Uniform, dense arrays of vertically aligned, large-diameter single-walled carbon nanotubes

Precisely controlled reactive chemical vapor synthesis of highly uniform, dense arrays of vertically aligned single-walled carbon nanotubes (SWCNTs) using tailored trilayered Fe/Al2O3/SiO2 catalyst is demonstrated. More than 90% population of thick nanotubes (>3 nm in diameter) can be produced by tailoring the thickness and microstructure of the secondary catalyst supporting SiO2 layer, which is commonly overlooked. The proposed model based on the atomic force microanalysis suggests that this tailoring leads to uniform and dense arrays of relatively large Fe catalyst nanoparticles on which the thick SWCNTs nucleate, while small nanotubes and amorphous carbon are effectively etched away. Our results resolve a persistent issue of selective (while avoiding multiwalled nanotubes and other carbon nanostructures) synthesis of thick vertically aligned SWCNTs whose easily switchable thickness-dependent electronic properties enable advanced applications in nanoelectronic, energy, drug delivery, and membrane technologies.

Plasma-enabled growth of single-crystalline SiC/AlSiC Core–Shell nanowires on porous alumina templates

We report the catalyst-free synthesis of the arrays of core–shell, ultrathin, size-uniform SiC/AlSiC nanowires on the top of a periodic anodic aluminum oxide template. The nanowires were grown using an environmentally friendly, silane-free process by exposing the silicon supported porous alumina template to CH4 + H2 plasmas. High-resolution scanning and transmission electron microscopy studies revealed that the nanowires have a single-crystalline core with a diameter of about 10 nm and a thin (1–2 nm) amorphous AlSiC shell. Because of their remarkable length, high aspect ratio, and very high surface area-to-volume ratio, these unique structures are promising for nanoelectronic and nanophotonic applications that require efficient electron emission, light scattering, etc. A mechanism for nanowire growth is proposed based upon the reduction of the alumina template to nanosized metallic aluminum droplets forming between nanopores. The subsequent incorporation of silicon and carbon atoms from the plasma leads to nucleation and growth from the top of the alumina template.

Control of density of self-organized carbon nanotube arrays by catalyst pretreatment through plasma immersion ion implantation

A simple and effective method of controlling the growth of vertically aligned carbon nanotube arrays in a lowerature plasma is presented. Ni catalyst was pretreated by plasma immersion ion implantation prior to the nanotube growth by plasma-enhanced chemical vapor deposition. Both the size distribution and the areal density of the catalyst nanoparticles decrease due to the ion-surface interactions. Consequently, the resulting size distribution of the vertically aligned carbon nanotubes is reduced to 50 ∼ 100 nm and the areal density is lowered (by a factor of ten) to 10 8 cm -2, which is significantly different from the very-high-density carbon nanotube forests commonly produced by thermal chemical vapor deposition. The efficiency of this pretreatment is compared with the existing techniques such as neutral gas annealing and plasma etching. These results are highly relevant to the development of the next-generation nanoelectronic and optoelectronic devices that require effective control of the density of nanotube arrays.

Graphene and carbon nanotubes from arc plasmas : experiment and plasma modeling

Graphene and carbon nanotubes are the most promising nanomaterials for application in various modern nanodevices. The successful production of the nanotubes and graphene in a single process was achieved by using a magnetically enhanced arc discharge in helium atmosphere between carbon and metal electrodes. A 3-D fluid model has been used to investigate the discharge parameters.

Thin single-walled carbon nanotubes with narrow chirality distribution : constructive interplay of plasma and Gibbs–Thomson effects

Multiscale, multiphase numerical modeling is used to explain the mechanisms of effective control of chirality distributions of single-walled carbon nanotubes in direct plasma growth and suggest effective approaches to further improvement. The model includes an unprecedented combination of the plasma sheath, ion/radical transport, species creation/loss, plasma–surface interaction, heat transfer, surface/bulk diffusion, graphene layer nucleation, and bending/lift-off modules. It is shown that the constructive interplay between the plasma and the Gibbs–Thomson effect can lead to the effective nucleation and lift-off of small graphene layers on small metal catalyst nanoparticles. As a result, much thinner nanotubes with narrower chirality distributions can nucleate at much lower process temperatures and pressures compared to thermal CVD. This approach is validated by a host of experimental results, substantially reduces the amounts of energy and atomic matter required for the nanotube growth, and can be extended to other nanoscale structures and materials systems, thereby nearing the ultimate goal of energy- and matter-efficient nanotechnology.

Simulation of carbon arc discharge for the synthesis of nanotubes

Arc discharge ablation with a catalyst-filled carbon anode in a helium background was used for the synthesis of graphene and carbon nanotubes. In this paper, we present the results of the numerical simulation of the distribution of various plasma parameters in discharge, as well as the distribution of carbon flux on the nanotube surface, for the typical discharge with an arc current of 60 A and a background gas pressure of 68 kPa.

Heating and plasma sheath effects in low-temperature, plasma-assisted growth of carbon nanofibers

Plasma sheath, nanostructure growth, and thermal models are used to describe carbon nanofiber (CNF) growth and heating in a low-temperature plasma. It is found that when the H2 partial pressure is increased, H atom recombination and H ion neutralization are the main mechanisms responsible for energy release on the catalyst surface. Numerical results also show that process parameters such as the substrate potential, electron temperature and number density mainly affect the CNF growth rate and plasma heating at low catalyst temperatures. In contrast, gas pressure, ion temperature, and the C2H2:H2 supply ratio affect the CNF growth at all temperatures. It is shown that plasma-related processes substantially increase the catalyst particle temperature, in comparison to the substrate and the substrate-holding platform temperatures.

3-Orders-of-magnitude density control of single-walled carbon nanotube networks by maximizing catalyst activation and dosing carbon supply

Tailoring the density of random single-walled carbon nanotube (SWCNT) networks is of paramount importance for various applications, yet it remains a major challenge due to the insufficient catalyst activation in most growth processes. Here we report on a simple and effective method to maximise the number of active catalyst nanoparticles using catalytic chemical vapor deposition (CCVD). By modulating short pulses of acetylene into a methane-based CCVD growth process, the density of SWCNTs is dramatically increased by up to three orders of magnitude without increasing the catalyst density and degrading the nanotube quality. In the framework of a vapor-liquid-solid model, we attribute the enhanced growth to the high dissociation rate of acetylene at high temperatures at the nucleation stage, which can be effective in both supersaturating the larger catalyst nanoparticles and overcoming the nanotube nucleation energy barrier of the smaller catalyst nanoparticles. These results are highly relevant to numerous applications of random SWCNT networks in next-generation energy, sensing and biomedical devices. © 2011 The Royal Society of Chemistry.

Real-time monitoring of nucleation-growth cycle of carbon nanoparticles in acetylene plasmas

Quantum cascade laserabsorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticlenucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generatingplasmas.

Control of dense carbon nanotube arrays via hierarchical multilayer catalyst

Effective control of dense, high-quality carbon nanotube arrays using hierarchical multilayer catalyst patterns is demonstrated. Scanning/transmission electron microscopy, atomic force microscopy, Raman spectroscopy, and numerical simulations show that by changing the secondary and tertiary layers one can control the properties of the nanotube arrays. The arrays with the highest surface density of vertically aligned nanotubes are produced using a hierarchical stack of iron nanoparticles and alumina and silica layers differing in thickness by one order of magnitude from one another. The results are explained in terms of the catalyst structure effect on carbon diffusivity.

Effect of hydrophilicity of carbon nanotube arrays on the release rate and activity of recombinant human bone morphogenetic protein-2

Novel nanostructures such as vertically aligned carbon nanotube (CNT) arrays have received increasing interest as drug delivery carriers. In the present study, two CNT arrays with extreme surface wettabilities are fabricated and their effects on the release of recombinant human bone morphogenetic protein-2 (rhBMP-2) are investigated. It is found that the superhydrophilic arrays retained a larger amount of rhBMP-2 than the superhydrophobic ones. Further use of a poloxamer diffusion layer delayed the initial burst and resulted in a greater total amount of rhBMP-2 released from both surfaces. In addition, rhBMP-2 bound to the superhydrophilic CNT arrays remained bioactive while they denatured on the superhydrophobic surfaces. These results are related to the combined effects of rhBMP-2 molecules interacting with poloxamer and the surface, which could be essential in the development of advanced carriers with tailored surface functionalities.

Controlled synthesis of a large fraction of metallic single-walled carbon nanotube and semiconducting carbon nanowire networks

Controlled synthesis of both single-walled carbon nanotube and carbon nanowire networks using the same CVD reactor and Fe/Al2O3 catalyst by slightly altering the hydrogenation and temperature conditions is demonstrated. Structural, bonding and electrical characterization using SEM, TEM, Raman spectroscopy, and temperature-dependent resistivity measurements suggest that the nanotubes are of a high quality and a large fraction (well above the common 33% and possibly up to 75%) of them are metallic. On the other hand, the carbon nanowires are amorphous and semiconducting and feature a controlled sp2/sp3 ratio. The growth mechanism which is based on the catalyst nanoisland analysis by AFM and takes into account the hydrogenation and temperature control effects explains the observed switch-over of the nanostructure growth modes. These results are important to achieve the ultimate control of chirality, structure, and conductivity of one-dimensional all-carbon networks.

Ion-assisted precursor dissociation and surface diffusion : enabling rapid, low-temperature growth of carbon nanofibers

Growth kinetics of carbon nanofibers in a hydrocarbon plasma is studied. In addition to gas-phase and surface processes common to chemical vapor deposition, the model includes (unique to plasma-exposed catalyst surfaces) ion-induced dissociation of hydrocarbons, interaction of adsorbed species with incoming hydrogen atoms, and dissociation of hydrocarbon ions. It is shown that at low, nanodevice-friendly process temperatures the nanofibers grow via surface diffusion of carbon adatoms produced on the catalyst particle via ion-induced dissociation of a hydrocarbon precursor. These results explain a lower activation energy of nanofiber growth in a plasma and can be used for the synthesis of other nanoassemblies. © 2007 American Institute of Physics.

Controlled electronic transport in single-walled carbon nanotube networks : selecting electron hopping and chemical doping mechanisms

The electronic transport in both intrinsic and acid-treated single-walled carbon nanotube networks containing more than 90% semiconducting nanotubes is investigated using temperature-dependent resistance measurements. The semiconducting behavior observed in the intrinsic network is attributed to the three-dimensional electron hopping mechanism. In contrast, the chemical doping mechanism in the acid-treated network is found to be responsible for the revealed metal-like linear resistivity dependence in a broad temperature range. This effective method to control the electrical conductivity of single-walled carbon nanotube networks is promising for future nanoscale electronics, thermometry, and bolometry. © 2010 American Institute of Physics.

Superhydrophobic amorphous carbon/carbon nanotube nanocomposites

Superhydrophobic amorphous carbon/carbon nanotube nanocomposites are fabricated by plasma immersion ion implantation with carbon nanotube forests as a template. The microstructure of the fabricated nanocomposites shows arrays of carbon nanotubes capped with amorphous carbon nanoparticles. Contact angle measurements show that both advancing and receding angles close to 180° can be achieved on the nanocomposites. The fabrication here does not require patterning of carbon nanotubes or deposition of conformal coatings with low surface energy, which are usually involved in conventional approaches for superhydrophobic surfaces. The relationship between the observed superhydrophobicity and the unique microstructure of the nanocomposites is discussed. © 2009 American Institute of Physics.