First-principles predictions of the geometries and electronic structures of tungsten ditelluride nanoribbons

First-principles calculations are carried out to predict the structures and electronic properties of 2H- and Td-WTe₂ nanoribbons with different termination edges. It is found that the 2H-WTe₂ nanoribbon along the armchair direction and the Td-WTe₂ nanoribbon along the X direction show semiconducting characters with tunable band gaps. The 2H-WTe₂ nanoribbon along the zigzag direction and the Td-WTe₂ nanoribbon along the Y direction show metallic characters.     

Structural, electronic, and magnetic properties of pristine and oxygen-adsorbed graphene nanoribbons

The structural, electronic and magnetic properties of pristine and oxygen-adsorbed (3,0) zigzag and (6,1) armchair graphene nanoribbons have been investigated theoretically, by employing the ab initio pseudopotential method within the density functional scheme. The zigzag nanoribbon is more stable with antiferromagnetically coupled edges, and is semiconducting. The armchair nanoribbon does not show any preference for magnetic ordering and is semiconducting. The oxygen molecule in its triplet state is adsorbed most stably at the edge of the zigzag nanoribbon. The Stoner metallic behaviour of the ferromagnetic nanoribbons and the Slater insulating (ground state) behaviour of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. However, the local magnetic moment of the edge carbon atom of the ferromagnetic zigzag ribbon is drastically reduced, due to the formation of a spin-paired C-O bond.     

Manipulating edge current spin polarization in zigzag MoS2 nanoribbons

The electronic structure and quantum transport of a zigzag monolayer molybdenum disulfide (MoS₂) nanoribbon are investigated using a six-band tight-binding model. For metallic edge modes, considering both an intrinsic spin–orbit coupling and local exchange field effects, spin degeneracy and spin inversion symmetry are broken and spin selective transport is possible. Our model is a three-terminal field effect transistor with a circular-shaped gate voltage in the middle of scattering region. One terminal measures the top edge current and the other measures the bottom edge current separately. By controlling the circular gate voltage, each terminal can detect a totally spin-polarized edge current. The radius of the circular gate and the strength of the exchange field are important, because the former determines the size of the channel in both S-terminated (top) and Mo-terminated (bottom) edges and the latter is strongly related to unbalancing of the density of spin states. The results presented here suggest that it should be possible to construct spin filters using implanted MoS₂ nanoribbons.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

A metal-semiconductor transition triggered by atomically flat zigzag edge in monolayer transition-metal dichalcogenides

Due to the structure of three stacked layers, monolayer transition-metal dichalcogenides (TMDs) is different from graphene. Creating atomically flat graphene-like edges in them has long been expected, which is crucial to the modulation of electronic structures in two-dimensional systems. Recently, by thermal annealing, Chen et al. [21] successfully synthesized atomically flat Mo-terminated edge in monolayer MoS₂. Inspired by this, through first-principles calculations, we studied the electronic and transport properties of typical TMD monolayers with transition atom-terminated flat zigzag edges, i.e., ScS₂, VS₂, CrS₂, FeS₂, NiS₂, MoS₂ and WS₂. It is found that the nanoribbons with and without flat edges are both metallic. Interestingly, the vacancy in the flat edge could open a transmission gap at the Fermi level in the ScS₂ ribbon, and trigger a metal-semiconductor transition. Further analysis shows that, the opening of bandgap around the Fermi level induced by the specific pattern of vacancies is the mechanism behind, which could be used as an modulating method for electronic structures. We believe our results are quite beneficial for the development of many other monolayer transition-metal dichalcogenides configurations, showing great application potential.     

First-principles study of electronic structure and magnetism of cubic Al1−xErxN using the LSDA+U approach

First-principles calculations, by means of the full-potential augmented plane wave method using the LSDA+U approach (local spin density approximation with Hubbard-U corrections), have been carried out for the electronic structure of the Al_0.75Er_0.25N. The LSDA+U method is applied to the rare-earth 4? states. We have investigated the electronic and magnetic properties.The Al_0.75Er_0.25N is shown to be a semiconductor, where the filled ? states are located in the valence bands and the empty ones above the conduction band edge. The magnetic interaction of the rare-earth ion with the host states at the valence and conduction band edges has been investigated and discussed.     

Electronic structure,optical properties,and phonon transport in Janus monolayer PtSSe via first-principles study

Motivated by the synthesis of a Janus monolayer, the new PtSSe transition-metal dichalcogenide (TMD) have attracted remarkable attention due to their characteristic properties. In this work, we calculated the electronic structure, optical properties, and the thermal conductivity of the PtSSe monolayers, and performed a detailed comparison with other TMDs (monolayer PtS₂ and PtSe₂) using first-principles calculations. The calculated band gaps of the PtS₂, PtSSe, and PtSe₂ monolayers were 1.76, 1.38, and 1.21?eV, respectively, which are in good agreement with experimental data. At the same time, we observed a larger spin-orbit splitting in the electronic structure of PtSSe monolayers. The optical properties were also calculated and a significant red shift was observed from the PtS₂ to PtSSe to PtSe₂ monolayers. The lattice thermal conductivity of the PtSSe monolayer at room temperature (36.19?W/mK) is significantly lower than that of the PtS₂ monolayer (54.25?W/mK) and higher than that of the PtSe₂ monolayer (18.07?W/mK). Our results show that the PtSSe monolayer breaks structural symmetry and has the same ability to reduce the thermal conductivity as MoSSe and ZrSSe monolayers due to the shorter group velocity and the lower converged phonon scattering rate. These results may stimulate further studies on the electronic structure, optical properties, and thermal conductivity of the PtSSe monolayer in both experimental synthesis and theoretical efforts.     

Structural and magnetic properties of MoS2 monolayer zigzag nanoribbon doped by Ti,V, Cr,and Mn

In order to find new functions of monolayer MoS₂ in nanoelectronics or spin electronic devices, using spin-polarized density functional theory (DFT) calculations with on-site coulomb interaction (U), we investigated substitutional doping of Mo atoms of monolayer zigzag MoS₂ nanoribbon (ZZ-MoS₂ NR) by transition metals (TM) (where TM = Ti, V, Cr, Mn) at the Mo-edge, S-edge, and the middle of the NRs. The results of this study indicate the NR widened irrespective of the doped TM position and type, and the Mo-edge was found as the easiest substitutional position. For ZZ-MoS₂ NR doped by Mn, Cr or V atoms, the preferred magnetic coupling state is the edge atoms of S at the S-edge, exhibiting the same spin polarization with TM (named the FM1 state), attributing the NR with metallic magnetism. For Ti-doped monolayer ZZ-MoS₂ NR, in addition to the FM1 state, other preferred magnetic coupling state was observed in which the edge atoms of S at the S-edge exhibit the opposite spin polarization with that of Ti (named the FM2 state). Thus, the NR doped by Ti atom possesses metallic (FM1 state) or half-metallic (FM2 state) magnetism. The total magnetic moments of the ZZ-MoS₂ NR doped by TM follows a linear relationship as a function of the TM dopants (Mn, Cr, V, and Ti). Under $>4\text{\%}$ applied strain, the NR doped by Ti atom only presents the characteristics of half-metallic magnetism as the initial one in the FM2 state, and its total magnetic moment always remained 0 μB, i.e., it was not affected by the width of the NR. This study provides a rational route of tuning the magnetic properties of ZZ-MoS₂ NRs for their promising applications in nanoelectronics and spin electronic devices.     

The electronic structure and magnetic properties of metallic antiferromagnetic Co[(CH3PO3)(H2O)] studied by first-principles calculations

The electronic structure, the metallic and magnetic properties of metal phosphonate Co[(CH₃PO₃)(H₂O)] have been studied by first-principles calculations, which were based on the density-functional theory (DFT) and the full potential linearized augmented plane wave (FPLAPW) method. The total energy, the spin magnetic moments and the density of the states (DOS) were all calculated. The calculations reveal that the compound Co[(CH₃PO₃)(H₂O)] has a stable metallic antiferromagnetic (AFM) ground state and a half-metallic ferromagnetic (FM) metastable state. Based on the spin distribution obtained from calculations, it is found that the spin magnetic moment of the compound is mainly from the Co²⁺, with some small contributions from the oxygen, carbon and phosphorus atoms, and the spin magnetic moment per molecule is 5.000μ_B, which is in good agreement with the experimental results.     

Effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon

The effect of the dangling bond on the electronic and magnetic properties of BN nanoribbon with zigzag edge (ZBNNR) and armchair edge (ABNNR) have been studied using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. Though ZBNNR or ABNNR with H atom terminated at both edges is nonmagnetic semiconductor, the dangling bond induces magnetism for the ZBNNR with bare N edge, bare B edge, bare N and B edges, the ABNNR with bare N edge and bare B edge. However, the ABNNR with bare N and B edges is still nonmagnetic semiconductor due to the strong coupling of the dangling bonds of dimeric N and B atoms at the same edge. The magnetic moment of ZBNNR with bare N(B) edge is nearly half the magnetic moment of ABNNR with bare N(B) edge. Such a half relationship is also existed in the number of the dangling bond states appeared around the Fermi level in the band structures. Furthermore, the dangling bond states also cause both ZBNNR and ABNNR with bare N edge a transition from semiconducting to half-metallic and thus a completely (100%) spin-polarization, while cause both ZBNNR and ABNNR with bare B edge as well as ABNNR with bare N and B edges only a decrease in their band gap.     

Atomic disorder and the magnetic, electrical, and optical properties of a Co2CrAl Heusler alloy

Two Co₂CrAl alloy samples subjected to different heat treatment regimes are studied. An exact distribution of atoms over the sublattices in the samples is determined by X-ray diffraction and neutron diffraction methods. These data are used to perform ab initio density of states calculations and to calculate the magnetic moments of the samples in a coherent potential approximation. The calculated magnetic moments are compared to the experimental values. The effect of atomic ordering on the electronic structure near the Fermi level is analyzed using optical methods. The possible causes of the detected temperature dependence of the electrical resistivity, unusual for metallic alloys, are discussed.     

Carbon nanoribbons and nanotubes based on δ-graphyne: A first-principles study

As a stable allotropy of two-dimensional (2D) carbon materials, δ-graphyne has been predicted to be superior to graphene in many aspects. Using first-principles calculations, we investigated the electronic properties of carbon nanoribbons (CNRs) and nanotubes (CNTs) formed by δ-graphyne. It is found that the electronic band structures of CNRs depend on the edge structure and the ribbon width. The CNRs with zigzag edges (Z-CNRs) have spin-polarized edge states with ferromagnetic (FM) ordering along each edge and anti-ferromagnetic (AFM) ordering between two edges. The CNRs with armchair edges (A-CNRs), however, are semiconductors with the band gap oscillating with the ribbon width. For the CNTs built by rolling up δ-graphyne with different chirality, the electronic properties are closely related to the chirality of the CNTs. Armchair (n, n) CNTs are metallic while zigzag (n, 0) CNTs are semiconducting or metallic. These interesting properties are quite crucial for applications in δ-graphyne-based nanoscale devices.     

Edge magnetization in Bernal-stacked trilayer zigzag graphene nanoribbons

We have used a tight-binding Hamiltonian of an ABA-stacked trilayer zigzag graphene nanoribbon with β-alignment edges to study the edge magnetizations. Our model includes the effect of the intralayer next-nearest-neighbor hopping, the interlayer hopping responsible for the trigonal warping and the interaction between electrons, which is considered by a single band Hubbard model in the mean field approximation. Firstly, in the neutral system we analyzed the two magnetic states in which both edge magnetizations reach their maximum value; the first one is characterized by an intralayer ferromagnetic coupling between the magnetizations at opposite edges, whereas in the second state that coupling is antiferromagnetic. The band structure, the location of the edge-state bands and the local density of states resolved in spin are calculated in order to understand the origins of the edge magnetizations. We have also introduced an electron doping so that the number of electrons in the ribbon unit cell is higher than in neutral case. As a consequence, we have obtained magnetization steps and charge accumulation at the edges of the sample, which are caused by the edge-state flat bands.     

Electronic and magnetic properties of SiC nanoribbons by F termination

By using the first-principles calculations, the electronic properties are studied for the F-terminated SiC nanoribbons (SiCNRs) with either zigzag edges (ZSiCNRs) or armchair edges (ASiCNRs). The results show that the broader F-terminated ZSiCNRs are metallic and the edge states appear at the Fermi level, while the F-terminated ASiCNRs are always semiconductors independent of their width but the edge states do not appear due to the Si-C dimer bonds at the edges. The charge density contours analyses shows that the Si-F and Si-C bonds are all ionic bonds due to the much stronger electronegativities of the F and C atoms than that of the Si atom. However, the C-F bonds display a typical non-polar covalent bonding feature because of the electronegativity difference between the F and C atoms of 1.5 is a much smaller than that of between the F and Si atoms of 2.2, as well as the tighter bounded C 2s ²2p ² electrons with smaller orbital radius than the Si 3s ²3p ² electrons. For both the F- and the H-terminated ZSiCNRs, the ground state is a ferromagnetic semiconductor.     

Vanadium sulfide nanoribbons: Electronic and magnetic properties

We report the structural, electronic and magnetic properties of zigzag-type 2H-VS₂ nanoribbons based on the first-principles calculations. Our results suggest that the zigzag-type 2H-VS₂ nanoribbons are intrinsic ferromagnetic or ferrimagnetic materials dependent on their edge structures. The S-terminated VS₂ nanoribbons with or without hydrogen saturation at the edges are ferromagnetic, whereas V-terminated VS₂ nanoribbons are ferrimagnetic at their ground states. The average magnetic moment per V atom of VS₂ nanoribbons increases monotonously with their width, but still smaller than that of perfect VS₂ monolayer. These results imply the great potential of VS₂ nanoribbons in spintronics application.     

First-principles study on the metallic antiferromagnet Co[PhPO3]·H2O

The electronic structure and the magnetic properties of transition metal phosphonate Co(PhPO₃)·H₂O have been studied by first-principles within the density-functional theory (DFT) and the full potential linearized augmented plane wave (FP-LAPW) method. The total energy, the total magnetic moment, the atomic spin magnetic moments and the density of states(DOS) of Co(PhPO₃)·H₂O were all calculated. The calculations reveal that the title compound is a metallic antiferromagnet and has a metallic ferromagnetic metastable state, which are in good agreement with the experiment. The spin magnetic moment of Co(PhPO₃)·H₂O is about 4.93 _μB${\mu }_{\mathrm{B}}$ per molecule, and it is mainly assembled at the cobalt atom, at the same time, with a little contribution from the P, O1, O2, O3.     

First principles study on the electronic structure of the two chain compounds of [ M(N3)2(HCOO)][(CH3)2NH2] (M=Fe and Co)

First principles calculations have been performed to study the electronic structure and the ferromagnetic properties on the two chain compounds of [M(N₃)₂(HCOO)][(CH₃)₂NH₂] (M=Fe and Co). The relative stability of the ground state, the density of states and the electronic band structure are examined. The results reveal that antiferromagnetism (AFM) state is the ground state and ferromagnetism (FM) state is the metastable one for both of them. The two compounds exhibit semiconductor character with small gap in the FM state, while metallic in the AFM state. In the FM state, the magnetic moments mainly arise from the Fe and Co ions with little contribution from the nearest-neighboring N and O atoms due to the hybridization between the Fe or Co 3d states and the nearest-neighboring N and O 2p states.     

A first-principle study of half-metallic ferrimagnetism in the Ti2CoGa Heusler compound

Electronic structure calculations based on density functional (DFT) theory within the generalized gradient approximation (GGA) for the Ti₂CoGa Heusler compound have been performed using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. The electronic band structures and density of states of the Ti₂CoGa compound show that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.5 eV, resulting in stable half-metallic ferrimagnetic behavior with a magnetic moment of 2μ_B.     

Electronic properties of MoS2 nanoribbons with disorder effects

A theoretical study of electronic properties on MoS₂ nanoribbon is made on focusing the calculation of zero bias transport in the presence of disorders. Disorders including intrinsic and extrinsic vacancies and also weak uniform scatter defects are considered. The calculations are based on the tight-binding Green's function formalism by including an iterative procedure. The Slater–Koster transformations are used to determine the parameters. This model reduces the numerical calculation time. The unsaturated atoms at the edge of armchair (zigzag) ribbon induce some mid-gap states with nearly high (low) localization, which act as scattering centers. The antiresonances of created quasi-localized states due to vacancy cause the conductance of the armchair nanoribbon to decrease. Finally, the zigzag ribbon provides the highest sensitivity as well as selectivity between the smaller energy range, in the presence of the single weak scatter with potential value of 2 eV at the edge of the ribbon.     

氢化与非氢化石墨烯纳米条带的密度泛函研究

基于密度泛函理论的第一性原理计算方法,研究了宽度N=8的边缘氢化和非氢化条带的结构和电子性质. 研究表明,扶手形无氢化石墨纳米条带的边缘碳原子是以三重键相互结合,它在边缘的成键强度比氢化时要高,具有更强的化学活性,可作为纳米化学传感器的基础材料. 能带结构计算表明,无论是扶手形条带还是锯齿形条带,它们都是具有带隙的半导体,且无氢化条带的带隙要比氢化的条带带隙宽度大,氢化对于条带的电子性质具有显著修饰作用. 通过锯齿形石墨纳米条带顺磁性、铁磁性和反铁磁性的计算,发现反铁磁的状态最稳定,并且边缘磁性最强,这有利于条带在自旋电子器件中的应用. 关键词： 石墨纳米条带 成键机理 电子结构 自旋分布