(Invited) Pristine Electronics Grade Semiconducting Carbon Nanotubes By Switching the Rigidity of the Wrapping Polymer Backbone
Authors: Gopalan Padma , Joo Yongho , Brady Gerald J , Shea Matthew J , Arnold Michael S. 
Published: February 2020

In order to widely use single-walled carbon nanotubes (SWCNTs) as the semiconducting material in electronic devices, it is essential to separate the desirable semiconducting SWCNTs from as-synthesized electronically heterogeneous mixtures of metallic (m-) and semiconducting (s-) SWCNTs. Among the sorting methods developed, polyfluorene polymers have been studied as semiconducting-selective agents with selectivity for chirality, diameter and electronic type. Developing an understanding for the factors that lead to strong selective interactions between the conjugated polymer and the semiconducting tubes is evolving but far from complete. We choose a commonly used commercially available polyfluorene derivative PFO-BPy as a model polymer to directly probe the effect of chain rigidity on the wrapping/unwrapping process on the s-SWCNTs. Our choice of this model system is based on its commercial availability, outstanding sorting selectivity, availability of functional groups in the backbone to alter rigidity, and recent device results. Recently, we demonstrated the extraordinarily electronic-type selectively of the polyfluorene derivative PFO-BPy by measuring the on/off ratio of FETs, where zero metallic nanotubes were encountered in the measurement of more than 5,000 s-SWCNTs.1-3 Using the similar polymer poly(9,9-dioctylfluorene-2,7-diyl) (PFO), Bindl et al., demonstrated that even after several aggressive rinsing steps to remove free or excess polymer using ultracentrifugation a significant amount of PFO remained bound to the SWCNT. 4 These remaining polymer residues are expected to increase the contact resistance at the metal-nanotube interface of SWCNT FETs, ultimately limiting the conductance of these FETs at sub-100 nm channel lengths. A more complete removal of adsorbed polymer residues from the SWCNT surface is one possible approach towards improving contact resistance and reducing variation in device performance.5

Recently we demonstrated6 an effective yet simple and mild method to remove post-sorting, and the wrapping PFO-BPy copolymer from the surface of s-SWCNTs by switching the backbone rigidity. We used chelation chemistry to complex pentacarbonylrhenium chloride (Re (CO)5Cl) to the bipyridine (BPy) moiety in the wrapping copolymer backbone. We showed the effectiveness of the chelation chemistry on both large and small diameter tubes. The chain-stiffness of the PFO-BPy changes upon complexation, likely providing the driving force to overcome the π-π and electronic interactions7 which adhere the polymer to the nanotube. Optical absorbance, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and large-scale molecular dynamics simulations were used to characterize the extent of PFO-BPy removal and configurational changes to the PFO-BPy after Re(CO)5Cl complexation (PFO-BPy:Re).The ability to  switch the rigidity of the PFO-BPy in order to unwrap from the nanotubes opens up the door to new design strategies where specific functional groups can be placed on the polymer backbone to chemically alter the rigidity.  Here, we present recent results on direct experimental measurements of changes in stiffness of the polymer backbone and its aggregation state upon metal complexation.

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