Nanotube bundle oscillators: Carbon and boron nitride nanostructures

In this paper, we investigate the oscillation of a fullerene that is moving within the centre of a bundle of nanotubes. In particular, certain fullerene–nanotube bundle oscillators, namely C₆₀-carbon nanotube bundle, C₆₀-boron nitride nanotube bundle, B₃₆N₃₆-carbon nanotube bundle and B₃₆N₃₆-boron nitride nanotube bundle are studied using the Lennard–Jones potential and the continuum approach which assumes a uniform distribution of atoms on the surface of each molecule. We address issues regarding the maximal suction energies of the fullerenes which lead to the generation of the maximum oscillation frequency. Since bundles are also found to comprise double-walled nanotubes, this paper also examines the oscillation of a fullerene inside a double-walled nanotube bundle. Our results show that the frequencies obtained for the oscillation within double-walled nanotube bundles are slightly higher compared to those of single-walled nanotube bundle oscillators. Our primary purpose here is to extend a number of established results for carbon to the boron nitride nanostructures.     

Electronic and elastic properties of expanded radialenes carbon nanotubes

The series of expanded radialenes of molecular formulae C_2n H _n and C_3n H _n are obtained by inserting acetylene or diacetylene groups between each pair of methylene units of radialenes C _n H _n . This paper reports on the new theoretical investigation of electronic and elastic properties of some carbon nanotubes based on expanded radialenes with different diameters and with the armchair and zigzag edges Using the ABINIT software package, which is based on the density functional theory (DFT), we found that all studied carbon nanotubes based on expanded radialenes possess metallic behavior. Also, the calculations show that the elastic properties of the nanotubes depend on the form of the rolling up (armchair (m,m) or zigzag (m,0)). The Young’s modulus is slightly smaller for the (m,0) nanotubes and a bit larger for (m,m) nanotubes.     

Molecular dynamics investigation into the electric charge effect on the operation of ion-based carbon nanotube oscillators

The fabrication of nanoscale oscillators working in the gigahertz (GHz) range and beyond has now become the focal center of interest to many researchers. Motivated by this issue, this paper proposes a new type of nano-oscillators with enhanced operating frequency in which both the inner core and outer shell are electrically charged. To this end, molecular dynamics (MD) simulations are performed to investigate the mechanical oscillatory behavior of ions, and in particular chloride ion, tunneling through electrically charged carbon nanotubes (CNTs). It is assumed that the electric charges with similar sign and magnitude are evenly distributed on two ends of nanotube. The interatomic interactions between carbon atoms and van der Waals (vdW) interactions between ion and nanotube are respectively modeled by Tersoff-Brenner and Lennard-Jones (LJ) potential functions, whereas the electrostatic interactions between ion and electric charges are modeled by Coulomb potential function. A comprehensive study is conducted to get an insight into the effects of different parameters such as sign and magnitude of electric charges, nanotube radius, nanotube length and initial conditions (initial separation distance and velocity) on the oscillatory behavior of chloride ion-charged CNT oscillators. It is shown that, the chloride ion frequency inside negatively charged CNTs is lower than that inside positively charged ones with the same magnitude of electric charge, while it is higher than that inside uncharged CNTs. It is further observed that, higher frequencies are generated at higher magnitudes of electric charges distributed on the nanotube.     

Optical studies of carbon nanotubes and nanographites

Resonance Raman and photoluminescence excitation (PLE) spectroscopies are used to study the optical properties of different types of carbon nanostructures such as carbon nanotube, nanoribbons, nanographites and graphite edges. In the resonance Raman experiments of carbon nanotubes, the (n,m) assignment is obtained by comparing the experimental and theoretical diameter and chirality dependence of the optical transitions. The influence of the environment on the optical transitions of the nanotubes is also obtained in the Raman experiments. The PLE measurements in different samples of carbon nanotubes show both direct and phonon-assisted optical transitions, and the results give new evidences that the optical transitions in nanotubes have an excitonic character, which is very strong for the low energy transitions. We also analyze the Raman spectra of nanoribbons and nanographites, showing that this technique is an important tool for defect characterization in graphitic materials, and can be used to distinguish the atomic structure of the graphite edges.     

Restricted Three Body Problems at the Nanoscale

In this paper, we investigate some of the classical restricted three body problems at the nanoscale, such as the circular planar restricted problem for three C₆₀ fullerenes, and a carbon atom and two C₆₀ fullerenes. We model the van der Waals forces between the fullerenes by the Lennard–Jones potential. In particular, the pairwise potential energies between the carbon atoms on the fullerenes are approximated by the continuous approach, so that the total molecular energy between two fullerenes can be determined analytically. Since we assume that such interactions between the molecules occur at sufficiently large distance, the classical three body problems analysis is legitimate to determine the collective angular velocity of the two and three C₆₀ fullerenes at the nanoscale. We find that the maximum collective angular velocity of the two and three fullerenes systems reach the terahertz range and we also determine the stationary points and the points which have maximum velocity for the carbon atom, for the carbon atom and the two fullerenes system.     

Derivation of a universal estimate for the stiffness of carbon nanotubes

An algorithm has been developed based on numerical simulation to relate physical geometry to the Young’s modulus of symmetric and asymmetric single-walled carbon nanotubes (SWCNTs). A large number of finite element results for the stiffness of SWCNTs has been categorized into three main classes (i.e., armchair, zigzag and chiral) and the best curve fitting function has been obtained to describe the relation between the geometry of SWCNTs and their stiffness. For two standard configurations of carbon nanotubes (i.e., armchair and zigzag), four equations referring to geometry parameters (n, m) and diameter (d) have been introduced. To find the size dependence of asymmetric nanotubes, three-dimensional surfaces of stiffness (E(n, m)) have been used. However, since the stiffness of asymmetric nanotubes depends upon n and m, it was impossible to define any diameter dependency. To account for the hidden mechanical behavior of asymmetric SWCNTs, a new physical factor (CF) was introduced as the major novelty in this work. The proposed CF converts any asymmetric geometry (n, m) into a value between 0 and 1. The CF for a chiral nanotube can imply the percentage of similarity in its mechanical properties to the two standard symmetric configurations. Based on the CF concept, a new equation is derived to predict the Young’s modulus of asymmetric carbon nanotubes based on the symmetric prediction of standard models. The new physical factor (CF) which is developed in this study can be useful for the characterization of SWCNTs and the selection of all configurations.     

Nanoemitter of giga- and terahertz ranges based on a carbon peapod: Numerical simulation

A mathematical model of a nanoemitter for the giga- and terahertz ranges based on a carbon nanopeapod formed by a (10, 10) nanotube with encapsulated C₆₀ fullerenes has been proposed. The fundamental possibility of the generation of giga- and terahertz radiation by a charged free fullerene oscillating in the potential well created by the atomic framework of the nanotube and several fullerenes polymerized with its walls and with each other has been proven. The radiation frequency is controlled by means of an external electric field. The dependence of the amplitude and frequency of oscillations on the charge of C₆₀ and on the external field strength has been revealed by the molecular dynamics method. If the fullerene has a charge of +1e or +2e, it emits electromagnetic waves only in the gigahertz range in strong external fields. At the same time, the physical conditions under which the frequency of radiation can be 0.36 GHz have been established: the charge of the fullerene must be +3e, the tube should be oriented strictly along the external field lines, and the strength should be 0.1 V/nm.     

Scaling of magnetoresistance of carbon nanomaterials in Mott-type hopping conductivity region

A method for analyzing data on Mott hopping conduction in a magnetic field, ρ ～ exp[(T ₀/T)^α], based on scaling relation ln[ρ(H)/ρ(0)] = (T ₀/T)^α F(H/T) for the spin-polarized contribution to the magnetore-sistance is proposed. This general approach is tested for a carbon nanomaterial synthesized from single-wall carbon nanotubes under high pressure (up to 7 GPa). The experiments confirmed the theoretical predictions over the temperature range 1.8–12.0 K in a magnetic field of up to 70 kOe and made it possible to correctly determine all parameters of the localized states involved in the model. The experimental data obtained for carbon nanomaterials synthesized from single-wall carbon nanotubes and a mixture of C_2N fullerenes indicate the possible renormalization of the magnetic moment of electrons involved in hopping transport.     

Environmentally benign nanomixing by sonication in high-pressure carbon dioxide

Due to the increased use of nanocomposites, mixing at nanoscale has become important. Current mixing techniques can be classified into: (a) dry mixing (mechanical mixing), (b) wet mixing, and (c) simultaneous production of mixed nanoparticles (when possible). Dry mixing is in general not effective in achieving desired mixing at nanoscale, whereas wet mixing suffers from different disadvantages like nanomaterial of interest should be insoluble, has to wet the liquid, and involves additional steps of filtration and drying. This paper examines the use of pressurized carbon dioxide having high density and low viscosity to replace the liquids (e.g., n-hexane, toluene). Ultrasound is applied to the suspension of nanopowders in gaseous and supercritical carbon dioxide where high impact collisions during sonication help mixing and the final mixture is obtained by simple depressurization. The method is tested for binary mixture of alumina/silica, silica/titania, MWNT (multiwalled carbon nanotubes)/silica, and MWNT/titania. The effects of sonication intensity and pressure on the degree of mixing are studied. Comparative study is also done with liquid n-hexane as a mixing media. Quantitative characterization (e.g., mean composition standard deviation, intensity of segregation) of mixing of alumina/silica and silica/titania is done with energy-dispersive X-ray spectroscopy, and that of MWNT/silica and MWNT/titania is done using field-emission scanning electron microscopy and day-light illumination spectrophotometry. Results show that mixing in carbon dioxide at higher ultrasound amplitudes is as good as in liquid n-hexane, and the final mixed product does not contain any residual media as in the case of liquid n-hexane.     

Template assisted formation of micro- and nanotubular carbon nitride materials

Micro- and nanotubes of an amorphous carbon nitride material were synthesized by metathesis reactions between cyanuric chloride (C₃N₃Cl₃) and different nitrogen sources, such as Li₂(CN₂) or Li₃(BN₂). The intermediate formation of needle-shaped crystals of N(C₃N₃Cl₂)₃ was always observed in our reactions, and investigated with respect to their role as a template in the formation of tubes. Chemical analyses of the micro- and nanotubes reveal carbon to nitrogen ratios near 3:4, consistent with the suspected material C₃N₄. Synthesized carbon nitride materials were thermally stable up to 600 ^°C in inert atmosphere. They were inspected by a number of physical measurements, mainly using TEM, EDX and IR investigations.     

Prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on surfaces of single-walled carbon nanotubes along their axes can lead to a modification of cylindrical carbon cores of these single-walled carbon nanotubes to carbon cores that have a nearly prismatic shape (prismatic modification). In faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. It has been established that the main characteristics of the single-walled carbon nanotubes thus modified are rather close to the corresponding characteristics of the related isostructural polymer conjugated systems (such as cis-polyenes, polyphenylenes, poly(periacenes), or polyphenantrenes). Fragments of model nanotubes of the (n, n) and (n, 0) types that contain up to 360 carbon atoms and their derivatives doped with fluorine atoms have been calculated using the semiempirical parametric method 3.     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Dimer-covering resonating-valence-bond treatment of single-walled zigzag carbon nanotubes

Single-walled zigzag carbon nanotubes with h hexagons around the carbon nanotube, h ranging from 3 to 19, have been investigated from a resonating-valence-bond point of view. The energies calculated for the undoped h = 3n–1 zigzag carbon nanotubes, n integer, suggest that the two lowest-lying phases are degenerate. Therefore, de-confined low-energy topological spin defects would occur. Then, these carbon nanotubes should be conductors, in analogy to polyacethylene. In clear contrast, no such degeneracy is obtained for either, h = 3n+1 or h = 3n, so bound pairs of topological spin defects are expected to occur in these cases. Our findings provide further insights into electron correlation and exchange effects in carbon nanotubes.     

Molecular-dynamics simulations of carbon nanotubes as gigahertz oscillators

Recently, Zheng and Jiang [Phys. Rev. Lett. 88, 045503 (2002)]] have proposed that multiwalled carbon nanotubes could be the basis for a new generation of nano-oscillators in the several gigahertz range. In this Letter, we present the first molecular dynamics simulation for these systems. Different nanotube types were considered in order to verify the reliability of such devices as gigahertz oscillators. Our results show that these nano-oscillators are dynamically stable when the radii difference values between inner and outer tubes are of approximately 3.4 A. Frequencies as large as 38 GHz were observed, and the calculated force values are in good agreement with recent experimental investigations.     

Preparation and luminescence study of Eu(III) titanate nanotubes and nanowires using carbon nanotubes as removable templates

Eu(III) titanate nanotubes and nanowires have been successfully synthesized by solvothermal method using carbon nanotubes (CNTs) as removable templates. The products were characterized by X-ray diffraction spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry, thermogravimetric and differential thermal analysis. It is demonstrated that CNTs are fully coated with an amorphous Eu₂(TiO₃)₃ layer, which is about 10 nm thick and almost continuous and uniform. After the Eu₂(TiO₃)₃/CNTs composites have been calcined at various temperatures, Eu₂(TiO₃)₃ nanotubes and nanowires are obtained by removing the CNTs templates. The diameter of the Eu₂(TiO₃)₃ nanotubes is 40–60 nm, which is consistent with that of CNTs. Both nanotubes and nanowires have a narrow distribution of diameters. The fluorescence properties of the Eu₂(TiO₃)₃ nanotubes and nanowires calcined at various temperatures have been investigated. The results indicate that when the Eu₂(TiO₃)₃/CNTs composites were calcined at 700 °C for 5 h, the Eu₂(TiO₃)₃ nanotubes obtained can be effectively excited by 395 nm light, and exhibit strong red emission around 616 nm. It is very interesting to discover that a few residual carbons doped in Eu₂(TiO₃)₃ nanotubes and many oxygen vacancies could promote the intensity of red emission peak of Eu³⁺ ions. In addition, Eu₂(TiO₃)₃ nanowires calcined at 900 °C for 5 h also have a strong red emission peak due to many oxygen vacancies and defects formed on the surface of the nanowires and inside them.     

Radiation Properties of Carbon Nanotubes Antenna at Terahertz/Infrared Range

The geometric structure and the terahertz/infrared radiation characteristics of carbon nanotubes dipole antenna arrays have been investigated by CST MICROWAVE STUDIO based on finite integral methods. In terahertz and infrared frequency span, the antenna properties such as electrical field distributions, scattering parameters, standing wave ratio, gain, and two dimension directivity patterns are discussed. Our results show that N × N antenna arrays have higher radiation efficiency than single carbon nanotube dipole antenna. This work was supported by National Natural Science Foundation of China (60571026, 10396160).     

Unravelling low lying phonons and vibrations of carbon nanostructures: The contribution of inelastic and quasi-elastic neutron scattering

We illustrate the contribution of inelastic neutron scattering to the understanding of the vibrations and lattice excitations of fullerenes and carbon nanotubes, through some significant experimental results. Particular emphasis is placed on the study of intra and inter-molecular modes of fullerene C₆₀, as well as on the order/disorder transition characteristic of these molecules. In addition, a significant part of this article is dedicated to various intercalation compounds of fullerenes and carbon nanotubes, such as the co-crystal ??fullerene-cubane?? consisting of an arrangement of molecules of spherical and cubic shapes, or the compound called ??peapods??, in which fullerene C₆₀ are inserted inside carbon nanotubes.     

Thermal conductance for single wall carbon nanotubes

We report a theoretical analysis of the phonon thermal conductance, κ(T), for single wall carbon nanotubes (SWCN). In a range of low temperatues up to 100 K, κ(T) of perfect SWCN is found to increase with temperature, approximately, in a parabolic fashion. This is qualitatively consistent with recent experimental measurements where the tube-tube interactions are negligibly weak. When the carbon-carbon bond length is slightly varied, κ(T) is found to be qualitatively unaltered which implies that the anharmonic effect does not change the qualitative behavior of κ(T). Received 12 June 2001     

碳纳米管振荡的分子动力学模拟

运用分子动力学模拟方法,模拟了三种碳纳米管振荡器内管的振荡运动.结果显示：振荡器的内管越短,振荡的频率越大,且受到的轴向回复力的波动也越大.内管在沿着管轴振荡的同时,还绕着管轴旋转,转动的动能有明显涨落并与内管管长密切相关.该研究对于开发碳纳米管的相关应用技术有指导意义. 关键词： 分子动力学模拟 多壁碳纳米管 振荡     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.