(Invited) Designer Polymer Dispersants for Sorting, and Assembly of Carbon Nanotube Arrays

Abstract

Conjugated polymers have been extensively utilized to isolate semiconducting single walled carbon nanotubes (s-CNTs) in organic solvents creating a high-purity s-CNT “ink”. To overcome inter-CNT π–π interactions to individualize and disaggregate s-CNTs, typically dispersing agents are necessary. Solution deposition of s-CNTs from these inks onto target substrates to create a monolayer of CNT arrays is actively being explored to fabricate FETs with superior performance compared to conventional silicon and gallium arsenide-based FETs. A wide range of deposition and alignment of s-CNTs on substrates has been explored by methods such as Langmuir-Blodgett/Schaefer, vacuum filtration, electric fields, shear, evaporation, 3D printing, and at liquid/liquid interfaces. While significant progress has been made in deposition and alignment of CNTs on wafer-scale, achieving a controllable pitch in a perfectly aligned dense array of CNTs is still a challenge.Following the initial discovery of dispersants such as aromatic conjugated polymers which interact non-covalently with CNTs and sort CNT soot into high-purity, electronics-grade s-CNT inks, the vast community now uses polymers such as polyfluorene and polythiophene homo and copolymers with other monomers. We have focused in this study on revisiting some of the design rules for these conjugated polymer dispersants to incorporate additional functions such as self-assembly dictated pitch control and improving the yield and selectivity of the sorting process. We have investigated ABA triblock copolymers where B is the conjugated block and A is a non-conjugated coil block to study its effect on dispersion and sorting of CNTs. Though these types of ABA di and triblocks have been investigated in the literature for modulation of the photophysical properties of the B block, their use for sorting of s-CNTs is largely unexplored. In this study, Arc CNT or HiPCO soot was mixed with the ABA triblock at various B block: CNT ratio’s and its dispersion and sorting characteristics were studied. Our studies show that the A coil block greatly improves the sorting efficiency while maintaining the semiconducting selectivity. The selectivity of the ABA triblocks towards s-CNT chiralities is similar to the conjugated B block alone. Furthermore, the ABA architecture allows for much lower degree of polymerization for the B block that what is commonly accepted in the literature. Our results show a generalizable polymer dispersant design where the pitch of the assembled CNT arrays can be potentially controlled.