Synthesis of carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics

The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs) synthesized by CVD on iron nanoparticles dispersed in a SiO₂ matrix as catalyst was investigated. The general growing conditions were: 110 cm³/min flow rate, 690 °C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C₂H₂/N₂. The catalyst and nanotubes were characterized by HR-TEM, SEM and DRX. TGA and DTA were also carried out to study degradation stages of synthesized CNTs. MWCNTs synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of amorphous carbon nanoparticles lead to a material with high decomposition temperature.     

High-density and well-aligned carbon nanotubes formed by surface decomposition of SiC

The structure and diameter of the densely aligned carbon nanotubes (CNTs) by surface decomposition of C-face in vacuum were investigated by cross-sectional, plan-view transmission electron microscopy and electron diffraction methods. Zigzag-type CNTs were confirmed to be selectively formed and the formation mechanism was proposed from crystallographic analysis. Furthermore, the wall number of CNTs was found to be directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of , and then constructed the CNT walls with the minimum diffusion distance at the interface.     

Magnetic field effects and renormalization of the long-range Coulomb interaction in carbon nanotubes

We develop two theoretical approaches for dealing with the low-energy effects of the repulsive interaction in one-dimensional electron systems. Renormalization Group methods allow us to study the low-energy behavior of the unscreened interaction between currents of well-defined chirality in a strictly one-dimensional electron system. A dimensional regularization approach is useful, when dealing with the low-energy effects of the long-range Coulomb interaction. This method allows us to avoid the infrared singularities arising from the long-range Coulomb interaction at D = 1. We can also compare these approaches with the Luttinger model, to analyze the effects of the short-range term in the interaction. Thanks to these methods, we are able to discuss the effects of a strong magnetic field B in quasi one-dimensional electron systems, by focusing our attention on Carbon Nanotubes. Our results imply a variation with B in the value of the critical exponent α for the tunneling density of states, which is in fair agreement with that observed in a recent transport experiment involving carbon nanotubes. The dimensional regularization allows us to predict the disappearance of the Luttinger liquid, when the magnetic field increases, with the formation of a chiral liquid with α = 0.     

Carbon nanotubes as reinforcement of styrene-butadiene rubber

This study reports an easy technique to produce cured styrene-butadiene rubber (SBR)/multi-walled carbon nanotubes (MWCNT) composites with a sulphur/accelerator system at 150 °C. Significant improvement in Young's modulus and tensile strength were achieved by incorporating 0.66 wt% of filler without sacrificing SBR elastomer high elongation at break. A comparison with carbon black filled SBR was also made. Field emission scanning electron microscopy was used to investigate dispersion and fracture surfaces. Results indicated that the homogeneous dispersion of MWCNT throughout SBR matrix and strong interfacial adhesion between oxidized MWCNT and the matrix are responsible for the considerable enhancement of mechanical properties of the composite.     

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     

Fabrication of bismuth telluride nanotubes via a simple solvothermal process

The unique Bi₂Te₃ tubes were obtained via a simple solvothermal reaction in the presence of ethylenediaminetetraacetic acid disodium salt. The product was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Bi₂Te₃ nanosheets are vertically grown off the tube wall to form Bi₂Te₃ tubes. A possible formation mechanism is proposed.     

Ferromagnetic resonance of cobalt nanoparticles used as a catalyst for the carbon nanotubes synthesis

Catalyst is considered to be the most crucial parameter for the growth of carbon nanotubes. In this work we study the ferromagnetic resonance (FMR) spectra of the catalyst nanoclusters. Moreover we report for the first time the angle FMR studies of catalyst particles with and without CNT layer. The dependencies of the FMR spectra, X-ray diffraction (XRD) patterns, Raman spectra and morphology of the CNT layers on the growth conditions are discussed.     

Surface characterization of the Zn-Ni-Al2O3 nanocomposite coating fabricated under ultrasound condition

Zn-Ni-Al₂O₃ nanocomposite coating, which was fabricated by eletrodeposition technique with the aid of ultrasound, was investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The results reveal that 7.2 wt.% nano-alumina particles uniformly dispersed in the matrix of the composite coating. The XPS analyses demonstrate that the outermost layer of Zn-Ni-Al₂O₃ coating was composed of nano-alumina and Zn(OH)₂, while the transition layer between the outermost layer and the Zn-Ni matrix consisted of nano-alumina, metallic Zn, ZnO and metallic Ni. In order to investigate the influences of ultrasonic agitation and the incorporation of nano-alumina on the composition and surface structure of Zn-Ni matrix, the comparison studies of Zn-Ni-Al₂O₃ nanocomposite coating with Zn-Ni coatings fabricated with and without ultrasound were conducted. The results indicate that ultrasonic agitation resulted in a decrease of Ni content in the Zn-Ni matrix and an increase of the thickness of surface oxide layer; while the incorporation of nano-α-Al₂O₃ increased the Ni content in the Zn-Ni matrix.     

Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light

A novel and simple method for preparing F-doped anatase TiO₂ (defined as FTO) film with high photocatalytic activity was developed using titanium-n-butoxide and NH₄F as TiO₂ and fluorine precursors under mild condition, i.e. low temperature (lower than 373 K) and ambient pressure. The prepared samples were characterized by XRD, SEM, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectrum (DRS), photoluminescence spectrum (PL) and TG-DSC analysis. The photocatalytic activity was evaluated by decomposing X-3B under artificial solar light. The results showed that the crystallinity of TiO₂ was improved by F-doping. F⁻ ions can prevent the grain growth, and the transformation of anatase to rutile phase was also inhibited. The doped fluorine atoms existed in two chemical forms, and the ones incorporated into TiO₂ lattice might take a positive role in photocatalysis. Compared with surface fluorination samples, FTO film exhibited better photocatalytic activity. The high photocatalytic activity of FTO may due to extrinsic absorption through the creation of oxygen vacancies rather than the excitation of the intrinsic absorption band of bulk TiO₂. Furthermore, the FTO can be recycled with little photocatalytic activity depression. Without any further treatment besides rinsing, after 6 recycle utilization, the photocatalytic activity of FTO film was still higher than 79%.     

Linear conductance of multiwalled carbon nanotubes at high temperatures

We have studied the electrical conductance of gas-desorbed multiwalled carbon nanotubes (MWNTs) at high temperatures, and found a peculiar linear temperature dependence of conductance over a wide temperature range from 100 to 800 K. We interpret this phenomenon by using a thermal activation picture of conduction channels below the gap in the vicinity of the Dirac points. The result also indicates a very short and temperature-independent electron mean free path in our MWNTs, and provides a way to determine the number of residual conduction channels in the MWNTs.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

Photo-catalytic H2 evolution over a series of Keggin-structure heteropoly blue sensitized Pt/TiO2 under visible light irradiation

We report the visible light-induced hydrogen generation over a series of Keggin-structure heteropoly blue (HPB) anions (PW₁₂O₄₀³⁻, phosphotungstic blue (PTB), GeW₁₂O₄₀⁴⁻ (GTB), SiW₁₂O₄₀⁴⁻ (STB), BW₁₂O₄₀⁵⁻ (BTB)) sensitized Pt/TiO₂ photo-catalysts. The sensitization of TiO₂ by HPB was certified using photo-electrochemical measurements and UV-vis absorption spectra. PTB showed the most pronounced sensitization effect for TiO₂ in those HPB anions and Pt/TiO₂-PTB showed the highest hydrogen generation activity. The sensitization of TiO₂ was significantly dependent on the reduction potential of HPA, which was determined by the kind of central atom in HPA.     

Broadband optical limiting performance of polymer-wrapped carbon nanotubes in the orange-NIR region

Soluble multi-walled carbon nanotubes (MWNTs) have been obtained by noncovalent modification with poly [2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV). For the composite MWNT/MEH-PPV, there is π-π interaction between the MEH-PPV and MWNTs in addition to the wrapping of the polymer. The nonlinear optical transmittance was measured using a nanosecond optical parametric oscillator pumped with a Nd:YAG system. Excellent optical limiting performance of the composite MWNT/MEH-PPV was observed both in the visible region of 590-680 nm and at the wavelength of 1064 nm. By means of time-correlated single-photon counting fluorescence measurement, an explanation based on the nonlinear absorption of MWNT dominated by the intermolecular energy transfer was proposed.     

Contacting single bundles of carbon nanotubes with alternating electric fields

Single bundles of carbon nanotubes have been selectively deposited from suspensions onto sub-micron electrodes with alternating electric fields. We show that it is possible to control the trapping of a single bundle by the use of Ag as electrode material which, unlike Au, strongly interacts with the carboxyl functionalized carbon nanotubes. Excellent alignment of the bundles between Au or Ag electrodes occurs at frequencies above 1 kHz, with superior contacts being formed with Ag electrodes. Received: 22 May 2002 / Accepted: 21 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-7247/82-6368, E-mail: ralph.krupke@int.fzk.de     

Growth of carbon nanofibers on aligned zinc oxide nanorods and their field emission properties

Carbon nanofibers were grown by electrodeposition technique onto aligned zinc oxide (ZnO) nanorods deposited by hybrid wet chemical route on glass substrates. X-ray diffraction traces indicated very strong peak for reflections from (0 0 2) planes of ZnO. The Raman spectra were dominated by the presence of G band at about 1597 cm⁻¹ corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band at about 1350 cm⁻¹ originating from disordered carbon. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peak at ∼511 cm⁻¹ for Zn-O stretching mode. Photoluminescence spectra indicated band edge luminescence of ZnO at ∼3.146 eV along with a low intensity peak at ∼0.877 eV arising out of carbon nanofibers. Field emission properties of these films and their dependence on the CNF coverage on ZnO nanorods are reported here. The average field enhancement factor as determined from the slope of the FN plot was found to vary between 1 × 10³ and 3 × 10³. Both the values of turn-on field and threshold field for CNF/ZnO were lower than pure ZnO nanorods.     

Electrical conductivity and electromagnetic interference shielding characteristics of multiwalled carbon nanotube filled polyacrylate composite films

Multiwalled carbon nanotubes (MWCNTs) were homogeneously dispersed in pure acrylic emulsion by ultrasonication to prepare MWCNT/polyacrylate composites applied on building interior wall for electromagnetic interference (EMI) shielding applications. The structure and surface morphology of the MWCNTs and MWCNT/polyacrylate composites were studied by field emission scanning microscopy (FESEM) and transmission electron microscopy (TEM). The electrical conductivity at room temperature and EMI shielding effectiveness (SE) of the composite films on concrete substrate with different MWCNT loadings were investigated and the measurement of EMI SE was carried out in two different frequency ranges of 100-1000 MHz (radio frequency range) and 8.2-12.4 GHz (X-band). The experimental results show that a low mass concentration of MWCNTs could achieve a high conductivity and the EMI SE of the MWCNT/polyacrylate composite films has a strong dependence on MWCNTs content in both two frequency ranges. The SE is higher in X-band than that in radio frequency range. For the composite films with 10 wt.% MWCNTs, the EMI SE of experiment agrees well with that of theoretical prediction in far field.     

Growth, modulation and photoresponse characteristics of vertically aligned ZnO nanowires

Vertically aligned, c-axis oriented zinc oxide (ZnO) nanowires were grown on Si substrate by metal organic chemical vapor deposition (MOCVD) technique, where sputtered aluminum nitride (AlN) film was used as an intermediate layer and thermally evaporated barium fluoride (BaF₂) film as a sacrificial layer. The aspect ratio and density of the nanowires were also varied using only Si microcavity without any interfacial or sacrificial layer. The UV detectors inside the microcavity have shown the higher on-off current ratio and fast photoresponse characteristics. The photoresponse characteristics were significantly varied with the aspect ratio and the density of nanowires.     

Effects of surface roughness on the electrical characteristics of ZnO nanowire field effect transistors

We have systematically investigated the effects of surface roughness on the electrical characteristics of ZnO nanowire field effect transistors (FETs) before and after passivation by poly (methyl metahacrylate) (PMMA), a polymer-insulating layer. To control the surface morphology of ZnO nanowires, ZnO nanowires were grown by the vapor transport method on two different substrates, namely, an Au-catalyzed sapphire and an Au-catalyzed ZnO film/sapphire. ZnO nanowires grown on the Au-catalyzed sapphire substrate had smooth surfaces, whereas those grown on the Au-catalyzed ZnO film had rough surfaces. Electrical characteristics such as the threshold voltage shift and transconductance before and after passivation were strongly affected by the surface morphology of ZnO nanowires.     

Titanium-induced germanium nanocones synthesized by vacuum electron-beam evaporation: growth mechanism and morphology evolution

High-vacuum electron-beam evaporation method is used for large area, metal-nucleated germanium (Ge) nanodots and nanocones on Si₃N₄/Si preparation. Nanodot and nanocone arrays with uniform size in bulk-quantity are synthesized using titanium (Ti) nanocrystals as nucleating center at 750 °C with different Ge deposition amount, respectively. The morphology evolution from nanodot to nanocone is studied by atomic force microscopy (AFM). The structure of the prepared sample is characterized by X-ray diffraction (XRD) and Raman scattering. Ge nanocones formed by this convenient fabrication process could have potential applications on nanoelectronics and vacuum electron field emission.