【作者】 孙媛媛；

【导师】 都有为； 汤怒江；

【作者基本信息】 南京大学 ， 物理学， 2017， 博士

【摘要】 二维材料诱导磁性的研究,对于基础物理学和自旋电子学器件应用两方面都具有及其重要的研究意义,近年来备受关注。石墨烯带动了一批类石墨稀二维材料的诱导磁性研究,比如过渡金属硫族化合物二硫化钨。完美的石墨烯和二硫化钨均为本征抗磁性,如何在其中引入高浓度的局域自旋并使其长程铁磁有序,是其能否实现在自旋电子学器件应用的关键。理论研究表明,石墨烯及二硫化钨的锯齿型边界有自旋极化的边界态,该边界态能够引入局域自旋可能诱导产生铁磁有序。本论文基于石墨烯和二硫化钨两种二维材料,围绕其边界态的磁性开展了系统的研究。由于纳米尺寸的石墨烯量子点边界比例大,其锯齿型边界的自旋极化边界态有望引入大量自旋而备受关注。大量理论研究表明,石墨烯量子点锯齿型边界自旋极化的边界态提供局域磁矩,其边界结构、尺寸和形状共同决定各磁团簇之间的磁耦合和整体的磁表现。令人遗憾的是:一直以来,实验上难以获得高产量、高纯度和结构本征的石墨烯量子点,导致不能通过宏观磁性测量来研究边界态诱导的磁。因此,目前还没有关于石墨烯量子点磁性的实验研究报道。石墨烯量子点的制备多采用氧化切割方法,得到的样品实为氧化石墨烯量子点,其中含有大量的氧官能团。理论研究表明,边界氧官能团对石墨烯量子点边界态的磁会产生不同程度的影响。同样,目前没有关于氧化石墨烯量子点磁性的实验报道。此外,值得注意的是:通过高温退火可以有效去除氧化石墨烯量子点中的多种氧官能团。由于键合在边界的多种氧官能团具有不同的热稳定性,于是边界氧官能团热稳定性的研究对于石墨烯量子点的边界修复就显得尤为重要。综上所述,实验上对氧化石墨烯量子点和石墨烯量子点的磁性、及石墨烯量子点边界氧官能团的热稳定性研究都具有重要意义。目前关于二维二硫化钨磁性的实验和理论研究均还相对较少。理论研究表明,二硫化钨的锯齿型边界与石墨烯相类似同样具有自旋极化的边界态,使其在自旋电子学方面同样具有潜在应用。值得注意的是,在二维二硫化钨的制备过程中往往容易形成一些空位缺陷或氧化,然而理论研究表明空位缺陷或氧化对二硫化钨磁性的影响较为复杂。因此,制备过程中产生的结构缺陷或氧化会导致其磁源分析的复杂性,从而难以辨别边界态诱导的磁。由此可见,如何获得结构完美的二硫化钨成为鉴别其边界态诱导磁的关键。本论文围绕二维石墨烯和二硫化钨边界态的磁展开研究。关于石墨烯边界态的磁,我们对边界比例较大的氧化石墨烯量子点及石墨烯量子点的磁性进行了研究。另外,基于氧化石墨烯量子点对影响边界态磁的边界氧官能团热稳定性进行了系统的研究。在二硫化钨边界态磁的研究中,采用硫氧离子交换法对含有缺陷的二硫化钨纳米片的结构进行了有效修复,进而对修复后的二硫化钨纳米片边界态的磁进行了研究。主要研究内容和结果如下:1.基于氧化切割方法制备得到高产量、高纯度的单层氧化石墨烯量子点。首次采用宏观磁性测量对氧化石墨烯量子点的磁性进行了研究,明确了氧化石墨烯量子点的磁性和磁源。氧化石墨烯量子点在低温2K下表现为弱的居里型顺磁性,局域磁矩约为3～4 μB。由于边界上的缺陷抑制了自旋极化的边界态,剩余的边界自旋之间又由于反铁磁耦合而相互抵消,导致边界态难以产生大的局域磁矩。因此,氧化石墨烯量子点的边界态并不是主要磁源,其局域磁矩主要来自于基板面上的羟基。2.首次从实验上对本征的石墨烯量子点的磁性进行了研究,明确了自旋极化的边界态对石墨烯量子点磁性的贡献。将高纯度的单层氧化石墨烯量子点在氩气氛围中1010℃高温退火,得到结构本征的石墨烯量子点。石墨烯量子点的平均粒径为2.04 nm。磁性结果表明,石墨烯量子点在2 K下表现为居里型顺磁,局域磁矩为1.2μB,仅有1/7的量子点贡献磁矩。由于经过高温退火,石墨烯量子点的锯齿型边界在高温下会重构成非磁的ZZ57结构,仅有少量σ键被酚羟基饱和的锯齿型边界未发生结构重构。石墨烯量子点边界的缺陷和ZZ57结构重构导致大多数的量子点不提供磁矩,只有少量酚羟基饱和的锯齿型边界提供石墨烯量子点的局域磁矩。因此,采用酚羟基饱和锯齿型边界的σ键,可能成为保护自旋极化边界态不被高温重构而消失的有效途径。3.基于氧化石墨烯量子点对石墨烯边界氧官能团的热稳定性进行了研究。由于氧化石墨烯量子点边界上键合大量的氧官能团,使其成为研究边界氧官能团热稳定性的最佳选择。将氧化石墨烯量子点在氩气中不同温度下进行退火,通过对样品中含有氧官能团的情况分析,明确了边界氧官能团的热稳定性。边界羧基最不稳定,退火温度高于～400℃可去除绝大多数的边界羧基。边界的醚、羰基和酚羟基表现出高的热稳定性,在～1010℃的高温下仍能稳定存在。退火温度高于400℃后,边界的氧官能团主要为醚。另外,随着退火温度的提高,其磁源逐渐由基板面的羟基和缺陷转变为边界自旋极化的边界态。4.提出硫氧离子交换法为修复二硫化钨结构缺陷的有效方法,并对结构修复后的二硫化钨纳米片的磁性进行了研究,明确了自旋极化边界态对磁性的贡献。研究表明硫氧离子交换法能够有效修复二硫化钨中含有的硫空位缺陷和六价钨缺陷,获得了微米尺寸、结构完美的少层二硫化钨纳米片。修复后二硫化钨纳米片的磁性结果表明,室温下表现为较弱的铁磁性,饱和磁矩为0.002 emu/g。由于修复后的二硫化钨纳米片结构完美,其室温铁磁有序来源于锯齿型边界自旋极化的边界态。更多还原

【Abstract】 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 3～4μB 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 1.2μB 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(ⅰ)edge carboxyl groups are thermally unstable,which can be removed above～400 0C,(ⅱ)ether,carbonyl and phenolic hydroxyl groups at the edge are thermally stable even at the high temperature of 1010 0C,and(ⅲ)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.更多还原