Band structure and optical transitions in semiconducting double-wall carbon nanotubes

The electronic structure of semiconducting double-wall carbon nanotubes (CNTs) is calculated using the linearized augmented cylindrical wave method. The consideration is performed in the framework of the local density functional theory and the muffin-tin (MT) approximation for the one-electron Hamiltonian. The electronic spectrum of a double-wall CNT is determined by the free motion of electrons in the interatomic space of the two cylindrical layers, scattering by the MT spheres, and tunneling through the classically impenetrable region. Calculated results for double-wall CNTs of the (n, 0)@(n′, 0) zigzag type indicate that the shift of the band-gap width depends on whether n and n′ are divided by 3 with a remainder of 1 or 2. It is found that, regardless of the type of the inner tube, the energy gap E _g of the outer tube decreases by 0.15–0.22 eV if the tube belongs to the sequence n = 2 (mod 3). For the outer tubes of the sequence n = 1 (mod 3), the shifts of the band gap ΔE _g are always negative ?0.15 ≤ ΔE _g ≤ ?0.05 eV. In both cases, the shifts ΔE _g weakly oscillate rather than decrease in going to tubes of a larger diameter d. For the inner tubes, the changes in the band gap ΔE _g are more sensitive to the diameter. At 10 ≤ n ≤ 16, the shifts ΔE _g are positive and the maximum value of ΔE _g equals 0.39 and 0.32 for the sequences n = 2 (mod 3) and n = 1 (mod 3), respectively. In going to the inner tubes of a larger diameter, ΔE _g rapidly drops and then oscillates in the range from ?0.05 to 0.06 eV. The calculated results indicate that the shifts of the optical band gaps in core and shell tubes upon the formation of double-wall CNTs are significant, which must hinder the identification of double-wall CNTs by optical methods. On the other hand, the obtained results open up possibilities for a more detailed classification of double-wall nanotubes.     

Simplified synthesis of double-wall carbon nanotubes

We demonstrate a simplified synthesis technique for double-wall carbon nanotubes that is an adaptation of chemical vapor deposition (CVD) techniques used previously for the production of single-wall nanotubes. Double-wall nanotubes (DWNTs) provide ideal geometries for numerous fundamental structural, electronic, thermal and vibrational studies, as well as providing a unique new platform for practical applications. The diameter distribution of DWNTs is broad, and it is possible that in previous studies using CVD-grown small-diameter nanotubes, presumed to be single-wall, there were significant numbers of DWNTs present.     

Phonon spectrum of double-wall carbon nanotubes

The results of studying the phonon spectra of carbon nanotubes within the framework of the periodic-cluster model are presented. Special attention is paid to the phonon spectrum of double-wall nanotubes. It is shown that the spectrum is separated into two subbands corresponding to the acoustic and optical vibrational modes. A specific feature of the phonon dispersion curves is their “doublet” character. The spectrum has an acoustic mode corresponding to longitudinal oscillations of the two walls of a double-wall carbon nanotube with respect to each other.     

Length-independent inter-tube conductance in double-wall carbon nanotubes

Conductance due to inter-tube transfer in incommensurate double-wall tubes is numerically studied. The conductance is usually much smaller than e²/π and its average and fluctuation are independent of the length. This behavior arises because the inter-tube transfer oscillate around zero in a complex plain as a function of position in a quasi-periodic manner and therefore cancel each other when being summed up except near tube edges.     

双壁纳米碳管的制备及其结构研究

用含有铁族金属硫化物的复合石墨棒作阳极，在氢气氛围下实施电弧放电，制备出了双壁纳米碳管.经透射电子显微镜观察与分析，发现在蒸发室内壁及阴极周围的附着物中，都含有双壁纳米碳管. 关键词： 双壁纳米碳管 透射电子显微镜     

Atomic correlation between adjacent graphene layers in double-wall carbon nanotubes

Atomic correlation between adjacent graphene layers was elucidated for double-wall carbon nanotubes (DWNTs) through a chiral index assignment of two nested nanotubes by high-resolution transmission electron microscopy. Our analysis provides a rather constant diameter difference close to 0.75 nm but no chiral angle correlation between the constituent nanotubes in the concentric DWNTs. The local atomic correlation as a commensurate graphene stacking was repeatedly found in eccentric DWNTs and circumscribed nanotubes, which should lead to elastic deformation and bundling of nanotubes.     

Structure and electronic properties of the double-wall nanotubes constructed from SiO2 nanotubes encapsulated inside zigzag carbon nanotubes

This paper presents ab initio self-consistent field crystal orbital calculations on the structures, stabilities, elastic and electronic properties of the double-wall nanotubes made of SiO(2) nanotubes encapsulated inside zigzag carbon nanotubes based on density functional theory. It is found that formation of the combined systems is energetically favorable when the nearest distance between the two constituents is in the area of the van der Waals effect. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. Based on the deformation potential theory and effective mass approximation, the mobilities of charge carriers are calculated to be in the range of 10(2)-10(4) cm(2) V(-1) s(-1), the same order of magnitude as those of the corresponding zigzag carbon nanotubes. The Young's moduli are also calculated for the combined systems.     

Structure, stability, and motion of dislocations in double-wall carbon nanotubes

In this paper,a novel double-wall carbon nanotube(DWCNT) with both edge and screw dislocations is studied by using the molecular dynamics(MD) method.The differences between two adjacent tubule indexes of armchair and zigzag nanotubes are determined to be 5 and 9,respectively,by taking into account the symmetry,integrality,and thermal stability of the composite structures.It is found that melting first occurs near the dislocations,and the melting temperatures of the dislocated armchair and zigzag DWCNTs are around 2600 K-2700 K.At the premelting temperatures,the shrink of the dislocation loop,which is comprised of edge and screw dislocations,implies that the composite dislocation in DWCNTs has self-healing ability.The dislocated DWCNTs first fracture at the edge dislocations,which induces the entire break in axial tensile test.The dislocated DWCNTs have a smaller fracture strength compared to the perfect DWCNTs.Our results not only match with the dislocation glide of carbon nanotubes(CNTs) in experiments,but also can free from the electron beam radiation under experimental conditions observed by the high resolution transmission electron microscope(HRTEM),which is deemed to cause the motion of dislocation loop.     

Determination of optical isomers for left-handed or right-handed chiral double-wall carbon nanotubes

The handedness relationship between adjacent layers in nested double-wall carbon nanotubes (DWNTs) has been investigated for the first time. Our high-resolution electron microscopy analysis on a series of specimen tilts can successfully tell the handedness of each constituent nanotube in a DWNT, and therefore the chiral indices (n, m) including their optical isomers [(n, m) or (m, n)] of inner and outer nanotubes can be uniquely determined. It is shown that right-handed and left-handed nanotubes are equally distributed for both the inner and outer nanotubes in the examined specimens and a preferable handedness relationship between the adjacent layers in DWNT may exist.     

拉伸形变下BC3纳米管的能带结构

运用紧束缚能带理论,研究拉伸形变下BC₃纳米管的能带结构. 研究表明:随着拉伸和压缩强度的不断增加,BC₃纳米管的导带能级和价带能级逐渐靠近,最终发生能带交叠. 压缩形变下能带的交叠程度可达05 eV,而拉伸形变下只有02 eV. 对于扶手椅型BC₃纳米管,随着拉伸和压缩的不断增加,BC₃纳米管首先由直接半导体转化为间接半导体,进而发生能带的交叠,表现出金属性. 在无形变时,扶手椅型BC₃纳米 关键词： 3纳米管')" href="#">BC₃纳米管 能隙 拉伸形变 半导体     

螺旋上升对自激发锯齿型双壁碳纳米管振荡行为的影响

运用分子动力学方法模拟了锯齿型双壁碳纳米管体系的振荡行为,其中旋转的内管施加了不同大小的螺旋上升长度.不同于以前关于扶手椅型碳纳米管的工作(Zeng Y H,et al.2016 Nanotechnology 27 95705),锯齿型的内管在施加了不同大小的螺旋上升长度之后,其管壁结构会产生畸变或缺陷.模拟过程中,锯齿型内管在施加一定的旋转速度以后保持自由,而固定的外管为无任何缺陷的理想锯齿型碳纳米管.分子动力学模拟结果显示锯齿型内管的轴向振荡行为与内管施加的螺旋上升长度密切相关.内管的振荡频率随着内管螺旋上升长度的增加而增加.但当内管的螺旋上升长度较大时,由于螺旋上升所引起的内管缺陷结构造成整个内管的破裂,从而导致其无法进行稳定的轴向振荡.模拟结果还显示,对于无螺旋上升的理想锯齿型碳管,虽然其轴向振荡效果非常微弱,但却可以作为一种具有恒定旋转频率的旋转致动器.此外,对螺旋上升长度为0.5 nm的内管在不同温度下的振荡性能进行了模拟分析,结果表明内管振荡的幅度随温度的升高而相应地增加,但当温度超过一定的临界值后,内管不能保持稳定的振荡.     

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional(2D) layered materials have potential applications in nanoelectronics, nanophotonics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes(LEDs), and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.     

Band engineering of B_2H_2 nanoribbons

Freestanding honeycomb borophene is unstable due to the electron-deficiency of boron atoms. B_2H_2 monolayer, a typical borophene hydride, has been predicted to be structurally stable and attracts great attention. Here, we investigate the electronic structures of B_2H_2 nanoribbons. Based on first-principles calculations, we have found that all narrow armchair nanoribbons with and without mirror symmetry(ANR-s and ANR-as, respectively) are semiconducting. The energy gap has a relation with the width of the ribbon. When the ribbon is getting wider, the gap disappears. The zigzag ribbons without mirror symmetry(ZNR-as) have the same trend. But the zigzag ribbons with mirror symmetry(ZNR-s) are always metallic. We have also found that the metallic ANR-as and ZNR-s can be switched to semiconducting by applying a tensile strain along the nanoribbon. A gap of 1.10 eV is opened under 16% strain for the 11.0-■ ANR-as. Structural stability under such a large strain has also been confirmed. The flexible band tunability of B_2H_2 nanoribbon increases its possibility of potential applications in nanodevices.     

Halloysite nanotubes template-induced fabrication of carbon/manganese dioxide hybrid nanotubes for supercapacitors

Ionics - Carbon/manganese dioxide (C/MnO2) hybrid nanotubes were prepared by the successive deposition of carbon and manganese dioxide using natural halloysite nanotubes as template. The...     

Band engineering of honeycomb monolayer CuSe via atomic modification

Two-dimensional monolayer copper selenide(Cu Se) has been epitaxially grown and predicted to host the Dirac nodal line fermion(DNLF). However, the metallic state of monolayer Cu Se inhibits the potential application of nanoelectronic devices in which a band gap is needed to realize on/off properties. Here, we engineer the band structure of monolayer Cu Se which is an analogue of a p-doped system via external atomic modification in an effort to realize the semiconducting state.We find that the H and Li modified monolayer Cu Se shifts the energy band and opens an energy gap around the Fermi level.Interestingly, both the atomic and electronic structures of monolayer Cu HSe and Cu Li Se are very different. The H atoms bind on top of Se atoms of monolayer Cu Se with Se–H polar covalent bonds, annihilating the DNLF band of monolayer Cu Se dominated by Se orbitals. In contrast, Li atoms prefer to adsorb at the hexagonal center of Cu Se, preserving the DNLF band of monolayer Cu Se dominated by Se orbitals, but opening band gaps due to a slight buckling of the Cu Se layer. The realization of metal-to-semiconductor transition from monolayer Cu Se to Cu X Se(X = H, Li) as revealed by first-principles calculations makes it possible to use Cu Se in future electronic devices.     

Band engineering of ZnS by codoping for visible-light photocatalysis

Codoping is demonstrated as an efficient approach to narrow the band gap of ZnS and enhance its photocatalytic activity. Herein, we perform the density-function theory calculations of ZnS by codoping of X (N, F) with transition metals (TM = V, Cu). The band gap is reduced in four different types of codoped ZnS. In particular, Cu_ZnF_S codoping, a charge-compensated donor–acceptor pair, leads to an about 32 % reduction of the energy gap, thus extending the absorption edge to visible-light region. The band gap reduction is due to the upshift of the top valence band comprised with the delocalized hybridizing levels of Cu 3d and S 3p states, and the downshift of the bottom conduction band consisting of F 2s states. Moreover, the larger value of m _e*/m _h* in Cu_ZnF_S–ZnS would result in a lower recombination rate of the electron–hole pairs. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in codoped ZnS. These findings raise the prospect of using codoped ZnS with specifically engineered electronic properties in a variety of photocatalytic applications.     

Band gap of carbon nanotubes under combined uniaxial–torsional strain

Within tight-binding model, the band gaps of armchair and zigzag carbon nanotubes (CNTs) under both uniaxial tensile and torsional strains have been studied. It is found that the changes in band gaps of CNTs depend strongly on the strain type. The torsional strain can induce a band gap for armchair CNTs, but it has little effect on band gap of the zigzag CNTs. While the tensile strain has great effect on band gap of zigzag CNTs, but it has no effect on that of the armchair CNTs. More importantly, when both the tensile and torsional strains are simultaneously applied to the CNTs, the band gap changes of armchair CNTs are not equal to a simple sum over those induced separately by uniaxial tensile and torsional strains. There exists a cooperative effect between two kinds of strains on band gap changes of armchair CNTs. But for zigzag CNTs, the cooperative effect was not found. Analytical expressions for the band gaps of armchair and zigzag CNTs under combined uniaxial–torsional strains have been derived, which agree well with the numerical results.     

Band offsets at semiconductor-oxide interfaces from hybrid density-functional calculations

Band offsets at semiconductor-oxide interfaces are determined through a scheme based on hybrid density functionals, which incorporate a fraction alpha of Hartree-Fock exchange. For each bulk component, the fraction alpha is tuned to reproduce the experimental band gap, and the conduction and valence band edges are then located with respect to a reference level. The lineup of the bulk reference levels is determined through an interface calculation, and shown to be almost independent of the fraction alpha. Application of this scheme to the Si-SiO2, SiC-SiO2, and Si-HfO2 interfaces yields excellent agreement with experiment.     

Transitional failure of hybrid carbon nanotubes under multiaxial loads

Transitional failure envelopes of hybrid single-walled carbon nanotubes functionalized by functional groups and filled with butane molecules under combined tension–torsion are predicted using classical molecular dynamics simulations. The observations reveal that while the tensile failure load decreases with combined torsion, the torsional buckling moment increases with combined tension. As a result, the failure envelopes under combined tension–torsion are definitely different from those under pure tension or torsion. In such combined loading, there is a multitude of failure modes (tensile failure and torsional buckling), and the failure therefore exhibits the feature of transitional failure envelopes. In addition, the functionalization by functional groups decreases both tensile failure load and torsional buckling moment, while filling with butane molecules increases only the torsional buckling moment. Consequently, the transitional failure envelopes of functionalized and filled nanotubes are absolutely different relative to what is predicted for pristine nanotubes.