In materials science, vertically aligned carbon nanotube arrays (VANTAs) are a unique microstructure consisting of carbon nanotubes oriented with their longitudinal axis perpendicular to a substrate surface. These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled. VANTAs are consequently widely useful in a range of current and potential device applications.
Synthesis There are a handful of experimental technologies available to align a single or an array of CNTs along a pre-determined orientation. The techniques rely on different mechanisms and therefore are applicable to different situations. These techniques are categorized into two groups pertaining to when the alignment is achieved: (a) in-situ techniques where alignment is achieved during the CNT growth process and (b) ex-situ techniques where CNTs are originally grown in random orientations and alignment is achieved afterwards such as during the device integration process.
Thermal chemical vapor deposition
Growth mechanism Thermal chemical vapor deposition is a common technique to grow aligned arrays of CNTs. In the CVD process, a hot carbonaceous gas decomposes on contact with catalytic surface (e.g. iron, cobalt, or nickel). Decomposition leaves behind elemental carbon which diffuses in or on the catalyst until nucleating nanotubes on certain crystallographic faces of catalysts. The size of catalyst particles controls the diameter of the resulting nanotubes. There are two primary growth models for the CVD growth of VANTAs: “tip-growth model” and the “base-growth model.”
Catalyst The catalyst enables the pyrolysis of carbon and subsequent growth of VANTA. Catalysts are typically metals that have high carbon solubility at high temperatures and that exhibit a high carbon diffusion rate, such as iron (Fe), cobalt (Co), and nickel (Ni). Other transition metals such as copper (Cu), gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) are also reported to catalyze CNT growth from various hydrocarbons but have lower carbon solubility and consequently lower growth rates. Solid organometallocenes such as ferrocene, cobaltocene, nickelocene are also common catalysts. It is found that the temperature and time of the thermal and reduction catalyst pre-treatment steps are crucial variables for optimized nanoparticle distribution with different average diameters, depending on the initial film thickness. For CNT growth by CVD, a sputtered thin film of catalyst (e.g. 1 nm of Fe) is applied. During heating, the film de-wets, creating islands of iron that then nucleate nanotubes. As the iron is mobile, islands can merge if left too long at the growth temperature before initiating nanotube growth. Annealing at the growth temperature reduces the site density #/mm2 and increases the diameter of the nanotubes. As the nanotubes grow from the catalyst islands, the crowding effects and van der Waals forces between other CNTs leave them no choice to grow in any direction but vertically to the substrate. The height of vertically aligned CNTs varies with catalyst particle spacing as well. Reports have indicated that for vertically aligned arrays of CNT bundles, the CNTs grow longer when there are other CNTs growing near them, indicated by longer CNTs grown on larger catalyst particles or when catalyst particles are spaced close together. Choi et al. reported good morphology and dense distribution of VANTAs grown from Ni nano powders and magnetic fluids mixed in polyvinyl alcohol spin-coated on Si and alumina. Xiong et al. demonstrated that single crystal magnesium oxide (MgO) is a capable substrate for growing VANTAs as long as 2.2 mm when catalyzed with a Fe catalyst. It has also been demonstrated that applying a monolayer of Mo with a Co catalyst suppressed the broadening of the SWNT diameter distribution in the as-grown VANTA, while both the composition and amount of Co and Mo affected the catalytic activity.
Support The substrate material, its surface morphology and textural properties greatly affect the resulting VANTA yield. Some examples of commonly used substrates in CVD are quartz, silicon, silicon carbide, silica, alumina, zeolite, CaCO3, and magnesium oxide. Most substrates are coated with an underlayer consisting of 10–20 nm of alumina before depositing the catalyst. This regularizes the dewetting of the catalyst into islands of predictable size, and is a diffusion barrier between the substrate and the metal catalyst. Li et al. have produced VANTA consisting of Y-shaped carbon nanotubes by the pyrolysis of methane over cobalt- covered magnesium oxide catalyst on branched nanochannel alumina templates. Qu et al. used a pitch-based carbon fiber as a support for the growth of VANTA using a FePc carbon source. The resulting array propagates radially on the surface of the carbon fiber. Zhong, et al. demonstrated the direct growth of VANTAs on metallic titanium (Ti) coatings with a Fe/Ti/Fe catalyst sputtered on SiO2/Si wafers. Alvarez et al. reports the ability to spin-coat an alumoxane solution as a catalyst support for VANTA growths via CVD. After a conventional Fe catalyst was evaporated onto the spin-coated support, the resulting VANTA growth yield was similar to conventional Al2O3 powder supports.
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