(Invited) Controlling Nanotube-Nanotube Interactions in Carbon Nanotube Networks: Implications for Energy Transport and Harvesting

Abstract

While the unique chemical and physical structure of semiconducting single-walled carbon nanotubes (s-SWCNTs) were identified decades ago, marking these nanoscale materials as promising candidates for constituents in a variety of optical and electronic applications, the ability to generate high-purity samples has hampered efforts to validate these expectations. The intervening years have played witness to a number of elegant enrichment strategies aimed at extracting tailored semiconducting SWCNT species from the raw soot, from the use of subtly tunable surfactant interactions to the exploitation of specific DNA sequences. In contrast, we employ an enrichment approach based on conjugated polymers, typically derived from the fluorene moiety, since they show great promise with regards to their high selectivity and viability for scalable manufacturing approaches.As alluded to above, these fluorene-based (co-)polymers confer solubility on the s-SWCNT species, allowing for the preparation of highly enriched s-SWCNT dispersions that can be deposited using a variety of solution-processing approaches. Unfortunately, the strong van de Waals forces between the π-electron systems of the polymer and SWCNT that enable the selective extraction of semiconducting SWCNTs with high purity also make removal of the polymer difficult. Since these polymers typically have a wide bandgap they can act as an insulating coating on the surface of the individual SWCNTs within functional networks, inhibiting the transport of energy in the form of excitons and/or charge carriers.Here we demonstrate approaches that allow us to replace the strongly-bound polymers with derivatives that can be removed using simple solution-based chemical strategies, resulting in networks with modified energy transport properties. We show that removal of the polymer results in a significant enhancement of the charge carrier mobility and electrical conductivity in doped s-SWCNT networks. Finally, we extend the approach to samples strongly enriched in a single chiral s-SWCNT species, which allows us to employ transient spectroscopic techniques to probe exciton transport within the s-SWCNT network with high spectral fidelity. We show that the efficiency of exciton transport is subtly dependent on the complex interplay between polymer removal and carbon nanotube bundling. Our studies highlight a methodology by which high-performance s-SWCNT thin films can be prepared and discuss the implications of our findings on the potential to realize their promise for electronic and optoelectronic applications.