Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields. Received: 25 April 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Effect of gas pressure on the growth and structure of carbon nanotubes by chemical vapor deposition

The effect of gas pressure on the structure of carbon nanotubes (CNTs) has been systematically investigated in the chemical vapor deposition process. The yield of CNTs (defined as the weight ratio of CNTs vs. catalyst) increases significantly with the gas pressure, reaches 600% at 600 Torr, then decreases with further increase of gas pressure. At low reacting gas pressure the CNTs have completely hollow cores, whereas at high pressure the CNTs have a bamboo structure. The density of the compartments in the bamboo-structured CNTs increases dramatically with the increase of the gas pressure. This result shows that the structure and yield of carbon nanotubes are strongly affected by the growth gas pressure. Received: 10 May 2001 / Accepted: 10 May 2001 / Published online: 20 June 2001     

Synthesis of aligned carbon nanotubes

Carbon nanomaterials seem to be most attractive because of their fascinating features. Carbon nanotubes emerged recently as unique nanostructures with remarkable mechanical and electronic properties. Future applications will require a fabrication method capable of producing uniform carbon nanotubes with well-defined and controllable reproducibility of their properties. In this review, recent results addressing rational and efficient methods to obtain aligned arrays of these one-dimensional carbon nanomaterials will be discussed. Received: 3 November 2000 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Growth of vertically aligned carbon-nanotube array on large area of quartz plates by chemical vapor deposition

A simple method has been developed for growth of well-aligned carbon nanotubes (CNTs) on nickel-film quartz plates by chemical vapor deposition (CVD) with organic ethylenediamine as a precursor. High-density carbon nanotubes were vertically aligned on a large area of the quartz plates. The height of the nanotube array could be controlled by varying the CVD time. High-resolution transmission electron microscopy analysis revealed that the multiwalled CNTs were composed of crystalline graphitic sheets with a bamboo structure. Received: 28 May 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

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     

Carbon nanotubes and nanostructures grown from diamond-like carbon and polyethylene

We report a simple way to synthesize carbon nanotubes and nanostructures from the solid phase. Vacuum annealing of diamond-like carbon (DLC) films or polyethylene mixed with catalyst in argon atmosphere leads to the formation of nanotubes and nanostructures. High-resolution transmission electron microscopy studies reveal highly graphitized multi-walled nanotubes (MWNTs) or amorphous fibre-like structures, depending on the catalyst amount. This synthesis process may give a new approach to understanding the phase transition of different carbon allotropes into nanotubes or nanostructures. Received: 3 July 2001 / Accepted: 3 July 2001 / Published online: 2 October 2001     

Template-based synthesis of nanomaterials

The large interest in nanostructures results from their numerous potential applications in various areas such as materials and biomedical sciences, electronics, optics, magnetism, energy storage, and electrochemistry. Ultrasmall building blocks have been found to exhibit a broad range of enhanced mechanical, optical, magnetic, and electronic properties compared to coarser-grained matter of the same chemical composition. In this paper various template techniques suitable for nanotechnology applications with emphasis on characterization of created arrays of tailored nanomaterials have been reviewed. These methods involve the fabrication of the desired material within the pores or channels of a nanoporous template. Track-etch membranes, porous alumina, and other nanoporous structures have been characterized as templates. They have been used to prepare nanometer-sized fibrils, rods, and tubules of conductive polymers, metals, semiconductors, carbons, and other solid matter. Electrochemical and electroless depositions, chemical polymerization, sol-gel deposition, and chemical vapour deposition have been presented as major template synthetic strategies. In particular, the template-based synthesis of carbon nanotubes has been demonstrated as this is the most promising class of new carbon-based materials for electronic and optic nanodevices as well as reinforcement nanocomposites. Received: 27 May 1999 / Accepted: 27 October 1999 / Published online: 8 March 2000     

Scalable bottom‐up assembly of suspended carbon nanotube and graphene devices by dielectrophoresis

Bottom‐up assembly by dielectrophoresis (DEP) has emerged in recent years as a viable alternative to conventional top–down fabrication of electronic devices from nanomaterials, particularly carbon nanotubes and graphene. Here, we demonstrate how this technique can be extended to fabricate devices containing carbon nanotubes and graphene suspended between two electrodes over a back‐gate electrode. The suspended device geometry is critical for the development of nano‐electromechanical devices and to extract maximum performance out of electronic and optoelectronic devices. This technique allows for parallel assembly of devices over large scale. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Role of the curvature of atomic layers in electron field emission from graphitic nanostructured carbon

Layers of oriented carbon nanotubes and nanometer-size plate-shaped graphite crystallites are obtained by chemical vapor deposition in a glow-discharge plasma. A structural-morphological investigation of a carbon material consisting of nanotubes and nanocrystallites is performed, and the field-emission properties of the material are also investigated. It is shown that electron field emission is observed in an electric field with average intensity equal to or greater than 1.5 V/μm. The low fields giving rise to electron emission can be explained by a decrease in the electronic work function as a result of the curvature of the atomic layers of graphitic carbon. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 381–386 (10 March 1999)     

Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions

x C_yN_z nanotubes and related heterojunctions have been studied using both ab initio and semi-empirical approaches. Pure BN nanotubes present a very stable quasiparticle band gap around 5.5–6.0 eV independent of the tube radius and helicity. The bottom of the conduction bands is controlled by a nearly-free-electronn state localized inside the nanotube, suggesting interesting properties under doping. In the case of nanotubes with BC₂N stoichiometry, we show that in the thermodynamic limit the system is driven towards segregation of pure C and BN sections. This demixing significantly affects the electronic properties of such materials. The same process of segregation into BC₃ islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions. In particular, C/BN superlattices or isolated junctions have been investigated as specific examples of the wide variety of electronic devices that can be realized using such nanotubes. Received: 27 November 1998 / Accepted: 14 December 1998     

The use of calcination in exposing the entrapped Fe particles from multi-walled carbon nanotubes grown by chemical vapour deposition

Multi-walled carbon nanotubes (MWCNTs) were synthesized a by chemical vapour deposition method. The effect of calcination at temperatures ranging from 300 to 550°C in exposing the metal nanoparticles within the nanotube bundles was studied. The degree of degradation of the structural integrity of the MWCNTs during the thermal process was studied by Raman spectroscopy, X-ray diffraction analysis, field-emission scanning electron microscopy, and transmission electron microscopy. The thermal behaviour of the as-prepared and calcined samples was investigated by thermogravimetric analysis. Calcination in air, at 400°C for 1 h, was found to be an efficient and simple method to extract metallic impurities from the amorphous carbon shells with minimal damage to the tube walls and lengths. The nanotubes were observed to be damaged at temperatures higher than 450°C.     

Remanence Enhancement Effect in Ni_(0.7)Zn_(0.3)Fe_2O_4/Co_(0.8)Fe_(2.2)O_4 Ferrite Multilayer Film

Ni_(0.7)Zn_(0.3)Fe_2O_4/Co_(0.8)Fe_(2.2)O_4(NZFO/CFO) multilayer films are fabricated on Si(100) substrates by the chemical solution deposition method.The microstructure and magnetic properties are systematically investigated.The results of field-emission scanning electronic microscopy show that the grain size of the NZFO/CFO multilayer film is quite uniform and the thickness is about 300 nm.The remanence enhancement effect of the NZFO/CFO multilayer film can be mainly attributed to the exchange coupling interaction between NZFO and CFO ferrite films,which is in favor of the design and fabrication of modern electronic devices.     

CVD synthesis and purification of single-walled carbon nanotubes on aerogel-supported catalyst

Single-walled carbon nanotubes (SWNTs) were synthesized by disproportionation of carbon monoxide on an aerogel-supported Fe/Mo catalyst. A simple acidic treatment followed by an oxidation process produced a high purity (>99%) of SWNTs. The nanotubes obtained are bundled SWNTs and free of amorphous-carbon coating. Several factors that affect the yield and the quality of the SWNTs were also studied. This method shows great promise for large-scale production of SWNTs. Received: 30 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Viscoelastic properties of multiwalled carbon nanotube solutions

We have prepared solutions of multiwalled carbon nanotubes in very low vapour pressure solvents (a mixture of chlorinated biphenyls). The solutions are stable and show no sign of precipitation for six months. Rheological measurements using a modified Birnboim apparatus with annular and Sogel-Pochetino geometries have been performed. Using time-temperature superposition we obtained the real and imaginary part of the complex viscosity coefficient in a frequency range covering eight orders of magnitude and a temperature range from 5 to 50 ^○C. The data shows unexpected changes in the solution with temperature: for T below 30 ^○C there appears to be some reorganization or clustering. This self-organization could result in a useful technique to improve the electronic properties of polymer/carbon nanotubes composites used in organic electronic devices.     

Electrochemical synthesis of polypyrrole/carbon nanotube nanoscale composites using well-aligned carbon nanotube arrays

Polypyrrole/carbon nanotube nanoscale composites were successfully fabricated by electrochemical deposition of polypyrrole over each of the carbon nanotubes in well-aligned large arrays. The thickness of the polypyrrole coating can be easily controlled by the value of the film-formation charge. For both thin (low film-formation charge) and thick (high film-formation charge) films, the polypyrrole coating on the surface of each nanotube is very uniform throughout the entire length, as observed by transmission electron microscopy. Received: 2 May 2001 / Accepted: 4 May 2001 / Published online: 20 June 2001     

Imaging as-grown single-walled carbon nanotubes originated from isolated catalytic nanoparticles

Discrete catalytic nanoparticles with diameters in the range of 1–3 nm are obtained by placing controllable numbers of metal atoms into the cores of apoferritin. With nanoparticles placed on transmission electron microscope (TEM) grids coated with ultra-thin alumina membranes, isolated single-walled carbon nanotubes are grown by chemical vapor deposition and directly examined by TEM. The characterizations, carried out at single-tube and single-particle level, obtain clear evidence that the diameters of the nanotubes are determined by the diameters of catalytic nanoparticles. For the first time, both ends of an as-grown single-walled nanotube are imaged by TEM, leading to a microscopic picture of the nanotube-growth mechanism. Received: 19 September 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Effect of temperature on growth and structure of carbon nanotubes by chemical vapor deposition

The effect of temperature on growth and structure of carbon nanotubes (NTs) using chemical vapor deposition (CVD) has been investigated. Iron embedded silica was used to grow NTs in large quantity at various temperatures from 600 to 1050 °C with gas pressure fixed at 0.6 and 760 Torr, respectively. The growth and structure of the NTs are strongly affected by the temperature. At low gas pressure, the NTs are completely hollow at low temperature and bamboo-like structure at high temperature. While at high gas pressure, all the NTs are bamboo-like structure regardless of temperature. The diameter of NTs increases significantly with temperature. At low gas pressure the diameter gets bigger by mainly increasing the number of graphene layers of the wall of NTs, whereas at high gas pressure the diameter gets bigger by increasing both the number of graphene layers of the wall and the inner diameter of the NTs. This result indicates that the growth temperature is crucial in synthesizing NTs with different structures. The findings here are important for realizing controlled growth of NTs for their applications in different fields. Received: 20 November 2001 / Accepted: 21 November 2001 / Published online: 4 March 2002     

Integration and characterization of aligned carbon nanotubes on metal/silicon substrates and effects of water

We report here a facile way to grow aligned multi-walled carbon nanotubes (MWCNTs) on various metal (e.g. gold, tungsten, vanadium and copper)/silicon electrically conductive substrates by aerosol-assisted chemical vapor deposition (AACVD). Without using any buffer layers, integration of high quality MWCNTs to the conductive substrates has been achieved by introducing appropriate amount of water vapor into the growth system. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) determination indicate tidy morphology and narrow diameter distribution of the nanotubes as well as promising growth rate suitable for industrial applications. Raman spectra analysis illustrates that the structural order and purity of the nanotubes are significantly improved in the presence of water vapor. The growth mechanism of the nanotubes has been discussed. It is believed that water vapor plays a key role in the catalyst-substrate interaction and nucleation of the carbon nanotubes on the conductive substrates. This synthesis approach is expected to be extended to other catalyst-conductive substrate systems and provide some new insight in the direct integration of carbon nanotubes onto conductive substrates, which promises great potential for applications in electrical interconnects, contacts for field emitters, and other electronic nanodevices.     

Simplified synthesis of double-wall carbon nanotubes

We demonstrate a simplified synthesis technique for double-wall carbon nanotubes that is an adaptation of chemical vapor deposition (CVD) techniques used previously for the production of single-wall nanotubes. Double-wall nanotubes (DWNTs) provide ideal geometries for numerous fundamental structural, electronic, thermal and vibrational studies, as well as providing a unique new platform for practical applications. The diameter distribution of DWNTs is broad, and it is possible that in previous studies using CVD-grown small-diameter nanotubes, presumed to be single-wall, there were significant numbers of DWNTs present.     

Electronic structure and field-emission characteristics of open-ended single-walled carbon nanotubes

The field-emission mechanism of open-ended single-walled carbon nanotubes (SWNTs) is studied. Owing to electronic effects that directly alter the bonding mode and remarkably influence the work function, an open-ended SWNT has much better field-emission properties than a closed SWNT; owing to geometrical effects that slightly influence the work function and the amplification factor, an open-ended SWNT with relaxation has higher threshold voltage and higher current density compared to one without relaxation. It is suggested that adjusting the localized electronic states of the emitting regions, by electronic and geometrical means, could improve the field-emission properties of carbon nanotubes.