Bandgap tuning in armchair MoS(2) nanoribbon

We report on the first-principles calculations of bandgap modulation in armchair MoS(2) nanoribbon (AMoS(2)NR) by transverse and perpendicular electric fields respectively. In the monolayer AMoS(2)NR case, it is shown that the bandgap can be significantly reduced and be closed by transverse field, whereas the bandgap modulation is absent under perpendicular field. The critical strength of transverse field for gap closure decreases as ribbon width increases. In the multilayer AMoS(2)NR case, in contrast, it is shown that the bandgap can be effectively reduced by both transverse and perpendicular fields. Nevertheless, it seems that the two fields exhibit different modulation effects on the gap. The critical strength of perpendicular field for gap closure decreases with increasing number of layers, while the critical strength of transverse field is almost independent of it.     

Spin polarized quantum pump effect in zigzag graphene nanoribbons

The spin polarized adiabatic quantum pump effect in zigzag graphene nanoribbons has been numerically analyzed. Since the ground state of such a ribbon is antiferromagnetic (the opposite spin electrons are located on the opposite edges of the ribbon), the spin currents can be generated in this system with the help of the quantum pump effect when symmetry between the opposite spin states is broken. Two methods of this breaking by means of defects at the ribbon edge and the transverse electric field have been proposed. It has been shown that the generation of not only the electron and spin currents, but also the purely spin current is possible in both cases.     

Energy structure and electromagnetically induced transparency of neutral donor in a multi-hilled GaAs quantum ribbon

The theoretical study of the energy spectrum and optical response of off-axis donor in a GaAs quantum ribbon with anisotropic rim height under the presence of constant crossed electric and magnetic fields is presented. The calculations are carried out within the effective mass and parabolic approximation, using an adiabatic approach combined with a diagonalization scheme. The rim height of the quantum ribbon has been modeled by including a phenomenological two-parametric function that accounts for realistic features of multi-hilled semiconductor nanostructure obtained from atomic force microscopy images. The first phenomenological parameter is introduced to control the number of quantum ribbon structural hills and the second one to deal with the height of the hills. It is shown that both the depth and the number of structural hills tend to substantially affect the Aharonov–Bohm oscillation pattern which can be quenched or restored by applying an electric field in an appropriate direction. The effect of the changes in the geometry and in the applied electric field onto the optical absorption and refractive index change are discussed. The phenomenon of electric-field-induced optical transparency in the system is particularly highlighted.     

Gap opening and tuning in single-layer graphene with combined electric and magnetic field modulation

The energy band structure of single-layer graphene under one-dimensional electric and magnetic field modulation is theoretically investigated. The criterion for bandgap opening at the Dirac point is analytically derived with a two-fold degeneracy second-order perturbation method. It is shown that a direct or an indirect bandgap semiconductor could be realized in a single-layer graphene under some specific configurations of the electric and magnetic field arrangement. Due to the bandgap generated in the single-layer graphene,the Klein tunneling observed in pristine graphene is completely suppressed.     

Electromechanical and light tunable cholesteric liquid crystals

A light-responsive cholesteric liquid crystal (CLC) mixture is tuned with light and applied DC electric field. Interestingly, the reflection of the CLC is red shifted with UV light and blue shifted with applied DC electric fields. UV light exposure induces a red shift in the reflection of the CLC bandgap by as much as 800 nm from the original spectral position. Spatial variations in pitch (and thus reflection color) are shown to blue shift with applied DC field regardless of the photohistory and restore the notch position or image upon removal of the field. The ability to tune the reflection of the CLC bandgap on demand to the red or blue with multiple stimuli is a never before demonstrated effect that could have potential utility in lasing, optical filtering, or data communication applications.     

A comparative investigation of an AB- and AA-stacked bilayer graphene sheet under an applied electric field: A density functional theory study

An AB- and AA-stacked bilayer graphene sheet(BLG) under an electric field is investigated by ab initio calculation.The interlayer distance between the two layers,band structures,and atomic charges of the system are investigated in the presence of different electric fields normal to the BLG.The AB- stacked BLG is able to tune the bandgap into 0.234 eV with the increase of the external electronic field to 1 V/nm,however,the AA-stacked BLG is not sensitive to the external electric field.In both the cases,the spacing between the BLG slightly change in terms of the electric field.The charges in the AB- stacked BLG are increased with the increase of the electric field,which is considered to be the reason that causes the bandgap opening in the AB- stacked BLG.     

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.     

Electric field effect in ultrathin zigzag graphene nanoribbons

The electric field effect in ultrathin zigzag graphene nanoribbons containing only three or four zigzag carbon chains is studied by first-principles calculations, and the change of conducting mechanism is observed with increasing in-plane electric field perpendicular to the ribbon. Wider zigzag graphene nanoribbons have been predicted to be spin-splitted for both valence band maximum(VBM) and conduction band minimum(CBM) with an applied electric field and become half-metal due to the vanishing band gap of one spin with increasing applied field. The change of VBM for the ultrathin zigzag graphene nanoribbons is similar to that for the wider ones when an electric field is applied. However, in the ultrathin zigzag graphene nanoribbons, there are two kinds of CBMs, one is spin-degenerate and the other is spin-splitted, and both are tunable by the electric field. Moreover, the two CBMs are spatially separated in momentum space. The conducting mechanism changes from spin-degenerate CBM to spin-splitted CBM with increasing applied electric field. Our results are confirmed by density functional calculations with both LDA and GGA functionals, in which the LDA always underestimates the band gap while the GGA normally produces a bigger band gap than the LDA.     

Electrically Tunable Energy Bandgap in Dual-Gated Ultra-Thin Black Phosphorus Field Effect Transistors

The energy bandgap is an intrinsic character of semiconductors, which largely determines their properties. The ability to continuously and reversibly tune the bandgap of a single device during real time operation is of great importance not only to device physics but also to technological applications. Here we demonstrate a widely tunable bandgap of few-layer black phosphorus(BP) by the application of vertical electric field in dual-gated BP field-effect transistors. A total bandgap reduction of 124 meV is observed when the electrical displacement field is increased from 0.10 V/nm to 0.83 V/nm. Our results suggest appealing potential for few-layer BP as a tunable bandgap material in infrared optoelectronics, thermoelectric power generation and thermal imaging.     

Interface effect on structural and electronic properties of graphdiyne adsorbed on SiO_2 and h-BN substrates:A first-principles study

We use the first-principles calculation method to study the interface effect on the structure and electronic properties of graphdiyne adsorbed on the conventional substrates of rough SiO2 and flat h-BN. For the SiO2 substrate, we consider all possible surface terminations, including Si termination with dangling bond, Si terminations with full and partial hydrogenation, and oxygen terminations with dimerization and hydrogenation. We find that graphdiyne can maintain a flat geometry when absorbed on both h-BN and SiO2 substrates except for the Si termination with partial hydrogenation(Si-H) SiO2 substrate. A lack of surface corrugation in graphdiyne on the substrates, which may help maintain its electronic band character, is due to the weak Van der Waals interaction between graphdiyne and the substrate. Si-H SiO2 should be avoided in applications since a covalent type bonding between graphdiyne and SiO2 will totally vary the band structure of graphdiyne.Interestingly, the oxygen termination with dimerization SiO2 substrate has spontaneous p-type doping on graphdiyne via interlayer charge transfer even in the absence of extrinsic impurities in the substrate. Our result may provide a stimulus for future experiments to unveil its potential in electronic device applications.     

Electronic properties of bearded graphene nanoribbons

We investigate the electronic properties of graphene nanoribbons with attachment of bearded bonds as a model of edge modification. The main effect of the addition of the beards is the appearance of additional energy subbands. The originally gapless armchair graphene nanoribbons become semiconducting. On the other hand, the originally semiconducting armchair graphene nanoribbons may or may not change to gapless systems depending on the width. With the inclusion of a transverse electric field, the band structures of bearded graphene nanoribbons are further altered. An electric field creates additional band-edge states, and changes the subband curvatures and spacings. Furthermore, the energy band symmetry about the chemical potential is lifted by the field. With varying width, the bandgap demonstrates a declining zigzag behavior, and touches the zero value regularly. Modifications in the electronic structure are reflected in the density of states. The numbers and energies of the density of state divergent peaks are found to be strongly dependent on the geometry and the electric field strength. The beard also causes electron transfer among different atoms, and alters the probability distributions. In addition, the electron transfers are modified by the electric field. Finally, the field introduces more zero values in the probability distributions, and removes their left–right symmetry.     

Polarization dependent guiding in liquid crystal filled photonic crystal fibers

A theoretical study of nematic liquid crystal filled photonic crystal fibers (LCPCFs) is presented. Detailed investigations including the polarization dependent bandgap formation and the modal properties are given for LCPCFs, in which alignment of the molecules could be controlled by external static electric field. The polarization dependent bandgap splitting caused by the high index difference between the ordinary and the extraordinary dielectric index of nematic liquid crystals provides the possibility of single-mode single-polarization guiding. A polarization operation diagram is proposed to describe the guiding behavior of LCPCFs. The influence of rotation angle ? of the director of liquid crystals on the modal properties is investigated. It is shown that the polarization axis of the guided mode is determined by the rotation angle ?, which could be controlled by external electric field.     

外场调控下硼磷烯量子点的电子结构和光学性质研究

采用紧束缚近似方法对锯齿状六边形硼磷烯量子点在平面电场和垂直磁场调控下的电子结构和光学性质进行了研究. 研究表明,硼磷烯量子点作为直接带隙半导体,在无外加电场和磁场作用时,能隙不随尺寸的改变而变化. 在平面电场调控下,能隙随电场强度的增加逐渐减小直至消失,平面电场方向几乎不会对硼磷烯量子点体系产生影响, 且随量子点尺寸的增大,能隙消失所需电场强度逐渐减小. 在垂直磁场调控下,表现为体态的能级在磁场作用下形成朗道能级,而能隙边缘处的朗道能级近似为一个平带,不随磁通量的改变而变化,态密度主要分布于朗道能级处. 另外,垂直磁场作用下的光吸收主要是由朗道能级之间的跃迁引起的.     

Effect of alkali metals and superalkali species on electronic properties of graphdiyne with open hexagonal edges

In the present work, we study the new two dimensional materials with tunable bandgap and high chemical reactivity via locating the alkali metals and superalkalis (Li, Na, K, Li3O, Na3O, and K3O) on the graphdiyne (GDY) sheets with open hexagonal edges. Our density functional theory (DFT) calculations reveal that the bandgap energy of GDY with a different open zigzag or armchair edges (ZGDY and AGDY) is decreased by interacting with alkali metals and superalkali species. We found that the alkali metals and superalkalis lowered the bandgap energy of AGDY and ZGDY by more than 170% and converted them into the semimetals. Our results also show that the M3O/ZGDY systems show higher chemical reactivity as compared to the other studied systems. Among the studied M(M3O)/ZGDY(AGDY) systems, the maximum binding energy, maximum means static polarisability, minimum bandgap energy, and minimum chemical hardness are related to the K3O/ZGDY complex. Consequently, the K3O/ZGDY is a promising chemically reactive material for applications in various fields such as gas detectors, catalysts in the oxygen reduction reactions, and drug delivery application.     

A full numerical calculation of the Franz--Keldysh effect on magnetoexcitons in a bulk semiconductor

We have performed a full numerical calculation of the Franz--Keldysh (FK) effect on magnetoexcitons in a bulk GaAs semiconductor. By employing an initial value method in combination with the application of a perfect matched layer, the numerical effort and storage size are dramatically reduced due to a significant reduction in both computed domain and number of base functions. In the absence of an electric field, the higher magnetoexcitonic peaks show distinct Fano lineshape due to the degeneracy with continuum states of the lower Landau levels. The magnetoexcitons that belong to the zeroth Landau level remain in bound states and lead to Lorentzian lineshape, because they are not degenerated with continuum states. In the presence of an electric field, the FK effect on each magnetoexcitonic resonance can be identified for high magnetic fields. However, for low magnetic fields, the FK oscillations dominate the spectrum structure in the vicinity of the bandgap edge and the magnetoexcitonic resonances dominate the spectrum structure of higher energies. In the moderate electric fields, the interplay of FK effect and magnetoexcitonic resonance leads to a complex and rich structure in the absorption spectrum.     

Local modification of thin oxide films on a titanium surface

It is shown that a controllable structural rearrangement of a thin titanium oxide film, accompanied by a change in the bandgap width, can be accomplished under the probe tip of a scanning tunneling microscope (STM). The mechanism of the rearrangement is considered to be the migration of cations and oxygen vacancies in the electric field of the STM.     

Inhomogeneous extended hollow cathode discharge for raising the current density in a forevacuum plasma source of a ribbon electron beam

The plasma parameters and the emissivity of a ribbon electron beam source based on a discharge with an inhomogeneous extended hollow cathode are measured. A constriction in the cathode cavity increases the plasma density near the emitting area boundary, which adds to the electron current density in the beam. The reason for the above effect is the formation of the plasma density distribution nonuniform across the cavity with a maximum in the middle. This maximum is caused primarily by a plasma electron flow from the constriction, which is generated by the electric field and is directed toward a slit emission-extracting aperture.     

外磁场与带轴夹角对非晶FeSiB/Cu/FeSiB三明治薄带巨磁阻抗特性的影响

采用层间胶合方法制备了淬态非晶FeSiB/Cu/FeSiB三明治薄带,研究了同尺寸单层薄带和三明治薄带的巨磁阻抗(giant magneto-impedance, GMI)随外磁场与带轴夹角β的变化特性.结果表明,FeSiB单层薄带在7.0 MHz最佳响应频率下,GMI仅约30%,外磁场与带轴夹角对单层薄带GMI几乎没有影响;三明治薄带的GMI效应则十分显著,在0.6 MHz最佳响应频率下,纵、横向GMI比分别达到272%和464%, GMI随β的增大而增强;所有β角的三明治薄带GMI曲线都出现各向异性峰,各向异性峰随β的增大而展宽.根据磁畴转动模型推导了薄带横向磁导率与各向异性场及β之间的函数关系式.结果显示,三明治薄带GMI随夹角β变化的特性与理论推算的横向磁导率变化有较好的一致性,而单层薄带则不然.该磁畴转动模型能定性解释三明治薄带GMI随外磁场方向变化特性.     

结构参数对p-i-n结构InGaN太阳能电池性能的影响及机理

研究了器件结构参数对p-i-n结构InGaN单结太阳能电池性能的影响及物理机制. 模拟结果发现: 随着InGaN禁带宽度的增加, InGaN电池的短路电流减小, 但同时开路电压增加, 当InGaN层的禁带宽度为1.5 eV左右时, 同质p-i-n结InGaN电池的效率最高, 并计算了不同厚度的i层对InGaN电池效率的影响. 进一步的计算表明, 适当采用带宽更大的p-InGaN层形成异质p-i-n结InGaN电池可以获得更高效率, 但是p-InGaN层带宽过大也会导致电池的效率急剧下降. 研究还发现, 采用禁带宽度更大的n-InGaN层可以形成背电场, 从而增加p-i-n结InGaN太阳电池的效率. 研究结果表明, 适当选择p-InGaN和n-InGaN禁带宽度形成异质p-i-n结可以提高InGaN太阳能电池效率.     

Mechanical properties of graphdiyne sheet

The influences of strain to the energetic and electronic properties of graphdiyne are investigated based on first-principles calculations. The elastic parameters of graphdiyne are determined by total energy calculation. Compared to graphyne, graphdiyne is softer because it has less C–C bonds. Moreover, the band gap of graphdiyne is tunable under uniform strain. It monotonously increases with increasing strain value, which originates from the decreased orbital overlap between C atoms when strain increases.