缺陷对碳纳米管摩擦与运动行为的影响

本文采用分子动力学方法研究了公度、无公度情况下含空位、Stone-Thrower-Wales(STW)型缺陷的单壁碳纳米管(SWCNT)在石墨基底上的摩擦与运动行为.结果表明,公度时缺陷的存在导致了界面局部无公度,减小了摩擦.随着碳纳米管底部STW缺陷的增多,碳纳米管变形增大,侧向力波动的幅值减小,局部无公度性增强,摩擦减小.含空位缺陷的碳纳米管所受的摩擦力明显大于含STW缺陷的碳纳米管,原因在于含空位缺陷的碳纳米管在运动的后期出现了明显的翻转现象,增大了能量耗散.无公度时,碳纳米管与石墨基底间的摩擦力很小,缺陷对其摩擦力影响不大,原因在于无论是否含有缺陷,碳纳米管与石墨组成的界面的无公度性差别不大.     

有限长双壁碳纳米管的电子输运性质

基于Landauer公式，研究了有限长的非公度和公度双壁碳纳米管的电子输运性顾，结果表明 ，双壁管的几何结构对其电子输运性质有显著的影响：非公度的双壁碳管的电导随能量的不 同，既可以是弹道型的，也可以是非弹道型的；由armchair管组成的公度的双壁碳管的电导 随能量变化呈现快速的电导振荡，并且此快速振荡叠加在背景慢振荡上，而zigzag管组成的 公度双壁管的电导随能量变化只有快速振荡、没有规则的慢振荡背景. 关键词： 碳纳米管 电子输运性质     

Helicity and electron-correlation effects on transport properties of double-walled carbon nanotubes

Starting from selection rules for intershell tunneling in double-walled nanotubes with commensurate (c-DWNTs) and incommensurate (i-DWNTs) shells, we show that for i-DWNTs the coupling is negligible between lowest energy subbands, but it becomes important as the higher subbands become populated. In turn the elastic mean-free path of i-DWNTs is reduced for increasing energy, with additional suppression at subband onsets and crossings. At low energies, a Luttinger liquid theory for DWNTs with metallic shells is derived. Interaction effects are more pronounced in i-DWNTs than in c-DWNTs.     

Band structure and CK α emission of ultrathin nanotubes

An attempt is made to calculate the energy bands and spectra of the characteristic CK _α emission of small-diameter carbon nanotubes. The calculated spectra for the nanotubes are compared with similar spectra for graphite monolayers used as a test object and with known experimental results for nanotubes. It is concluded that the x-ray emission spectra can be used to identify thin carbon nanotubes. A classification of solid-phase carbon is proposed which takes into account the position of carbon nanotubes in the family of allotropic carbon forms. The type of hybridization of the electrons in the carbon atom is used as the criterion for classification.     

Soliton density and intensity of commensurate magnetic resonance lines in incommensurate systems

The intensity of the “commensurate” lines appearing in the magnetic resonance spectra of incommensurate systems in the presence of a multisoliton lattice is evaluated as a function of the soliton density. It is shown that the apparent soliton density obtained from a comparison of the intensities of the commensurate lines in the incommensurate and commensurate phases is different from the soliton density obtained by a comparison of the theoretical and experimental lineshapes and defined as the volume fraction of the incommensurate domain walls.     

Regular chemisorption of hydrogen on achiral single-walled carbon nanotubes

Regular chemisorption of hydrogen on achiral single-walled carbon nanotubes has been investigated with the use of AM1 quantum-chemical semiempirical method. It has been found that regular hydrogen chemisorption deforms nanotubes, in some cases leading to stable prismatic modifications. The dependence of the adsorption energy on the density of hydrogen coverage has been found. A procedure for determining the adsorption energy by the spectra of thermally stimulated desorption has been proposed.     

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.     

Scattering of light in a ferroelectric with incommensurate phase: K2ZnCl4

We presented and analyzed spectra of Raman scattering in various phase states of K₂ZnCl₄ crystals, including the incommensurate phase. On the basis of this analysis we determined the parameters of modes associated with internal and external motions of ions. We found that the phase mode reveals itself in the Raman scattering spectra; this mode is responsible for the instability of the crystal lattice in the vicinity of the incommensurate phase transition.     

Influence of the Pseudogap on the Inelastic Neutron Scattering Spectra of the Underdoped Lanthanum Cuprate

The influence of pseudogap on the inelastic neutron scattering spectra of the underdoped lanthanum cuprate is studied on the basis of the model which incorporates both the superconducting state and pseudogap state.It is found that the striking effects of the influence of the pseudogap on the incommensurability of the spin excitation spectrum are that in the superconducting state the pseudogap makes the intensity of the incommensurate peak increase,in the normal state the pseudogap not only makes the intensity of the incommensurate peak increase,but also sharpens the incommensurate peak and increases incommensurability.     

Two energy scales in the magnetic resonance spectrum of electron and hole doped pnictide superconductors

We argue that a multiband superconductor with sign-changing gaps may have multiple spin resonances. We calculate the RPA-based spin resonance spectra of a pnictide superconductor by using the five-band tight-binding model or angle-resolved photoemission spectroscopy Fermi surface (FS) and experimental values of superconducting gaps. The resonance spectra split in both energy and momenta due to the effects of multiband and multiple gaps in s(±) pairing; the higher energy peak appears around the commensurate momenta due to scattering between α-FS to γ/δ-FS pockets. The second resonance is incommensurate, coming from β-FS to γ/δ-FS scatterings, and its q vector is doping-dependent and, hence, on the FS topology. Energies of both resonances ω(res)(1,2) are strongly doping-dependent and are proportional to the gap amplitudes at the contributing FSs.     

Defect electron states in carbon nanotubes and graphite from the NEXAFS spectroscopy data

Experimental results of studying the electronic structure of single-walled and multi-walled carbon nanotubes as well as graphite by X-ray absorption spectroscopy (or NEXAFS spectroscopy) are presented. The C1s absorption spectra are measured with high energy resolution using the equipment of the Russian-German beamline of the BESSY electron storage ring. Features found in absorption spectra of carbon nanotubes and graphite for the first time are interpreted in the case of carbon nanotubes as the contribution of electron states appearing due to the imperfection of their structure both under the nonequilibrium synthesis conditions and during the subsequent producing manipulations.     

Dynamics and spectra of excited states of water-micellar suspensions of single-walled carbon nanotubes

The dynamics of the photoinduced differential absorption and excited-state bleaching spectra of single-walled carbon nanotubes suspended in a micellar solution were studied in the spectral range from 40 to 1000 nm within a time interval from 70 fs to 150 ps under excitation by 50-fs pulses with photon energies 2 and 4 eV. The bleaching and absorption bands were observed in the spectra; the positions of the bleaching peaks were independent of the photon energy of the exciting femtosecond pulse in the range 2–4 eV. It was established that, for delay times shorter than 1 ps, the shape of the differential spectrum of excited nanotubes coincided with the shape of the second derivative of the absorption spectrum of unexcited nanotubes in the frequency range of exciting pulse above 18000 cm^?1 (the range of absorption bands of metallic nanotubes). In the frequency range below 16000 cm^?1 (the range of absorption peaks of semiconducting nanotubes), the bleaching peaks in the differential spectrum of excited nanotubes undergo a high-frequency shift of 200–300 cm^?1 with respect to the second-derivative spectrum of unexcited nanotubes. The excited-state relaxation rate constants were measured. They are well approximated by the exponential dependences and depend on the probe-pulse wavelength. An assumption was made about the nature of the observed spectra of excited nanotubes and about the excitation relaxation.     

Statics and dynamics of the modulation in incommensurate Rb2ZnBr4 as detected by NMR

The dynamics of the incommensurate modulation of Rb₂ZnBr₄ is investigated near the transition to the normal high-temperature phase using first-order quadrupole effects in nuclear magnetic resonance (NMR).⁸⁷Rb NMR spectra and two-dimensional⁸⁷Rb NMR exchange spectra are reported. All results can be described consistently in terms of a static modulation in the incommensurate phase without any indication for “floating” or large-scale fluctuations of the modulation wave. The spectra taken about 135 K below T_i in the lower incommensurate phase well above the soliton regime show no indication for the existence of a higher-order commensurate modulation in Rb₂ZnBr₄.     

Charge-carrier separation in rolled heterostructures

The strains in rolled InAs/GaAs heterostructures of nanometer sizes are calculated. It is shown that the strain distributions in nanotubes (structures with coherently bounded rolls) and nanoscrolls are essentially different, resulting in different energy spectra of the charge carriers. Photogenerated electrons and holes in nanotubes can be spatially separated across the wall width.     

One-dimensional and two-dimensional quantum systems on carbon nanotube bundles

We report the first measurement of the structure of 4He atoms adsorbed on bundles of single-walled carbon nanotubes. Neutron diffraction techniques and nanotube samples closed at the end were used. At low coverage, 4He forms a 1D, single line lattice along the grooves between two nanotubes on the surface of the nanotube bundles. As coverage is increased, additional lines of 1D lattices form along the grooves. This is followed by an incommensurate, 2D monolayer covering the whole nanotube bundle surface. The lattice constants of these 1D and 2D systems are largely independent of filling once a single 1D line is formed. No occupation of the interstitial channels between nanotubes is observed in the present sample.     

Super-slippery carbon nanotubes

The friction between the walls of multi-wall carbon nanotubes is shown to be extremely low in general, with important details related to the specific choice of the walls. This is governed by a simple expression revealing that the phenomenon is a profound consequence of the specific symmetry breaking: super-slippery sliding of the incommensurate walls is a Goldstone mode. Three universal principles of tribology, offering a recipe for lubricant selection are emphasized. Received 8 August 2001     

Energy of K-momentum dark excitons in carbon nanotubes by optical spectroscopy

Phonon sideband optical spectroscopy determines the energy of the dark K-momentum exciton for (6,5) carbon nanotubes. One-phonon sidebands appear in absorption and emission, split by two zone-boundary (K-point) phonons. Their average energy locates the E11 K-momentum exciton 36 meV above the E11 bright level, higher than available theoretical estimates. A model for exciton-phonon coupling shows the absorbance sideband depends sensitively on the K-momentum exciton effective mass and has minimal contributions from zone-center phonons, which dominate the Raman spectra of carbon nanotubes.     

硼掺杂单壁碳纳米管吸附甲醛的电子结构和光学性能研究

应用基于第一性原理的密度泛函理论研究了硼原子取代掺杂的（8,0）碳纳米管吸附甲醛气体分子的束缚能、电子结构以及光吸收和反射性质.研究发现,硼原子掺杂的碳纳米管对甲醛气体具有较强的敏感性,其束缚能大大增加,电荷转移更加显著,吸收峰和反射峰增多,峰值减小,且在低能区发生蓝移现象,在能量约为172 eV处均出现一特征峰.对计算结果进行了讨论,期望利用硼掺杂碳管来制作检测甲醛的纳米传感器和光电器件. 关键词： 碳纳米管 甲醛 硼原子取代掺杂 光学性能     

Manifestation of Spatial Modulation in the Vibration Spectra of (N(CH3)4)2CuCl4 Crystals

The specific manifestations of the spatial modulation in the vibration spectra of (N(CH₃)₄)₂CuCl₄ crystals have been investigated. It has been shown that because of the narrow temperature range of existence of the incommensurate phase and the complexity of the spectra being analyzed, reliable resolution of additional modes arising under the influence of both incommensurate and commensurate modulation is only possible using a computer approximation of the spectra obtained. In accordance with the theoretical models considered, additional modes have been observed mainly in the region of low-frequency skeletal vibrations exhibiting a marked dispersion.     

拓扑缺陷对单壁碳纳米管电子结构及其光学光谱的影响

应用密度泛函理论计算了半导体型单壁碳纳米管(7,0)和(8,0)以及其发生镜像对称和非镜像对称Stone-Wales形变、形成异质结(7,0)—(8,0)情况下的能带结构、吸收光谱、反射光谱,并对计算结果进行了比较。研究发现： 引入拓扑缺陷态后,碳纳米管的能带结构发生了明显的变化,费米能级在不同缺陷情况下移动方向不一致;碳管的吸收和反射明显减弱且吸收峰和反射峰在低能区发生红移现象;在光子能量约为E=13 eV处各碳管的吸收谱和反射谱中均出现一特征峰,并且在引入缺陷以后该特征峰向高能区移动。文章对计算结果进行了分析和探讨,可望利用这种拓扑缺陷的引入而产生的光电特性来设计碳管光电器件。