Theoretical And Experimental Study On Magnetism Of Graphene And Graphdiyne

Carbon material has sp3,sp2 and sp three kinds of hybrid state,which can form a variety of carbon allotropes.These allotropes can present varied structures as well as many novel properties and can be widely used in many fields.Among these allotropes,graphene and graphdiyne are two typical 2D carbon structures,which respectively composed of sp2 hybrid and sp2-sp hybrid structures.Theoretically,both of them have excellent electron transportation properties.Especially,the long spin diffusion length derived from the weak spin-orbit coupling of the carbon atoms,makes the spin diffusion scattered very small.However,due to the interlaced symmetry of the lattice structure,the perfect 2D carbon materials are intrinsically non-magnetic,which limits its application in spintronics devices.Therefore,introducing local magnetic moments in them,and making them ferromagnetic coupling and realization of the ferromagnetic long-range ordering is key for the applications in spintronic devices.Breaking the symmetry of the structure,such as by creating the vacancy,cutting the edge and absorbing heterogeneous atoms or functional groups,is an effective routine to introduce local magnetic moments.Especially,the adsorption of heterogeneous atoms or functional groups is considered as the main route to introduce magnetic moment.Adatoms such as fluorine atoms absorbed on the basal plane of the graphene sheet is a hot issue for carbon-based spintronics devices.So far,fluorinated graphene(FG)has been demonstrated to be paramagnetism.However,how to obtain a high saturation magnetization and how to clarify the magnetic sources still need to study further.As for graphdiyne,although it has an analogous structure with graphene,the natural large-pore structure affords it the strong chemical activity.As a result,graphdiyne can easy adsorb functional groups,which may afford the great opportunity to tune its magnetism.Up to now,as a new artificial 2D carbon material,the study on the magnetism of graphdiyne remains completely unknown.In this thesis,I focus on the magnetism of those two kinds of 2D carbon allotropes,including theoretically studying the magnetism of FG and experimentally studying the magnetism of graphdiyne.A new method to introduce magnetic moments in FG is proposed.We explored the magnetism behavior variation under anneal condition and studied its magnetic source and magnetism translate.Moreover,I also discussed the application of graphdiyne in photoluminescence(PL).The main results are below:(1)It is suggested that the structural fluorination can effectively induce magnetism in graphene,viz.,the double-sided asymmetric fluorination can effectively introduce magnetic moment.The results show that non-equilibrium fluorination can effectively introduce magnetic moments,whilst the equilibrium fluorination can not do.(2)The thermal stability of non-equilibrium FG and its effect on magnetism are systematically studied.The results show that with the increase of non-equilibrium degree,the stability of the non-equilibrium FG decreases,indicating that the efficiency of the introduction of magnetic distortion is limited.(3)The mechanism of the magnetism of non-equilibrium FG is studied.It is found that the non-equilibrium fluorinated structure has a corrugated shape,and midgap state was introduced in the corresponding corrugated region.The midgap state causes the splitting of the energy level,resulting in the appearance of the local magnetic moment.Spin charge density and projected band structure showed that the origin of non-equilibrium fluorination-induced magnetic moments mainly results from the carbon atoms around fluorination region,barely from internal fluorinated region carbon atoms and fluorine atoms.(4)It is proposed that the magnetic moment of non-equilibrium FG can be modulated by applying tensile stress.It is found that increasing the tensile stress can increase the local magnetic moment.The mechanism is that the tensile stress changes the hybridization degree of sp2 and sp3 on the fluorinated area,which affects the redistrlibution of the spin electrons and exacerbates the width spin splitting.(5)The graphdiyne film and powder were successfully prepared by cross-coupling reaction.It is found that the as-prepared graphdiyne has different functional groups,such as hydroxyl groups,pyridine-N and vacancy defects.After annealing at different temperatures,some of the functional groups can be reduced.(6)The magnetic properties of the graphdiyne samples annealed at different temperatures were systematically studied.It was found that the samples present paramagnetic behavior after annealing below 400 ?.After annealing at a higher temperature,it shows clear antiferromagnetism.(7)The magnetic source of graphdiyne was studied.It is proposed that the magnetic sources may be the hydroxyl groups and the vacancies.The magnetism introduced by hydroxyl is quasi-local,while the vacancy-induced magnetism is local.The other oxygen groups can't introduce magnetic moment.After annealing at a high temperature,the residual hydroxyl groups at the chains of graphdiyne sheets may contribute to the antiferromagnetim.The natural ring combined chain structure of graphdiyne can effectively prevent the hydroxyl clustering on the surface,which can contribute to the antiferromagnetism.(8)The luminescent properties of graphdiyne obtained by ultrasonic filtration method were studied.The results showed that the graphdiyne nanosheet exhibit excellent blue PL,and the emission wavelength is-320 nm.The mechanism of PL is band transfer due to oxygen-containing functional groups caused by the charge transfer in graphdiyne.