Binding Configuration and Surface Coverage of Poly(9,9-dioctylfluorene-2,7-diyl) on Electronic Type-Sorted Carbon Nanotubes

Several schemes have been demonstrated to successfully extract semiconducting (s-) single-walled carbon nanotubes (SWCNTs) from heterogeneous as-grown mixtures containing metallic (m-) SWCNTs. The polyfluorene family of conjugated polymers, specifically poly(9,9-dioctylfluorene-2,7-diyl) (PFO), have been shown to extract s-SWCNTs from mixtures of s- and m-SWCNTs with high selectivity (>99% s-SWCNT) by preferentially wrapping the s-SWCNT components. In addition to electronic type, the selectivity is heavily dependent on the nanotube diameter and chiral angle. To date, there is an inadequate understanding of the polymer-CNT interactions that lead to the high preference for s- over m-SWCNTs in the wrapping process. It is vital to understand these parameters to design separation schemes to reduce the proportion of m-SWCNT impurities to technologically relevant ppm and ppb levels.

In this work, we explore the chemistry of non-covalent nanotube-polymer wrapping processes and describe the effects of surface coverage and binding configuration on the separation efficiency. We disperse SWCNTs grown by the HiPCO process, which contain many chiral species of SWCNT with diameters near 1 nm, in toluene solutions of varying PFO concentration. The resulting dispersions are analyzed with optical absorption spectroscopy, excitation-emission photoluminescence spectroscopy, and photoluminescence anisotropy. First, we experimentally quantify the amount of PFO wrapping the SWCNTs by measuring exciton energy transfer from PFO to SWCNTs via photoluminescence spectroscopy. We characterize the dependence of the surface coverage on PFO concentration, finding that the surface coverage increases by a factor of 7 as PFO concentration increases from 0.2 to 2 mg/mL. Second, we demonstrate that the wrapping of nanotubes by PFO is described by a Langmuir adsorption isotherm, in which surface coverage of PFO on the nanotubes follows an S-shaped curve as a function of PFO concentration. The shape of the curve is related to the polymer-nanotube binding energy, which depends on the nanotube electronic type, diameter, chiral angle, solvent, and polymer molecular weight. Third, we determine the binding configuration of the PFO on the nanotube surface as a function of PFO concentration and demonstrate that the PFO becomes more ordered as the surface coverage decreases. In the highly-ordered PFO regime we estimate the wrapping angle of individual PFO strands around nanotubes in solution. Finally, we find that the metal-semiconductor separation efficiency is lowest when the concentration of PFO is high, surface coverage is high, and surface ordering is low.

Understanding the PFO wrapping process will be an important step toward attaining high-purity s-SWCNT from this and similar sorting procedures for a range of polymer-CNT systems. These advances will enable further development of technologically relevant high purity s-SWCNTs for next-generation electronic devices.