Patterned growth of carbon nanotube forests using Cu and Cu/Ag thin film reservoirs as growth inhibitors

Lithography and lift-off of the catalyst is the established way to pattern forests of carbon nanotubes (CNTs). We demonstrate an alternative technique where we pattern a thin film reservoir residing below the Al₂O₃ underlayer to partially or fully deactivate the iron catalyst on the surface, thus leading to short CNTs or no CNTs. High-resolution imaging suggests that mass transfer from the Cu or Cu/Ag alloy reservoir to the surface promotes interalloying with the catalyst, which is the mechanism for deactivation. The lower melting temperature of the Cu/Ag alloy is more effective than Cu in inhibiting CNT growth since it becomes partially liquid at the processing temperature, to faster alloy with the iron catalyst. We show how the lithographic patterning at the level of the reservoir translates into lithographic patterning of the CNT forest. This technique, besides avoiding contamination of the catalytic surface, is an important building block towards fabricating patterned forests with areas of different heights where the reservoir material will modulate the CNT height. Additionally, this research opens the door for testing additional materials as reservoirs and to analyze their effects on CNT growth to achieve fine modulation of the third dimension (height) in patterned CNT forests.