Evaluation of imaging in planar anisotropic and isotropic dendritic left-handed materials

Carbon nanotubes (CNTs) with totally hollow channels and/or totally filled copper nanowires have been fabricated by methane decomposition using copper microgrid as a catalyst at 1173 K. The formation mechanism of CNTs with totally hollow channels is carbon precipitation at carbon-metal interface via the preferable surface diffusion mode of carbon. The selectivity of these CNTs can be improved by increasing the purity of copper catalysts and adding hydrogen in the feed gas. To form long and continuous copper nanowires up to 8–10 μm the filling of copper in the CNT channel requires the liquid or quasi-liquid state capillary adsorption of nanosized copper at 1173 K under the thermal driving force. The filling volume ratio of copper to total nano-channel of the CNTs is firstly increased to about 50%. The copper inside the CNTs is of single crystalline form and face centered cubic (fcc) structure. The method is useful for further controlled synthesis of CNTs with totally hollow channels and/or totally copper filled nanowires. PACS 81.07.De; 82.33.Ya     

The interaction between unique hyperbranched polyaniline and carbon nanotubes,and its influence on the dielectric behavior of hyperbranched polyaniline/carbon nanotube/epoxy resin composites

Novel hybridized multi-walled carbon nanotubes (CNTs), consisting of a unique hyperbranched polyaniline (HSiPA) and CNTs, were prepared. The interaction between HSiPA and CNTs was investigated by many techniques, and results show that there are strong π–π and electrostatic interactions between HSiPA and CNTs, so HSiPA can stack firmly onto the surface of CNTs to form a coating. Based on this, a new kind of ternary composites made up of hybridized CNTs and epoxy (EP) resin was prepared, the influence of the ratio of HSiPA to CNTs on the structure and properties of the HSiPA/CNT/EP composites was intensively studied. The percolation threshold of HSiPA/CNT/EP composites is very low (1.26 wt%); besides, with a suitable ratio of HSiPA to CNTs, the HSiPA/CNT/EP composite has much higher dielectric constant and lower dielectric loss than the CNT/EP composite with the same loading of CNTs. When the ratio of HSiPA to CNTs is 0.5:1, the dielectric constant and loss at 100 Hz of the resultant HSiPA/CNT0.5/EP composite are 711 and 1.53, about 7.1 and 4.3 × 10^?3 times the corresponding value of CNT0.5/EP composite, respectively. In addition, compared with traditional CNT/EP composites, the HSiPA/CNT0.5/EP composites have different equivalent circuit models. These attractive results are attributed to unique structure of hybridized CNTs, and thus leading to greatly different structures between the CNT0.5/EP and HSiPA/CNT0.5/EP composites. This investigation reported herein suggests a new approach to prepare new CNTs and related composites with controllable dielectric properties.     

Structure of Lennard–Jones nanowires encapsulated by carbon nanotubes

Molecular dynamics simulations have been performed to investigate the structures of Lennard–Jones(LJ) nanowires(NWs) encapsulated in carbon nanotubes(CNTs). We find that the structures of NWs in a small CNT only adopt multi-shell motifs, while the structures of NWs in a larger CNT tend to adopt various motifs. Among these structures, three of them have not been reported previously. The phase boundaries among these structures are obtained regarding filling fractions, as well as the interaction between NWs and CNTs.     

Nanowire formation by coalescence of small gold clusters inside carbon nanotubes

The coalescence of Au₁₃, Au₅₅ and Au₁₄₇ icosahedral clusters encapsulated inside single walled carbon nanotubes (CNTs) of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.     

Enhanced field emission characteristics of zinc oxide mixed carbon nano-tubes films

A composite material of Zinc oxide and carbon nano-tubes (ZnO-CNTs) paste was synthesized by mixing multi-wall CNTs, ZnO nano-grains and organic vehicles. The microstructures and the morphologies of screen-printed films were characterized by field-emission scanning electron microscope. Results show that ZnO flakes geometrically matched with CNTs by filling into the interspaces of CNTs or directly covering upon CNTs. The field emission characteristics of films are found to be greatly effected by ZnO nano-grains. Especially, the turn-on electric field of ZnO-CNT film (1.17 V/μm) which is far lower than that of usual CNT films (1.70 V/μm). Furthermore, except that better emission stability is achieved, brightness and emission uniformity are notably enhanced as well. It can be speculated that the special microstructures of ZnO mixed CNT films dominate the enhanced electrical conductivity, thermal conductivity, and effective emitters.     

Smallest carbon nanotube is 3 a in diameter

Previous energetic considerations have led to the belief that carbon nanotubes (CNTs) of 4 A in diameter are the smallest stable CNTs. Using high-resolution transmission electron microscopy, we find that a stable 3 A CNT can be grown inside a multiwalled carbon nanotube. Density functional calculations indicate that the 3 A CNT is the armchair CNT(2,2) with a radial breathing mode at 787 cm(-1). Each end can be capped by half of a C12 cage (hexagonal prism) containing tetragons.     

Release potential of single-wall carbon nanotubes produced by super-growth method during manufacturing and handling

We investigated the release potential of single-wall carbon nanotubes (CNTs) produced by the super-growth method during their manufacturing and handling processes at a research facility. We generally sampled air at points both outside and inside of protective enclosures such as a glove box and fume hood. Sampling the air outside of the enclosures was intended to evaluate the actual exposure of workers to CNTs, while sampling the air inside the enclosures was performed to quantify the release of CNTs to the air in order to estimate the potential exposure of workers without protection. The results revealed that airborne CNTs were generated when (1) CNTs were separated from the substrates using a spatula and placed in a container in a glove box; (2) an air gun was used to clean the air filters (containing dust that included CNTs) of a vacuum cleaner; (3) a vacuum cleaner was used to collect CNTs (emission with exhaust air from the cleaner); (4) the container of CNTs was opened; and (5) CNTs in the bin of the cleaner were transferred to a container. In these processes, airborne CNTs were only found inside the enclosures, except for a small amount of CNTs released from the glove box when it was opened. Electron microscopic observations of aerosol particles found CNT clusters, which were fragments of CNT forests, with sizes ranging from submicrometers to tens of micrometers.     

Individuals, grasses, and forests of single- and multi-walled carbon nanotubes grown by supported Co catalysts of different nominal thicknesses

The relationships among the nominal thickness of Co catalyst, the structure of the catalyst particles, and the structure of carbon nanotubes (CNTs) growing from the catalyst during chemical vapor deposition were investigated. Various morphologies of CNTs such as individuals, random networks parallel to the surface of the substrate (‘grasses’), and vertically aligned forests of single- and multi-walled carbon nanotubes were grown by only varying the nominal thickness of catalyst under the same reaction condition. These different morphologies at the same growth time were due to the different areal density rather than to the length of CNTs. With increasing nominal thickness of catalyst, the catalyst particles changed in diameter while their areal density remained relatively almost constant. The change in diameter possibly affected the number ratio of active catalyst particles to the whole particles, which in turn affected the areal density of CNTs and yielded the various morphologies. Longer growth time increased the CNT length, which caused further change in CNT morphologies from individuals to grasses and grasses to forests.     

On the mechanism of carbon nanotube formation: the role of the catalyst

This work examines the recent developments in non-traditional catalyst-assisted chemical vapour deposition of carbon nanotubes (CNTs) with a view to determining the essential role of the catalyst in nanotube growth. A brief overview of the techniques reliant on the structural reorganization of carbon to form CNTs is provided. Additionally, CNT synthesis methods based upon ceramic, noble metal, and semiconducting nanoparticle catalysts are presented. Experimental evidence is provided for CNT growth using noble metal and semiconducting nanoparticle catalysts. A model for CNT growth consistent with the experimental results is proposed, in which the structural reorganization of carbon to form CNTs is paramount.     

Thermal conductivity and diffusion behaviour of lauric acid confined in carbon nanotubes as heat storage materials

The thermal conductivity and diffusion behaviour of lauric acid (LA) confined in single-walled carbon nanotube (CNT) with the filling ratio of 80% are investigated by molecular dynamics (MD) simulations. It is found that the concentric multilayer LA tubes are clearly observed under the interaction of CNT and LA and the hydrogen bonds (HB) among LA molecules in a confined environment. Due to the phonon scattering in low-frequency and the high-thermal conductivity characteristics of CNT, the axial thermal conductivity of CNT/LA is 49–57% lower than that of empty CNT and 115–188 times higher than that of crystal LA at the temperature range of 280 and 360?K. The confined LA molecules move as a whole cluster due to the long-lasting HB action and travel much faster than the bulk.     

Theoretical Semiempirical Study of the Biomolecules Interaction with Carbon Nanotubes

Modeling of the quantum interaction properties of glycine radicals on the sidewalls of the single-walled carbon nanotubes (CNTs) is investigated by MINDO/3 (Modified Intermediate Neglect of Differential Overlap version 3) calculations. It is found that the interaction potential of the N-centered glycine radical with the tubes results in stable complexes when it reacts with the nitrogen atom (N² centered) and metastable conformations with C² atoms. We have studied the effect of the diameter–length characteristics of the CNT on binding the amino acid. Our results suggest that the binding energy is lower as the CNT diameter increases, while as the CNT length increases, the binding energy initially increases and then slightly fluctuates.     

Effect of micro and nanoparticle inorganic fillers on the field emission characteristics of photosensitive carbon nanotube pastes

We successfully fabricated field emitter arrays of carbon nanotube (CNT) dots of 10 μm diameter with excellent field emission properties by using photosensitive CNT paste. The CNT paste was investigated in terms of morphologies, current-voltage properties, and luminous uniformities by varying the mixing ratios of micro and nanoparticle inorganic fillers and the amount of CNTs added into the paste. The 3:1 mixing of micro and nanoparticle fillers and the addition of 5% CNTs in the paste brought about the best field emission characteristics of dot-patterned CNT field emitter arrays.     

A comparative analysis of thin-film transistors using aligned and random-network carbon nanotubes

The purpose of this project is to investigate the characterization of carbon nanotube (CNT) thin-film transistors based on two solution-based fabrication methods: dielectrophoretic deposition of aligned CNTs and self-assembly of random-network CNTs. The electrical characteristics of aligned and random-network CNT transistors are studied comparatively. In particular, the selection effect of metallic and semiconducting CNTs in the dielectrophoresis process is evaluated experimentally by comparing the output characteristics of the two transistors. Our results demonstrate that the self-assembly method produces a stronger field effect with a much higher on/off ratio (I _on /I _off ). This phenomenon provides evidence that the metallic CNTs are more responsive to dielectrophoretic forces than their semiconducting counterparts under common deposition conditions. In addition, the nanotube–nanotube cross-junctions in random-network CNT films create additional energy barriers and result in a reduced electric current. Thus, additional consideration must be applied when using different fabrication methods in building CNT-based electronic devices.     

Comparative investigation on decorating carbon nanotubes with different transition metals

The diffusion dynamics and structure evolvement of the transition metal (TM=Ni, Cu, Au, and Pt) atoms decorating carbon nanotubes (CNTs) with differences have been systematically studied by Monte Carlo (MC) simulation. The studies show that TM atoms can be encapsulated inside, aggregated and even wrapped on the surface of the CNT, which depend on the interactions among TM–TM and TM–C during the spontaneous diffusion process. The decorating effect is greatly influenced by the diameters of CNTs, TM atoms tend to be encapsulated inside the tube in the relatively large CNTs, while they are inclined to stack on the surface for the small ones. More interestingly, Au and Pt atoms would wrap around the smaller CNT, whereas Ni and Cu atoms are still clustering outside of the CNTs with the increase of the number of TM atoms. Simulation results indicate that Pt and Au possess a better wetting effect with CNT than Ni and Cu.     

Carbon nanotube as the core of conical carbon fiber: fabrication,characterization and field emission property

Conical carbon fibers (CCFs) with carbon nanotubes (CNTs) as their cores were grown on graphite substrates with a chemical vapor deposition method. The CNT-cored CCFs were about several tens of micrometers in length. In the interaction with a probe system, the CNT core filaments demonstrated good mechanical strength. Furthermore, field emission currents around 100 μA were also attained from individual CNTs. The growth mechanism of this CNT/CCF combined structure is briefly discussed. Our method has provided a convenient and inexpensive approach to mass assembling CNTs to a carbon substrate. PACS 81.05.Uw; 81.15.Gh; 79.70.+q     

Molecular dynamic study on contact angle of water droplet on a single-wall carbon nanotube (SWCNT) plate

Molecular dynamics (MD) simulations are carried out to study the interaction between a carbon nanotube (CNT) plate and nano-sized water droplet. The cases with or without a quadrupole term acting on the charge sites of the water molecule, are directly compared. The wettability of the CNT plate with different separation distances is studied, and the contact angle on the plates with various separation distances is measured and analyzed. The simulation indicates that the contribution from quadrupole potential is negligible for wetting between twin CNTs and liquid water, while it is significant for holding a sphere-like water droplet and forming a reasonable contact angle.     

Influence of oxygen on the growth of carbon nanotubes by the SiC surface decomposition method

The influence of oxygen on the development of carbon nanotubes (CNTs) during the annealing process of the surface decomposition method on SiC(000−1) surfaces was investigated. In the case of annealing a SiC substrate under ultra-high vacuum conditions, carbon nanofibers (CNFs) form between the CNT layer and the substrate. However, CNTs form without CNFs by annealing the substrate in an oxygen atmosphere. The mean length of CNTs is longer than those formed without an oxygen atmosphere. From cross-sectional transmission electron microscopy images, it was found that oxygen plays an important role in CNT growth by the surface composition method.     

Improved emission uniformity and stability of printed carbon nanotubes in electrolyte

In this paper, we studied the effect of NaCl electrolyte as a surface treatment on improving the uniformity and stability of field emission of screen-printed carbon nanotubes (CNTs). A short period of the electrolyte treatment of CNT films remarkably increase emission uniformity and stability. Furthermore, the field emission characteristics of screen-printed carbon nanotubes (CNTs) such as low turn-on field, high emission current density and strong adhesion of the CNT film on the substrate were also reinforced after post-treated. SEM, TEM and Raman spectrum study revealed that uniformity and stability of field emission is enhanced by two factors. Firstly, the electrolyte treatment appeared to render the CNT surfaces more actively by exposing more CNTs form the CNT paste, which dominates initial uniformity and stability of field emission. Secondly, the number of opened CNTs and defects CNTs of CNT film were increased by electrical current energy.     

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.     

Sn/carbon nanotube composite anode with improved cycle performance for lithium-ion battery

Anode material for lithium-ion battery based on Sn/carbon nanotube (CNT) composite is synthesized via a chemical reduction method. The Sn/CNT composite is characterized by thermogravimetry, X-ray diffraction, and transition electron microscopy. The Sn/CNT composite delivers high initial reversible capacity of 630.5 mAh g^?1 and exhibits stable cycling performance with a reversible capacity of 413 mAh g^?1 at the 100th cycle. The enhanced electrochemical performance of the Sn/CNT composite could be mainly attributed to the well dispersion of Sn nanoparticles on CNT and partially filling Sn nanoparticles inside the CNT. It is proposed that the chemical treatment of CNT with concentrated nitric acid, which cuts carbon nanotube into short pieces and increases the amount of oxygen-functional groups on the surface, plays an important role in the anchoring of Sn nanoparticles on carbon nanotube and inhibiting the agglomeration of Sn nanoparticles during the charge–discharge process.