Design of surfactant-substrate interactions for roll-to-roll assembly of carbon nanotubes for thin-film transistors.

Controlled assembly of single-walled carbon nanotube (SWCNT) networks with high density and deposition rate is critical for many practical applications, including large-area electronics. In this regard, surfactant chemistry plays a critical role as it facilitates the substrate-nanotube interactions. Despite its importance, detailed understanding of the subject up until now has been lacking, especially toward tuning the controllability of SWCNT assembly for thin-film transistors. Here, we explore SWCNT assembly with steroid- and alkyl-based surfactants. While steroid-based surfactants yield highly dense nanotube thin films, alkyl surfactants are found to prohibit nanotube assembly. The latter is attributed to the formation of packed alkyl layers of residual surfactants on the substrate surface, which subsequently repel surfactant encapsulated SWCNTs. In addition, temperature is found to enhance the nanotube deposition rate and density. Using this knowledge, we demonstrate highly dense and rapid assembly with an effective SWCNT surface coverage of ~99% as characterized by capacitance-voltage measurements. The scalability of the process is demonstrated through a roll-to-roll assembly of SWCNTs on plastic substrates for large-area thin-film transistors. The work presents an important process scheme for nanomanufacturing of SWCNT-based electronics.