Studies on structural defects in carbon nanotubes

Structural defects in carbon nanotubes (CNTs) have been paid much attention, because they influence the properties of the CNTs to some extent. Among various defects in CNTs, both single vacancies and Stone-Wales (SW) defects are the simple and common ones. In this paper, we review the progress of research in these two kinds of defects in CNTs. For single vacancies, we first address their different structural features in both zigzag and armchair CNTs, and their stabilities in CNTs with different sizes and different symmetries systematically. The presence of the single vacancies in CNTs not only influences the electronic structures of the systems, but also affects the vibrational properties of the tubes. Nevertheless, being active chemically, the single vacancies in the tubes prefer to interact with adsorbates nearby, of which the interaction of the defects with hydrogen atom, hydrogen molecule and some small hydrocarbon radicals (-CH, -CH₂ and -CH₃) are discussed. The former is associated with H storage and the latter is of merit to improve the local structure of the defect in a CNT. For the Stone-Wales defect, we mainly focus on its stability in various CNTs. The influence of the SW defects on the conductance of CNTs and the identification of such a defect in CNT is described in brief.      

Superconductivity in thin films of boron-doped carbon nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with Tc=12 K and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.     

Pulsed laser deposition of multiwalled carbon nanotubes thin films

The physical/chemical properties of multiwalled carbon nanotubes have attracted much interest for applications in different fields, from micro-electronic to coating technology due, in particular, to their peculiar conductivity properties, to their hardness and high resistance to thermal stress. The technology to produce carbon nanotubes thin films with the desired properties, however, is still under development. In this work, we report on multiwalled carbon nanotubes thin films deposited by pulsed laser deposition techniques ablating commercially polystyrene-nanotubes pellets on alumina substrates. MicroRaman spectroscopy and high resolution Transmission Electron Microscopy provide the experimental confirmation that carbon nanotubes-like structures are present on the alumina surface with both minimal morphological damage of the tubes and structural changes induced by laser beam.     

Influence of structural defects on 1/f in thin niobium films

The 1/f noise in thin niobium films manufactured by various technologies and exposed to γ-quantum irradiation and current annealing has been studied. The evaporation conditions (the sublayer material and the substrate temperature) are shown to strongly affect the noise level. Irradiation adds to the film noise, whereas current annealing leads to decrease of the latter. The current dependence of the voltage fluctuations is found to be nonlinear. The fluctuations are proportional to the current in the region of low currents, and they are proportional to the square of the current flowing through the specimen in the region of high currents. The results are interpreted based on the relation of the 1/f noise to the mobile defects of the crystal structure.     

空位结构缺陷对C纳米管弹性性质的影响

用分子动力学方法对不同空位缺陷的扶手椅型与锯齿型单壁C纳米管杨氏弹性模量进行了计算和分析. 结果表明：扶手椅型（5, 5）, （10,10）和锯齿型（9, 0）, （18, 0） 纳米管在无缺陷时其杨氏模量分别为948,901和804,860 GPa. 随管径的增大,扶手椅型和锯齿型单壁C纳米管弹性模量分别减小和增大,表现出完全不同的变化规律. 随着C纳米管中单点空位缺陷的均匀增加,杨氏模量下降,当缺陷比率增加到一定程度时,杨氏模量下降骤然趋缓,形成一下降平台;双空位缺陷对C纳米管杨氏模量的影响与其分布方向有关;随单点空位缺陷间原子数的增加,在轴向上,杨氏模量下降到某一值小幅波动,而在周向上杨氏模量先下降,然后上升到某一稳定值. 随两单点空位缺陷的空间距离进一步增大,杨氏模量又呈微降趋势. 通过分子间σ键与π键特征及缺陷间近程电子云耦合作用规律与空位缺陷内部5-1DB缺陷的形成特点等理论对上述规律进行了分析. 关键词： 空位缺陷 C纳米管 分子动力学 杨氏模量     

On the role of structural defects during particle channeling through carbon nanotubes

The formation of structural defects during particle-flux channeling in an array of nanotubes is studied by the molecular dynamic method. The approach accounts for the formation of defects by means of the AIREBO potential. The characteristic dependences of the energy of the channeling particles on time are considered. Data are obtained on the types of effects formed and the energy of their formation.     

Carbon ad-dimer defects in carbon nanotubes

The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications. Barriers for the formation of carbon dimer induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect. The electronic states introduced by the ad-dimers depend on defect structure and tube type and size. Multiple carbon ad-dimers provide a route to structural engineering of patterned tubes that may be of interest for nanoelectronics.     

Preparation and structural properties of thin carbon films by very-high-frequency magnetron sputtering

Growth and structural properties of thin a-C films prepared by the 60 MHz very-high-frequency(VHF) magnetron sputtering were investigated. The energy and flux of ions impinging the substrate were also analyzed. It is found that the thin a-C films prepared by the 60 MHz sputtering have a lower growth rate, a smooth surface, and more sp~3 contents.These features are related to the higher ion energy and the lower ions flux onto the substrate. Therefore, the 60 MHz VHF sputtering is more suitable for the preparation of thin a-C film with more sp~3 contents.     

Optical properties of carbon nanotubes and BaTiO3 composite thin films

Multiwalled carbon nanotubes and BaTiO₃ composite films have been prepared by pulsed-laser deposition technique at room temperature and high temperature of 600℃, separately. The structures of the composite films are investigated by using scanning electron microscopy and x-ray diffraction. The optical behaviours of the samples produced at different temperatures are compared with Raman spectroscopy, and UV-visible absorption. And the observation by Z-scan technique reveals that the composite films have a larger optical nonlinearity, and the samples prepared at high temperatures have better transmittance and opposite sign imaginary part of optical third-order nonlinearity.     

Role of deposition conditions,impurities and structural defects in the epitaxial growth of thin films

A summary of the theoretical and experimental investigations carried out to understand the problem of epitaxy is given. The effect of deposition conditions and the nature of the substrate-deposit combination on the orientation of the final film is discussed. The importance of the nucleation and growth stages, reorientation and recrystallization effects and the influence of the impurities and defects in the substrate surface on the quality of the films deposited are presented.     

Synthesis and crystallinity of carbon nanotubes produced by a vapor-phase growth method

We have studied the effect of temperature on the growth and crystallinity of carbon nanotubes (CNTs), synthesized by a vapor-phase growth method using a catalytic reaction of iron pentacarbonyl (Fe(CO)₅) and acetylene (C₂H₂) gas. By increasing the growth temperature from 750 °C to 950 °C, both the growth rate and the diameter of the CNTs increase. Moreover, the crystallinity of the graphite sheets improves progressively with increasing growth temperature. Adjustment of the growth temperature gives potential for controlled growth of CNTs in a large-scale synthesis of CNTs. PACS 61.46.+w; 68.37.-d; 81.07.De     

Effect of crystallinity on proton conductivity in yttrium-doped barium zirconate thin films

The effect of crystallinity on proton conductivity in amorphous, single crystal and polycrystal yttrium-doped barium zirconate (BYZ) thin films grown 120 nm in thickness on amorphous (quartz) and single crystal MgO(100) substrates has been studied. The conductivity was measured in the temperature range of 150 ~ 350 °C. By altering the film deposition temperature, varying degrees of crystallization and microstructure were observed by x-ray diffraction and transmission electron microscopy. The epitaxial BYZ film grown on MgO(100) substrate at 900 °C showed the highest proton conductivity among other samples with an activation energy of 0.45 eV, whereas polycrystalline and amorphous BYZ films showed lower conductivities due to grain boundaries in their granular microstructure.     

Transparent conducting hybrid thin films fabricated by layer-by-layer assembly of single-wall carbon nanotubes and conducting polymers

The effect of conducting polymers on the performance of transparent conducting SWNT thin films fabricated by the layer-by-layer (LBL) assembly has been investigated. Transparent conducting SWNT thin films were fabricated by the LBL assembly in two ways, one using conducting polymers, PEDOT-PEG, and the other using non-conducting polymers, PAH, and their electrical and optical properties were compared. The sheet resistance of (PSS-SWNT/PEDOT-PEG) _n films is more than thrice lower than that of (PSS-SWNT/PAH) _n films for the same n while the decrease of optical transmittance due to the absorbance of PEDOT-PEG is fairly small (ca. 2?% at n=30). The conductivity ratio of the (PSS-SWNT/PEDOT-PEG)₃₀ is 3.3 times larger than that of the (PSS-SWNT/PAH)₃₀. These figures indicate that the performance of the transparent conducting SWNT thin films fabricated by the LBL assembly is highly improved by using conducting polymers instead of non-conducting ones.     

The effect of chemical treatment on the crystallinity of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were chemically treated using nitric acid solution for different time. Quantitative analysis of the crystallinity of the MWCNTs was performed by wide-angle X-ray diffraction (WAXD). The WAXD patterns were deconvoluted into the crystalline diffraction peaks and the amorphous scattering peaks. The introduction of a correction factor for the integrated peak intensity can enhance the computational accuracy of the crystallinity. With increasing the chemical treatment time, the crystallinity of MWCNTs first increases, and then decreases. When the chemical treatment time is equal to 2 h, the crystallinity of MWCNTs reaches the maximum of 85.9%. Moreover, the degree of order in the structures of chemically treated MWCNTs was further studied by thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM). It was found that the external walls of chemically treated MWCNTs with high crystallinity consist of a series of perfectly continuous and straight graphite layers.     

On defects in aluminium single crystal thin films

Defects in Al single crystal thin films prepared by epitaxial growth on mica were investigated by transmission electron microscopy. Dislocations, subgrain boundaries, grain of different orientation and separated polycrystalline regions were observed in the single crystal matrix with density dependent on the pressure of residual gases.     

Edge States and magnetism in carbon nanotubes with line defects

We apply first-principles calculations to investigate the interplay between electronic and magnetic properties of carbon nanotubes with line defects. We consider three types of defects: lines of C--O--C epoxy groups, and defects resulting from the substitution of the oxygen atoms by CH2 or C2H4 divalent radicals. We find that the line defects behave as pairs of coupled graphene edge states, and a variety of electronic and magnetic ground states is predicted as a function of defect type, nanotube diameter, and a possibly applied transverse electric field.     

Modeling structural metastability of irradiated thin films

The nucleation of crystalline or amorphous phases in irradiated binary metallic, as well as non-metallic compound thin films is interpreted using the atomistic interface migration-charge transfer (IMCT) model. The space and time evolution of dense collision cascades, which form upon ion bombardment of the target, lead to non-equilibrium compositional and electronic density profiles at the interface between each cascade and the surrounding crystalline matrix. This occurs because one of film constituents preferentially migrates to the interface, which becomes enriched in it. Cascade relaxation to metastable equilibrium is simulated by local charge transfer reactions (CTR), each involving a pair of dissimilar atoms of the initial compound, which constitute a dimer of an effective compound. The energy cost to introduce an effective compound dimer into the matrix, the difference of formation enthalpy between effective and initial compound and the local strain suffered by the film as a consequence of ion formation by a CTR are evaluated. Threshold values are found in the above structure stability parameters that allow for a qualitative separation of amorphised from crystalline compounds, both metallic and non-metallic.     

Donor-like defects in ZnO substrate materials and ZnO thin films

We have investigated donor-like defects in ZnO substrate material grown by three different methods, and in epitaxial ZnO thin films grown on sapphire by pulsed laser deposition. Temperature dependent Hall effect measurements yield information about dominant donors. The thermal activation energies lie in a wide range from about 20 meV to about 370 meV. Deep level transient spectroscopy is used to obtain parameters of deep donor-like defects. For that, a high-speed diode contact configuration was laid out for the epitaxial thin films in order to determine the defect parameters with high precision. The identified levels are E1, E3 and E4, though the level E4 is observed only in single crystals grown by seeded chemical vapor transport. PACS 72.10.Fk; 72.80.Ey; 73.50.-h     

Elastic interaction between defects in thin and 2D films

Elastic interactions between defects is investigated at the surface of thin layers, a question on which we have given a brief account [P. Peyla et al. Phys. Rev. Lett. 82, 787 (1999)]. Two isotropic defects do not interact in an unlimited medium, regardless of the spatial dimension, a result which can be shown on the basis of the Gauss theorem in electrostatics. Within isotropic elasticity theory, defects interact only (i) if they are, for example, at a surface (or at least if they feel a boundary), or if their action on the material is anisotropic (e.g. they create a non central force distribution, though the material elasticity is isotropic). It is known that two identical isotropic defects on the surface of a semi-infinite material repel each other. The repulsion law behaves as 1/r ³(r = defects separation). We first revisit the Lau-Kohn theory and extend it to anisotropic defects. Anisotropy is found to lead to attraction. We show that in thin films defects may either attract or repel each other depending on the local geometric force distribution caused by the defect. It is shown that the force distribution (or more precisely the forces configuration symmetry) fixes the exponent in the power law 1/r ⁿ (e.g. for a four-fold symmetry n = 4). We discuss the implication of this behaviour in various situations. We treat the interactions in terms of the symmetries associated with the defect. We argue that if the defects are isotropic, then their effective interaction in an unlimited 2D (or a thin film) medium arises from the induced interaction, which behaves as 1/r ⁴ for any defect symmetry. We shall also comment on the contribution to the interaction which arises from flexion of thin films. Received 7 October 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: chaouqi.misbah@ujf-grenoble.fr