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.

Carbon source The carbon source for the CVD of VANTAs is most commonly a carbon gas such as methane, ethylene, acetylene, benzene, xylene, or carbon monoxide. Other examples of carbon precursors include cyclohexane, fullerene, methanol, and ethanol. The pyrolysis of these gases into carbon atoms varies based on the decomposition rate at growth temperatures, the carbon content of the gas molecules, and the growth catalyst. Linear hydrocarbons such as methane, ethylene, acetylene, thermally decompose into atomic carbons or linear dimers/trimers of carbon, and generally produce straight and hollow CNTs. On the other hand, cyclic hydrocarbons such as benzene, xylene, cyclohexane, fullerene, produce relatively curved/hunched CNTs with the tube walls often bridged inside. Aligned arrays of MWNTs have been synthesized through the catalytic decomposition of ferrocene-xylene precursor mixture onto quartz substrates at atmospheric pressure and relatively low temperature (~675 °C).

Eres et al. found that the addition of ferrocene into the gas stream by thermal evaporation concurrently with acetylene enhanced carbon nanotube growth rates and extend the VANTA thickness to 3.25 mm. Ferrocene was introduced into the gas stream by thermal evaporation concurrently with the flow of acetylene. Qu et al. reported a low-pressure CVD process on a SiO2/Si wafer that produces a VANTA consisting of CNTs with curly entangled ends. During the pyrolytic growth of the VANTAs, the initially formed nanotube segments from the base growth process grew in random directions and formed a randomly entangled nanotube top layer to which the underlying straight nanotube arrays then emerged. Zhong et al. studied purely thermal CVD process for SWNT forests without an etchant gas, and demonstrated that acetylene is the main growth precursor, and the conversion of any feedstock to C2H2 is of key importance to SWNT VANTA growth. 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.

Dasgupta et al. synthesized a free-standing macro-tubular VANTA with a spray pyrolysis of ferrocene-benzene solution in a nitrogen atmosphere, with the optimum condition for the formation of macro-tubular geometry was found to be 950 °C, 50 mg/ml ferrocene in benzene, 1.5 ml/min pumping rate of liquid precursor and 5 lpm of nitrogen gas flow rate.

Temperature At a too low temperature, the catalyst atoms are not mobile enough to aggregate together into particles to nucleate and grow nanotubes and the catalytic decomposition of the carbon precursor may be too slow for the formation of nanotubes. If the temperature is too high, the catalyst becomes too mobile to form particles small enough to nucleate and grow CNTs. A typical range of growth temperatures amenable to the CVD growth of VANTA is 600–1200 °C. The individual CNT structure is impacted by the growth temperature; a low-temperature CVD (600–900 °C) yields MWCNTs, whereas high-temperature (900–1200 °C) reaction favors SWCNT since they have a higher energy of formation. A critical temperature exists for each CVD system where the growth rate plateaus at a maximum value.

The temperature dependence of the carbon nanotube growth with ferrocene exhibits a steep drop at high substrate temperatures and a loss of vertical alignment at 900 °C. Zhang et al. conducted VANTA growths on a series of Fe/Mo/vermiculite catalysts and reported that with the increasing growth temperature, the alignment of CNTs intercalated among vermiculites became worse.

Flow assisted growth A key to high growth yields is a proper introduction of oxidative agents under the gas ambient so that the catalyst particle surfaces remain active for the longest possible period, which is presumably achieved by balancing the competition between amorphous carbon growth and sp2 graphitic crystal formation on the catalyst particles. Oxidants can not only remove or prevent amorphous carbon growth, but may also etch into graphite layers when used at higher than favorable concentrations. Hata et al. reported millimeter-scale vertically aligned 2.5 mm long SWCNTs using the water assisted ethylene CVD process with Fe/Al or aluminum oxide multilayers on Si wafers. It was proposed that controlled supply of steam into the CVD reactor acted as a weak oxidizer and selectively removed amorphous carbon without damaging the growing CNTs.

Field-assisted growth Since CNTs are all electrically conductive they have a tendency to align with the electric field lines. Various methods have been developed to apply a strong enough electric field during the CNT growth process to achieve uniform alignment of CNTs based on this principle. The orientation of the aligned CNTs is mainly dependent on the length of CNTs and the electric field besides the thermal randomization and van der Waals forces. This technique has been employed to grow VANTAs by positively biasing the substrate during CVD growth.

Another modified approach to grow VANTAs is to control the orientation of ferromagnetic catalysts that have one crystallographic magnetic easy axis. The magnetic easy axis tends to be parallel to the magnetic field. As a result, an applied magnetic force can orient these magnetic catalytic nanoparticles, like catalytic iron nanoparticles and Fe3O4 nanoparticles. Because only a certain nanocrystalline facet of catalytic nanoparticles is catalytically active and the diffusion rate of carbon atoms on the facet is the highest, the CNTs preferentially grow from the certain facet of the catalytic nanoparticles and the grown CNTs are oriented at a certain angle.

Individually addressable nanostructures