Stability and electronic structure of InN nanotubes from first-principles study

The stability and electronic structure of hypothetical InN nanotubes were studied by first-principles density functional theory. It was found that the strain energies of InN nanotubes are smaller than those of carbon nanotubes of the same radius. Single-wall zigzag InN nanotubes were found to be semiconductors with a direct band gap while the armchair counterparts have an indirect band gap. The band gaps of nanotubes decrease with increasing diameter, similar to the case of carbon nanotubes.     

Common electronic band gaps and similar optical properties of ZnO nanotubes

Electronic and optical properties of single-walled zinc oxide （ZnO） nanotubes are investigated from the firstprinciples calculations. Electronic structure calculations show that ZnO nanotubes are all direct band gap semiconducting nanotubes and the band gaps are relatively insensitive to the diameter and chirality of tubes. The origin of the common electronic band gaps of ZnO nanotubes is explained in terms of band-folding from the two-dimensional band structure of graphite-like sheet. Moreover, the optical properties such as dielectric function and energy loss function spectra of different ZnO nanotubes are very similar, relatively independent of diameter and chirality of tubes. The calculated dielectric function and loss function spectra show a moderate optical anisotropy with respect to light polarization.     

Energy and electronic properties of non-carbon nanotubes based on silicon dioxide

The geometric, energy, and electronic characteristics of new non-carbon nanotubes based on silicon dioxide are investigated in the framework of the local electron density functional formalism. Nanotubes are classified according to the type of rolling-up of the SiO₂ sheet. It is shown that, among the entire set of considered nanotubes with different symmetries, the (6, 0) nanotubes are energetically more favorable. The densities of states for nanotubes are calculated. It is established that all nanotubes are dielectrics with a wide band gap. The band gap varies over a wide range with a change in the longitudinal strain of the nanotube.     

A full spd tight-binding treatment for electronic bands of graphitic tubes

A tight-binding (TB) treatment with the inclusion of d orbitals is applied to the electronic structures of graphitic tubes. The results show that the high angular moment bases in TB scheme are necessary to account the severe curvature effect in ultra-thin single wall carbon nanotubes, especially for properly reproducing the band edge overlap behavior in (5, 0) tube, predicted by the existing ab initio calculations. In the large diameter limit, the participation of two symmetry-allowed d bases provides a natural replication to the recent measured electronic dispersions of valence band of graphene when the strong anisotropy due to the two-dimensional planar hexagonal sheet structure is dealt with properly. In addition, the detailed relation between the two sets of quantum numbers of screw symmetry and that of zone folding is formulated in appendix.     

TiO2纳米管的力学和电子学性质

基于密度泛函理论研究了纤铁矿和锐钛矿型TiO₂纳米管的原子结构、稳定性、Young模量以及电子能带结构.计算结果显示：在纳米管直径较小时,锐钛矿型TiO₂纳米管的稳定性要好于纤铁矿型纳米管,随着管径的增大,纤铁矿型纳米管变得比锐钛矿型纳米管要更稳定.纤铁矿型TiO₂纳米管具有比锐钛矿型纳米管更大的Young模量,力学性能比较优异.另外,通过对电子能带结构的研究发现,手性对TiO₂纳米管的电子结构影响较大,纤铁矿（0,n）型和锐钛矿（n,0）型纳米管为间接带隙半导体,而纤铁矿（n,0）型和锐钛矿（0,n）型纳米管却具有直接带隙. 关键词： 2纳米管')" href="#">TiO₂纳米管 Young模量 间接带隙 直接带隙     

Encapsulation of small fullerenes into nitrogenated holey nanotubes: a density functional theory study

Encapsulation of fullerene into nanotubes based on a C₂N sheet, known as nitrogenated holey graphene, was investigated using density functional theory. The structural and electronic properties of these carbon hybrid materials, consisting of nitrogenated holey nanotubes and a small C₂₀ fullerene, were studied. The formation energies showed that encapsulation of the fullerene into the nitrogenated holey nanotube is an exothermic process. To characterise the electronic properties, the electronic band structure and density of states of armchair and zigzag nitrogenated holey nanotubes were calculated. Filling these nanotubes with the C₂₀ fullerene resulted in a p-type semiconducting character. The energy band gap of the nitrogenated holey nanotubes decreased with fullerene encapsulation. The results are indicative of the possibility of band gap engineering by encapsulation of small fullerenes into nitrogenated holey nanotubes.     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

Structure and properties of BeO nanotubes

The structure of a new non-carbon (beryllium oxide BeO) nanotube consisting of a rolled-up graphene sheet is proposed, and its physical properties are described. Ab initio calculations of the binding energy, the electronic band structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y for BeO nanotubes of different diameters are performed in the framework of the density functional theory (DFT). From a comparison of the binding energies calculated for BeO nanotubes and crystalline BeO with a wurtzite structure, it is inferred that BeO nanotubes can be synthesized by a plasma-chemical reaction or through chemical vapor deposition. It is established that BeO nanotubes are polar dielectrics with a band gap of ～5.0 eV and a stiffness comparable to that of the carbon nanotubes (the Young’s modulus of the BeO nanotubes Y _BeO is approximately equal to 0.7Y _C, where Y _C is the Young’s modulus of the carbon nanotubes). It is shown that, for a nanotube diameter D > 1 nm, the (n, n) armchair nanotubes are energetically more favorable than the (n, 0) zigzag nanotubes.     

Narrow band gap group III-nitride alloys

Recent results on the properties of narrow gap group III-nitrides and their alloys are reviewed. It is shown that InN with the energy gap of 0.7 eV exhibits classical characteristics of a narrow gap semiconductor with strongly nonparabolic conduction band and an energy dependent electron effective mass. With the new discovery, the direct band gaps of the group III-nitride alloys span an extremely wide energy range from near infrared in InN to deep ultraviolet in AlN offering possibilities for new device applications of these materials. We also discuss properties of dilute group III-N–V alloys in which incorporation of a small amount of nitrogen results in a dramatic band gap reduction. All the unusual properties of the alloys are well described by a band anticrossing model that considers an interaction between localized nitrogen states and the extended states of the conduction band.     

Variation of the electronic properties of zigzag boron nitride nanotubes by Al-doping: a DFT study

Boron nitride nanotubes (BNNTs) are semiconductors with a wide band gap. In comparison with carbon nanotubes (CNTs), BNNTs have higher chemical stability, excellent mechanical properties and higher thermal conductivity. In this paper, we study the effect of diameters and substituting B and N atoms of various zigzag BNNTs with Al, on structural and electronic properties of BNNTs in solid state using the density functional theory method. The results of calculations of density of states and band structure (band) showed that the band gap between the valence and conduction level increases as a result of the enhancement of tube diameter of BNNTs. Finally, the results showed that the electronic properties of the pristine BNNTs can be improved by doping Al atom in the zigzag configuration of tubes.     

TiO$lt;sub$gt;2$lt;/sub$gt;纳米管电子结构和光学性质的第一性原理研究

运用基于密度泛函理论的第一性原理方法, 系统研究了小尺寸锐钛矿相(n,0)型TiO₂纳米管(D<16 Å)的几何构型、电子结构和光学性质. 结果表明: 随着管径增大, 体系单位TiO₂分子的形成能降低, 体系趋于稳定; 在管径14 Å左右, (n,0)型TiO₂纳米管会发生一次构型的转变. 能带分析显示, TiO₂纳米管的电子态比较局域化, 小管径下(D<14 Å)其导电性更好; 随着构型的转变, TiO₂纳米管由直接带隙转变为间接带隙, 并且带隙值随着管径的增大而增大, 这是由于π轨道重叠效应的影响大于量子限域效应所导致的结果. 两种效应的竞争, 使得TiO₂纳米管的介电函数虚部ε₂ (ω)谱的峰值位置随管径增大既可能红移也可能蓝移, 管径大于9 Å (即(8, 0)管)之后, TiO₂纳米管的光吸收会出现明显的增强. 关键词： 2纳米管')" href="#">TiO₂纳米管 第一性原理 电子结构 光学性质     

New class of non-carbon AlP nanotubes: Structure and electronic properties

A new class of non-carbon nanotubes based on Group III and Group V elements (aluminum and phosphorus, respectively) is considered. The equilibrium geometry, energy characteristics, and electronic structure of the AlP nanotubes were calculated using the density functional theory. These calculations demonstrated that the AlP nanotubes are energetically stable structures. It was found that a low strain energy (approximately 0.01–0.07 eV) is required for rolling a two-dimensional hexagonal AlP structure into a tube. The AlP nanotubes are found to be wide-band-gap semiconductors with a band gap of 2.05–3.73 eV with direct (for the zigzag type) or indirect (for the armchair type) transitions between the top of the valence band and the bottom of the conduction band. The band gap of these nanotubes increases with the tube diameter, approaching the band gap of a two-dimensional hexagonal AlP layer.     

First-principles study of narrow single-walled GaN nanotubes

Structural and electronic properties of narrow single-walled GaN nanotubes with diameter from 0.30 to 0.55 nm are investigated using the density functional method with generalized-gradient approximation. The calculations of total energies predict that the most likely GaN nanotubes in our calculation are (2,2), (3,2) and (3,3) nanotubes. From a detailed analysis we find that these narrow single-walled GaN nanotubes are all semiconductors, of which the armchair and chiral tubes are indirect-band-gap semiconductors whereas the zigzag ones have a direct gap except for (4,0) tube. The indirect band gap of (4,0) tube can stem from band sequence change induced by curvature effect. Our results show that the π-π hybridization effect and the formation of benign buckling separations play a key role in the band sequence changes of (4,0) tube.     

Influence of sputtering power and Ar–N2 flow on the structure and optical properties of indium nitride films prepared by magnetron sputtering

ABSTRACT

This paper discusses the deposition of indium nitride (InN) thin films on Si (100) substrates by using pulsed DC magnetron sputtering. Effects of varying sputtering power and Ar–N₂ flow ratio on the structural, morphological, and optical properties of indium nitride (InN) films were investigated. The structural characterization indicated nanocrystalline InN film with preferred orientation towards (101) plane that exhibited the optimum crystalline quality at 130?W and for 40:60 Ar–N₂ ratio. The surface morphology of InN, as observed through FESEM, contained irregularly shaped nanocrystals with size that increases with higher sputtering power and Ar:N₂ flow ratio. The optical properties of InN films were studied using ellipsometer at room temperature. The band gap of InN was decreased with the increase of sputtering power to 130?W, whereas an increase in the band gap was noticed with the increase of the Ar:N₂ flow ratio.     

Reduction in the electronic band gap of titanium oxide nanotubes

The structural and electronic properties of individual titanium oxide nanotubes have been studied using both empirical and ab initio calculations. Two different types of titanium oxide nanotubes (A-nanotube and B-nanotube) have been constructed and energy-minimized by molecular mechanics calculations. We found that the A-nanotubes are energetically more favorable than the B-nanotubes. The electronic band structure of the titanium oxide nanotubes was also calculated with respect to the tubule diameter and the tubule type using the ab initio method. The band gap of the A-nanotube was reduced by up to 60% as the tubule diameter decreases from 1.2 nm to 0.5 nm.     

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides(TMDs) with different chalcogens on each side(X-TM-Y) have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes(DWNTs) from metal to semiconductor. Using density-function theory(DFT) with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs(X = O and S, inside a tube; Y = S and Se, outside a tube) depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms(X) of outer tube and outer atoms(Y) of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.     

Theory of B(2)O and BeB(2) nanotubes: new semiconductors and metals in one dimension

We describe two new boron-based nanotubes: B(2)O and BeB(2). Both are isoelectronic to graphite, have reasonable curvature energies, and have already been made in their bulk planar forms. The lowest energy allotrope of planar single-layer B(2)O is a semiconductor with a moderate band gap. The local density approximation band gap of the corresponding (3,0) B(2)O nanotube [similar in size to (9,0) carbon nanotube tube] is direct and around 1.6 eV, within a range inaccessible to previous C or BN nanotubes. Single-layer BeB(2) has a fascinating structure: the Be atoms rest above the boron hexagonal faces, nearly coplanar to the boron sheet. The unusual K-point pi-pi(*) Fermi-level degeneracy of graphite survives, while a new nearly pointlike Fermi surface appears at the M point. As a result, BeB(2) nanotubes are uniformly metallic.     

Mie resonances, infrared emission, and the band gap of InN

Mie resonances due to scattering or absorption of light in InN-containing clusters of metallic In may have been erroneously interpreted as the infrared band gap absorption in tens of papers. Here we show by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV. Microcathodoluminescence studies complemented by the imaging of metallic In have shown that bright infrared emission at 0.7-0.8 eV arises in a close vicinity of In inclusions and is likely associated with surface states at the metal/InN interfaces.     

Anisotropy of the dielectric function for wurtzite InN

A -plane InN film grown by molecular beam epitaxy on -plane sapphire substrate with an AlN nucleation layer and a GaN buffer was studied by spectroscopic ellipsometry. The data analysis yields both the ordinary and the extraordinary dielectric tensor components perpendicular and parallel to the optical axis, respectively. Strong optical anisotropy is demonstrated over the whole energy range from 0.72 up to 9.5 eV. The line shapes of the tensor components and the polarisation behaviour are in very good agreement with the results of recently published band structure and dielectric function calculations. Above the band gap, five van Hove singularities are evidenced from the ordinary component, while three are resolved from the extraordinary part. The polarisation dependence below 1 eV can be interpreted in terms of optical selection rules for three energetically split valence bands around the Γ-point of the Brillouin zone, similar to the well known behaviour of wurtzite GaN. This emphasises a band gap of hexagonal InN of about 0.7 eV.     

Band structure of InN

The energy band structure of wurtzite-structure semiconductive InN is predicted using empirical nearest-neighbor tight-binding theory. The tight-binding parameters are extrapolated from those of zincblende InP, InAs, and InSb by using empirical rules for the dependences of the parameters on bond length and on row of the Periodic Table. The predicted band gap is direct and agrees well with the data for this potential orange light-emitter. It is suggested that zincblende InN, if it can be grown, also will have a band-gap near 2 eV.