Sign inversion in the terahertz photoconductivity of single-walled carbon nanotube films

TL;DR

This study clarifies that the sign of terahertz photoconductivity in single-walled carbon nanotube films varies with nanotube length and is mainly due to heat-induced changes in electron relaxation, impacting optoelectronic applications.

Contribution

It demonstrates that CNT length, not chirality, primarily influences the sign of THz photoconductivity, resolving conflicting reports and clarifying the underlying mechanism.

Findings

Photoconductivity sign varies with CNT length.

Heat modifies electron relaxation rates, affecting conductivity.

Chirality has little influence on photoconductivity sign.

Abstract

In recent years, there have been conflicting reports regarding the ultrafast photoconductive response of films of single walled carbon nanotubes (CNTs), which apparently exhibit photoconductivities that can differ even in sign. Here, we observe explicitly that the THz photoconductivity of CNT films is a highly variable quantity which correlates with the length of the CNTs, while the chirality distribution has little influence. Moreover, by comparing the photo-induced change in THz conductivity with heat-induced changes, we show that both occur primarily due to heat-generated modification of the Drude electron relaxation rate, resulting in a broadening of the plasmonic resonance present in finite-length metallic and doped semiconducting CNTs. This clarifies the nature of the photo-response of CNT films and demonstrates the need to carefully consider the geometry of the CNTs, specifically the length, when considering them for application in optoelectronic devices.