Physical Mechanical Properties And Mechanical-electrical Modulation Study In Low Dimensional Nanomaterials

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.