These VANTAs effectively preserve and often accentuate the unique anisotropic properties of individual carbon nanotubes and possess a morphology that may be precisely controlled.
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.
[4] 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.
[5] 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.
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.
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).
[11] 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.
[12] 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.
[13] 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.
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.
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.
[17] 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.
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.
This technique, which accomplishes precise placement and configuration of individual nanotubes, unlocks and enhances a wide range of applications for VANTA's: diagnostic testing for many analytes simultaneously, high energy density supercapacitors, field effect transistors, etc.
The DC-PECVD process for vertically aligned CNT arrays includes four basic steps: evacuation, heating, plasma generation, and cooling.
H2-to-CH4 ratios greater than 5 in the feed gas result in high hydrogen concentrations in the plasma and strongly reducing conditions, which prevents the conversion of Fe to Fe3C and cause poorly-graphitized nanofibers to grow with thick walls.
Jiang et al. demonstrated a spinning and twisting method that forms a CNT yarn from a VANTA that gives rise to both a round cross-section and a tensile strength of around 1 GPa.
Qu et al. was able to demonstrate VANTA films that exhibited macroscopic adhesive forces of ~100 newtons per square centimeter, which is almost 10 times that of a gecko foot.
[27] This was achieved by tuning the growth conditions of the VANTA to form curls at the end of the CNTs, which provide stronger interfacial interactions even with a smooth surface.
[34] VANTAs of SWNTs with perfectly linear geometries are applicable as high-performance p- and n-channel transistors and unipolar and complementary logic gates.
Measurements on p- and n-channel transistors that involve as many as about 2,100 SWNTs reveal device-level mobilities and scaled transconductance approaching about 1,000 cm2 V-1 s-1 and $3,000 S m-1, respectively, and with current outputs of up to about 1 A in devices that use interdigitated electrodes.
Palladium supported on vertically aligned multi-walled carbon nanotubes (Pd/VA-CNTs) is used as catalyst for the C-C coupling reactions of p-iodonitrobenzene with styrene and ethyl acrylate under microwave irradiation.
Gong et al. reported that VANTAs doped with nitrogen can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells.
This effect, coupled with aligning the nitrogen-doped CNTs, provides a four-electron pathway for the oxygen reduction reactions on VANTAs with a superb performance.
Like ordinary capacitors, VANTA supercapacitors and electromechanical actuators typically comprise two electrodes separated by an electronically insulating material, which is ionically conducting in electrochemical devices.
In Pitkänen et al., on-chip energy storage is demonstrated using architectures of highly aligned vertical carbon nanotubes acting as supercapacitors, capable of providing large device capacitances.
The efficiency of these structures is further increased by incorporating electrochemically active nanoparticles such as MnOx to form pseudocapacitive architectures thus enhancing areal specific capacitance to 37 mF/cm2.
In order to obtain better electrode performance than networks of random CNTs and CNT composites, VANTAs are used as to provide better electron transport and higher surface area.
Owing to the high tensile strength and large aspect ratio of carbon nanotubes, VANTAs are a potential tether material for the Space Elevator concept.