Ultralong Carbon Nanotubes Research Articles

Fabrication of Ultralong and Electrically Uniform Single-Walled Carbon Nanotubes on Clean Substrates

We report the controlled growth of ultralong single-wall carbon nanotube (SWNT) arrays using an improved chemical vapor deposition strategy. Using ethanol or methane as the feed gas, monodispersed Fe-Mo as the catalyst, and a superaligned carbon nanotube (CNT) film as the catalyst supporting frame, ultralong CNTs over 18.5 cm long were grown on Si substrates. The growth rate of the CNTs was more than 40 mum/s. No catalyst-related residual material was found on the substrates due to the use of a CNT film as the catalyst supporting frame, facilitating any subsequent fabrication of SWNT-based devices. Electrical transport measurements indicated that the electrical characteristics along a single ultralong SWNT were uniform. We also found that maintaining a spatially homogeneous temperature during the growth process was a critical factor for obtaining constant electrical characteristics along the length of the ultralong SWNTs.

Enhancement of Electrical Performance of Ultra-Long Multi-Wall Carbon Nanotube Arrays by Au/Pd Coating and High-Bias Treatment

Enhancement of Electrical Performance of Ultra-Long Multi-Wall Carbon Nanotube Arrays by Au/Pd Coating and High-Bias Treatment

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.

Study of Growth Kinetics of Ultra-Long Carbon Nanotube Arrays through Wall Number Statistical Investigation

AbstractThe unique properties of carbon nanotubes (CNTs) strongly depend on their structures. In this study, the growth kinetics of ultra-long multi-walled CNT (MWCNT) arrays by water-assisted chemical vapor deposition (WACVD) has been investigated based on the statistical studies of CNT wall number. It was found that the kinetics of MWCNT arrays in WACVD demonstrated a lengthening and thickening growth. In the linear lengthening stage, CNT wall number remains constant and catalysts preserve the activity; while in the thickening stage, CNTs thicken substantially through the gas phase-induced thickening process and catalysts start to deactivate. The effects of ethylene and hydrogen flow rates on the MWCNT array growth have also been studied. It was found that by changing ethylene flow rate, different linear lengthening stages corresponding to different CNT wall numbers could be obtained. These findings provide experimental solutions to fabrication MWCNT arrays with both selective heights and controllable wall numbers by WACVD.

Air-assisted growth of ultra-long carbon nanotube bundles

We report an air-assisted chemical vapor deposition (CVD) method for the synthesis ofsuper-long carbon nanotube (CNT) bundles. By mixing a small amount of air in the vaporphase catalyst CVD process, the catalyst lifetime can be dramatically increased, and extremelylong dense and aligned CNT bundles up to 1.5 cm can be achieved. Electron microscopycharacterization shows that the injection of air does not damage the CNT structures. Further,we have estimated that individual ultra-long CNTs can carry moderate current densities∼105 A cm−2, indicating their possible use in nanoelectronic devices.

Formation of Highly Dense Aligned Ribbons and Transparent Films of Single-Walled Carbon Nanotubes Directly from Carpets

Utilizing aligned carbon nanotube arrays grown from chemical vapor deposition, we present a highly scalable route toward the formation of ribbons and ultrathin transparent films directly from vertically aligned single-walled carbon nanotube arrays (SWNT carpets). To "lay-over" the aligned nanotubes to form a film, we use a roller which acts to compress the film and preserve the alignment of nanotubes within the film. As we demonstrate, we can control the nanotube-catalyst interaction, leading to highly efficient transfer of the film to virtually any host substrate by following growth with a controlled H(2)O vapor etch. In addition, we demonstrate our ability to grow carpets on patterned substrates leading to upright carpet lines, which can be rolled over to form transparent films composed of ultralong carbon nanotubes. This work demonstrates a highly scalable technique to form homogeneous, transparent films of aligned SWNTs that can be ultralong with absolutely no need for liquid phase SWNT processing.

Ultralong Aligned Multi-Walled Carbon Nanotube for Electrochemical Sensing

We have investigated electrochemical sensing properties of electrodes fabricated with ultralong aligned multi-walled carbon nanotube (MWNT) bundles synthesized using water-assisted chemical vapor deposition on aluminum (Al) and iron (Fe) coated silicon wafer with ethylene and argon/hydrogen gas as carbon source and buffer gas respectively. Cyclic voltammograms performed on these electrodes show diffusion-controlled-reversible reaction. The dominance of radial diffusion mass transport at these electrodes was also indicated by sigmoidal-shaped voltammograms obtained at various scan rates. These electrodes were able to sense very low concentration of ascorbic acid (approximately 0.7 microM) and dopamine (approximately 1.87 microM), two model species often used in electro-analysis. The excellent electrochemical properties along with good single species detection ability suggest that these MWNTs are promising electrode materials for developing very sensitive chemical and/or biological sensors.

Direct synthesis of ultralong carbon nanotube bundles by spray pyrolysis and investigationof growth mechanism

A mass of ultralong aligned carbon nanotube (CNT) bundles up to about 8 mm in lengthwas synthesized by utilizing the spray pyrolysis of ferrocene/xylene solutions in theabsence of promoters, such as thiophene or pure sulfur, and etching agents, suchas plasma, oxygen, ethanol or water. Scanning electron microscopy (SEM) andhigh-resolution transmission electron microscopy (HRTEM) results indicated thecontinuous and multi-walled nanotube structure for the CNTs in the macroscopicalbundles. A graph demonstrating the correlation between growth time and lengthof CNT arrays revealed that the maximum growth rate reaches approximately4612 nm s−1. Based on the experimental phenomena and results, a continuous rapid growthmechanism in both directions was proposed, which suggests that longer CNTarrays can be synthesized at this rapid growth rate if the growth time is extended.

Temperature effect on the substrate selectivity of carbon nanotube growth in floatingchemical vapor deposition

An easy approach to control the carbon nanotube (CNT) array selectivity onSi/SiO2 in the floating chemical vapor deposition (CVD) method by adjusting the operationtemperature is demonstrated. Aligned CNTs can be obtained directly on Si using astandard procedure at a relatively low temperature. After analyzing the temperatureinfluence on different substrates, an attempt to grow an ultra-long CNT forest at theoptimized temperature and without any feedstock diffusion limitation is discussed.

Mechanical behavior of ultralong multiwalled carbon nanotube mats

Carbon nanotubes (CNTs) have been a subject of great interest partially due to their potential for exceptional material properties. Improvements in synthesis methods have facilitated the production of ultralong CNT mats, with lengths in the millimeter range. The increased length of these ultralong mats has, in return, opened the way to greater flexibility to probe their mechanical response. In this work, mats of dense, well-aligned, multiwalled carbon nanotubes were grown with a vapor-phase chemical vapor deposition technique using ferrocene and benzene as reactants, and subsequently tested in both tension and compression using two methods, in a thermomechanical analyzer and in situ inside a scanning electron microscope. In compression, measured stiffness was very low, due to buckling of the nanotubes. In tension, the nanotube mats behaved considerably stiffer; however, they were still more compliant than expected for nanotubes (∼1TPa). Analysis of both the growth method used and the nanotube mat fracture surface suggests that the mats grown in this method are not composed of continuous nanotubes and their strengths actually closely match those of woven nanotube yarns and ropes.

Mesoscale modeling of mechanics of carbon nanotubes: Self-assembly, self-folding, and fracture

Using concepts of hierarchical multiscale modeling, we report development of a mesoscopic model for single-wall carbon nanotubes with parameters completely derived from full atomistic simulations. The parameters in the mesoscopic model are fit to reproduce elastic, fracture, and adhesion properties of carbon nanotubes, in this article demonstrated for (5,5) carbon nanotubes. The mesoscale model enables modeling of the dynamics of systems with hundreds of ultralong carbon nanotubes over time scales approaching microseconds. We apply our mesoscopic model to study self-assembly processes, including self-folding, bundle formation, as well as the response of bundles of carbon nanotubes to severe mechanical stimulation under compression, bending, and tension. Our results with mesoscale modeling corroborate earlier results, suggesting a novel self-folding mechanism, leading to creation of racket-shaped carbon nanotube structures, provided that the aspect ratio of the carbon nanotube is sufficiently large. We find that the persistence length of the (5,5) carbon nanotube is on the order of a few micrometers in the temperature regime from 300 to 1000 K.

Quasi-Continuous Growth of Ultralong Carbon Nanotube Arrays

We report the growth of ultralong (>10 cm) multi-walled and single-walled carbon nanotubes such that the length is limited by the size of the furnace rather than by the termination of growth. The disturbance of microscale laminar flows results in disordered or shorter growth of carbon nanotubes. By downsizing reaction pipes, reaction gas flows are stabilized with low Reynolds numbers. In this way, the catalyst nanoparticles at the end of growing carbon nanotubes can travel a longer distance to grow ultralong nanotubes.

Raman Spectroscopy and Imaging of Ultralong Carbon Nanotubes

Raman spectroscopy and confocal Raman imaging with 514 nm excitation was performed on recently developed ultralong carbon nanotubes grown by the "fast-heating" chemical vapor deposition (CVD) method. The ultralong nanotubes are found to consist of both semiconducting and metallic types, with spectra that are consistent with the nanotubes being single walled. Characterization of nanotube diameters shows that short nanotubes appearing near the sample catalyst region have a broader distribution than is observed for the ultralong nanotubes. The narrow diameter distribution is determined by uniformity of catalyst particle size and gives additional evidence for the proposed "kite" mechanism for long nanotube growth. Raman imaging was performed over large length scales (up to 140 microm). Imaging reveals the ultralong nanotubes to be of high quality, with a very low defect density. Variations in G-band frequencies and intensity demonstrate the occurrence of minor structural changes and variations in nanotube-substrate interaction along the length of the nanotubes. Evidence also demonstrates that larger structural changes resulting in a full chirality change can occur in these nanotube types to produce a metal-to-semiconductor intramolecular junction.

Nanotech goes to new lengths: Scientists create ultralong carbon nanotubes

Nanotech goes to new lengths: Scientists create ultralong carbon nanotubes