Uniaxial-stress effects on electronic structures of nanographite ribbons

The uniaxial-stress effects on the low-energy electronic properties of nanographite ribbons are studied by the tight-binding model. The dependence on the strain, the edge structure, the ribbon width, and the stacking sequence is strong. The strain could induce the alternation of energy dispersions, the destruction of state degeneracy, the variation of energy gap, the semiconductor–metal transition, and the change of special structures in density of states. The effects of strain are important for the AB- and AA-stacked armchair ribbons. However, they are negligible for the AB- and AA-stacked zigzag ribbons. Armchair ribbons could exhibit the semiconductor–metal transition. Such transition is mainly determined by the strain and the ribbon–ribbon interactions.     

石墨带的晶格动力学研究

基于晶格动力学理论,采用力常数模型,计算了石墨带的声子色散关系、振动模式密度和比热.计算结果表明,石墨带的声子谱特征介于一维碳纳米管和二维石墨片之间.扶手椅型和锯齿型石墨带的中、高频声子支分别与锯齿型和扶手椅型碳纳米管的类似.由于声子限域效应,低频声子支随着石墨带带宽的改变出现明显的频移现象.振动模式密度在高频区几乎不敏感于带宽,而低频区的峰位随着带宽的增加而逐渐向低频移动.此外,无论是在低温还是高温,比热都随着带宽的增加而逐渐降低,呈现量子尺寸效应.在300K时,比热可以拟合成C_V=C_Vg+A/n,其中C_Vg为石墨片的热容,而A/n项反映了石墨带中边缘效应对比热的影响. 关键词： 石墨带 声子色散关系 比热     

Calculations of electric capacitance in carbon and BN nanotubes, and zigzag nanographite (BN, BCN) ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The electronic states with the opposite electric charges separated along both edges are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano-functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons, the electronic structure is always like that of semiconductors. The calculated capacitance is inversely proportional to the distance between the positive and negative electrodes.     

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.     

Effect of oxidation of graphene nanoribbons on electronic and magnetic properties

The electronic properties, band gap and ionization potential as well as the energies of the singlet and triplet states of zigzag and armchair graphene nanoribbons are calculated as a function of the number of oxygen atoms on the ribbon employing density functional theory at B3LYP/6-31G* level. The calculated band gaps indicate that both structures are semiconducting. The band gap of the armchair ribbons initially decreases followed by an increase with oxygen number. For zigzag ribbons the band gap decreases with increasing oxygen number whereas the ionization potential increases with oxygen content. In both armchair and zigzag ribbons the ionization potential shows a gradual increase with the number of oxygen atoms. Some of the oxygenated ribbons calculated have triplet ground states and have the density of states at the Fermi level for spin down greater than spin up suggesting the possibility they may be ferromagnetic semiconductors.     

Electronic structures of the F-terminated AlN nanoribbons

Using the first-principles calculations, electronic properties for the F-terminated AlN nanoribbons with both zigzag and armchair edges are studied. The results show that both the zigzag and armchair AlN nanoribbons are semiconducting and nonmagnetic, and the indirect band gap of the zigzag AlN nanoribbons and the direct band gap of the armchair ones decrease monotonically with increasing ribbon width. In contrast, the F-terminated AlN nanoribbons have narrower band gaps than those of the H-terminated ones when the ribbons have the same bandwidth. The density-of-states (DOS) and local density-of-states (LDOS) analyses show that the top of the valence band for the F-terminated ribbons is mainly contributed by N atoms, while at the side of the conduction band, the total DOS is mainly contributed by Al atoms. The charge density contour analyses show that Al–F bond is ionic because the electronegativity of F atom is much stronger for F atom than for Al atom, while N–F bond is covalent because of the combined action of the stronger electronegativity and the smaller covalent radius.     

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.     

Quantum transport in honeycomb lattice ribbons with armchair and zigzag edges coupled to semi-infinite linear chain leads

We study quantum transport in honeycomb lattice ribbons with either armchair or zigzag edges. The ribbons are coupled to semi-infinite linear chains serving as the input and output leads and we use a tight-binding Hamiltonian with nearest-neighbor hops. The input and output leads are coupled to the ribbons through bar contacts. In narrow ribbons we find transmission gaps for both types of edges. The appearance of this gap is due to the enhanced quantum interference coming from the multiple channels in bar contacts. The center of the gap is at the middle of the band in ribbons with armchair edges. This particle-hole symmetry is because bar contacts do not mix the two sublattices of the underlying bipartite honeycomb lattice when the ribbon has armchair edges. In ribbons with zigzag edges the gap center is displaced to the right of the band center. This breakdown of particle-hole symmetry is the result of bar contacts now mixing the two sublattices. We also find transmission oscillations and resonances within the transmitting region of the band for both types of edges. Extending the length of a ribbon does not affect the width of the transmission gap, as long as the ribbon’s length is longer than a critical value when the gap can form. Increasing the width of the ribbon, however, changes the width of the gap. In ribbons with zigzag edges the gap width systematically shrinks as the width of the ribbon is increased. In ribbons with armchair edges the gap is not well-defined because of the appearance of transmission resonances. We also find only evanescent waves within the gap and both evanescent and propagating waves in the transmitting regions.     

The electronic properties tuned by the phase transition between the semiconducting and metallic phase of monolayer MoS2/WS2

Using first-principles methods, we systematically investigate the electronic properties and atomic mechanism of the monolayer MoS₂/WS₂ homo-junction structure, which contains different phase structures, either the semiconducting hexagonal (H) structure or metallic trigonal (T) structure. Through tuning the size of the lateral homo-junction structure of either MoS₂ or WS₂, it can produce different boundaries which induce different phase transferred styles. More interestingly, the electronic structures of homo-junction structures can also be tuned by changing the size of the armchair and zigzag shapes of nanoribbons. The homo-junction structure of either MoS₂ or WS₂ exhibits alterable band structure and band edge position with the changing of the size. The strong dependence of the band offset on the sizes of the homo-junction monolayer also implicates a possible way of patterning quantum structures with size engineering.     

单空位缺陷对石墨纳米带电子结构和输运性质的影响

基于第一性原理电子结构和输运性质计算,研究了单空位缺陷对单层石墨纳米带(包括zigzag型和armchair型带)电子性质的影响.研究发现,单空位缺陷使石墨纳米带在费米面上出现一平直的缺陷态能带;单空位缺陷的引入使zigzag型半导体性的石墨纳米带变为金属性,这在能带工程中有重要的应用价值;奇数宽度的armchair型石墨纳米带表现出金属特性,有着很好的导电性能,同时,偶数宽度的armchair型石墨带虽有金属性的能带结构,但却有类似半导体的伏安特性;单空位缺陷使得奇数宽度的armchair石墨纳米带导电 关键词： 石墨纳米带 单空位缺陷 电子结构 输运性质     

Anisotropy of the Raman spectra of nanographite ribbons

A polarized Raman study of nanographite ribbons on a highly oriented pyrolytic graphite substrate is reported. The Raman peak of the nanographite ribbons exhibits an intensity dependence on the light polarization direction relative to the nanographite ribbon axis. This result is due to the quantum confinement of the electrons in the 1D band structure of the nanographite ribbons, combined with the anisotropy of the light absorption in 2D graphite, in agreement with theoretical predictions.     

Quasi-one dimensional graphite ribbon structures in the presence of a magnetic field and the on-site Coulomb correlation at half-filling

We have presented the role of the Coulomb interaction (U) and the magnetic field [(B)\vec]\vec{B} on the ground state properties of the quasi-one dimensional graphite ribbon structures at half-filling. Mean field Hartree-Fock Approximation is used to study the systems. To understand the boundary effects in graphite structures, we have compared the results of these systems with those of the square lattice ribbon structures. Studying the density of states, the Drude weight and the charge gap, we have drawn the U – B phase diagrams for the zigzag and the armchair graphite ribbons.     

Detection of valley currents in graphene nanoribbons

There are two valleys in the band structure of graphene zigzag ribbons, which can be used to construct valleytronic devices. We studied the use of a T junction formed by an armchair ribbon and a zigzag ribbon to detect the valley-dependent currents in a zigzag graphene ribbon. A current flowing in a zigzag ribbon is divided by the T junction into the zigzag and armchair leads and this separation process is valley dependent. By measuring the currents in the two outgoing leads, the valley-dependent currents in the incoming lead can be determined. The method does not require superconducting or magnetic elements as in other approaches and thus will be useful in the development of valleytronic devices.     

Doping effects on metallic and semiconductor single-wall carbon nanotubes

In this paper we investigate nitrogen- and boron-doped zigzag and armchair single-wall carbon nanotubes (SWNTs) with theoretical models based on the density functional theory. We take into account nitrogen and boron doping for two isomers in which substitutive atoms are on opposite sides of the tube, but only in one isomer the impurity sites are symmetrical with respect to the diameter. The band structures show a strong hybridization with impurity orbitals that change the original band structure. Although the two isomers of armchair SWNT exhibit the same formation energy, their band structures are different. Indeed asymmetrical isomers are gapless and exhibit a crossing of valence and conduction bands at k?=?π/c, leading to metallic SWNTs. Band structures of symmetrical isomers, on the other hand, exhibit an energy gap of 0.4?eV between completely filled valence and empty conduction bands. We use density of charge in order to understand this difference. In zigzag SWNT an impurity band is introduced in the energy gap and for N doping this band is just partially occupied in such a way that the electronic behaviour is reversed from semiconductor to metallic. Whereas for a given isomer armchair SWNT shows similar behaviours of N- and B-doped structures, B-doped zigzag SWNTs present different band structure and occupation compared to the N-doped case.     

Localized electronic states on graphite edge

Examining the band structure of graphite ribbons with a typical edge shapes of armchair or zigzag, we find that minute graphite in a nanometer scale shows a striking contrast in the π electronic states depending on the edge shape. A wide armchair ribbon can reproduces the electronic state of graphite, but a zigzag ribbon shows a pair of partly flat bands which gives a remarkable peak of density of states at the Fermi level. We derive the analytic solution of this peculiar Edge State, disclosing the puzzle of its emergence.     

Electronic properties of α-graphyne nanoribbons under the electric field effect

In this paper, we investigate the electronic structure of both armchair and zigzag α-graphyne nanoribbons. We use a simple tight binding model to study the variation of the electronic band gap in α-graphyne nanoribbon. The effects of ribbon width, transverse electric field and edge shape on the electronic structure have been studied. Our results show that in the absence of external electric field, zigzag α-graphyne nanoribbons are semimetal and the electronic band gap in armchair α-graphyne nanoribbon oscillates and decreases with ribbon's width. By applying an external electric field the band gap in the electronic structure of zigzag α-graphyne nanoribbon opens and oscillates with ribbon width and electric field magnitude. Also the band gap of armchair α-graphyne nanoribbon decreases in low electric field, but it has an oscillatory growth behavior for high strength of external electric field.     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

Magnetic properties of nanographite with modified zigzag edges

Newly proposed aromatic molecules and graphene fragments are shown to have the high-spin ground state by the first-principles electronic structure calculations. Our strategy to predict magnetic carbon materials is based on our previous conclusion that mono-hydrogenated, di-hydrogenated or mono-fluorinated zigzag edges of honeycomb networks are magnetic. Structural optimization as well as determination of the electronic states was performed for various nanographite ribbons and high-spin molecules, e.g. 1,8,9-di-hydro-anthracene, C₁₉H₁₄ and C₁₄F₁₃. For hydrogenated molecules and ribbons, the total spin S determined by the LSDA calculation coincides with the value expected from a counting rule for the total spin on a bipartite network. However, S depends on structures of fluorinated nanographite.     

Stability and electronic states of NC3 nanoribbons

We apply the first-principles method to investigate the electronic and structural properties NC₃ nanoribbons. The calculation results show that the stability does not depend on the ribbon width but depends on the edge type, where armchair structures are the more stable ones. The present nanostructures always have a metallic behavior. Such feature is connected with the spatial arrangement of N and C atoms, where the conducting behavior is associated to the contribution of p _z -like orbitals of carbon atoms and the presence of a carbon stripe. In addition, no net magnetization is observed for the calculated structures.     

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.