Atmospheric pressure atomic layer deposition of iron oxide nanolayer on the Al₂O₃/SiO₂/Si substrate for mm-tall vertically aligned CNTs growth

Atmospheric pressure atomic layer deposition (AP-ALD) of iron oxide, as a catalyst for water-assisted chemical vapor deposition (WA-CVD) growth of vertically aligned carbon nanotubes (VA-CNTs), was studied. Fe₂O₃ film was deposited on the porous Al₂O₃/SiO₂/Si substrate using ferrocene and O₂ as precursors. The self-liming film growth was obtained at the temperature window of about 180–250 °C with the optimum dosing and purging duration of about 20 min. The rather longer dosing time here compared to the conventional ALD is attributed to the high pressure as well as high porosity of the substrate. Considering the lower deposition rate at the beginning of the growth, it is suggested that the substrate-inhibited growth mode takes place in this work. The grown films have been characterized by SEM, AFM, Raman, and XRR. Raman spectroscopy peaks were consistent with the formation of Fe₂O₃. SEM images showed that the uniform coating of Fe₂O₃ follows the Volmer–Weber growth mode. The catalyst thickness-dependent growth of VACNTs revealed that the deposition through 12 cycles at 250 °C provides sufficient thickness, which resulted in the growth of mm-tall VA-CNTs within a relatively short time of WA-CVD. The changes in the catalyst, i.e. surface roughness and particle size, as a function of WA-CVD time were investigated. The time-dependent evolutions were mainly due to the subsurface diffusion and Ostwald ripening that ultimately led to the growth termination of CNTs.