Excitonic optical response of carbon chains confined in single-walled carbon nanotubes

Abstract

It has been recently shown that long linear carbon chains (carbyne) can be formed inside multiwalled carbon nanotubes (CNTs). Encapsulation of carbyne inside the CNT affects the electronic structure of the chain by the long-range Coulomb interaction. This introduces an indirect band gap in the combined CNT-chain system and results in a change in the optical band gap. We study the excitonic optical response of the combined system using the Bethe-Salpeter and Wannier equations based on density functional theory and tight-binding band structures. The optical properties of isolated CNTs and chains are strongly affected by excitonic effects and the CNT-chain system follows a similar trend. The interaction between the CNT and chain results in new bright excitons as well as charge transfer excitons, where electrons are localized on the CNT and holes on the chain, yielding new dark excitons in the combined system.