Preparation and characterization of lanthanum borate nanowires

Lanthanum borate nanowires had been fabricated by reacting lanthanum oxide and boron with carbon nanotubes at 1100 °C. Electron microscopy studies show that the nanowires have a single-crystal structure with uniform diameters of ∼ 15 nm and lengths up to several micrometers. The growth mechanism of LaBO₃ nanowires could be basically attributed to a CNTs template-confined reaction process; carbon nanotubes confine the reaction in a local space during the reaction. The morphology of lanthanum borate nanowires depends on the shape of the carbon nanotubes at high reaction temperatures.     

Room temperature sensor based on carbon nanotubes and nanofibres for methane detection

Carbon nanotubes and nanofibres deposited by an electrodeposition technique were utilized to fabricate sensor material for the detection of methane. Carbon nanotubes (CNT) and nanofibres were grown on Si(0 0 1) substrate using acetonitrile (1% v/v) and water as electrolyte at an applied d.c. potential ∼20 V. Sensing properties were studied with as-deposited CNT films. It was found that the films showed good sensing properties at room temperature.     

Growth of vertically aligned arrays of carbon nanotubes for high field emission

Vertically aligned multi-walled carbon nanotubes have been grown on Ni-coated silicon substrates, by using either direct current diode or triode plasma-enhanced chemical vapor deposition at low temperature (around 620 °C). Acetylene gas has been used as the carbon source while ammonia and hydrogen have been used for etching. However densely packed (∼ 10⁹ cm^− 2) CNTs were obtained when the pressure was ∼ 100 Pa. The alignment of nanotubes is a necessary, but not a sufficient condition in order to get an efficient electron emission: the growth of nanotubes should be controlled along regular arrays, in order to minimize the electrostatic interactions between them. So a three dimensional numerical simulation has been developed to calculate the local electric field in the vicinity of the tips for a finite square array of nanotubes and thus to calculate the maximum of the electron emission current density as a function of the spacing between nanotubes. Finally the triode plasma-enhanced process combined with pre-patterned catalyst films (using different lithography techniques) has been chosen in order to grow regular arrays of aligned CNTs with different pitches in the micrometer range. The comparison between the experimental and the simulation data permits to define the most efficient CNT-based electron field emitters.     

Structural modification and enhanced electron emission from multiwalled carbon nanotubes grown on Ag/Fe catalysts coated Si-substrates

Multiwalled carbon nanotubes and carbon nano-filaments were grown using Fe as the main catalyst and Ag as a co-catalyst by microwave plasma enhanced chemical vapour deposition. In this work we demonstrate the growth behaviour of carbon nanotubes (CNTs) grown on pure Fe-film and Ag–Fe films. We find that using Ag film beneath Fe film significantly abate the catalyst–substrate interactions by acting as a barrier layer as well as enhances the nucleation sites for the growth of CNTs due to the limited solubility with Fe and silicon. Scanning electron microscopy and transmission electron microscopy studies were carried out to image the microstructures of the samples. It was observed that the length of Fe catalyzed CNTs was ∼500 nm and Ag–Fe catalyzed CNTs varied from ∼600 nm to 1.7 μm. Micro Raman spectroscopy confirmed the improved crystalline nature of Ag–Fe CNTs. It was found that I_D/I_G ratio for Fe catalyzed CNTs was ∼1.08 and for Ag–Fe catalyzed CNTs was ∼0.7. The Ag–Fe catalyzed CNTs were found to be less defective as compared to Fe catalyzed CNTs. Field emission measurements using diode configuration, showed that electron emission from Ag–Fe catalyzed CNTs was much stronger as compared to Fe catalyzed CNTs. The threshold field for Ag–Fe catalyzed CNTs was (2.6 V μm⁻¹) smaller as compared to Fe catalyzed CNTs (3.8 V μm⁻¹) and thus shows better emission properties. This enhancement in electron emission mechanism as a result of introduction of Ag underlayer is attributed to the increased emitter sites and improved crystallinity.     

Low temperature growth of carbon nanotubes in a magnetic field

We report on the growth of carbon nanotubes on a glass substrate at a low temperature of 450 °C by plasma-enhanced chemical vapor deposition in the presence of a magnetic field. The growth of carbon nanotubes can be realized at 450 °C only when a magnetic field is applied to the substrate. Carbon nanotubes cannot be grown in the absence of a magnetic field at the same temperature. An NH₃ plasma pretreatment significantly improved the uniformity of the grain size of the Ni catalyst under the magnetic field. The enhancement in the growth of CNTs at low temperature can be attributed to the magnetic moment pre-alignment of the ferromagnetic catalyst film under high magnetic field. A high emission current density of 20 mA/cm² was obtained at 6 V/μm and a stable emission current was observed. This method permits the growth of carbon nanotubes directly on glass substrate at much more reliable low temperatures for the fabrication of high-density field emitter arrays.     

Magnetic field-guided orientation of carbon nanotubes through their conjugation with magnetic nanoparticles

Low intensity magnetic fields (22mT) rendered by a pair of bar magnets have been used to achieve in situ precise orientation of multiwalled carbon nanotubes (MWCNTs) and their directional deposition on solid substrates. The nanotubes were imparted magnetic characteristics through Fe₃O₄ (magnetite) nanoparticles covalently attached to their surface. The side walls of nanotubes were first acid oxidized with H₂SO₄/HNO₃ (3:1 v/v) mixture and amine-functionalized magnetic nanoparticles were then interfaced to ends and side walls of the nanotubes through covalent linkages in the presence of a zero length cross linker, 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide. Fourier transformed infrared spectroscopic investigations affirmed the functionalization of nanostructures and formation of a magnetic nanohybrid. Transmission electron microscopy results revealed the attachment of nanoparticles along the side walls of MWCNTs. A flow cell was utilized to orient magnetic nanohybrid in the desired direction and also to create thin films of aligned MWCNTs. Further, directional assembly of magnetic MWCNTs at different orientation angles on solid substrates was studied by field emission scanning electron microscopy and optical microscopy. The procedure can be scaled to align CNTs on large surface areas for numerous applications, e.g., nanosensors, field emitters, and composites.     

Statistical analysis of mechanical properties of pressureless sintered multiwalled carbon nanotube/alumina nanocomposites

Mechanical properties of pressureless sintered 0.15–1.2 vol.% multiwalled carbon nanotube reinforced alumina matrix nanocomposites have been analyzed using the 2-parameter Weibull statistics. Electron microscopy and phase analysis of nanocomposites sintered at 1700 °C for 2 h in Argon revealed existence of interpenetrating network of nanotubes in alumina, formation of thin interface resembling stoichiometric aluminum monoxycarbide and matrix grain refinement by nanotubes. Statistical analyses indicated that with increasing Vickers hardness testing load (4.9–19.6 N) and flexural strength measurement temperature (room temperature to 1100 °C), Weibull modulus of nanocomposites increased significantly suggesting improved consistency at higher load and temperature. The highest Weibull moduli were obtained for nanocomposites containing either 0.15 or 0.3 vol.% nanotube which were ∼40% and ∼15% higher than single phase alumina for hardness and strength, respectively, supporting the specimen size effect on reliability of present brittle ceramic matrix nanocomposites. Superior mechanical reliability of nanocomposites over pure alumina was primarily attributed to the presence of structurally intact nanotubes forming effective interface region to ensure proper load sharing, matrix grain refinement, and especially, at higher testing load and temperature, overall averaging effect of flaws to yield higher Weibull moduli.     

A structure model and growth mechanism for novel carbon nanotubes

The growth of carbon nanotubes from catalytic thermal decomposition of acetylene on fine iron particles has been studied. Electron microscopic images of the carbon nanotubes as formed and after annealing treatment are presented. Besides the ordinary carbon nanotubes which have been reported (S. Iijima, Nature, 354 (1991) 56), we have found, at first time, two other new kinds of carbon nanotubes: one is straight or curved nanotube with many irregular multi-layered diaphragms in the hollow core, in particular, the fringes of the wall of the tubes are not parallel to the axis of the tube; another one has many regular diaphragms (bamboo-like) which keep almost constant distance with each other. A model that postulates two steps growth of nanotubes from catalyst particles is proposed to explain the microstructure of the novel carbon nanotubes.     

Influence of single wall carbon nanotubes and thermal treatment on the morphology of polymer thin films

Homogeneous and stable thin films of poly(butylene terephthalate) PBT and its nanocomposites based on single wall carbon nanotubes (SWCNTs) were prepared by spin coating. PBT thin films show crystalline structures for thicknesses above 40 nm, consisting of submicrometer size 2D-spherulites. In the case of nanocomposites, carbon nanotubes act as nucleating agents and provide a template for the crystallization of PBT. This gives rise to hybrid shish-kebab structures, even in the thinnest films (∼10 nm thick). Melting and recrystallization provoke the crystallization of PBT and its nanocomposites, and can be used to control morphology. For PBT thin films, the orientation of crystalline lamellae undergoes a transformation, changing from a disposition perpendicular to the substrate (“edge-on”) to a parallel arrangement (“flat-on”) after recrystallization. In the case of the nanocomposites, the CNT influence on the polymer crystallization morphology in thin films is less significant than in the bulk due to the effect of the substrate interactions. Using Raman microscopy it is possible to directly observe both, the degree of dispersion and the location of carbon nanotubes in the films. The results reveal that bigger agglomerates act as more effective nucleating points than isolated bundles of SWCNTs during crystallization of the polymer matrix.     

Enhanced electron emission from titanium coated multiwalled carbon nanotubes

Enhanced field emission properties and improved crystallinity of titanium (Ti) coated multiwalled carbon nanotubes (MWCNTs), prepared by microwave plasma enhanced chemical vapour deposition have been observed. Ti films of extremely low thicknesses (0.5 nm, 1.0 nm and 1.5 nm) were coated over carbon nanotubes (CNTs) and their field emission behaviour was investigated. The turn on field of Ti coated CNTs was found to be low (~ 0.8 V/μm) as compared to pristine CNTs (~ 1.8 V/μm). The field enhancement factor for Ti coated CNTs was quite large (~ 1.14 × 10⁴) as compared to pristine CNTs (~ 6 × 10³). This enhancement in electron emission is attributed to the passivation of defects and improved crystallinity of CNTs. Surface morphological and microstructural studies were carried out to investigate the growth of pristine and Ti coated CNTs. It was observed that Ti nanoclusters adsorb on the edges of MWCNTs and increase their crystallinity. This increase is directly correlated with the thickness of Ti film deposited. Micro Raman spectroscopy confirmed the improved crystallanity of Ti coated CNTs.     

ESR investigations on polyethylene-single wall carbon nanotube composites

Electron spin resonance investigations on single wall carbon nanotubes dispersed in polyethylene are reported. Three resonance lines were observed; a wide line assigned to magnetic iron clusters (catalyst residues), a broad and intense line originating from uncoupled electrons delocalized over the conducting domains of carbon nanotubes (in interaction with the electronic spins assigned to magnetic impurities), and a faint line superimposed on the broad one, assigned to paramagnetic impurities. The temperature dependence of resonance line parameters (resonance line position, width, and double integral) in the range 150–450 K has been analyzed. It was observed that the parameters of the broad and narrow lines are sensitive to the glass and melting relaxations occurring within the polymeric matrix.     

Impedance response of carbon nanotube-titania electrodes dried under modified gravity

The synthesis and impregnation of porous titania films by commercial multiwalled carbon nanotubes and nanotube rich carbon soot are reported. The samples were dried under terrestrial gravity g and in a centrifuge accelerated at 13 g. X-Ray Diffraction data and Scanning Electron Microscopy images indicated differences in the crystal structure and tendency to agglomeration in both carbon types, providing different microstructures of functionally graded electrodes. Drying the samples in a centrifuge helps to the distribution of carbon nanoparticles and to the decrement of the impedance at the contact interfaces. The presence of titania weakens the differences observed in both drying protocols, but not the differences due to the carbon source. Superior capacitance and network conductivity were observed in electrodes based on commercial carbon nanotubes.     

Synthesis and characterization of indium phosphide films prepared by co-evaporation technique

Polycrystalline indium phosphide films were successfully deposited on glass and Si substrates by co-evaporating indium and phosphorus from appropriate crucibles. Microstructural studies indicated the average crystallite size to be ∼78 nm. X-ray diffraction pattern indicated reflections from (111), (220) and (311) planes only. The surface roughness of the films was estimated to be 30 nm and the band gap as determined from the transmittance versus wavelength traces was found to be ∼1.42 eV. The PL spectrum measured at 300 K was dominated by a strong peak located ∼1.41 eV. The intensity of this peak increased significantly when recorded at lower (10 K) temperatures and shifted towards higher energy (∼1.54 eV). XPS studies indicated two peaks ∼444.5 eV and ∼451.9 eV, corresponding to peaks of 3d_5/2 and 3d_3/2 of In 3d core while the P 2p peak at ∼128.8 eV was assigned to only P in InP. Characteristics Raman peaks for InP at ∼303 cm⁻¹ (TO) and ∼342 cm⁻¹ (LO) were observed.     

Co-synthesis of single-walled carbon nanotubes and carbon fibers

A new vertical floating catalytic technique is developed and used to prepare both single-walled carbon nanotubes (SWNTs) and carbon fibers (CFs). Scanning electron microscopy (SEM) observation shows a clear separation of these two materials. Thin films of SWNTs can be peeled easily from the CF substrate which just acts as a catalyst support for the SWNT growth. The production process is also semicontinuous, resulting in a yield of ∼1.0 g h⁻¹ of SWNTs film with high purity. Structure and vibrational properties of these materials are investigated by electron microscopy and Raman spectroscopy, respectively.     

Cell viability and adhesion on as grown multi-wall carbon nanotube films

This work studies cell viability and cell adhesion on as-grown dense films of vertically aligned carbon nanotubes. Microwave plasma CVD reactor was used to produce quite pure carbon nanotubes. Fibroblast L929 mouse cells were used. MTT assay was used for the study of cell viability. The results show very high cell viability, close to 100% after 96 h of incubation. Cell adhesion was observed by scanning electron microscopy, which shows a first cell layer spreading flat on the surface formed by the nanotube tips. This first layer seems to block cell interaction with the nanotubes since next layers present normal globular growth.     

Magnetic and field emission studies of atom beam sputtered Ni:SiO2 granular films

Ni:SiO₂ granular films have been prepared by atom beam sputtering technique under ambient conditions. These films have been subsequently annealed at 200-600 °C temperature. GAXRD and TEM analyses show the growth of Ni particles and improvement in crystallinity with increase in annealing temperature. Selected area electron diffraction and XPS analyses show the presence of a small quantity of NiO phase in addition to metallic Ni. Room temperature magnetic measurements indicate that the films annealed at lower temperatures (≤400 °C) are superparamagnetic and the film annealed at 600 °C is ferromagnetic. Magnetic results at 5 K are explained on the basis of exchange bias between Ni particles and surrounding nickel oxide. Systematic field emission studies on as-deposited and annealed films show a turn-on field ∼6.2-13.5 V/μm corresponding to an emission current density of ∼1 A/m². Field emission results are explained on the basis of electrical inhomogeneity effects.     

Morphology of carbon nanotubes synthesized by thermal CVD under high magnetic field up to 10 T

High magnetic fields up to 10 T are applied to thermal chemical vapor deposition (CVD) for carbon nanotube synthesis in order to control the nanotube morphology. Although changes of the nanotube morphology in the presence of magnetic field were not obviously discernible in the SEM images, an increase in the onion-like nano-carbons due to magnetic field was observed by TEM. It suggests that the magnetic field influences formation processes of nano-carbons including carbon nanotubes, via magnetic effects on metal catalysts.     

Bulk preparation and characterization of mesoporous carbon nanotubes by catalytic decomposition of cyclohexane on sol–gel prepared Ni–Mo–Mg oxide catalyst

Multiwalled carbon nanotubes were synthesized using Ni–Mo–Mg oxide catalyst prepared by sol–gel technique. Carbon nanotubes were formed in situ by the reduction of nickel oxide (NiO) and molybdenum oxide (MoO₃) to Ni and Mo by a gas mixture of nitrogen, hydrogen and cyclohexane at 750 °C. Scanning Electron Microscopy (SEM) was used to confirm the formation of carbon nanotubes (CNTs). The pore size distribution of carbon nanotubes (CNTs) was investigated by N₂ adsorption and desorption. It was found that the pore size fell into the mesopore range: 2 < d < 50 nm. Interpretation was also made using Raman spectroscopy, Diffuse reflectance spectroscopy, X-ray diffraction and ESR spectra. This method is found to produce a very high yield weighing over 20 times of the catalyst. Based on the experimental conditions and results obtained a possible growth mechanism of the carbon nanotubes is proposed.     

水或液氮中电弧放电制备炭纳米材料

利用特制的电弧放电装置，研究了水或液氮中碳电弧放电形成炭纳米材料的机理。借助高分辨率透射电子显微镜对电弧放电生成的产物进行了观察和分析。结果表明：在水或液氮中碳电弧放电可以生成多壁碳纳米管和碳纳米洋葱结构，液氮中碳电弧放电可以生成单壁碳纳米角，水中钴催化碳电弧放电可以生成碳包裹的纳米钴颗粒。横向低频交变磁场会影响碳纳米材料的形核过程，并且可以推测磁场交变的频率5Hz与纳米管、纳米洋葱等结构的生长周期存在某种拟合。根据实验现象，提出了一种解释液体中碳电弧放电过程纳米材料生成的理论模型。     

Synthesis, electrical and magnetic characterization of core-shell silicon carbo-nitride coated carbon nanotubes

We demonstrate synthesis, electrical and magnetic characterization of silicon carbo-nitride (SiCN) coated multiwalled carbon nanotubes in a core-shell structure. The core formed by a carbon nanotube had a diameter in the range of 10-100 nm. The shell was synthesized by pyrolysis of an SiCN precursor on the surface of carbon nanotubes. Electrical resistivity of an individual composite nanotube was measured to be ~ 2.55 × 10³ Ω cm. The magnetic measurements performed by a superconducting quantum interference device on the composite nanotubes in the temperature range of 5-300 K show a reduced coercive field with increasing temperatures. The monolayer thick coating of an ultra high temperature multifunctional ceramic SiCN makes these composite nanotubes very promising for sensing applications in harsh environments.