The Edge States Magnetism Of Graphene Quantum Dots And WS₂ Nanosheets

The magnetism of two-dimensional(2D)materials composed by nonmagnetic elements has aroused tremendous interest in the area of fundamental physics for their potential applications in spintronics.Researches on the magnetism of graphene have led to exploration of other 2D materials,such as 2D transition metal dichalcogenides materials layered WS2.As is known to all,the 2D graphene and WS2 are intrinsically non-magnetic.Thus,to induce localized magnetic moments or even ferromagnetic ordering is very important for their potential application in spintronics.The zigzag edges of graphene and WS2 have spin polarized edge states by theoretical reports,which would introduce localized magnetic moments and even ferromagnetic ordering In this work,I focus on the spin polarized edge states induced magnetism of graphene and WS2.Graphene quantum dots(GQDs)with a size of only several nanometers have high edge-to-area ratio and possible substantial spin-polarized edge states at the zigzag segments,which attract a great deal of interest lately.Theoretically,the magnetism of GQDs is associated with the edge configuration,size and geometric shape.However,the low yield and purity of GQDs all along,which is insufficient for magnetism detection by macro-magnetic measurement,limit experimental research on the magnetism of GQDs.Generally,the GQDs synthesized via oxidative cutting methods usually contain oxygen functional groups,which are actually GQDs oxides(GOQDs).It is reported by theoretical research that the oxygen functional groups of GOQDs have complex effects on its edge states magnetism.Despite this,experimentally,the magnetism of GOQDs is also completely unknown.Notably,thermal annealing is effective in deoxygenation with no introduction of any contaminants.The oxygen functional groups in GOQDs may be removed by thermal annealing.Thus,clarifying the thermal stability of these edge oxygen functional groups is important to the edge recovery of GQDs.In a word,the experimental research on the magnetism of GOQDs and GQDs,and the thermal stability of edge oxygen functional groups are of great significance.Up to now,the study on the magnetism of 2D transition metal dichalcogenides materials WS2 is still very few.Theoretically,similar to the case of graphene,there are also spin polarized edge states at the zigzag edge of WS2,implying that it may have potential application in spintronics.However,2D WS2 synthesized by mechanical exfoliation or chemical vapor deposition methods generally contains vacancy defects and oxidized impurities,and both of which may have complex influence to the magnetism of WS2.Thus,to identify the edge states induced magnetism,recovering the defects in WS2 is necessary and important.In this work,I focus on the edge states induced magnetism of 2D graphene and WS2.The magnetic properties of GOQDs and GQDs with high edge-to-area ratio are experimentally studied,meanwhile the thermal stability of oxygen functional groups at the edge of GOQDs are systematically analysized.The structure of WS2 nanosheets was recoverd by sulfuration process,and then its magnetic properties induced by edge states are studied.The results of our work are outlined as follows:1.The magnetic properties of GOQDs are studied.Monolayer GOQDs with high yield and high purity were obtained by oxidative cutting.The results show that most of GOQDs are nonmagnetic,and only few of GOQDs are weakly paramagnetic.The localized magnetic moments of GOQDs is 3?4?B at 2 K.It is proposed that the edge states magnetism is suppressed by the edge defects and/or the magnetic correlation induced spins cancellation between magnetic fragments of the boundary,and hydroxyl groups on the basal plane are the major magnetic source of magnetic GOQDs.2.The intrinsic magnetism of GQDs are studied.Our GQDs with the average diameter of ca.2.04 run show the purely Curie-like paramagnetism with the local moment of 1.2?B at 2 K.It is proposed that the magnetic moment of GQDs may mainly origin from the residual zigzag edges passivated by hydroxyl groups.The ratio of nonmagnetic graphene quantum dots is approximately 6/7,with most of the magnetic edge states suppressed by edge defects and/or edge reconstruction arising from the high-temperature annealing.Our study experimentally unveils the intrinsic magnetism of GQDs.3.The thermal stability of oxygen functional groups at the edge of GQDs are systematically studied.GOQDs in nanometer have plenty of oxygen groups at the edge,which is favorable to detect the thermal stability of the edge oxygen groups.In this study,detailed annealing experiments of GOQDs were performed.The oxygen configurations of the samples were detected,and then the thermal stability of the edge oxygen groups are clarified.Our study showed that(?)edge carboxyl groups are thermally unstable,which can be removed above?400 0C,(?)ether,carbonyl and phenolic hydroxyl groups at the edge are thermally stable even at the high temperature of 1010 0C,and(?)the stable oxygen groups at the edge are apt to be mainly in the form of ether over?400?.Magnetic properties of the samples were investigated,which are consistent with the oxygen configuration analyses.4.The sulfuration process is demonstrated to be effective method to repair the defects in WS2 nanosheets,and then the edge states magnetism of WS2 nanosheets was clarified.The sulfuration process is effective in reparing the sulfur vacancies and converting the W6+ impurities to WS2 structure.The perfect WS2 nanosheets were obtained after sulfuration process,which show weakly ferromagnetism at the room temperature with the saturation magnetic moment of 0.002 emu/g.Since the WS2 nanosheets have perfect basal-plane strucure,the ferromagnetism may only origin from the spin polarized zigzag edges.