Infrared Study of Charge Carrier Confinement in Doped (6,5) Carbon Nanotubes

TL;DR

This study explores how doping affects charge carrier behavior and vibrational coupling in (6,5) carbon nanotubes, revealing charge confinement at low doping and delocalization at high doping through infrared spectroscopy and simulations.

Contribution

It provides new insights into charge carrier confinement and vibrational-electronic coupling in doped (6,5) carbon nanotubes using infrared spectroscopy and Monte-Carlo simulations.

Findings

Drude plasmon shifts to lower wavenumbers with increased doping.

Charge carriers are confined at low doping and delocalize at high doping.

Fano antiresonances reveal vibrational-electronic coupling in doped nanotubes.

Abstract

Electronic degrees of freedom and their coupling to lattice vibrations in semiconductors can be strongly modified by doping. Accordingly, the addition of surplus charge carriers to chirality-mixed carbon nanotube samples has previously been found to give rise to a Drude-type plasmon feature as well as Fanotype antiresonances in the far- to mid-infrared spectral range (FIR/MIR). Here we investigate the FIR/MIR response of redox-chemically doped semiconducting (6,5) carbon nanotubes (s-SWNTs). We find that, contrary to expectations, the Drude-type plasmon shifts to lower wavenumbers with increasing doping level. By means of Monte-Carlo simulations of the optical response, we attribute this behavior to the confinement of excess charge carriers at low doping levels and their progressive delocalization when approaching degenerate doping. The coupling of vibrational modes to intraband excitations in the doped s-SWNTs can be probed via a double resonance process similar to that responsible for the Raman D-band. The resulting Fano antiresonances shed new light onto the character and coupling of electronic and vibrational degrees of freedom in these one-dimensional semiconductors.