CNT Forest Research Articles

Batch-mode micropatterning of carbon nanotube forests using UV-LIGA assisted micro-electro-discharge machining

This paper reports batch-mode, three-dimensional micropatterning for arrays of vertically aligned carbon nanotubes, also known as CNT forests, based on dry micro-electro-discharge machining (μEDM). The process employs an array of copper electrodes microfabricated through an advanced UV-LIGA process enabled with a new photoresist system in combination with electroplating, providing a low-cost path to constructing high-density arrays of μEDM electrodes for high-throughput parallel processing. The fabricated arrays of 85-μm-tall electrodes are utilized to demonstrate and characterize planar dry μEDM for post-growth patterning of CNT forests in air. Die sinking and scanning processes are tested to show pattern transfers with a 4-μm tolerance and an average surface roughness of 230nm. An elemental analysis suggests that contamination of the electrode material on the produced patterns is minimal. Key characteristics in the use of planar electrodes for batch processing of CNT forests are revealed through experimental analysis and discussed in detail. The results suggest that the investigated process is a promising approach toward offering a cost-effective manufacturing technology for future products functionalized with custom-designed microstructures of CNT forests.

Formation of Carbon Nanotube Forest over Spin-coated Fe2O3 Reduced Thin-film by Chemical Vapor Deposition

A dense carbon nanotube [CNT] forest was formed by chemical vapor deposition [CVD] using spin-coated iron oxide (Fe2O3) thin-film on Si substrate. The nanoforest contains carbon nanotubes was obtained by feeding a mixture of acetylene, hydrogen and nitrogen [N2:H2: C2H2 = 27: 27: 5 mL/min] for 45 min at 700°C in the CVD chamber. The CNT forest was subjected to analyze the phase, crystalline quality and microstructure using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy [SEM], respectively. The elemental analysis and mapping were also studied using energy dispersive X-ray spectroscopy [EDX]. The morphology, shape and size of the nanotubes were characterized by transmission electron microscopy. The obtained carbon nanotubes were of length 13–14 μm and diameter of 10–100 nm over Fe thin-film coated on Si substrate, which makes this CNT/Fe/Si hybrid material could be a better candidate for field emission applications.

Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction

Cost effective hydrogen evolution reaction (HER) catalyst without using precious metallic elements is a crucial demand for environment-benign energy production. Molybdenum sulfide is one of the promising candidates for such purpose, particularly in acidic condition, but its catalytic performance is inherently limited by the sparse catalytic edge sites and poor electrical conductivity. We report synthesis and HER catalysis of hybrid catalysts composed of amorphous molybdenum sulfide (MoSx) layer directly bound at vertical N-doped carbon nanotube (NCNT) forest surface. Owing to the high wettability of N-doped graphitic surface and electrostatic attraction between thiomolybdate precursor anion and N-doped sites, ∼2 nm scale thick amorphous MoSx layers are specifically deposited at NCNT surface under low-temperature wet chemical process. The synergistic effect from the dense catalytic sites at amorphous MoSx surface and fluent charge transport along NCNT forest attains the excellent HER catalysis with onset overpotential as low as ∼75 mV and small potential of 110 mV for 10 mA/cm(2) current density, which is the highest HER activity of molybdenum sulfide-based catalyst ever reported thus far.

Enzyme-free glucose biosensor based on low density CNT forest grown directly on a Si/SiO2 substrate

A highly sensitive nonenzymatic amperometric glucose sensor was fabricated by using Ni nanoparticles homogeneously dispersed within and on the top of a vertically aligned CNT forest (CNT/Ni nanocomposite sensor), which was directly grown on a Si/SiO2 substrate. The surface morphology and elemental analysis were characterized using scanning electron microscopy and energy dispersive spectroscopy, respectively. Cyclic voltammetry and chronoamperometry were used to evaluate the catalytic activities of CNT/Ni electrode. The CNT/Ni nanocomposite sensor exhibited a great enhancement of anodic peak current after adding 5mM glucose in alkaline solution. The sensor can also be applied to the quantification of glucose content with a linear range covering from 5μM to 7mM, a high sensitivity of 1433μAmM−1cm−2, and a low detection limit of 2μM. The CNT/Ni nanocomposite sensor exhibits good reproducibility and long-term stability, moreover, it was also relatively insensitive to commonly interfering species, such as uric acid, ascorbic acid, acetaminophen, sucrose and d-fructose.

Efficient growth of millimeter-long few-walled carbon nanotube forests and their oil sorption

Millimeter-long few-walled carbon nanotube (FWNT) forests were grown by a water-assisted chemical vapor deposition method. In only 10 min, the forests grew to over 1.2 mm in height, with a growth yield around 2.76±0.30 mg/cm2. Transmission electron microscopy observation and Raman analysis showed high crystallinity of the nanotubes. A carbon purity better than 98.8 wt% was further determined by thermogravimetric analysis measurement. The FWNT forests were evaluated as an adsorbent for oil sorption and exhibited reasonable oil sorption capacity of 35 g/g. It is inferred that the oil sorption for CNT forests does not depend on their surface area.

Dyes in Vertically Aligned Carbon Nanotube Arrays for Solar Cell Applications

ABSTRACTThe infiltration of dissolved dyes into vertically aligned carbon nanotube arrays (va-CNT) is reported. The ultra hydrophobic surface of the CNT forest can be wetted and hence infiltrated for an appropriate choice of solvent. The dye-infiltrated CNT array forms a well ordered bulk-heterojunction structure for organic solar cells in which the CNT can act as a large electrode or, for appropriate energy levels, as an acceptor material. Derivatives of the small molecule copper phthalocyanine or the polymer poly(3-hexylthiophene) were used as dyes. Drop coating was chosen as the infiltration technique resulting in a completely embedded CNT forest. Field emission secondary electron microscopy analysis illustrates the final layer quality. Common electrical characterization under AM1.5 illumination proves photosensitivity and implies photovoltaic behavior of the composite.

Characterizations of contact and sheet resistances of vertically aligned carbon nanotubeforests with intrinsic bottom contacts

Comprehensive studies on the sheet and contact resistances of vertically alignedcarbon nanotube (CNT) forests with as-grown bottom contacts to the metallayer have been conducted. Using microfabrication and four distinct methods:(1) the transfer length method (TLM), (2) the contact chain method, (3) theKelvin method, and (4) the four point probe method, we have designed multipletesting devices to characterize the resistances of CNT-forest-based devices.Experimental results show that devices based on stripe-shaped CNT forests100 µm in heightand 100 µm in width have a sheet resistance of approximately . The corresponding specific contact resistance to the molybdenum layer is roughly5 × 104 Ω µm2. Consistency of the results from the four different methods validates the study. After twomonths of storage of the CNT forest samples in open air, less than 0.9% deviations in theresistance values were observed. We further demonstrated one application of CNT forests as anNH3 gas sensor and measured 0.5 ppm of sensing resolution with a detection response time of1 min.

Morphology control of a rapidly grown vertically aligned carbon‐nanotube forest for fiber spinning

AbstractCarbon nanotube (CNT) fibers have interesting applications because of their good electrical and mechanical properties. CNT fiber is fabricated in a simple way, that it is spun from a vertically aligned CNT forest (VACNF). Spinning occurs by joining of CNT when the CNT at the edge of a VACNF is drawn horizontally. However, the mechanism of spinning is still not clear. To investigate it, we fabricated a spinnable VACNF by thermal chemical vapor deposition (CVD) and compared it with unspinnable VACNF by scanning electron microscope (SEM) observation and polarized Raman spectroscopy. It was found that there were a few big differences between spinnable and unspinnable VACNFs. In the case of spinnable VACNF, most of the CNTs are straight and well aligned. Spinnable VACNF also contains CNT bundles consisting of 8–10 CNTs that are not vertically aligned. This CNT bundle was shared by a few adjacent thicker CNT bundles. These points are very important for continuous fiber spinning.

Synthesis of Bimetallic Nanoparticles and Their Application to Growth of Multiwalled Carbon Nanotube Forest

We synthesized bimetallic nanoparticles and used them to grow CNTs with an atmospheric-pressure plasma-enhanced chemical vapor deposition system. FePt and FeCo bimetallic nanoparticles were synthesized by the bottom-up high-temperature polyol process using a simple centrifugal method. The diameter of synthesized nanoparticles ranged from 2 to 5 nm with high uniformity, as measured with a transmission electron microscope. The CNTs synthesis was carried out on bimetallic nanoparticles as a catalyst using acetylene as a carbon source gas. The CNT forests grown exhibited a maximum height of 96 microm, which is relatively high compared with other growth technologies. A forest of highly pure multiwalled CNTs with diameters of 10-20 nm was successfully synthesized on a bimetallic nanoparticle catalyzed substrate. As the growth temperature increased, the quality of CNTs improved remarkably, indicating that the graphitization could be controlled by varying the growth temperature. Furthermore, the oxygen plasma pre-treatment of dispersed bimetallic nanoparticles before CNTs growth could contribute to the production of highly pure CNTs. By TEM measurements, we observed that pure multiwall CNTs without defects or amorphous carbon could be synthesized.

Reversible transformation of hydrophobicity and hydrophilicity of aligned carbon nanotube arrays and buckypapers by dry processes

Dry treatment using a combination of UV and ozone can readily change the surface of vertically aligned carbon nanotubes from superhydrophobic to superhydrophilic. This treatment is also effective for buckypapers. Heating in a vacuum at an elevated temperature (650–750 °C) can reverse the surface state from superhydrophilic to superhydrophobic. The UV & ozone treatment causes the least amount of damage to the stripe-like carbon nanotube patterns. The effect of rough surface on apparent contact angles of CNT forests was discussed to explain the origin of superhydrophilicity and superhydrophobicity.

Catalyst Distribution and Carbon Nanotube Morphology in Multilayer Forests by Mixed CVD Processes

Carbon nanotubes can be grown as forests of aligned fibers on a substrate with a catalyst coated prior to or added during synthesis. A major process interruption can initiate the growth of second and successive layers of forest on top or at the base of the existing layers which are thereby lifted up. We report on the generation of multilayer CNT forests where the first forest is generated either by catalyst coinjection (CCI) of ferrocene with hydrocarbon (xylene) or by catalyst predeposition (CPD) of iron followed with hydrocarbon (acetylene). Subsequent layers are then produced by CCI alone to give uniform (all CCI) or mixed (CPD and CCI) structures to study the distribution of the iron catalyst and CNT morphology and to determine whether the CPD forest templates or otherwise influences the growth of subsequent CCI forests. The bottom-up base growth of second and subsequent CCI forests is reaction rate controlled. CCI multilayer forests accumulate catalyst (iron) in a variety of distinct locations. A pre-existing CPD forest modifies subsequent CCI forest initiation, morphology, and catalyst distribution but does not itself accumulate catalyst or change appearance.

The Variation of Surface Contact Angles According to the Diameter of Carbon Nanotubes

The shape of CNTs is affected by various growth parameters such as reaction time, temperature, working power, and pressure as well as the type of catalytic layer and synthesis method. In this work, the thickness of Ni catalyst layer was varied to control the diameter of synthesized CNT. Ni catalyst layer was prepared using a DC magnetron sputtering method and the layer thickness was varied from 40 nm to 100 nm with the increment of 20 nm. And CNTs were grown on Ni catalyst layer using the hot-filament plasma enhanced chemical vapor deposition (HF-PECVD) with ammonia (NH3) gas for pretreatment and acetylene (C2H2) gas for the synthesis. The shape of the resulting CNTs was analyzed using field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). The surface contact angle of well-aligned CNTs was correlated with the diameter of CNT. As determined by contact angle measurement, the surface of CNT forests became more hydrophilic as the diameter of CNT increased.

Fabrication and characterization of ultrahigh-volume- fraction aligned carbon nanotube-polymer composites.

Aligned CNT nanocomposites with variable volume fraction, up to 20%, are demonstrated. Biaxial mechanical densification of aligned CNT forests, followed by capillarity-driven wetting using unmodified aerospace-grade polymers, creates centimeter-scale specimens. Characterizations confirm CNT alignment and dispersion in the thermosets, providing a useful platform for controlled nanoscale interaction and nanocomposite property studies that emphasize anisotropy.