The PLUMED consortium (2019). Promoting transparency and reproducibility in enhanced molecular simulations. Nature Methods, 16(8), 670-673. https://doi.

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.

Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 degrees C and a substrate temperature of 500 degrees C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 kOmega for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.