Electronic structures of Stone–Wales defective chiral (6,2) silicon carbide nanotubes: First-principles calculations

By using first-principle calculations based on density functional theory, the geometries and electronic structures of the Stone–Wales defective chiral (6,2) silicon carbide nanotubes (SiCNTs) are investigated. Independent on their orientations, Stone–Wales defects form two asymmetric pentagons and heptagons coupled in pairs (5-7-7-5) and a defect energy level in the band gap of the SiCNT. By applying transverse electric fields, significant differences in the electronic structures of the defective (6,2) SiCNTs are achieved, which may provide the foundation of identifying the orientation of Stone–Wales defects in chiral SiCNTs.     

Effect of vacancy defect on electrical properties of chiral single-walled carbon nanotube under external electrical field

Ab initio calculations demonstrated that the energy gap modulation of a chiral carbon nanotube with mono-vacancy defect can be achieved by applying a transverse electric field. The bandstructure of this defective carbon nanotube varying due to the external electric field is distinctly different from those of the perfect nanotube and defective zigzag nanotube. This variation in bandstructure strongly depends on not only the chirality of the nanotube and also the applied direction of the transverse electric field. A mechanism is proposed to explain the response of the local energy gap between the valence band maximum state and the local gap state under external electric field. Several potential applications of these phenomena are discussed.     

Tuning the electronic properties of single-walled SiC nanotubes by external electric field

The electronic properties of SiC nanotubes (SiCNTs) under external transverse electric field were investigated using density functional theory. The pristine SiCNTs were semiconductors with band-gaps of 2.03, 2.17 and 2.25 eV for (6,6), (8,8) and (10,10) SiCNTs, respectively. It was found the band gaps was reduced with the external transverse electric filed applied. The (8,8) and (10,10) SiCNTs changed from semiconductor to metals as the intensity of electric field reached 0.7 and 0.5 V/Å. The results indicate that the electronic properties of SiCNTs can be tuned by the transvers electric field with integrality of the nanotubes.     

外加横向电场作用下石墨烯纳米带电子结构的密度泛函紧束缚计算

采用基于密度泛函理论的紧束缚方法计算研究了外加横向电场对边缘未加氢/加氢钝化的扶手椅型石墨烯纳米带的电子结构及电子布居数的影响.计算结果表明,石墨烯纳米带的能隙变化受其宽带影响.当施加沿其宽度方向的横向外加电场时,纳米带的能带结构及态密度都会产生较大的变化.对于具有半导体性的边缘未加氢纳米带,随着所施加电场强度的增加,会发生半导体-金属的转变.同时,电场也会对能级分布产生显著影响.外加电场导致纳米带内原子上电子布居数分布失去对称性,电场强度越大,其布居数不对称性越明显.边缘加氢钝化可以显著改变纳米带内原子上的布居数分布.     

Tunable electronic and optical properties of SnC/BAs heterostructure by external electric field and vertical strain

Based on the first-principles method, we investigate the electronic structure of SnC/BAs van der Waals (vdW) heterostructure and find that it has an intrinsic type-II band alignment with a direct band gap of 0.22 eV, which favors the separation of photogenerated electron–hole pairs. The band gap can be effectively modulated by applying vertical strain and external electric field, displaying a large alteration of band gap via the strain and experiencing an indirect-to-direct band gap transition. Moreover, the band gap of the heterostructure varies almost linearly with external electric field, and the semiconductor-to-metal transition can be realized in the presence of a strong electric field. The calculated band alignment and the optical absorption reveal that the SnC/BAs heterostructure could present an excellent light-harvesting performance. Our designed heterostructure is expected to have great potential applications in nanoelectronic devices and photovoltaics and optical properties.     

电场作用下染料掺杂手性向列相液晶器件激光辐射谱研究

研究了电场作用下染料掺杂手性向列相液晶器件激光辐射谱。设计了两种电极结构,分别对正性和负性液晶器件施加横向和纵向电场,采用532 nm的Nd∶YAG脉冲固体激光器泵浦样品。对正性液晶器件施加电场,在 630～660 nm范围获得多波长的激光输出。对负性液晶激光器件施加电场,获得调谐范围为18.5 nm的激光输出。由器件织构和光子禁带的变化,进行了深入的分析。正性液晶器件,在电场力矩与扭曲力矩相互竞争过程中,引起液晶的流动,光子禁带上下浮动,因此不仅在禁带边沿,禁带内也出现激光辐射。而负性液晶器件随着电场强度增大,液晶螺距收缩,禁带蓝移,输出激光波长从681.0 nm蓝移到662.5 nm,出射激光波长为光子禁带边沿处。负性液晶器件在电场作用下的稳定性较好。     

Effect of electric field on the electrical conductivity of defected carbon nanotube: Multifractal properties of the wavefunctions

A theoretical analysis of controllable metal–insulator transition is performed by carrying out a quantum chaos analysis for a single-walled carbon nanotube which is affected by topological Stone–Wales defect. Nanotubes have recently attracted attention as promising materials for flexible nanoelectronic devices. Individual topological Stone–Wales defects have been identified experimentally in carbon nanotubes (CNTs) and graphene. The findings reveal that defected CNT displays a gradual crossover from metal to insulator phase in a longitudinal electric field. By determining the threshold value of the electric field for metal–insulator transition, CNT may be used as a switch in electronic devices. Our results are obtained by calculating the singularity spectrum of a nearest-neighbor tight-binding model. Also, quantum chaos theory is used for obtaining a detailed understanding of a dynamic phase transition from delocalized states (chaotic) to localized states (Poisson). More interestingly, the appearance of negative differential resistance for pure CNT suggests potential applications in nanoelectronic devices.     

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.     

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.     

The giant Stark effect in armchair-edge phosphorene nanoribbons under a transverse electric field

We study the variation of electronic properties for armchair-edge phosphorene nanoribbons (APNRs) modulated by a transverse electric field. Within the tight-binding model Hamiltonian, and by solving the differential Schrödinger equation, we find that a band gap closure appears at the critical field due to the giant Stark effect for an APNR. The gap closure has no field polarity, and the gap varies quadratically for small fields but becomes linear for larger ones. We attribute the giant Stark effect to the broken edge degeneracy, i.e., the charge redistributions of the conduction band minimum and valence band maximum states localized at opposite edges induced by the field. By combined with the Green's function approach, it is shown that in the presence of the critical field a gap of density of states (DOS) disappears and a high value DOS turns up at the energy position of the band gap closure. Finally, as the field increases, we find the band gap decreases more rapidly and the gap closure occurs at smaller fields for wider ribbons. Both the band gap and DOS variations with the field show an insulator-metal transition induced by a transverse electric field for the APNR. Our results show that wider APNRs are more appreciable to design field-effect transistors.     

Effects of an electric field on the electronic and optical properties of zigzag boron nitride nanotubes

We have investigated the electro-optical properties of zigzag BNNTs, under an external electric field, using the tight binding approximation. It is found that an electric field modifies the band structure and splits the band degeneracy. Also the large electric strength leads to coupling the neighbor subbands which these effects reflect in the DOS and JDOS spectrum. It has been shown that, unlike CNTs, the band gap of BNNTs can be reduced linearly by applying a transverse external electric field. Also we show that the larger diameter tubes are more sensitive than small ones. The semiconducting metallic transition can be achieved through increasing the applied fields. The number and position of peaks in the JDOS spectrum are dependent on electric field strength. It is found that at a high electric field, the two lowest subbands are oscillatory with multiple nodes at the Fermi level.     

Band gap tuning of Ge/SiC bilayers under an electric field: a density functional study

The structure and electronic properties of Ge/SiC van der Waals (vdW) bilayer under the influence of an electric field have been investigated by the first-principles method. Without an electric field, the system shows a small band gap of 126 meV at the equilibrium state. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the negative E-field changes from 0.0 to ?0.40 V/Å, the band gap first increases to a maximum of about 378 meV and then decreases to zero. A similar trend is exhibited for the positive E-field, ranging from 0.0 to +0.40 V/Å. The band gap reaches a maximum of about 315 meV at +0.10 V/Å. The significant variations of band gap are owing to different states of Ge, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the Ge/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Modulation of band gap by normal strain and an applied electric field in SiC-based heterostructures

The structure and electronic properties of the WS₂/SiC van der Waals (vdW) heterostructures under the influence of normal strain and an external electric field have been investigated by the ab initio method. Our results reveal that the compressive strain has much influence on the band gap of the vdW heterostructures and the band gap monotonically increases from 1.330 to 1.629 eV. The results also imply that electrons are likely to transfer from WS₂ to SiC monolayer due to the deeper potential of SiC monolayer. Interestingly, by applying a vertical external electric field, the results present a parabola-like relationship between the band gap and the strength. As the E-field changes from to ?0.50 +0.20 V/Å, the band gap first increases from zero to a maximum of about 1.90 eV and then decreases to zero. The significant variations of band gap are owing to different states of W, S, Si, and C atoms in conduction band and valence band. The predicted electric field tunable band gap of the WS₂/SiC vdW heterostructures is very promising for its potential use in nanodevices.     

Electronic properties of telescoping carbon nanotubes under external fields

In this work, we use the tight-binding model to study the low-energy electronic properties of telescoping double-walled carbon nanotubes subject to the influences of a transverse electric field and a parallel magnetic field. The state energy and energy spacings are found to oscillate significantly with the overlapping length. External fields would modify the state energies, alter the energy gaps, and destroy the state degeneracy. Complete energy gap modulations can be accomplished either by varying the overlapping length, or by applying an electric field or a magnetic field. The variations of state energies with the external fields will be directly reflected in the density of states. The numbers, heights, and frequencies of the density of states peaks are strongly dependent on the external fields.     

Full electrical control of the electron spin relaxation in GaAs quantum wells

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by time-resolved photoluminescence spectroscopy. By applying an external electric field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under a transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.     

Mechanism of Carbon Nanotubes Aligning along Applied Electric Field

The mechanism of single-walled carbon nanotubes （SWCNTs） aligning in the direction of external electric field is studied by quantum mechanics calculations. The rotational torque on the carbon nanotubes is proportional to the difference between the longitudinal and transverse polarizabilities and varies with the angle of SWCNTs to the external electric field. The longitudinal polarizability increases with second power of length, while the transverse polarizability increases linearly with length. A zigzag SWCNT has larger longitudinal and transverse polarizabilities than an armchair SWCNT with the same diameter and the discrepancy becomes larger for longer tubes.     

Transverse conductivity behavior at threshold electric field for CDW sliding in NbSe3

We have measured the dynamical properties which occur in the transverse direction to the conducting chains in NbSe₃ single-crystals, when the CDW slides along the chains. A sharp decrease in transverse conductivity takes place above an electric field less than the longitudinal threshold one for CDW sliding; that may result from induced phase shifts between CDW chains. Under the joint application of dc and rf driving fields voltage Shapiro steps for longitudinal transport are observed as usual but also pronounced current Shapiro steps in transverse direction. The possible mechanisms of this effect as well as a tentative new view on the origin of the narrow band noise in CDW compounds are discussed.     

Influence of strain and electric field on the properties of silicane

We investigate the influence of strain and electric field on the properties of a silicane sheet. Some elastic parameters of silicane, such as an in-plane stiffness of 52.55 N/m and a Poisson’s ratio of 0.24, are obtained by calculating the strain energy. Compared with silicene, silicane is softer because of its relatively weaker Si-Si bonds. The band structure of silicane is tunable by a uniform tensile strain, with the increase of which the band gap decreases monotonously. Moreover, silicane undergoes an indirect-direct gap transition under a small strain, and a semiconductor-metal transition under a large strain. The electric field can change the Si-H bond length of silicane significantly. When a strong field is applied, the H atom at the high potential side becomes desorbed, while the H atom at the low potential side keeps bonded. So an external electric field can help to produce single-side hydrogenated silicene from silicane. We believe this study will be helpful for the application of silicane in the future.     

Plasmon resonance coupling in strongly coupled gold nanotube arrays with structural defects

We theoretically investigate the transmission spectra and the field distributions with different defects in the gold nanotube arrays by using the finite-difference time-domain method.It is found that the optical properties of the nanotube arrays are strongly influenced by different defects.When there are no defects in the central nanotube,the values of peaks located at both sides of the photonic band gap have their maxima.Based on the distributions of electric field component E x and the total energy distribution of the electric and the magnetic field,we show that mainly a dipole field distribution is exhibited for the plasmon mode at the long-wavelength edge of the band gap but higher order modes of the composite are excited at the short-wavelength edge of the band gap.The plasmon resonant modes can also be controlled by introducing defects.