Enrichment of 1.0 Nanometer Diameter Single-Chirality Single-Walled Carbon Nanotubes Dictated by Conjugated Polymer Characteristics: Implications for High-Performance Thin-Film Transistors

Abstract

In conjugated polymer extraction (CPE), the polymer characteristics impact the chiral selectivity and yield in the enrichment of single-chirality, single-walled carbon nanotubes (SWCNTs). Polyfluorene is one of the most efficient conjugated polymers to enrich semiconducting or single-chirality SWCNTs. We found that copolymers of fluorene and pyridine (PF12-Py2,5) or oxadiazole (P(F₄-alt-Ox)) exhibited a prevailing selectivity toward (8,7) (diameter 1.032 nm). The chiral purity of (8,7) was improved by storage and ultracentrifugation. In addition, p-xylene is better than toluene to achieve a higher selectivity; as a result, 66% (8,7) purity was reached in p-xylene. Furthermore, the length of the alkyl side chain in PF-Py2,5 also influences the selectivity, with the medium length (n-dodecylfluorene, PF12-Py2,5) showing higher (8,7) selectivity than a shorter length (n-octylfluorene, PF8-Py2,5) and a longer length (n-tetradecylfluorene, PF14-Py2,5). Besides, copolymers of fluorene and benzothiadiazole (PFO-BT) and anthracene (PFO=A) enriched (10,5) (diameter 1.050 nm) in p-xylene and (9,5) (diameter 0.976 nm) in toluene with their chiral purity up to 94.5% and 86%, respectively. The chiral purity was calculated based on the area percentage of the optical absorption S₁₁ peak and also evaluated by photoluminescence excitation (PLE) mapping and Raman scattering spectroscopy. All three (n,m) achieved the highest purity within the CPE methodology. The quality of enriched (8,7) was examined by the performance of thin-film transistors (TFT), which exhibited hole and electron mobilities of 2.6 ± 0.7 and 1.5 ± 0.6 cm²/(V s), respectively, and high on/off ratios of ∼10⁴. 1.0 nm diameter was suggested to reach balanced drive current and leakage in high-performance TFTs. Density functional theory calculation shows that polyfluorene exhibits increasing wrapping stability as the tube diameter increases, and the geometric model of polymer wrapping suggests that P(F₄-alt-Ox) has better coverage of nanotube surface toward (8,7).