By controlling the timing and duration of hydrogen exposure in a fixed thermal process, we tuned the diameters of carbon nanotubes (CNTs) within a vertically aligned film by a factor of 2, and tuned the areal densities by an order of magnitude. The CNT structure is correlated with the catalyst morphology, suggesting that while chemical reduction of the catalyst layer is required for growth, prolonged H2 exposure not only reduces the iron oxide and enables agglomeration of the Fe film, but also leads to catalyst coarsening. Control of this coarsening process allows tuning of CNT characteristics.

In the accepted mechanisms of carbon nanotube (CNT) growth by catalytic chemical vapor deposition (CCVD), the catalyst support is falsely considered as a passive material whose only role is to prevent catalytic particles from coarsening. The chemical changes that occur to the carbon source molecules on the surface are mainly overlooked. Here, we demonstrate the strong influence of the support on the growth of CNTs and show that it can be tuned by controlling the acid-base character of the support surface. This finding largely clarifies the CCVD growth mechanism. The CNTs' growth stems from the support where the presence of basic sites catalyzes the aromatization and reduces the complexity of CNT precursor molecules. On basic supports, the growth is activated and CNTs are more than 1000 times longer than those produced on acidic supports. These results could be the bedrock of future development of more efficient growth of CNTs on surfaces of functional materials. Finally, the modification of the aciditiy of the catalyst support during the super growth process is also discussed.

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Vertically aligned carbon nanotubes and nanofibers are grown on metallic Ta and Pd underlayers at temperatures below 500 °C. Controlling the size of the grains of the underlayer film is critical because this leads to a more uniform distribution of catalyst dots, which in turn results in vertical alignment of the carbon nanostructures. Rapid and limited heating and appropriate materials selection can also be used to limit catalyst/underlayer reactions that hinder or suppress carbon nanostructure growth or that lead to entangled growth. Control of catalyst reactivity with metallic underlayers is significant because growth on conductive substrates is notoriously difficult, but needed for many applications such as the use of carbon nanostructures in microelectronic circuits.

We present the fabrication and high frequency characterization of a capacitive nanoelectromechanical system (NEMS) switch using a dense array of horizontally aligned single-wall carbon nanotubes (CNTs). The nanotubes are directly grown onto metal layers with prepatterned catalysts with horizontal alignment in the gas flow direction. Subsequent wetting-induced compaction by isopropanol increases the nanotube density by one order of magnitude. The actuation voltage of 6 V is low for a NEMS device, and corresponds to CNT arrays with an equivalent Young’s modulus of 4.5–8.5 GPa, and resistivity of under 0.0077 Ω⋅cm. The high frequency characterization shows an isolation of −10 dB at 5 GHz.