Single-walled Carbon Nanotube Forests Research Articles

Single‐walled carbon nanotube networks growth optimization

AbstractIn recent years there have been many improvements in quality and yield of single‐walled carbon nanotubes (SWCNTs) obtained by chemical vapour deposition (CVD). Mainstream in research is focused on growing thick vertically aligned forests of SWCNTs. At the same time SWCNT films – usually obtained by spraying and filtration – are one of the most promising candidates to replace indium tin oxide (ITO) as transparent electrodes in solar cells and flat displays. The main limit of those techniques is that usually carbon nanotubes (CNTs) are deposited in the form of thick bundles and are mixed with insulating molecules, such as surfactants, used to try to unbundle the CNTs in solution. Moreover, ultrasonication is in general used to help dispersion, inducing defects in the tubes; the overall effect is to considerably reduce the conductivity of such CNT films. It is our firm belief that a considerable improvement in film quality can come by directly synthesizing the networks onto the substrate via CVD, but this requires achieving a control of the density and quality of the SWCNT networks without losing the planar arrangement. We show here our latest results in enhancing the quality of SWCNT percolating network synthesis by alcohol CVD on silicon oxide substrates using cobalt and nickel acetates as precursors.

Process intensification by CO2 for high quality carbon nanotube forest growth: Double-walled carbon nanotube convexity or single-walled carbon nanotube bowls?

Introduction of CO2 is a facile way to tune the growth of vertically aligned double- or single-walled carbon nanotube (CNT) forests on wafers. In the absence of CO2, a double-walled CNT convexity was obtained. With increasing concentration of CO2, the morphologies of the forests transformed first into radial blocks, and finally into bowl-shaped forests. Furthermore, the wall number and diameter distribution of the CNTs were also modulated by varying the amount of CO2. With increasing CO2 concentration, CNTs with fewer wall number and smaller diameter were obtained. The addition of CO2 is speculated to generate water and serve as a weak oxidant for high quality CNT growth. It can tune the growth rate and the morphologies of the forests, prevent the formation of amorphous carbon, and reduce the wall number of the CNTs.

Single-wall carbon nanotube forest arrays for immunoelectrochemical measurement of four protein biomarkers for prostate cancer.

Protein arrays that measure multiple protein cancer biomarkers in clinical samples hold great promise for reliable early cancer detection. Herein, we report a prototype 4-unit electrochemical immunoarray based on single-wall carbon nanotube forests for the simultaneous detection of multiple protein biomarkers for prostate cancer. Immunoarray procedures were designed to measure prostate specific antigen (PSA), prostate specific membrane antigen (PSMA), platelet factor-4 (PF-4), and interleukin-6 (IL-6) simultaneously in a single serum sample. All of these proteins are elevated in serum of patients with prostate cancer, but they have widely different relative levels of serum concentration. Horseradish peroxidase (HRP) was used as label on detection (secondary) antibodies in a sandwich immunoassay scheme. Biotinylated secondary antibodies (Ab(2)) that bind specifically to streptavidin-HRP conjugates provided 14-16 labels per antibody and gave the necessary higher sensitivity required for PF-4 and IL-6 detection at physiological levels. Conventional singly labeled Ab(2)-HRP conjugates were sufficient for PSA and PSMA detection. Immunoarrays were used to measure four biomarkers in clinical human serum samples of prostate cancer patients and controls with excellent correlation to referee enzyme-linked immunosorbent (ELISA) assays.

Acetylene: A Key Growth Precursor for Single-Walled Carbon Nanotube Forests

The growth of tall, single-walled carbon nanotube (SWNT) forests is often believed to require the use of specific feedstocks (e.g., ethanol) or etchants (e.g., water). Here, on the basis of mass spectrometry for hydrogen-diluted methane in a microwave plasma-enhanced chemical vapor deposition (PECVD) reactor, we develop a purely thermal CVD process for SWNT forests without an etchant gas. The thermally grown SWNT forests are found to have a similar high quality, purity, growth rate, areal density, diameter, and chirality distribution as in the PECVD case. We show that C2H2 is the main growth precursor, and suggest more generally that feedstock conversion to C2H2 is of key importance to SWNT forest CVD. A reactive etchant, such as water, atomic hydrogen, or hydroxyl radicals, can widen the SWNT forest deposition window but is not required in cold-wall reactors at low pressures.

Effects of Metal Underlayer Grain Size on Carbon Nanotube Growth

In this paper we demonstrate that the nucleation density of single-walled carbon nanotubes (SWNTs), formed by thermal catalytic chemical vapor deposition, strongly depends on the grain size of Al underlayers covered with a native oxide (Al/Al2O3). By varying the Substrate temperature during Al sputter deposition it was possible to investigate the effect of Al grain size on growth without inducing changes in the underlayer thickness, surface chemistry, or any other growth parameter. The resulting SWNT growth structures ranged from low-density 2D nanotube networks that lay across the surface of the substrate to high density 3D nucleation which gave rise to vertical forest growth. The height of the SWNT forest was observed to increase with increasing Al deposition temperature as follows, 200 > 100 > 60 > 20 degrees C on Si/Al but in the order 100 > 200 > 60 > 20 degrees C on SiO2/Al substrates for fixed growth conditions. The differences in the SWNT growth trends on Si and SiO2 substrates are believed to be due to the existence of an optimal Al/Al2O3 underlayer grain size for the formation of active catalytic nanoparticles, with larger Al/Al2O3 grains forming on SiO2 than Si at a fixed substrate temperature. Numerous surface analysis techniques including AFM, XPS, FESEM, TEM, and Raman spectroscopy have been employed to ascertain that the observed changes in nanotube growth for this system are related primarily to changes in underlayer morphology.

60]-Fullerene and Single-Walled Carbon Nanotube-Based Ultrathin Films Stepwise Grafted onto a Self-Assembled Monolayer on ITO

A step-by-step method was used to prepare homogeneous ultrathin films composed of [60]-fullerene (C60) and single-walled carbon nanotubes (SWNTs), grafted onto the functional surface of an alkylsilane self-assembled monolayer (SAM) on an ITO substrate with an ITO-C60-SWNT sequence using amine addition across a double bond in C60 followed by amidation coupling with acid-functionalized SWNTs. Atomic force microscope and scanning electron microscope images of the resulting composite film showed two-component ball-tube microstructures with high-density coverage, where C60 was homogeneously distributed in the SWNT forest. The attachment of SWNTs to the residual amine units in the SAM on the ITO substrate (SAM-ITO) as well as on the C60 sphere results in the C60 molecules in the aggregated clusters being more separately dispersed, which forms a densely packed composite film as a result of the pi-pi interaction between the C60 buckyballs and the SWNT walls. It was found using ferrocene as an internal redox probe that the oxidative and reductive processes at the film-solution surface were effectively retarded because of obstruction from the densely packed film and the electronic effect of SWNT and C60. In addition, the electrochemical properties of C60 on SAM-ITO plates observed by cyclic voltammetry were significantly modified by chemical anchorage using SWNTs. X-ray photoelectron spectroscopy (XPS) analysis also indicated the successful grafting of C60 and SWNT. The XPS chemical shift of the binding energy showed the presence of electronic interactions between C60, SWNT, and ITO components. Such a uniformly distributed C60-SWNT film may be useful for future research in electrochemical and photoactive nanodevices.

Thermal Diffusivity of Single-Walled Carbon Nanotube Forest Measured by Laser Flash Method

The science and technology of carbon nanotubes are very interesting topics in the field of nanotechnology. It is considered that such nanotubes have excellent properties of electrical conduction, tensile strength, and thermal conduction. However, these properties are not well understood yet. Techniques for handling a nanotube and measuring these properties have not yet been established. Recently, a technique for synthesizing supergrowth carbon nanotubes, which form a highly pure single-walled carbon nanotube (SWNT) forest, has been developed. These supergrowth carbon nanotubes grow to as long as a millimeter scale. As-grown supergrowth carbon nanotubes include about 5×1011 SWNTs per square centimeter. The solid of supergrowth carbon nanotubes is prepared by reducing the distance between SWNTs. We have investigated the thermal conduction of such carbon nanotubes and were successful in measuring the thermal diffusivity of self-standing samples of as-grown supergrowth carbon nanotubes and their solid using the laser flash method. It was found that the carbon nanotube samples show a comparable thermal diffusivity to isotropic graphite. We have also measured the temperature dependence of the thermal diffusivity of the supergrowth carbon nanotube samples.

Designing nanomaterial-enhanced electrochemical immunosensors for cancer biomarker proteins

Detection of multiple cancer biomarker proteins in human serum and tissue at point-of-care is a viable approach for early cancer detection, but presents a major challenge to bioanalytical device development. This article reviews recent approaches developed in our laboratories combining nanoparticle decorated electrodes and multilabeled secondary antibody labeled particles to achieve high sensitivity for the detection of cancer biomarker proteins. Two nanomaterial-based sensor platforms were used: (a) upright single wall carbon nanotube forests and (b) layers of densely packed 5 nm gold nanoparticles. Both platforms feature pendant carboxylate groups for easy attachment of enzymes or antibodies by amidization. In quality performance tests, the biocatalytic responses for determination of hydrogen peroxide of AuNP layers with attached horseradish peroxidase (HRP) on electrodes gave somewhat better detection limit and sensitivity than single wall carbon nanotube (SWNT) forest platforms with HRP attached. Evaluation of these sensors as platforms for sandwich immunoassays for cancer biomarker prostate specific antigen (PSA) in serum showed that both approaches gave accurate results for human serum samples from cancer patients. The best detection limit (0.5 pg mL − 1 ) and sensitivity were obtained by combining the AuNP immunosensors with binding of 1 μm diameter magnetic particles decorated with secondary antibodies and 7500 HRP labels.

Electrochemiluminescent immunosensor for detection of protein cancer biomarkers using carbon nanotube forests and [Ru-(bpy)3]2+-doped silica nanoparticles

We report the first electrochemiluminescent immunosensor combining single-wall carbon nanotube forests with RuBPY-silica-secondary antibody nanoparticles for sensitive detection of cancer biomarker prostate specific antigen.

Growth Window and Possible Mechanism of Millimeter-Thick Single-Walled Carbon Nanotube Forests

Our group recently reproduced the water-assisted growth method, so-called "SuperGrowth", of millimeter-thick single-walled carbon nanotube (SWNT) forests by using C2H4/H2/H2O/Ar reactant gas and Fe/Al2O3, catalyst. In this current work, a parametric study was carried out on both reaction and catalyst conditions. Results revealed that a thin Fe catalyst layer (about 0.5 nm) yielded rapid growth of SWNTs only when supported on Al2O3, and that Al2O3 support enhanced the activity of Fe, Co, and Ni catalysts. The growth window for the rapid SWNT growth was narrow, however. Optimum amount of added H2O increased the SWNT growth rate but further addition of H2O degraded both the SWNT growth rate and quality. Addition of H2 was also essential for rapid SWNT growth, but again, further addition decreased both the SWNT growth rate and quality. Because Al2O3 catalyzes hydrocarbon reforming, Al2O3 support possibly enhances the SWNT growth rate by supplying the carbon source to the catalyst nanoparticles. The origin of the narrow window for rapid SWNT growth is also discussed.

Multiple “optimum” conditions for Co–Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co–Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.

Diagnostics and growth control of single-walled carbon nanotube forests using a telecentric optical system for in situ height monitoring

An in situ telecentric height monitoring system was employed as a standard characterization tool for in situ height monitoring of nanotube forests. We demonstrated that the system possesses a wide dynamic range beyond the centimeter scale, high resolution of 1 μm, free from routine maintenance, simple installation, and use in contrast to previous methods. These features were highlighted by the monitoring of a 5 mm tall forest, the examination of the effect of gas species on growth rates, and the automatic control forest height.

Single-Walled Carbon Nanotube Forest with Hydrogenase for Electrochemical Hydrogen Evolution

not Available.

Water-Assisted Highly Efficient Synthesis of Single-Walled Carbon Nanotubes Forests from Colloidal Nanoparticle Catalysts

Highly efficient single-walled carbon nanotube (SWNT) growth from Fe−Mo nanoparticle catalysts made by colloidal synthesis is demonstrated by water-assisted chemical vapor deposition. In a 10 min growth time, SWNT forests with heights up to 1.5 mm were synthesized possessing a 2.8 nm average diameter, carbon purity above 99.99%, and Brunauer−Emmett−Teller (BET) surface area of 1200 m2/g, which rivals SWNTs grown from sputtered Fe thin films. Realization of high efficiency SWNT growth using catalysts prepared by economical and scalable wet processes opens up a cost-effective route toward the mass production of SWNT forests.

Oriented Growth and Subsequent Handling of Vertical Single-walled Carbon Nanotube Forest Characterized by Electron and Probe Microscopy

Extract Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Effective properties for plain weave composites through-thickness reinforced with carbon nanotube forests

Abstract In this paper, an analytical approach is proposed to study the elastic properties of plain weave unit cells with reinforcing single-walled carbon nanotube forests of different growing patterns on the warp and weft carbon yarns. For the carbon nanotube-reinforced polymer matrix in which the carbon fibers are embedded, its properties can be evaluated using a modified rule of mixture. A numerical study using the present analytical approach reveals that for the cases considered, only a small increase in the nanotube volume fraction can result in a remarkable increase in the out-of-plane Young’s modulus and a slight increase in the out-of-plane shear modulus but a decrease in the out-of-plane Poisson’s ratios for the plain weave composite. The existence of carbon nanotube forests does not significantly affect the effective in-plane properties of plain weave composites. It is also noted from the present numerical study that the effective properties of plain weave composites with nanotube forest reinforcement are closely related to the nanotube volume fraction, growing or distribution patterns, but slightly affected by the orientations of carbon nanotubes.

Angle-resolved x-ray absorption near edge structure study of vertically aligned single-walled carbon nanotubes

Vertically aligned single-walled carbon nanotube (SWNT) forest was studied by using angular-dependent C K-edge x-ray absorption near edge structure (XANES) with linearly polarized x-ray beam. The XANES analysis found a crust of entangled nanotubes on top of the forest formed at the first stage of the forest growth, which shapes the morphology of the entire forest and constricts the nanotubes to grow to the same length. It indicates that this type of SWNT forest has a different growth mechanism from the multiwalled carbon nanotube forest.

Influence of a Top Crust of Entangled Nanotubes on the Structure of Vertically Aligned Forests of Single-Walled Carbon Nanotubes

The time evolution of the growth process of vertically aligned single-walled carbon nanotubes (or V-SWNTs) on a flat substrate was examined by scanning electron microscopy (SEM), resonant Raman spectroscopy, and angle-resolved X-ray absorption near-edge structure (XANES). This detailed characterization gives evidence for the growth of a thin layer (crust) of randomly oriented single-walled carbon nanotubes during the first stages of the growth process. This crust is responsible for the unique forest-like morphology exhibited by this type of SWNT structure.

Carbon nanotube amplification strategies for highly sensitive immunodetection of cancer biomarkers.

We describe herein the combination of electrochemical immunosensors using single-wall carbon nanotube (SWNT) forest platforms with multi-label secondary antibody-nanotube bioconjugates for highly sensitive detection of a cancer biomarker in serum and tissue lysates. Greatly amplified sensitivity was attained by using bioconjugates featuring horseradish peroxidase (HRP) labels and secondary antibodies (Ab(2)) linked to carbon nanotubes (CNT) at high HRP/Ab(2) ratio. This approach provided a detection limit of 4 pg mL(-)(1) (100 amol mL(-)(1)), for prostate specific antigen (PSA) in 10 microL of undiluted calf serum, a mass detection limit of 40 fg. Accurate detection of PSA in human serum samples was demonstrated by comparison to standard ELISA assays. PSA was quantitatively measured in prostate tissue samples for which PSA could not be differentiated by the gold standard immunohistochemical staining method. These easily fabricated SWNT immunosensors show excellent promise for clinical screening of cancer biomarkers and point-of-care diagnostics.

84% Catalyst Activity of Water-Assisted Growth of Single Walled Carbon Nanotube Forest Characterization by a Statistical and Macroscopic Approach

We propose a statistical and macroscopic analysis to estimate the catalyst activity of water-assisted growth (super-growth) of single-walled nanotubes (SWNT) and to characterize SWNT forests. The catalyst activity was estimated to be 84% (+/-6%), the highest ever reported. The SWNT forest was found to be a very sparse material where SWNTs represent only 3.6% of the total volume. This structural sparseness is believed to play a critical role in achieving highly efficient growth.