(Invited) Understanding Chemical Doping and Transport in Dispersions of Isolated Carbon Nanotubes

Abstract

Recently we demonstrated that the chemical structure of the redox charge-carrier dopant plays a significant role in the delocalization and transport properties of the injected charge-carriers in networks of enriched semiconducting single-walled carbon nanotubes (s-SWCNT). Here, we seek to gain a deeper understanding of the charge-carrier doping and transport processes for isolated polymer-wrapped s-SWCNT in a low dielectric constant solution-phase environment. We compare solution-phase s-SWCNT samples in toluene that are p-type doped by either 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) or (2,3,4,5,6-pentafluoro)benzyloxy-substituted icosahedral dodecaborane cluster (DDB-F60). We show that DDB-F60 is a more potent charge-carrier dopant than F4-TCNQ at low concentrations but that the ultimate injected hole charge-carrier density is limited by the large size of the DDB-F60 counteranion, due to saturation of the carbon nanotube surface area that is accessible to the dopant molecules. Using a non-contact spectroscopy technique in the microwave frequency region (ca. 9 GHz), we show that the holes injected by DDB-F60 are more delocalized and mobile, consistent with calculations that suggest a smaller coulombic binding energy between the hole on the carbon nanotube and the electron on the DDB-F60 counteranion. However, samples doped by F4-TCNQ reach a higher peak conductance than is achieved by DDB-F60 doping, where the conductance in the latter case is limited by the maximum achievable hole charge-carrier density and hole scattering processes that reduce the carrier mobility at high hole densities. Our observations point to the need to understand the subtle balance between the carrier-counterion interactions and the injected charge-carrier density to optimize charge-carrier transport in chemically doped s-SWCNT systems.