Monitoring structural defects and crystallinity of carbon nanotubes in thin films

We report the influence of catalyst formulation and reaction temperature on the formation of carbon nanotube (CNT) thin films by the chemical vapour deposition (CVD) method. Thin films of CNTs were grown on Fe-Mo/Al₂O₃-coated silicon wafer by thermal decomposition of methane at different temperatures ranging from 800 to 1000°C. The electron microscopic investigations, SEM as well as HRTEM, of the as-grown CNT thin films revealed the growth of uniform multi-walled CNTs in abundance. The intensity ratio of D-band to G-band and FWHM of G-band through Raman measurements clearly indicated the dependency of structural defects and crystallinity of CNTs in thin films on the catalyst formulation and CVD growth temperature. The results suggest that thin films of multi-walled CNTs with negligible amount of defects in the nanotube structure and very high crystallinity can be obtained by thermal CVD process at 925°C.     

Growth of carbon nanotubes from C<Subscript>60</Subscript>

Carbon nanotubes can be obtained from a multitude of molecular precursors in chemical vapor deposition (CVD) processes. Here we demonstrate that the use of C₆₀ as the carbon feedstock gas in an iron-catalyzed thermal CVD experiment leads to the formation of films of multi-walled carbon nanotubes. The critical role of the diameter of the catalyst particles in determining the efficiency of nanotube growth is clearly demonstrated. Electron microscopy and Raman spectroscopy were employed for the characterisation of the nanotube material. The structural properties of the individual nanotubes show distinctive differences to acetylene-grown multi-walled nanotubes. PACS 81.07.De; 81.10.Bk     

Hydrogen-free spray pyrolysis chemical vapor deposition method for the carbon nanotube growth: Parametric studies

Spray pyrolysis chemical vapor deposition (CVD) in the absence of hydrogen at low carrier gas flow rates has been used for the growth of carbon nanotubes (CNTs). A parametric study of the carbon nanotube growth has been conducted by optimizing various parameters such as temperature, injection speed, precursor volume, and catalyst concentration. Experimental observations and characterizations reveal that the growth rate, size and quality of the carbon nanotubes are significantly dependent on the reaction parameters. Scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy techniques were employed to characterize the morphology, structure and crystallinity of the carbon nanotubes. The synthesis process can be applied to both semiconducting silicon wafer and conducting substrates such as carbon microfibers and stainless steel plates. This approach promises great potential in building various nanodevices with different electron conducting requirements. In addition, the absence of hydrogen as a carrier gas and the relatively low synthesis temperature (typically 750 °C) qualify the spray pyrolysis CVD method as a safe and easy way to scale up the CNT growth, which is applicable in industrial production.     

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.     

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.     

Silicon enhanced carbon nanotube growth on nickel films by chemical vapor deposition

Silicon enhances carbon nanotube growth on nickel films by chemical vapor deposition using methane and hydrogen. Nanotube growth characteristic is significantly improved on nickel films patterned by argon plasma etching on silicon oxide layers. Auger electron spectroscopy shows that a reduced silicon phase forms in the surface silicon oxide layer by Ar ion bombardment used for patterning. The enhanced growth of carbon nanotubes could be ascribed to an oxygen removal effect by silicon in the process of synthesis.     

Soft limiting circuit implementable with a single multi-walled carbon nanotube

Dispersed growth techniques are applied to grow carbon nanotubes between metal pads on silicon wafers. Process conditions are tuned to yield nanotube devices having only one multi-walled carbon nanotube connecting the metal pads. The nonlinear transfer characteristics of these devices are often accompanied with high impedance and low conduction current. These attributes can be utilized for a soft limiting circuit with impedance much higher than conventional silicon-based implementations.     

Realization, characterization and functionalization of lipidic wrapped carbon nanotubes

Mass-produced carbon nanotubes (CNTs) are strongly aggregated and highly hydrophobic, and processes to make them water soluble are required for biological applications. Both covalent and non-covalent strategies are pursued for obtaining stable, highly concentrated CNT aqueous dispersions. Covalent functionalization has the great disadvantage of producing an irreversible chemical modification of nanotubes, thus alterating their mechanical, chemical and electric properties. On the other hand, non-covalent functionalization is often obtained by employing surfactants that sensibly affect cell viability. Moreover, derivatization with biological moieties is often impossible through non-covalent CNT dispersion. This paper proposes a non-covalent dispersion of multi-wall CNT based on a lipidic mixture that can guarantee high concentration and high stability as well as high cytocompatibility. Moreover, CNTs wrapped with a lipid membrane are realized to demonstrate that the proposed CNTs can be functionalised with a dodecapeptide that specifically recognizes activated platelets without chemical modification of the nanotube itself.     

Morphology of carbon nanotubes prepared via chemical vapour deposition technique using acetylene: A small angle neutron scattering investigation

Small angle neutron scattering (SANS) has been utilized to study the morphology of the multi-walled carbon nanotubes prepared by chemical vapour deposition of acetylene. The effects of various synthesis parameters like temperature, catalyst concentration and catalyst support on the size distribution of the nanotubes are investigated. Distribution of nanotube radii in two length scales has been observed. The number density of the smaller diameter tubes was found more in number compared to the bigger one for all the cases studied. No prominent scaling of the structure factor was observed for the different synthesis conditions.      

Thinning and diluting aligned carbon nanotube films for uniform field emission

Aligned carbon nanotube (CNT) films are potential field emitters for large-area flat panel displays. However, the distribution of emission areas in the CNT films is quite non-uniform because of inhomogeneous nanotube growth, which is hard to avoid using the conventional chemical vapor deposition (CVD) method. Here we show that the emission uniformity of CNT films can be improved simply by reducing the film thickness (thinning) or the nanotube density (diluting). The thickness and density of CNT films could be controlled by controlling the CNT growth time and temperature. Received: 12 June 2001 / Accepted: 27 October 2001 / Published online: 23 January 2002     

Temperature enhanced growth of ultralong multi-walled carbon nanotubes forest

This work demonstrates a method to synthesize ultralong multi-walled carbon nanotubes (MW-CNTs) forest with high growth rate using the three-zone temperature chemical vapor deposition (TZT-CVD). The effect of temperature difference in the TZT-CVD is to sustain the lifetime of the catalyst and hence increase the growth efficiency. By optimizing the growth variables, TZT-CVD produced MW-CNTs forest with a height up to 4.27 mm in 60 min. The CNT synthesis ratio, an ultimate figure of merit is 712 times and the carbon source consumption is 4% compared to that of single-zone temperature CVD.     

Nanofabrication by electrochemical routes of Ni-coated ordered arrays of carbon nanotubes

Ordered arrays of carbon nanotubes (CNT) have been coated by Ni nanoparticles and Ni thin films by using the chronoamperometry technique for nickel reduction. Two different kinds of nanotube arrays have been used: aligned bundles of CNT grown on Si substrates by chemical vapour deposition (CVD) and networks of CNT bundles positioned via a dielectrophoretic post-synthesis process between the electrodes of a multifinger device. The morphology and structure of the Ni-coated CNT bundles have been characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). By changing the parameters of the electrochemical process, it is possible to modulate the morphological characteristics of the Ni deposits, which can be obtained in form of nanoparticles uniformly distributed along the whole length of the CNT bundles or of Ni thin films. A qualitative study of the nucleation and growth mechanism of Ni onto CNT has been performed using the theoretical model for diffusion-controlled electrocrystallization, and a correlation between growth mechanism and samples morphology is presented and discussed. The possibility to maintain the architecture of the pristine nanotube deposits after the Ni coating process opens new perspectives for integration of CNT/Ni systems in magnetic and spintronics devices.     

Electric field aligned growth of single-walled carbon nanotubes

Electric field aligned, single-walled carbon nanotubes are grown between electrodes using thermal chemical vapour deposition (CVD) of methane. The growth occurs on a thin film layered catalyst of aluminium, iron and molybdenum patterned on top of electrodes. The nanotubes bridge 10 μm sized electrode gaps and have a typical diameter of less than 2 nm as measured by Raman spectroscopy and atomic force microscopy. We present electrical transport measurements on a directly grown nanotube which shows p-type semiconducting behaviour.     

Ultra-long carbon nanotube growth on catalyst

Ultra-long carbon nanotube growth with Fe particles sitting at tip end as a function of reaction time, reaction temperature, diameter of the carbon nanotube, damping factor of the system, and the type of catalyst in chemical vapor deposition is investigated by using a theoretical analysis on the phonon vibration of the system. Simulations demonstrate that metal cluster makes and keeps the carbon atoms at tip end reactive. So carbon nanotube grows more than 4 cm. In addition, results show carbon nanotubes with larger diameter grow lesser owing to higher damping factors. In addition, effect of temperature on growth is discussed and it is shown that there is an optimum temperature for growth process. Lastly, dependence of type of catalyst on growth process is investigated.     

In situ measurements and modeling of carbon nanotube array growth kinetics during chemical vapor deposition

Direct measurements of carbon nanotube growth kinetics are described based upon time-resolved reflectivity (TRR) of a HeNe laser beam from vertically aligned nanotube arrays (VANTAs) as they grow during chemical vapor deposition (CVD). Growth rates and terminal lengths were measured in situ for VANTAs growing during CVD between 535 °C and 900 °C on Si substrates with evaporated Al/Fe/Mo multi-layered catalysts and acetylene feedstock at different feedstock partial pressures. Methods of analysis of the TRR signals are presented to interpret catalyst particle formation and oxidation, as well as the porosity of the VANTAs. A rate-equation model is developed to describe the measured kinetics in terms of activation energies and rate constants for surface carbon formation and diffusion on the catalyst nanoparticle, nanotube growth, and catalyst over-coating. Taken together with the TRR data, this model enables basic understanding and optimization of growth conditions for any catalyst/feedstock combination. The model lends insight into the main processes responsible for the growth of VANTAs, the measured number of walls in the nanotubes at different temperatures, conditions for growth of single-wall carbon nanotube arrays, and likely catalyst poisoning mechanisms responsible for the sharp decline in growth rates observed at high temperatures. PACS  61.46.+w; 81.07.De; 81.16.Hc     

Adjustable thermal conductivity in carbon nanotube nanofluids

Homogeneous and stable nanofluids have been produced by suspending well dispersible multi-walled carbon nanotubes (CNTs) into ethylene glycol base fluid. CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. Thermal conductivity enhancement was adjusted by ball milling and cutting the treated CNTs suspended in the nanofluids to relatively straight CNTs with an appropriate length distribution. Our findings indicate that the straightness ratio, aspect ratio, and aggregation have collective influence on the thermal conductivity of CNT nanofluids.     

A chemical kinetic model for chemical vapor deposition of carbon nanotubes

Carbon nanotubes (CNTs) are classified among the most promising novel materials due to their exceptional physical properties. Still, optimal fabrication of carbon nanotubes involves a number of challenges. Whatever be the fabrication method, a process optimization can be evolved only on the basis of a good theoretical model to predict the parametric influences on the final product. The work reported here investigates the dependence of the deposition parameters on the controllable parameters for carbon nanotube growth during Chemical vapor deposition (CVD), through a chemical kinetic model. The theoretical model consisted of the design equations and the energy balance equations, based on the reaction kinetics, for the plug flow and the batch reactor, which simulate the CVD system. The numerical simulation code was developed in-house in a g++ environment. The results predicted the growth conditions for CNT: the deposition temperature, pressure and number of atoms, which were found to be influenced substantially by the initial controllable parameters namely the temperature, volumetric flow rate of the carbon precursor, and the reaction time. An experimental study was also conducted on a CVD system developed in the laboratory, to benchmark the computational results. The experimental results were found to agree well with the theoretical predictions obtained from the model.     

Microstructural investigations of zirconium oxide—on core–shell structure of carbon nanotubes

Single-walled carbon nanotubes and multi-walled carbon nanotubes/ZrO₂ nanocomposites were obtained by isothermal hydrolyzing and chemical precipitation method for both the carbon nanotubes. The coating was taken place by dispersion of both the carbon nanotubes in ZrOCl₂·8H₂O aqueous solution. However, a highly conformal and uniform monoclinic zirconia coating was deposited on multi-walled carbon nanotubes rather than single-walled carbon nanotubes by this new and simple method. Also, it has been observed that the thickness of the individual carbon nanotube after zirconia coating was increased by isothermal hydrolyzing process rather than traditional chemical precipitation method and it has been confirmed by high-resolution transmission electron microscopy study.     

Nonlinear finite-element modeling of graphene and single- and multi-walled carbon nanotubes under axial tension

An effective finite-element (FE) approach for modeling the structure and the deformation of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) is presented. An individual tube was modeled using a frame-like structure with beam elements. The effect of van der Waals forces, crucial in MWCNTs, was modeled by spring elements. The success of this new carbon nanotube (CNT) modeling approach was verified by comparing the simulation results for single- and multi-walled nanotubes and graphene with other experimental and computational results available in the literature. Simulations of final deformed configurations were in excellent agreement with the atomistic models for various deformations. The proposed approach successfully predicts the experimentally observed values for mechanical behavior of SWCNTs and MWCNTs. The results demonstrated that the proposed FE technique could provide a valuable tool for studying the mechanical behavior of different types of nanotubes, as well as their effectiveness as load-bearing entities in nanocomposite materials.     

Acetic acid effects on enhancement of growth rate and reduction of amorphous carbon deposition on CNT arrays along a growth window in a floating catalyst reactor

The mm-long carbon nanotube (CNT) arrays were grown in a floating catalyst reactor, using xylene-ferrocene and a small amount of acetic acid as the feed. The CNT arrays deposited on a quartz substrate at several positions along the reactor were extensively characterized using Raman spectroscopy, scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy, and optical microscopy. Various characterization methods consistently reveal that the acetic acid additive to the feed alleviates deposition of amorphous carbon layer, which gradually thickens CNTs along the reactor. The acetic acid also resulted in a higher growth rate along the so-called growth window, where CNT arrays are deposited on the quartz substrate. High-performance liquid chromatography of extracted byproducts (PAHs) confirmed the presence of some polycyclic aromatic hydrocarbons. The solid weight of PAHs decreased upon addition of ferrocene as the catalyst precursor, as well as of acetic acid to xylene feed. The results suggest that primary light products of xylene pyrolysis can be competitive reactants for both catalytic and subsequent pyrolytic reactions. They may also be more efficient feeds for CNT growth than xylene itself.