The Magnetic Properties Of Hydroxylated Graphene

As an ideal two-dimensional material,graphene is now one of the biggest concerns for its extraordinary performances in many fields.Due to the special symmetry of its bipartite structure,the interaction between the spin and orbit is very weak,and thus the spins could diffuse on the basal plane of graphene to a long relaxation length,which foundates the favorable conditions for its spintronics applications.However,pristine graphene could be only used as spin-transport channel because it is intrinsically non-magnetic and lack of local magnetic moments,and therefore,to induce localized magnetic moments in graphene is very important for spin generation,spin injection,spin transport and spin recognization,etc.The emergence of coupled magnetic moments,or even magnetic ordering would further expand its application prospect.Furthermore,light-weighted graphene ferromagnet would bring new revolution in the traditional fields of magnetic recording,magnetic memory and magnetic switch,etc.In essence,to induce localized magnetic moments in graphene is to break down its ?-symmetric electronic structure.Such methods mainly include high-energy paticle irradiation to generate vacancy,zigzag edge tailoring and adatom chemisorption,etc.However so far,the weak magnetization,poor controllability and repeatability,and ambiguous magnetic source make the whole topic of magnetic graphene conflicting and hinder its deeper research.Motivated by the inherent impetus to address the situation,in this work,we choose the most commonly used starting graphene derivative,graphene oxide(GO)as the object to systemtically study its magnetic properties.Through altering the preparation conditions and seperating different oxidation-degree parts of the same-batch GO by ammonia washing,we found the oxidative degree is the key factor to influence the magnetic properties of GO,and the type,content and location of oxygen groups would as well depend on the oxidation.Next,by thermal annealing to remove unstable oxygen groups to leave stable hydroxyl groups(OH),we obtained hydroxylated graphene(OHG),and found OH groups is a superior magnetic source to induce magnetic moments on the basal plane of the graphene sheet.In the end,we measured the magnetic transport properties of OHG,and provided a glimpse on the promising graphene derivatives on the spintronics.Our results in this work are outlined as follows:1.We systematically studied the factors which could influence the magnetic properties of GO,and divided GO into two parts with different oxidation degree by ammonia wash.The oxygen groups are identified as a primary reason to influence the magnetic properties of GO.The supersonication duration,acid treatment and centrifugal speed could exert influence on it as well.By controlling the oxidation time to prepare lightly oxidative sample,we obtained the large-sheeted GO with a magnetization as high as 1.2 emu/g.2.We put forward a significant conclusion for the first time:sp3-type defects on the basal plane of the graphene sheet is more competitive on inducing magnetic moments in graphene than edge-type defects.For GO,the heavily oxidative debris which rich in edge-type defects has a much lower magnetization than the relative lightly oxidative large sheet,which has more sp-type OH groups.By thermal annealing to remove unstable oxygen groups to leave stable OH group,the saturated magnetization of GO could be increased further.3.Compared OH with other sp3-type defect inducer,we considered OH is more effective to induce magnetic moments in graphene.The magnetic inducing efficiency of OH could be as high as 217 ?B/1000 OH,and the corresponding spin density could be 4.42?B/1000 C.A part of magnetic moments could survive at the high temperature as 900 ?.OH is superior to other sp3type defect inducers on both the inducing efficiency and stablity,for example,the highest inducing efficiency of F is?3?B/1000 OH and the magnetic moments could only withstand the temperature no more than 400 ?.4.We studied the coupling properties of the magnetic moments induced by OH in graphene.In GO,the magnetic cluster with 4-5 ?B is usually induced as elementary unit by OH groups.A highly symmetric(OH)7 cluster was sketched to explain the phenomenon.The stability of magnetic cluster was enhanced by the existence of ripples,surface topology and the coincidence of the structure symmetry.Moreover,epoxy or other groups on GO would increase the migration barrier to contribute the stablity as well.In OHQ the average space between the adjacent magnetic cluster is larger than 6 nm,which exceeds the space that obvious inter-cluster interaction could apply(3 nm).This could be the main reason why no signal of magnetic ordering was observed,and at the same time,the weak interaction ensures the magnetic cluster exist independently to not collapsing into nonmagnetic or low-spin state.5.The magnetic properties of OHG could be tuned by importing extra OH groups or nitrogen doping,and the saturated magnetization of nitrogen-doped OHG could be further increased to above 4 emu/g.However,the co-contribution of N and OH did not lead to the existence of controllable magnetic ordering.6.By spinning GO solution on SiO2/Si substrate followed by hypothermal annealing,we obtained OHG thin film with the thickness of?100 nm.The electronsin the film is strongly localized,and the transport could be described as a two-dimensional viriable range hopping(2D-VRH)model.No colossal magnetoresistance which appears in the diluted fluorinated graphene was observed.This may relate to the scattering centers such as boundaries,vcancies,multi-layer structure and other oxygen groups in the OHG thin film.In summary,we successfully identified the magnetic properties of GO,and drew a conclusion that sp3-type defect prevail over other type defects on inducing magnetic moments in graphene.We also found that,OH groups could highly effectively contribute robust magnetic clusters to graphene.Our findings would push forward the research of graphene light-element ferromagnets and spintronics.