Oxygen Groups-induced Magnetism In Two-dimensional Materials

With the continuous development of material science,numerous two-dimensional(2D)materials aroused researchers’ attention.In these materials,2D magnetic semiconductor is the most special topic due to the two facts that(i)the height of Z direction is near zero,and(ii)the electrostatic shield effect decreased greatly comparing with their bulk state.Thus,2D materials show their advantage in building next generation field-effect tube devices.In 2018,research groups in UC Berkeley and Washington University discovered the ferromagnetism in 2D Van De Waals crystal Cr2Ge2Te and CrI3 when their height decreases to monoatomic-level.However,these 2D materials show poor instability under ambient condition,which makes the experimental study very hard.On the other hand,the instability of these 2D system limits the device applications in the future.Therefore,many researchers are trying to realize ferromagnetism in 2D nonmagnetic(d0)system with air stability.In another word,since these original bulk ferromagnet are unstable after exfoliated into 2D system,can ferromagnetism be introduced into the inherently stable 2D materials?Obviously,this topic is very attractive,but it is no doubt that the study faces great challenges.For this reason,this topic has attracted many research interests.Our group has focused on the research of magnetism in graphene for several years.Based on the.previous study of our group,we studied the magnetic properties of 2D d0 systems,including graphene and black phosphorene.As to graphene or phosphorene,their weak spin-orbit coupling and long spin-diffusion length make them great potential in spintronics.However,intrinsic graphene and phosphorene do not have localized spins,which limits their applications in spintronics devices.Introduction of high-density,ferromagnetic coupling spins is crucial for the realization of ferromagnetic graphene or phosphorene with high magnetization for the novel spintronic applications.For graphene,both theoretical and experimental research showed that a single hydroxyl group can effectively induce localized spin on the basal plane of the graphene sheet.Similarly,there are many theoretical studies which predicted that defects in BP such as point vacancy,zigzag(ZZ)edge,or doping heteroatoms would create local spins and they can in principle couple(anti)ferromagnetically.For instance,each ZZ edge or point vacancy can theoretically generate 1 μB magnetic moment in phosphorene.Apparently,oxygen groups,which are one of the most common functional groups in 2D materials,can affect the magnetism of these materials.In this study,the effect of the oxygen group for the magnetism of 2D materials was systematically studied.This dissertation mainly includes the following results:1.Micro-sized graphene oxide(GO)sheet was obtained by chemical exfoliation of graphite powder.The hydroxylated graphene(OHG)with high-content hydroxyl groups was obtained by treating GO using NaBH4.High-content hydroxyl groups have been effectively generated.Thus,one can create high-density spins on the basal plane of GO sheets by hydroxyl groups generated via ring opening of epoxy groups.Our results clearly indicate that by generating high-content hydroxyl groups via ring opening of epoxy groups,high-density localized spins can be created on the basal plane of OHG sheets.Moreover,by annealing the OHG at low temperature(～300℃),we show that the magnetization of GO can be increased from 0.136 to 3.110 emu/g and the spin density can be increased from 0.40 μB/1000 C to 8.72 μB/1000 C.OHG with high magnetization and spin density was successfully obtained.2.Black phosphorus has aroused great interest due to its direct and layer-dependent tunable bandgap and anisotropy.The key step towards the real applications of BP is the low-cost and mass production of high-quality BP sheets with large lateral size,few-layer number,and low oxidization.Electrochemical exfoliation method has been successfully performed in obtaining high-quality graphene.Thus we have modified the electrochemical and sonication condition,and exfoliated the black phosphorus in alkaline(NH4)2SO4 solution,and sonicated in isopropyl alcohol(IPA).This method overcame the long-time and troublesome poor quality problems of BP when exfoliated in water.The obtained BP sheets combine the superiorities of the large size,few-layer number,low oxidization,and excellent processability and,thus paves the way for the industrial production and applications of BP sheets.3.Black phosphorus(BP)has aroused great research interests due to its layer-dependent bandgap and high mobility,which offers it distinct advantages over other 2D materials in electronic and optoelectronic applications.What is especially exciting is BP’s nanosecond spin lifetimes.Such long spin lifetimes afford BP new opportunities in spin-based electronics.However,it has been widely reported that BP may undergo a fast degradation in hours under ambient conditions,and the degradation can largely change its magnetism.This work systematically studied the magnetic properties of phosphorene under ambient conditions.Firstly discovered and confirmed that the oxygen group affect the magnetism of phosphorene and gave constructive guidance for the forthcoming study on BP’s magnetism.