Ballistic and resonant negative photocurrents in semiconducting carbon nanotubes

TL;DR

This study investigates ultrafast photocurrents in semiconducting carbon nanotubes, revealing ballistic charge transport, thermionic emission dominance, and polarity changes linked to charge injection and population inversion.

Contribution

It demonstrates the role of thermionic emission and population inversion in photocurrent behavior in carbon nanotubes under resonant excitation.

Findings

Photocurrent is dominated by thermionic emission processes.

Charge transport occurs ballistically after exciton dissociation.

Polarity of photocurrent changes with charge injection levels.

Abstract

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.