Vertically aligned carbon nanotube arrays

Vertically Aligned Carbon Nanotube Arrays or VANTAs are a unique microstructure consisting of carbon nanotubes oriented along their longitudinal axes normal 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.

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 is a common technique to grow aligned arrays of CNTs. In the CVD process, a hot carbonaceous gas decomposes, leaving carbon to diffuses into or around the catalyst particles, and then nucleates a graphitic nanotube sidewall at one crystalographic face of the catalyst. The catalyst diameter directly controls the diameter of the nanotubes that are grown. There are two primary growth models for the CVD growth of VANTAs: “tip-growth model” and the “base-growth model.” In the case of the tip-growth model, hydrocarbon decomposes on the top surface of the metal, carbon diffuses down through the metal, and CNT precipitates out across the metal bottom, pushing the whole metal particle off the substrate, and continues to grow until the metal is fully covered with excess carbon and its catalytic activity ceases. In the case of the base-growth model, the initial hydrocarbon decomposition and carbon diffusion take place similar to that in the tip-growth case, but the CNT precipitation emerges out from the metal particle’s apex and forms a hemispherical dome, which then extends up in the form of seamless graphitic cylinder. Subsequent hydrocarbon decomposition takes place on the lower peripheral surface of the metal, and as-dissolved carbon diffuses upward. Most thermal CVD processes grow nanotubes by the root or base growth method. The morphology of both the individual CNTs and the CNT array is dictated by various CVD growth parameters, which may be tuned to yield vertically aligned arrays of CNTs with various structures.

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. 1nm 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.

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