Environmental dielectric screening effect on exciton transition energies in single-walled carbon nanotubes

TL;DR

This study quantitatively examines how environmental dielectric screening influences exciton transition energies in single-walled carbon nanotubes, revealing a predictable redshift behavior and the impact of surfactant wrapping.

Contribution

It provides a simple empirical model describing the dielectric dependence of exciton energies and explores surfactant effects on SWNT optical properties.

Findings

Exciton energies redshift with increasing dielectric constant.

The energy shifts follow a power-law dependence on dielectric constant.

Surfactant wrapping introduces additional shifts similar to uniaxial stress effects.

Abstract

Environmental dielectric screening effects on exciton transition energies in single-walled carbon nanotubes (SWNTs) have been studied quantitatively in the range of dielectric constants from 1.0 to 37 by immersing SWNTs bridged over trenches in various organic solvents by means of photoluminescence and the excitation spectroscopies. With increasing environmental dielectric constant ($\epsilon_{\rm env}$), both $E_{11}$ and $E_{22}$ exhibited a redshift by several tens meV and a tendency to saturate at a $\epsilon_{\rm env} \sim 5$ without an indication of significant ($n$,$m$) dependence. The redshifts can be explained by dielectric screening of the repulsive electron-electron interaction. The $\epsilon_{\rm env}$ dependence of $E_{11}$ and $E_{22}$ can be expressed by a simple empirical equation with a power law in $\epsilon_{\rm env}$, $E_{\rm ii} = E_{\rm ii}^{\infty} + A\epsilon_{\rm env}^{-\alpha}$. We also immersed a sample in sodium-dodecyl-sulfate (SDS) solution to investigate the effects of wrapping SWNTs with surfactant. The resultant $E_{11}$ and $E_{22}$, which agree well with Weisman's data [Nano Lett. {\bf 3}, 1235 (2003)], are close to those of $\epsilon_{\rm env}$ of 2. However, in addition to the shift due to dielectric screening, another shift was observed so that the ($2n+m$)-family patterns spread more widely, similar to that of the uniaxial-stress-induced shift.