Investigation of substrate and hydrogen pretreatment time to modulate SWCNT diameter and growth yield

Synthesizing high-quality single-walled carbon nanotubes (SWCNT) requires innovative approaches that surpass traditional catalysts supported on amorphous alumina or silica. This study investigates SWCNT growth on various substrates, including SiO₂/Si, quartz, C-sapphire, and R-sapphire, with and without a 5 nm amorphous alumina buffer layer, using different pretreatment times via hot-filament catalytic chemical vapor deposition (HF-CCVD). A 30-seconds atomic hydrogen pretreatment effectively promotes SWCNT growth on all plain substrates. The addition of the thin alumina buffer layer further enhances yield, resulting in a dense network of SWCNT. However, extending the pretreatment to 120 s leads to catalyst particle coarsening via Ostwald ripening, significantly reducing SWCNT yield and favoring multi-walled carbon nanotubes (MWCNT) formation, regardless of the presence of a buffer layer. Our results demonstrate that Fe catalyst activity and the resulting CNT structure is tightly linked to substrate characteristics, including crystallinity, porosity, and surface roughness. Importantly, high-yield SWCNT growth is achievable directly on plain substrates without the need for an alumina buffer layer. This work highlights the critical role of substrate-catalyst interactions in CNT synthesis and highlights the potential for fine-tuning SWCNT growth under short atomic hydrogen pretreatment conditions.