Single-walled Carbon Nanotube Forests Research Articles

Increasing the length of a single-wall carbon nanotube forest by adding titanium to a catalytic substrate

The effect of titanium (Ti) added to the top layer of an aluminum (Al)/iron (Fe)/Al (bottom) sandwich catalytic substrate was studied. The Ti caused a significant lengthening in the single-wall carbon nanotube forest produced by chemical vapor deposition (CVD). In general, particles of iron oxide on the Al/Fe/Al catalytic substrate are formed by exposure to the atmosphere during deposition process of the substrate. The particles of iron oxide are metalized during pretreatment under a reductive gas before the growth of the nanotubes with nano-sized dispersion stabilized. On this process, the metallization and the stabilization of nano-sized iron oxide particles occur on the basis of the oxygen affinity in the top aluminum oxide layer, with the ionization tendency Al>Fe. It is thought that the addition of Ti increases the oxygen affinity of the catalytic substrate, since Ti has a stronger ionization tendency than Al. After optimizing the quantity of Ti added to the top layer, we successfully fabricated a millimeter-long, small-diameter, single-wall carbon nanotube forest.

Vertical Single-Wall Carbon Nanotube Forests as Plasmonic Heat Pipes

High thermal conductivity of carbon nanotubes (NTs) is attractive for the heat removal applications. However, the problem of efficient thermal coupling to the heater/cooler still needs to be resolved. We study near-field electromagnetic tunneling as a mechanism of heat transfer across the interface. We report interface thermal (Kapitza) conductance between a low-density vertical metallic single-wall NT forest and a quartz substrate on the order of 50 MW/Km(2) and explain it by strong electromagnetic interaction and near-field entanglement between the surface phonon-polaritons in the polar dielectric and the NT plasmons. We predict that the thickness of the NT film can be tweaked to the resonance wavelength of these entangled modes for performance optimization of nanocarbon thermal interconnects.

Growth of Ultrahigh Density Single-Walled Carbon Nanotube Forests by Improved Catalyst Design

We have grown vertically aligned single-walled carbon nanotube forests with an area density of 1.5 × 10(13) cm(-2), the highest yet achieved, by reducing the average diameter of the nanotubes. We use a nanolaminate Fe-Al(2)O(3) catalyst design consisting of three layers of Al(2)O(3), Fe, and Al(2)O(3), in which the lower Al(2)O(3) layer is densified by an oxygen plasma treatment to increase its diffusion barrier properties, to allow a thinner catalyst layer to be used. This high nanotube density is desirable for using carbon nanotubes as interconnects in integrated circuits.

Role of Subsurface Diffusion and Ostwald Ripening in Catalyst Formation for Single-Walled Carbon Nanotube Forest Growth

Here we show that essentially any Fe compounds spanning Fe salts, nanoparticles, and buckyferrocene could serve as catalysts for single-walled carbon nanotube (SWNT) forest growth when supported on AlO(x) and annealed in hydrogen. This observation was explained by subsurface diffusion of Fe atoms into the AlO(x) support induced by hydrogen annealing where most of the deposited Fe left the surface and the remaining Fe atoms reconfigured into small nanoparticles suitable for SWNT growth. Interestingly, the average diameters of the SWNTs grown from all iron compounds studied were nearly identical (2.8-3.1 nm). We interpret that the offsetting effects of Ostwald ripening and subsurface diffusion resulted in the ability to grow SWNT forests with similar average diameters regardless of the initial Fe catalyst.

High quality single-walled carbon nanotube synthesis using remote plasma CVD

Single-walled carbon nanotube (SWNT) forest synthesis using antenna-type remote plasma chemical vapor deposition (ARPCVD) is presented. A series of synthesis using carbon monoxide gas as carbon feedstock reveals that the remote conditions, in other words, distance between an antenna and a substrate affects the quality of nanotube significantly. That is, far distance geometry on ARPCVD creates high-quality SWNTs. It motivates us to use same methodology for the synthesis using CH4/H2 previously proven to make long SWNT forests. Finally, along the methodology, we achieve to make SWNT forest with high-quality and small diameter successfully. This study offers an important suggestion to embody SWNT forests with both length and quality using remote plasma CVD.

Gas Dwell Time Control for Rapid and Long Lifetime Growth of Single-Walled Carbon Nanotube Forests

The heat history (i.e., "dwell time") of the carbon source gas was demonstrated as a vital parameter for very rapid single-walled carbon nanotube (SWNT) forest growth with long lifetime. When the dwell time was raised to 7 s from the 4 s used for standard growth, the growth rate increased to 620 μm/min: a benchmark for SWNT forest growth on substrates. Importantly, the increase in growth rate was achieved without decreasing either the growth lifetime or the quality of the SWNTs. We interpret that the conversion rate of the carbon feedstock into CNTs was selectively increased (versus catalyst deactivation) by delivering a thermally decomposed carbon source with the optimum thermal history to the catalyst site.

Carbon Nanotube Microwell Array for Sensitive Electrochemiluminescent Detection of Cancer Biomarker Proteins

This paper describes fabrication of a novel electrochemiluminescence (ECL) immunosensor array featuring capture-antibody-decorated single-wall carbon nanotube (SWCNT) forests residing in the bottoms of 10-μL wells with hydrophobic polymer walls. Silica nanoparticles containing [Ru(bpy)(3)](2+) and secondary antibodies (RuBPY-silica-Ab(2)) are employed in this system for highly sensitive two-analyte detection. Antibodies to prostate specific antigen (PSA) and interleukin-6 (IL-6) were attached to the same RuBPY-silica-Ab(2) particle. The array was fabricated by forming the wells on a conductive pyrolytic graphite chip (1 in. × 1 in.) with a single connection to a potentiostat to achieve ECL. The sandwich immunoassay protocol employs antibodies attached to SWCNTs in the wells to capture analyte proteins. Then RuBPY-silica-Ab(2) is added to bind to the captured proteins. ECL is initiated in the microwells by electrochemical oxidation of tripropyl amine (TprA), which generates excited state [Ru(bpy)(3)](2+) in the 100-nm particles, and is measured with a charge-coupled device (CCD) camera. Separation of the analytical spots by the hydrophobic wall barriers enabled simultaneous immunoassays for two proteins in a single sample without cross-contamination. The detection limit (DL) for PSA was 1 pg mL(-1) and for IL-6 was 0.25 pg mL(-1) (IL-6) in serum. Array determinations of PSA and IL-6 in patient serum were well-correlated with single-protein ELISAs. These microwell SWCNT immunoarrays provide a simple, sensitive approach to the detection of two or more proteins.

Moderating carbon supply and suppressing Ostwald ripening of catalyst particles to produce 4.5-mm-tall single-walled carbon nanotube forests

Millimeter-tall single-walled carbon nanotube (SWCNT) forests were grown by chemical vapor deposition (CVD) from C 2H 2/H 2O/Ar using Fe/Al–Si–O catalysts. Using combinatorial catalyst libraries coupled with real-time monitoring of SWCNT growth, the catalyst and CVD conditions were systematically studied. The keys for this growth are to maintain the C 2H 2 pressure below its upper limit to prevent the killing of the catalysts and to grow the SWCNTs before the catalyst particles lose their activity because of coarsening through Ostwald ripening. Lower temperatures lead to lower limits for the C 2H 2 pressure which result in lower growth rates but also lead to even lower coarsening rates which result in even longer growth lifetimes. Using these principles, we grew 4.5-mm-tall SWCNT forests in 2.5 h at 750 °C.

Growth control of single-walled, double-walled, and triple-walled carbon nanotube forests by a priori electrical resistance measurement of catalyst films

We present the wall number control of carbon nanotube (CNT) forests grown on metal catalyst films in a water-assisted chemical vapor deposition (CVD) by measuring the sheet resistances of metal catalyst films. Catalyst film thicknesses and thickness variations are monitored using a 2-point-based electrical characterization methodology. The electrical characterization and high-resolution transmission electron microscopy analysis showed that single-, double-, and triple-walled CNT forests were grown on iron (Fe) catalyst films with mean sheet resistances of 646.63, 75.40, and 27.84 MΩ/sq, respectively. The average wall number and outer diameter of CNT forests were found to linearly depend on the logarithm of the mean sheet resistances of Fe catalyst films.

Direct electron transfer to hydrogenase for catalytic hydrogen production using a single-walled carbon nanotube forest

The production of hydrogen through the direct electron transfer from electrode to hydrogenase was successfully performed using hydrogenase combined with a single-walled carbon nanotube forest (SWNT-forest). The SWNT-forest has a unique structure comprising dense and vertically aligned SWNTs with millimeter-scale height. The vertically aligned SWNTs became rearranged into a wall-like architecture after immersing in water. Hydrogenase was spontaneously incorporated within the SWNT-forest and confined on the sidewall of the rearranged architecture. The SWNT-forest was a very stable protein carrier in aqueous solution, and an SWNT-forest integrated with hydrogenase could easily be constructed. With a hydrogenase assembled SWNT-forest we were able to electrochemically produce hydrogen with an electron transfer efficiency exceeding 30% without the use of any chemical mediator. Furthermore, a reverse oxidative reaction of hydrogen was also successfully performed using the same device. Thus, the SWNT-forest can work as an effective and direct electron mediator for the oxidation/reduction reaction of hydrogenase. The usefulness of the SWNT-forest as a beneficial material for electrochemistry was demonstrated, underlining the potential of protein assembled SWNT-forests in fabricating highly efficient biofuel cell devices.

X-ray photoelectron spectroscopy study on Fe and Co catalysts during the first stages of ethanol chemical vapor deposition for single-walled carbon nanotube growth

Optimized chemical vapor deposition processes for single-walled carbon nanotube (SWCNT) can lead to the growth of dense, vertically aligned, mm-long forests of SWCNTs. Precise control of the growth process is however still difficult, mainly because of poor understanding of the interplay between catalyst, substrate and reaction gas. In this paper we use x-ray photoelectron spectroscopy (XPS) to study the interplay between Fe or Co catalysts, SiO2 and Al2O3 substrates and ethanol during the first stages of SWCNT forest growth. With XPS we observe that ethanol oxidizes Fe catalysts at carbon nanotube (CNT) growth temperatures, which leads to reduced carbon nanotube growth. Ethanol needs to be decomposed by a hot filament or other technique to create a reducing atmosphere and reactive carbon species in order to grow vertically aligned single-walled carbon nanotubes from Fe catalysts. Furthermore, we show that Al2O3, unlike SiO2, plays an active role in CNT growth using ethanol CVD. From our study we conclude that metallic Fe on Al2O3 is the most optimal catalyst/substrate combination for high-yield SWCNT forest growth, using hot filament CVD with ethanol as the carbon containing gas.

Micropatterning of single-walled carbon nanotube forest

In this work, high-aligned single-walled carbon nanotube (SWCNT) forest have been grown using a high-density plasma chemical vapor deposition technique (at room temperature) and patterned into micro-structures by photolithographic techniques, that are commonly used for silicon integrated circuit fabrication. The SWCNTs were obtained using pure methane plasma and iron as precursor material (seed). For the growth carbon SWCNT forest the process pressure was 15 mTorr, the RF power was 250 W and the total time of the deposition process was 3 h. The micropatterning processes of the SWCNT forest included conventional photolithography and magnetron sputtering for growing an iron layer (precursor material). In this situation, the iron layer is patterned and high-aligned SWCNTs are grown in the where iron is present, and DLC is formed in the regions where the iron precursor is not present. The results can be proven by Scanning Electronic Microscopy and Raman Spectroscopy. Thus, it is possible to fabricate SWCNT forest-based electronic and optoelectronic devices.

Low Temperature Growth of Single-walled Carbon Nanotube Forest

Forest of single-walled carbon nanotubes (SWNTs) was grown at 450 °C by water-plasma chemical vapor deposition using ultrathin iron on alumina supporting film. The growth rate of the SWNT forest is ∼ 0.9 μm/min, and the diameters of nanotubes are mainly in a range of 3.0 ∼ 3.5 nm. The low intensity ratio of D- to G-band (I D /I G ∼ 0.098) in Raman spectra indicates that our SWNT forest grown at 450 °C is fairly pure and crystalline. This low temperature growth of SWNT forest may enable variable applications requiring the vertically-aligned nanotubes to obtain large surface area.

Sequential Layer Analysis of Protein Immunosensors Based on Single Wall Carbon Nanotube Forests

Electrochemical immunosensors using vertically aligned single wall carbon nanotube (SWNT) forests can provide ultrasensitive, accurate cancer biomarker protein assays. Herein we report a systematic investigation of the structure, thickness, and functionality of each layer of these immunosensors using atomic force microscopy (AFM), quartz crystal microbalance (QCM), and scanning white light interferometry (SWLI). This provides a detailed picture of the surface morphology of each layer along with surface concentration and thickness of each protein layer. Results reveal that the major reasons for sensitivity gain can be assigned to the dense packing of carboxylated SWNT forest tips, which translate to a large surface concentration of capture antibodies, together with the high quality of conductive SWNT forests.

Outer-specific surface area as a gauge for absolute purity of single-walled carbon nanotube forests

We present a simple approach to estimate the absolute purity of carbon nanotube (CNT) forests based on outer-specific surface area (SSA). The outer-SSA and absolute purity for diverse forms of CNT forests to increased levels of carbonaceous impurities showed similar behavior and revealed a method to distinguish between CNTs and carbonaceous impurities. Furthermore, these experiments showed a direct proportionality between the outer-SSA and absolute purity which led to a simple analytical model to use outer-SSA as a gauge for absolute purity analysis.

Integration of SWNT film into MEMS for a micro-thermoelectric device

This paper reports the top-down fabrication of carbon nanotube (CNT) thin film onmicroelectrical mechanical systems (MEMS) and the characterization of the thermoelectriccoefficients of aligned single-walled carbon nanotube (SWNT) forest film. The film wassynthesized by water-assisted chemical vapor deposition (CVD), a process known as‘super-growth’. CNT film was then condensed, manually maneuvered, and convenientlypatterned by electron beam (EB) lithography to form desirable shapes. We demonstratethis process by patterning an array of Au–CNT thermocouples; measurement under roomtemperature conditions reveals the Seebeck coefficient of aligned SWNT film to be 18–20 µV C − 1.

Nanotubes Oxidation Temperature Controls the Height of Single-Walled Carbon Nanotube Forests on Gold Micropatterned Thin Layers

We developed a simple methodology for a direct control of the height of carboxylated single-walled carbon nanotube (SWNT) forests. We found that the important step is a good control of the oxidation temperature of the nanotubes. SWNTs oxidation at different temperature was followed by Raman and X-ray photoelectron spectroscopies. Atomic force microscopy images showed that micropatterned self-assembled monolayers forests have average height from 20 to 80 nm using SWNTs oxidized in the temperature ranging from 323 to 303 K, respectively.

Millimeter-tall single-walled carbon nanotube forests grown from ethanol

Millimeter-tall vertically-aligned carbon nanotubes (VA-CNTs) were grown from ethanol under ambient pressure by Co-catalyzed chemical vapor deposition (CVD), with systematic optimization of the CVD temperature and catalytic conditions using combinatorial catalyst libraries. We investigated the use of both aluminum oxide and silicon oxide as underlayers for the Co catalyst and found that VA-CNTs grew to millimeter heights in 15–30min when the pyrolysis of ethanol was carried out at high temperatures (⩾850°C) and long residence times (⩾10s). Thick Co catalytic layers (⩾1.3nm) produced (sub)millimeter-tall multi-walled VA-CNTs on both the aluminum oxide and silicon oxide underlayers. However, thin Co catalytic layers (0.62–1.0nm) produced (sub)millimeter-tall VA-CNTs, which consisted mainly of single-walled CNTs, only on the aluminum oxide underlayers. Stripe patterns were found in the VA-CNTs near the substrate on both aluminum oxide and silicon oxide, indicating some instability prior to growth termination. The possible roles of aluminum oxide in growing millimeter-tall single-walled VA-CNTs were discussed.

A Simple Combinatorial Method Aiding Research on Single-Walled Carbon Nanotube Growth on Substrates

Establishing fabrication methods of carbon nanotubes (CNTs) is essential to realize many applications expected for CNTs. Catalytic growth of CNTs on substrates by chemical vapor deposition (CVD) is promising for direct fabrication of CNT devices, and catalyst nanoparticles play a crucial role in such growth. We have developed a simple method called “combinatorial masked deposition (CMD)”, in which catalyst particles of a given series of sizes and compositions are formed on a single substrate by annealing gradient catalyst layers formed by sputtering through a mask. CMD enables preparation of hundreds of catalysts on a wafer, growth of single-walled CNTs (SWCNTs), and evaluation of SWCNT diameter distributions by automated Raman mapping in a single day. CMD helps determinations of the CVD and catalyst windows realizing millimeter-tall SWCNT forest growth in 10 min, and of growth curves for a series of catalysts in a single measurement when combined with real-time monitoring. A catalyst library prepared using CMD yields various CNTs, ranging from individuals, networks, spikes, and to forests of both SWCNTs and multi-walled CNTs, and thus can be used to efficiently evaluate self-organized CNT field emitters, for example. The CMD method is simple yet effective for research of CNT growth methods.

Improved and Large Area Single-Walled Carbon Nanotube Forest Growth by Controlling the Gas Flow Direction

A gas shower system was introduced to improve the growth of single-walled carbon nanotube (SWNT) forests by controlling the gas flow direction. Delivery of gases from the top of the forest enabled direct and precise supply of ethylene and water vapor to the Fe catalysts. As such, this approach solved one of the limiting factors of water-assisted chemical vapor deposition method (CVD), that is, delivery of the very small optimum water level to the catalysts. Consequently, this approach improved SWNT forests growth stability, uniformity, reproducibility, carbon efficiency (32%), and catalyst lifetime. With this improved growth, we could synthesize a 1 cm tall forest with 1 x 1 cm size. Also we employed this approach to grow an A4 size SWNT forest to highlight the scalability of water-assisted CVD.