoms and contributes to a delocalized ?bonding network.Such configurations lead to the intrinsic diamagnetic properties,which limits its applications in spintronic devices.Traditional magnetic materials have been associated with the elements containing partially filled 3d or 4f subshells.Magnetism appearing in graphene-based materials has attracted much attention because graphene contains only sp electrons.Localized magnetic moments are believed to evolve upon introduction of defects into the crystal lattice of graphene.The defects include local topology perturbations,vacancies,noncarbon atoms in the graphene lattice,mixed sp2-sp3 hybridization,and zigzag-type edges.Recently,fluorinated graphene and graphene oxide quantum dots have been reported to exhibit paramagnetic behavior without ferromagnetism.How to introduce high-density defects in graphene and its derivatives to induce the ferromagnetic ordering,is extremely important for spintronic applications.Ferromagnetism of reduced graphene oxide(rGO),three-dimensional graphene and.N2 plasma treated graphene oxide are investigated in the thesis.Main contents are listed as following:1.We studied the magnetic properties of w-rGO obtained by high-temperature annealing of graphene oxide(GO)prepared in weak oxidizing conditions.The w-rGO samples exhibit strong room-temperature ferromagnetism,with saturated magnetization(Ms)of 0.24 emu/g,about 24 times of the value of rGO prepared in strong oxidizing conditions.Hysteresis negative magnetoresistance(MR)has been observed in w-rGO,which confirms the intrinsic nature of the observed ferromagnetism.The MR value keeps nearly unchanged below 15 K,and decreases with further increasing temperature,and finally disappears at T>50 K.At T= 5 K,the maximum MR ratio of[R(H)-R(0)]/R(0)is about-2.5%.The strong ferromagnetism is attributed to the preserved graphitic structure of w-rGO,and the negative MR is due to the spin dependent scattering of defects.2.In graphene powders(graphene oxide and rGO powders),aggregation between individual graphene sheets driven by the strong ?-? interaction and van der Waals force greatly destroy the intrinsic unique properties of two-dimensional material,which limits its practical applications.2D graphene sheets can be self-assembled into complex three dimensional(3D)macrostructures.3D graphene not only possesses intrinsic properties of 2D graphene sheets,but also provides advanced functions with improved performance in various applications.Up to now,experimental researches on magnetic properties of 3D graphene have never been reported.We study the magnetic properties of 3D graphene framework(3DGF),which was prepared by a facile one-step hydrothermal method,without any additives except for graphene oxide suspension.Weak ferromagnetism with saturated magnetization(Ms)of 0.005 emu/g at 300 K has been observed in 3DGF.The reduction of graphene oxide with high concentration of sp3-type hydroxyl groups attached to the basal plane in the interconnected 3D porous network stiructure has been confirmed,which is believed to be the origin of magnetism.3.The ferromagnetism in graphene and its derivatives as light nonmetallic magnets is very important in spintronics.However,the researches and related applications are strongly hindered due to long reaction time,unwanted ferromagnetic impurities and complicated processes involved in the current synthesis methods of ferromagnetic graphene.We find the efficient production of rGO with simultaneous N doping(NrGO)by direct reduction of graphene oxide in N2 plasma environment at room temperature,and significantly enhanced room temperature ferromagnetism has been observed.The N2 plasma exposure time plays a crucial role in controlling the ferromagnetism in graphene.After the GO being exposed in N2 plasma for 0-30 min,Ms increases with increasing the reaction time.NrGO30(exposed in N2 plasma for 30 min)has the highest Ms(0.010 emu/g),which is 17 times higher than that of GO(0.0006 emu/g).However,exposing at a relatively long time(30-60 min)can significantly decrease the magnetization,feeble Ms of 0.001 emu/g is found for NrGO60(exposed in N2 plasma for 60 min),which is only one tenth of the value of NrGO30(0.01 emu/g).The pyrrolic N ratio of NrGO30(ca.45.8%)is higher than that of NrGO60(ca.24.7%)while the pyridinic N ratio of NrGO30(ca.51.3%)is lower than that of NrGO60(ca.69.0%).The ratio of graphitic N in NrGO30(ca.2.9%)and NrGO60(ca.6.3%)is quite small.The higher concentration of pyrrolic N in NrGO30 with larger Ms compared with NrGO60 suggests that the observed ferromagnetism in NrGO mainly comes from pyrrolic N.