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.     

碳掺杂硼氮纳米管电子场发射的第一性原理研究

对闭口硼氮纳米管（BNNT）顶层掺碳体系,运用第一性原理研究了电子场发射性能.结果表明,掺碳的BNNT体系电子结构变化显著;外电场愈强,体系态密度向低能端移动幅度愈大,且最高占据分子轨道（HOMO）/最低未占据分子轨道（LUMO）能隙愈小.体系态密度和局域态密度,HOMO和LUMO及其能隙分析一致表明,各种碳掺杂体系中C_eqBNNT的场发射性能最佳. 关键词： 硼氮纳米管 碳掺杂 第一性原理     

Electron transport in nanotube-ribbon hybrids

The electronic and transport properties of nanotube-ribbon hybrids subject to the influences of a transverse electric field are investigated theoretically. The energy dispersion relations are found to exhibit rich dependence on the nanotube-ribbon interactions, the field strength, and the geometry of the hybrids. The nanotube-ribbon coupling will modify the subband curvature, create additional band-edge states, and change the subband spacing or energy gap. The bandstructures are asymmetric and symmetric about the Fermi energy when the interactions are turned on and off, respectively. The inclusion of a transverse electric field will further alter the bandstructures and lift the degeneracy of the partial flat bands in hybrid (IV). The chemical-potential-dependent electrical and thermal conductance exhibit a stepwise increase behavior. Variations in the electronic structures with field strength will be reflected in the electrical and thermal conductance. Prominent peaks, as well as single-shoulder and multi-shoulder structures in the electrical and thermal conductance are predicted when varying the electric field strength and the nanotube location. The features of the conductance are found to be strongly dependent on the field strength, the geometry and the temperature.     

Transport properties of AB-stacked bilayer graphene nanoribbons in an electric field

The electronic and thermal properties of AB-stacked bilayer graphene nanoribbons subject to the influences of a transverse electric field are investigated theoretically, including their transport properties. The dispersion relations are found to exhibit a rich dependence on the interlayer interactions, the field strength, and the geometry of the layers. The interlayer coupling will modify the subband curvature, create additional band-edge states, change the subband spacing or energy gap, and separate the partial flat bands. The bandstructures will be symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The inclusion of a transverse electric field will further alter the bandstructures and lift the degeneracy of the partial flat bands. The chemical-potential-dependent electrical and thermal conductance exhibit a stepwise increase behavior. Variations in the electronic structures with field strength will be reflected in the electrical and thermal conductance. Prominent peaks, as well as single-shoulder and multi-shoulder structures in the electrical and thermal conductance are predicted when varying the electric field strength. The features of the conductance are found to be strongly dependent on the field strength, the geometry, interlayer interactions and temperature.     

p—HgCdTe反型层中子能带电子的基态能量

用变分自洽方法求解了ｐ－ＨｇＣｄＴｅ金属－绝缘体－半导体（ＭＩＳ）结构Ｎ型反型层子能带的基态能量Ｅ０及其与表面电子浓度的关系。计算中考虑了窄禁带半导体Ｈｇ１－ｘＣｄｘＴｅ带间相互作用所引起的非抛物带结构、波函数平均效应、共振缺陷态、Ｚｅｎｅｒ隧穿、以及电场在屏蔽长度内的衰减等因素，导出了子能带基态能量Ｅ０的计算公式并获得了与实验符合较好的结果。     

p-HgCdTe反型层中子能带电子的基态能量

用变分自洽方法求解了ｐ－ＨｇＣｄＴｅ金属－绝缘体－半导体（ＭＩＳ）结构Ｎ型反型层子能带的基态能量Ｅ₀及其与表面电子浓度的关系，计算中考虑了窄禁带半导体Hg_1-xCd_xTe带间相互作用所引起的非抛物带结构、波函数平均效应、共振缺陷态、Ｚｅｎｅｒ隧穿、以及电场在屏蔽长度内的衰减等因素，导出了子能带基态能量Ｅ₀的计算公式并获得了与实验符合较好的结果。     

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.     

氮掺杂及水分子吸附碳纳米管电子场发射第一性原理研究

对闭口碳纳米管(CNT)顶端分层掺氮及吸附不同数目水分子体系,运用第一性原理研究了有电场存在时的电子场发射性能.结果表明：掺氮并吸附水分子的CNT结构稳定;外电场愈强、水分子数愈多,体系态密度(DOS)向低能端移动幅度愈大且最高分子占据轨道(HOMO)/最低分子空轨道(LUMO)能隙愈小.吸附能,DOS/LDOS,HOMO/LUMO及其能隙分析一致表明,第三层氮掺杂CNT吸附不同数目水分子体系的场发射性能最佳. 关键词： 氮掺杂 水吸附 密度泛函理论 电子场发射     

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.     

First-principle calculation on the defect energy level of carbon vacancy in 4H--SiC

First, electronic structures of perfect wurtzite 4H-SiC were calculated by using first-principle ultra-soft pseudo- potential approach of the plane wave based on the density functional theory; and the structure changes, band structures, and density of states were studied. Then the defect energy level of carbon vacancy in band gap was examined by substituting the carbon in 4H-SiC with carbon vacancy. The calculated results indicate the new defect energy level generated by the carbon vacancy, and its location in the band gap in 4H-SiC, which has the character of deep acceptor. A proper explanation for green luminescence in 4H-SiC is given according to the calculated results which are in good agreement with our measurement results.     

Band mixing in narrow gap semiconductors

The interaction of conduction and valence bands in narrow gap semiconductors such as InSb and HgCdTe influences the position and width of subband energy levels in space-charge layers. While a nonzero width can only occur if electrons from the conduction band can tunnel into approximately degenerate states of the valence band the level shifts due to band mixing are always present. We present a Green's function treatment which allows in a simple way to discuss the dependence of band mixing effects on the parameters of thek·p-Hamiltonian in particular the band gap. The essential qualitative feature of the level shifts is adecrease of subband energy separation withdecreasing effective mass. This agrees with recent experimental results for Hg_1-x Cd _x Te.     

Tunable band structure effects on ballistic transport in graphene nanoribbons

Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry Ek=E−k of the subband dispersion. When applied together, they reverse the dispersion parity to be odd and gives Ee,k=−Eh,−k and mix the electron and hole subbands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range.     

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.     

The atomic and electron structure of ZrO2

The atomic structure of amorphous and crystalline zirconium dioxide (ZrO₂) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO₂ is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO₂ consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO₂, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO₂ phase point to the existence of both light (0.3m ₀) and heavy (3.5m ₀) holes, where m ₀ is the free electron mass. The effective masses of band electrons in ZrO₂ fall within (0.6–2.0)m ₀.     

Electronic properties of zero-dimensional finite-sized nanographene

In this work, we use the tight-binding model to study the low-energy electronic properties of zero-dimensional finite-sized nanographene subject to the influence of an electric field. State energies and energy spacings are found to oscillate significantly with the field strength. The state energies and band gaps also rely upon the type of the nanographene. The electric field will modify state energies, alter energy gaps, and induce the complete energy gap modulations. The band gap of the type-IV nanographene is always zero regardless of the value of the field strength. The variations of the state energies will be directly reflected in the density of states. The numbers and frequencies of the density of states’ divergent peaks are strongly dependent on the field strength and the type of the nanographene. Finally, the electron wave functions are found to be localized at certain zigzag lines at zero electric field.     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

电场调控双层石墨烯纳米带的电子结构和光学性质

利用基于密度泛函理论的第一性原理方法,研究了外加电场作用下双层AA堆垛的Armchair边缘石墨烯纳米带(BAGNRs)的电子结构和光学性质. BAGNRs具有半导体特性,其带隙随带宽(宽度为4～12个碳原子)的增加而振荡性减小.当施加电场后,BAGNRs的带隙随着电场强度的增加而逐渐减小,带隙越大对电场值的变化越敏感.当电场值为0.5 V/?时,所有BAGNRs的带隙都为零. BAGNRs具有各向异性的光学性质,其介电函数在垂直极化方向为半导体特性,而在平行极化方向为金属特性.在外加电场的作用下,BAGNRs的介电函数、吸收系数、折射系数、反射系数、电子能量损失系数和光电导率,其峰值向低能量区域移动,即产生红移现象.电场增强了能带间的跃迁几率.纳米带宽度对这些光学性质参数具有不同程度的影响.研究结果解释了电场调控BAGNRs光学性质的规律和微观机理.     

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/AlxGa1-xN heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/Al x Ga 1 x N heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation.The variations of Stark energy shift with electric field,impurity position,Al component and areal electron density are discussed.Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states.For a given impurity position,the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect.The weakening of the blue and red shifts,induced by the screening effect,strengthens gradually with the increase of electric field.Furthermore,the screening effect weakens the mixture crystal effect,thereby influencing the Stark effect.The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法，对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构，态密度以及分态密度的计算和比较，发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据；而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加，决定带隙宽度的CBM的位置下降，同时VBM的位置上升，从而导致了带隙的变窄，出现了红移现象. 此外，Cd掺杂会使晶胞发生膨胀，这种张应变也是导致Cd     

Cd掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法,对掺杂Cd导致ZnO禁带宽度下降的机理进行了研究. 通过对掺杂前后电子能带结构,态密度以及分态密度的计算和比较,发现Cd_xZn_1-xO价带顶端(VBM)始终由O-2p占据;而导带顶端(CBM)则由Cd-5s与Zn-4s杂化轨道控制. 随着掺杂浓度的增加,决定带隙宽度的CBM的位置下降,同时VBM的位置上升,从而导致了带隙的变窄,出现了红移现象. 此外,Cd掺杂会使晶胞发生膨胀,这种张应变也是导致Cd 关键词： 密度泛函理论 电子结构 Cd掺杂ZnO