Effects of doping, Stone Wales and vacancy defects on thermal conductivity of single-wall carbon nanotubes

The thermal conductivity of carbon nanotubes with certain defects (doping, Stone-Wales, and vacancy) is investigated by using the non-equilibrium molecular dynamics method. The defective carbon nanotubes (CNTs) are compared with perfect tubes. The influences of type and concentration of the defect, length, diameter, and chirality of the tube, and the ambient temperature are taken into consideration. It is demonstrated that defects result in a dramatic reduction of thermal conductivity. Doping and Stone-Wales (SW) defects have greater effect on armchair tubes, while vacancy affects the zigzag ones more. Thermal conductivity of the nanotubes increases, reaches a peak, and then decreases with increasing temperature. The temperature at which the thermal conductivity peak occurs is dependent on the defect type. Different from SW or vacancy tubes, doped tubes are similar to the perfect ones with a sharp peak at the same temperature. Thermal conductivity goes up when the tube length grows or diameter declines. It seems that the length of thermal conductivity convergence for SW tubes is much shorter than perfect or vacancy ones. The SW or vacancy tubes are less sensitive to the diameter change, compared with perfect ones.     

Radiation loss in a compound plasma system with high and low temperature regions

A code has been developed for calculating the non-coronal radiation of impurity in a compound plasma system which consists of high and low temperature regions. The radiation loss of impurity carbon in this system is calculated and analysed. The cooling rate in this system is smaller than that in a homogeneous plasma system due to the particle exchange between the two regions of the system. The volumetric radiation is much bigger in the low temperature region than in the high temperature region because of the relatively low temperature and high electron density, despite the much smaller volume.     

Enhancement of water permeation across nanochannels by partial charges mimicked from biological channels

In biological water channel aquaporins （AQPs）, it is believed that the bipolar orientation of the single-file water molecules inside the channel blocks proton permeation but not water transport. In this paper, the water permeation and particularly the water-selective behaviour across a single-walled carbon nanotube （SWNT） with two partial charges adjacent to the wall of the SWNT are studied by molecular dynamics simulations, in which the distance between the two partial charges is varied from 0.14 nm to 0.5 nm and the charges each have a quantity of 0.5 e. The two partial charges are used to mimic the charge distribution of the conserved non-pseudoautosomal （NPA） （asparagine/proline/alanine） regions in AQPs. Compared with across the nanochannel in a system with one ＋1 ε charge, the water permeation across the nanochannel is greatly enhanced in a system with two ＋0.5 e charges when charges are close to the nanotube, i.e. the two partial charges permit more rapid water diffusion and maintain better bipolar order along the water file when the distance between the two charges and the wall of SWNT is smaller than about 0.05 nm. The bipolar orientation of the single-file water molecules is crucial for the exclusion of proton transfer. These findings may serve as guidelines for the future nanodevices by using charges to transport water and have biological implications because membrane water channels share a similar single-file water chain and positive charged region at centre and provide an insight into why two residues are necessitated in the central region of water channel protein.     

Hydrogen storage in BC3 composite single-walled nanotube:a combined density functional theory and Monte Carlo investigation

This paper applies a density functional theory(DFT) and grand canonical Monte Carlo simulations(GCMC) to investigate the physisorptions of molecular hydrogen in single-walled BC 3 nanotubes and carbon nanotubes.The DFT calculations may provide useful information about the nature of hydrogen adsorption and physisorption energies in selected adsorption sites of these two nanotubes.Furthermore,the GCMC simulations can reproduce their storage capacity by calculating the weight percentage of the adsorbed molecular hydrogen under different conditions.The present results have shown that with both computational methods,the hydrogen storage capacity of BC 3 nanotubes is superior to that of carbon nanotubes.The reasons causing different behaviour of hydrogen storage in these two nanotubes are explained by using their contour plots of electron density and charge-density difference.     

Mechanism of Carbon Nanotubes Aligning along Applied Electric Field

The mechanism of single-walled carbon nanotubes （SWCNTs） aligning in the direction of external electric field is studied by quantum mechanics calculations. The rotational torque on the carbon nanotubes is proportional to the difference between the longitudinal and transverse polarizabilities and varies with the angle of SWCNTs to the external electric field. The longitudinal polarizability increases with second power of length, while the transverse polarizability increases linearly with length. A zigzag SWCNT has larger longitudinal and transverse polarizabilities than an armchair SWCNT with the same diameter and the discrepancy becomes larger for longer tubes.     

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.     

Characteristics of Microwave Discharge in a Modified Surfaguide with a Large Diameter

The characteristics of the microwave discharge are studied in a modified surfaguide with a large diameter. Ex-perimental results show that there exist three discharging modes, one is the plasma mode, and the others are the waveguide modes. The discharge can jump between one of the wavegudde modes and the plasma mode, and the corresponding hysteresis loop is influenced by the discharging pressure. In the higher pressure region, the hysteresis loop is wide enough so that the discharge in each mode is stable. In the middle pressure region, the discharge becomes unstable as a result of the hysteresis loop being sufficiently narrow. When the gas pressure is further decreased, the plasma mode disappears, while the mode jumps between the two wavegulde modes always appear and are stable in the discharge region we have explored.     

A low-outgassing-rate carbon fiber array cathode

In this paper, a new carbon fiber based cathode — a low-outgassing-rate carbon fiber array cathode — is investigated experimentally, and the experimental results are compared with those of a polymer velvet cathode. The carbon fiber array cathode is constructed by inserting bunches of carbon fibers into the cylindrical surface of the cathode. In experiment, the diode base pressure is maintained at 1×10~(-2) Pa–2×10~(-2) Pa, and the diode is driven by a compact pulsed power system which can provide a diode voltage of about 100 kV and pulse duration of about 30 ns at a repetition rate of tens of Hz.Real-time pressure data are measured by a magnetron gauge. Under the similar conditions, the experimental results show that the outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode and that this carbon fiber array cathode has better shot-to-shot stability than the velvet cathode. Hence, this carbon fiber array cathode is demonstrated to be a promising cathode for the radial diode, which can be used in magnetically insulated transmission line oscillator(MILO) and relativistic magnetron(RM).     

Ability of the radial basis function approach to extrapolate nuclear mass

The ability of the radial basis function(RBF) approach to extrapolate the masses of nuclei in neutron-rich and superheavy regions is investigated in combination with the Duflo-Zuker(DZ31), Hartree–Fock-Bogoliubov(HFB27), finite-range droplet model(FRDM12) and Weizs?cker-Skyrme(WS4) mass models. It is found that when the RBF approach is employed with a simple linear basis function, different mass models have different performances in extrapolating nuclear masses in the same region, and a single mass model may have different performances when it is used to extrapolate nuclear masses in different regions. The WS4 and FRDM12 models(two macroscopic–microscopic mass models), combined with the RBF approach, may perform better when extrapolating the nuclear mass in the neutron-rich and superheavy regions.     

Quantum diffusion in bilateral doped chains

In this paper,we quantitatively study the quantum diffusion in a bilateral doped chain,which is randomly doped on both sides.A tight binding approximation and quantum dynamics are used to calculate the three electronic characteristics:autocorrelation function C(t),the mean square displacement d(t) and the participation number P (E) in different doping situations.The results show that the quantum diffusion is more sensitive to the small ratio of doping than to the big one,there exists a critical doping ratio q 0,and C(t),d(t) and P (E) have different variation trends on different sides of q 0.For the self-doped chain,the doped atoms have tremendous influence on the central states of P (E),which causes the electronic states distributed in other energy bands to aggregate to the central band (E=0) and form quasi-mobility edges there.All of the doped systems experience an incomplete transition of metal-semiconductor-metal.     

Multiwalled carbon nanotubes mass-produced by dc arc discharge in He–H2 gas mixture

Uniform cathode deposits (longer than 15 mm), containing multiwalled carbon nanotubes (MWNTs) inside, were produced by dc arc discharge evaporation with a computer-controlled feeder of a pure-carbon electrode without a metal catalyst in a He–H₂ gas mixture. The purification of MWNTs was carried out to remove amorphous carbon and carbon nanoparticles. High-resolution transmission electron microscopy observations and Raman scattering studies show that the MWNTs possess a high crystallinity and a mean outermost diameter of ∼ ∼10 nm. It has been confirmed that the current density in the electron field emission from a purified MWNT mat can reach 77.92 mA/cm², indicating that the purified MWNTs are a promising candidate electron source in a super high-luminance light-source tube or a miniature X-ray source.     

Effect of van der Waals interactions on the Raman modes in single walled carbon nanotubes

We have measured the Raman spectrum of individual single walled carbon nanotubes in solution and compare it to that obtained from the same starting material where the tubes are present in ordered bundles or ropes. Interestingly, the radial mode frequencies for the tubes in solution are found to be approximately 10 cm (-1) higher than those observed for tubes in a rope, in apparent contradiction to lattice dynamics predictions. We suggest that there is no such contradiction, and propose that the upshift is due rather to a decreased energy spacing of the Van Hove singularities in isolated tubes over the spacings in a rope, thereby allowing the same laser excitation to excite different diameter tubes in these two samples.     

Effect of nickel,iron and cobalt on growth of aligned carbon nanotubes

The effect of pure nickel, iron and cobalt on growth of aligned carbon nanotubes was systematically studied by plasma-enhanced hot-filament chemical vapor deposition. It is found that the catalyst has a strong effect on the nanotube diameter, growth rate, wall thickness, morphology and microstructure. Ni yields the highest growth rate, largest diameter and thickest wall, whereas Co results in the lowest growth rate, smallest diameter and thinnest wall. The carbon nanotubes catalyzed by Ni have the best alignment and the smoothest and cleanest wall surface, whereas those from Co are covered with amorphous carbon and nanoparticles on the outer surface. The carbon nanotubes produced from Ni catalyst also exhibit a reasonably good graphitization. Therefore, Ni is considered as the most suitable catalyst for growth of aligned carbon nanotubes. Received: 30 November 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Field-induced evaporation of carbon nanotubes

We report here an experimental observation of field emission from arrays of multiwall carbon nanotubes. Current densities in the range 10–30 mA/cm² with excellent long-term stability were recorded. A detailed study of the destruction of nanotubes at extreme operation conditions is performed. We established that field evaporation of nanotubes accompanies field emission from a cold cathode at electric fields higher than 2 V/?. Electron microscopy of the evaporation products reveals irregularly shaped carbon nanoparticles with a hollow core. The diameter of the particles is ∼20 nm. A mechanism of the process is proposed and discussed. Received: 6 October 2000 / Accepted: 28 April 2001 / Published online: 27 June 2001     

Carbon linear chains inside multiwalled nanotubes

Multiwalled carbon nanotubes have been deposited on graphite cathodes by using an arc discharge technique in He atmosphere, with the insertion of a catalytic Ni-Cr mixture as well as without catalysers. The topography of such deposition has been investigated by SEM, while a parallel micro-Raman study has revealed, in particular regions of the deposited cathodes, strong bands in the range 1780-1860 cm⁻¹, assignable to linear carbon chains inside the nanotubes. The variation of intensity, frequency and bandwidth of such bands has been investigated, in relation with the spectral characters of the host multiwalled carbon nanotube. In the cathode deposited without catalyst a quite ordered configuration of multiwalled carbon nanotubes is obtained in the central zone, while the maximum concentration of linear carbon chains is found in a ring shaped zone just inside the border. In sample obtained with catalyst the deposited multiwalled carbon nanotubes appear always more disordered, and a remarkable concentration of carbon chains appears in some zones, with a more casual distribution.     

Unusual high degree of unperturbed environment in the interior of single-wall carbon nanotubes

Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with C60. Strong evidence is provided for a highly defect free and unperturbed environment in the interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to 0.35 cm(-1) which is almost 10 times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is 484 cm(-1) corresponding to a tube diameter of only 0.50 nm. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.     

Bamboo-shaped carbon nanotubes generated by methane thermal decomposition using Ni nanoparticles synthesized in water-oil emulsions

Ni nanoparticles were synthesized using two water-in-oil emulsions formulated with different surfactants and using n-heptane as the organic phase and aqueous nickel acetate as the catalytic metallic precursor. Characterization by transmission electron microscopy showed that the Ni nanoparticles have diameters ranging from 3 to 12 nm, and that the surface is lightly oxidized. The decomposition of diluted methane catalyzed by the as-prepared Ni nanoparticles was studied in a thermogravimetric analyzer (TGA), operated in the 25–930 °C range. The weight gains measured during the analysis showed that the Ni nanoparticles decomposed methane above 480 °C, producing similar g.C/g.cat ratios (6–7) at the end of the tests. High resolution transmission electron microscopy (HRTEM) confirmed that the carbons collected at 930 °C were bamboo-shaped carbon nanotubes (BSCNTs) with well defined conical compartments. The average outside diameter of the tubes was between 30 and 60 nm.     

Carbon nanostructures synthesized by arc discharge between carbon and iron electrodes in liquid nitrogen

An idea of using pure iron and graphite electrodes was employed for synthesizing carbon nanoparticles by arc discharge in liquid nitrogen. The synthesized products consist of multiwalled carbon nanotubes (MW–CNT), carbon nanohorns (CNH), and carbon nanocapsules (CNC) with core–shell structure. Effect of metallic cathode and discharge current on product structure and yield had been experimentally investigated. Typical evidence of transmission electron microscopic images revealed that under some certain conditions of discharge in liquid nitrogen the synthesized products mainly consisted of CNCs with mean diameter of 50–400 nm. When conventional graphitic electrodes were employed, CNHs with some MW–CNTs were mainly synthesized. Meanwhile, MW–CNTs with diameter of 8–25 nm and length 150–250 nm became less selectively synthesized as cathode deposit under the condition of discharge in liquid nitrogen with higher arc current. The production yield of carbon nanoparticles synthesized by either carbon–carbon or carbon–iron electrodes became also lower with an increase in the arc current.     

Interaction between concentric tubes in DWCNTs

A detailed investigation of the Raman response of the inner tube radial breathing modes (RBMs) in double-wall carbon nanotubes is reported. It revealed that the number of observed RBMs is two to three times larger than the number of possible tubes in the studied frequency range. This unexpected increase in Raman lines is attributed to a splitting of the inner tube response. It originates from the possibility that one type of inner tubes may form in different types of outer tubes. In this case, a splitting of lines results since the inner tube RBM frequency depends on the diameter of the outer tube. Finally, a comparison of the inner tube RBMs and the RBMs of tubes in bundles gave clear evidence for a stronger interaction between tubes in a bundle as compared to the interaction between inner and outer tubes.Received: 15 September 2004, Published online: 23 December 2004PACS: 81.07.De Nanotubes - 81.05.Tp Fullerenes and related materials - 78.30.Na Fullerenes and related materials