【作者】 陆鹏；

【导师】 郭万林；

【作者基本信息】 南京航空航天大学 ， 力学， 2012， 博士

【摘要】 外场(电场、力场等)作用下低维纳米材料和器件的力电磁性能一直是是纳米科技的研究热点之一。低维纳米材料由于量子限制效应的存在，具有与块体材料迥异的物理力学性质、化学性质和稳定性能，使得低维纳米材料成为受人瞩目的明星材料，其在下一代电子器件、逻辑器件以及光学器件等方面的应用被寄予厚望。本文使用基于密度泛函理论的第一性原理模拟的方法，在不受外场和受到外加力场、电场或者有缺陷情况下，深入研究了低维石墨烯材料、硼碳氮掺杂材料和二硫化钼材料的电磁性能，以及低维石墨烯材料中线性磁电效应的调控机制。1.石墨烯相关材料的电性和磁性能调控研究：石墨烯相关材料是近年来研究进展最为迅速的一种材料，其独特的电子学性质和丰富的由边缘态或缺陷态导致的磁性尤其重要。这里，我们通过第一性原理计算的方法，首先深入研究了有Stone-Wales缺陷对锯齿形石墨烯纳米条带的电子性质和磁性的影响，发现Stone-Wales缺陷在条带中的对称性缺失可以导致条带出现非零的总磁矩。随着位于条带边缘的缺陷往中心移动，条带的总磁矩逐渐减小到零，同时条带会从金属性变化到半金属性直到最后表现出半导体的性质。更为重要的是，本研究组最近在硅基底上的石墨烯纳米带体系中发现了以前只有在多铁材料中才有的电调控磁性。在这里，我们进一步揭示了磁性石墨烯纳米片放置在具有不同化学势基底上面的时候，出现的非线性到线性磁电效应转换的机制。我们发现当在石墨烯纳米片和石墨烯基底之间放上一层氮化硼的时候，磁电效应会从非线性到线性耦合转化。因此，石墨烯纳米片和基底之间的电子轨道作用是磁性石墨烯材料中出现线性和非线性磁电效应的关键。这对于制备和操作高质量的电子和自旋器件提供了新的思路和方法。有机分子与石墨烯成键作为一种调制石墨烯相关材料电磁性能的常用方法，对这种方法进一步的探索在实验和理论研究中都具有很重要的意义。我们研究了将氨基分子[CON(CH3)2]对石墨烯或者纳米管功能化的情况。发现当石墨烯的两个子格均衡的被氨基分子钝化的时候，表现半导体性质的功能化石墨烯带隙会随着氨基分子密度的增加而增大。而此时对于金属性的功能化石墨烯，其电子性质则几乎不受氨基分子密度增加的影响。当石墨烯两个子格不均衡的被氨基分子钝化的时候，功能化石墨烯会具有内禀的磁性。随着氨基分子密度的增加，功能化石墨烯的电子性质会从半导体变化到半金属，直到最后出现金属性质。对于锯齿形碳纳米管，当其两个子格不均衡的与氨基分子成键的时候，功能化的纳米管会根据分子的覆盖方式而表现金属或者半导体的性质。半径大于一个极限值的功能化锯齿形碳纳米管还可以表现出内禀的磁性。当其两个子格不均衡地与氨基分子成键的时候，功能化的纳米管都是半导体，其带隙随着两个相邻氨基分子沿管径方向距离的增大而增大。而所有功能化扶手椅形碳纳米管都表现金属非磁性质。2.BC2N纳米条带的电子和磁性质研究:硼碳氮材料具有带隙可调节的电子性质和自发的磁性质，一直以来备受大家关注。我们研究了氢原子边缘钝化BC2N纳米条带(BC2N的二维结构是由B-N键与C-C键间隔排列的六圆环作为基本单元构成)的电子性质、磁性质以及稳定性。锯齿形边缘的BC2N纳米条带(z-BC2NNRs)依赖于边缘原子的排布情况，可以表现出内禀的半导体或者金属性质。特别是，当条带宽度足够宽的时候，磁性甚至半金属性质在一些种类的z-BC2NNRs中都可以出现。取决于条带中硼碳氮原子的比例，扶手椅形的BC2N纳米条带(a-BC2NNRs)可以显示半导体或者金属性。半导体a-BC2NNRs的带隙随着条带宽度的增加而逐渐减小。而在条带宽度大于一个极值的时候，金属性的a-BC2NNRs会表现出内禀的磁性，这是第一次报道扶手椅形纳米条带中发现内禀磁性。所有半导体BC2N纳米条带的带隙起源都可以用条带两边缘直接电荷的极化来解释。该研究发现对于硼碳氮掺杂低维结构的应用提供了一个有效的方法。3.应变导致的二硫化钼单层、双层、纳米带和纳米管电磁性质的变化:低维二硫化钼具有不同于其三维块体结构的直接带隙性质和电子输运性质，最近在光学领域和逻辑器件领域被大家广泛关注和研究。在这里我们系统研究了二硫化钼单层、双层、纳米带和纳米管的电磁性质随着应变变化的规律。首先，对于二维的单层二硫化钼分别施加了等向性应变和单轴应变，在拉伸应变下直接带隙的二硫化钼会变成间接带隙半导体，带隙随着应变的增大而逐渐减小；而在压缩应变下，其直接带隙的性质不会改变，而带隙则会先增大然后减小。对于双层二硫化钼在受到等向性拉伸应变的时候，其间接的带隙值会线性减小直至达到6%的应变；而在等向性压缩应变下，间接的带隙值会首先增加，然后减小，当应变大于-4%的时候会成为直接带隙半导体。一维锯齿形二硫化钼纳米条带在生长方向受到单轴应变的时候，其总磁矩随着应变从-5%变化到5%而逐渐增大，当压缩应变大于-5%的时候磁矩会减小为零，或者拉伸应变大于5%的时候，磁矩会迅速的指数增加。对于一维扶手椅形二硫化钼纳米管，拉伸或者压缩的单轴应变会线性的减小或者增加其带隙，在纳米管的管径比较小或者拉伸应变比较大的时候，纳米管的带隙会被完全关闭。对于锯齿形二硫化钼纳米管，应变导致带隙的变化表现非线性形式，拉伸应变可以减小其带隙，而压缩应变则会首先增大其带隙，然后再逐渐的使得带隙减小。以上结果表明单轴应变或者等向性应变均可以作为调节低维二硫化钼材料电子性质和磁性质的有效手段，为设计相关材料在光学、自旋电子学方面的器件应用提供了方法。更多还原

【Abstract】 Mechanical, electronic and magnetic properties of low dimensional materials or devices havebeen widely investigated with external filed (electric field, mechanical field). Due to the existence ofquantum effect, the low dimensional materials have distinct different physical, chemical and stabilityproperties with the original materials and have attracted a great deal of attentions. Lots of studies showthat the low dimensional material can play an important role in the next generation of electronic, logicaland optical devices. In this work, based on the first principles calculations, we have investigated theelectronic and magnetic properties of low dimensional graphene, BC2N and MoS2materials with orwithout mechanical, electric field and defect. The mechanism for linear magnetoelectric effect ingraphene materials has also been studied.1. Investigations of modulating graphene materials electronic and magnetic properties：Graphene materials attracte more and more interstings during recent years. The electronic and magneticproperties of zigzag graphene nanoribbons (ZGNRs) with Stone-Wales defects are studied by extensivefirst-principles calculations. It is shown that the asymmetry distribution of the Stone–Wales defects caninduce finite magnetic moment in the defective ZGNRs. As the defect near one of the ribbon edgesmoving to the center region, the magnetic moment of the defective ZGNRs gradually decreases to zero,following a transition from metal to semi-half-metal and eventually to semiconductor. More importantly,our group has found the graphene nanoribbons with silicon substrat owning magnetoelectric effectrecently, which had only reported in3d metal materials before. Here, using density functional theorycalculations, we futherly reveal a novel nonlinear-linear transition of the ME effect in graphenenanoflakes (GNFs) placed on substrates with different chemical activities. We show that the ME effect isnonlinear in a magnetic GNF on graphene substrate. Interestingly, the ME effect in the same GNFbecomes highly linear with markedly increasedME coefficient when an h-BN sheet is inserted betweenthe GNF and graphene layer. We reveal that the weak electronic hybridization between the GNFs andsubstrate is the essential mechanism for the linear ME behavior in the graphene-based magnets. Theinvestigations open up new opportunities and ideas to fabricate and manipulate electronic and spindevices. Organic molecule bonding with graphene is one of the important way to modulte the electronicand magnetic properties of graphene, it is interesting to develop the way in both experimental andtheoretical. Next, we investigate electronic, magnetic, and electron transport properties of covalentlyfunctionalized graphene and carbon nanotubes (CNTs) by the amide groups [CON(CH3)2] using densityfunctional theory calculations. We find that when both sublattices of the graphene are evenlyfunctionalized with the amide groups, the band gap of the modified (semiconducting) graphene can besubstantially enlarged by increasing the coverage of amide groups. If the modified graphene is metallic,however, its electronic properties are little affected by increasing the coverage. When the two sublatticesof the graphene are functionalized unevenly, the decorated graphene exhibits magnetism. As the coverageof amide groups is increased, the electronic properties of the functionalized graphene can be transformedfrom semiconducting to half metallic and to metallic. For zigzag CNTs (ZCNTs), when the twosublattices are unevenly functionalized by the amide groups, the functionalized CNTs can be eithermetallic or semiconducting, depending on the pattern of decoration. ZCNTs with large diameters mayexhibit magnetism as well. When the two sublattices are unevenly functionalized, the functionalizedZCNTs are always semiconducting with their band gap increasing with the distance between twoneighboring amide groups in the radial direction. For armchair CNTs, however, all functionalized systemsare metallic without showing magnetism, regardless of the coverage or pattern of amide groups.2. Electronic and magnetic properties of BC2N nanoribbons investigation: BCN materialshave attracted lots of attention for the adjustable electronic properties and spontaneous magneticproperties. We reveal a rich variety of electronic and magnetic properties of H-terminated BC2Nnanoribbons (BC2NNRs) by using extensive first-principles calculations (hexagons in monolayer BC2Nare constituted by the B-N and C-C bonds). Zigzag edged BC2NNRs (z-BC2NNRs) can besemiconducting or metallic depending on the alignment of edge atoms. In particular, magnetic and evenhalf-metallic behaviors can appear in some edged z-BC2NNRs when the ribbon width is over a criticalvalue. Armchair-edged BC2NNRs also can be semiconducting or metallic but determined by theproportion of carbon, nitrogen, and boron atoms in the ribbons. The a-BC2NNRs with B and N atomscoordinated have band gaps decreasing with increasing ribbon width. In particular, a-BC2NNRs with theB and N atoms uncoordinated can be either p-or n-doped semiconductors, and the wide ones ownspontaneous magnetization. The band gaps of all semiconducting BC2NNRs can be explained by a universal mechanism that is due to the charge polarization between the opposite edges, which is impairedwith increasing ribbon width. The investigation provides a new way for applications of low dimentionalBCN materials.3. Strain-dependent electronic and magnetic properties of MoS2monolayer, bilayer,nanoribbon and nanotubes: Low dimensional MoS2has attracted lots of attentions in optical andlogical devise, because of the direct band gap and quickly charge transfer properties, which are differentfrom its bulk material. We investigate the strain-dependent electronic and magnetic properties oftwo-dimensional (2D) monolayer and bilayer MoS2, as well as1D MoS2nanoribbons and nanotubesusing first principles calculations. For2D monolayer MoS2subjected to isotropic or uniaxial tensile strain,the direct band gap of MoS2changes to an indirect gap that decreases monotonically with increasingstrain; while under the compressive strain, the original direct band gap is enlarged first, followed by gapreduction when the strain is beyond-2%. For bilayer MoS2subjected to isotropic tensile strain, its indirectgap reduces monotonically to zero at strain about6%; while under the isotropic compressive strain, itsindirect gap increases first and then reduces and turns into direct gap when the strain is beyond-4%. Forstrained1D metallic zigzag MoS2nanoribbons, the net magnetic moment increases slightly with axialstrain from about-5%to5%, but drops to zero when the compressive strain is beyond-5%or increaseswith a power law beyond5%. For1D armchair MoS2nanotubes, tensile or compressive axial strainreduces or enlarges the band gap linearly, and the gap can be fully closed for nanotubes with relativelysmall diameter or under large tensile strain. For zigzag MoS2nanotubes, the strain effect becomesnonlinear and the tensile strain can reduce the band gap, whereas compressive strain can initially enlargethe band gap and the decrease it. The results show that both isotropic and unixal strain can modulateelectronic and magnetic properties of low dimenstional materials effectively, provides new ways for lowdimensional materials application in optical and spin devices.更多还原