Nanotubes in polar media: polarons and excitons on a cylinder

TL;DR

This paper investigates how electrons and holes form polaronic states on semiconducting nanotubes in polar media, revealing a significant polaronic effect that influences exciton binding and charge separation, with implications for nanostructure behavior.

Contribution

It provides a detailed evaluation of polaron and exciton binding energies on cylindrical nanotubes in polar media, highlighting a non-monotonic ratio and potential effects on charge separation.

Findings

Polaron binding energy can reach about 35% of exciton energy.

The ratio of polaron to exciton binding energy varies non-monotonically with radius.

Strong polaronic effects can enhance charge separation in nanostructures.

Abstract

Electrons and holes on semiconducting nanotubes immersed in sluggish polar media can undergo self-localization into polaronic states. We evaluate the binding energy $\bpol$ of adiabatic Fr\"{o}hlich-Pekar polarons confined to a cylindrical surface and compare it to the corresponding exciton binding energy $\bexc$. The ratio $\bpol/\bexc$ is found to be a non-monotonic function of the cylinder radius $R$ that can reach values of about 0.35, substantially larger than values of about 0.2 for 2$d$ or 3$d$ systems. We argue that these findings represent a more general crossover effect that could manifest itself in other semiconductor nanostructures in 3$d$ polar environments. As a result of the strong polaronic effect, the activation energy of exciton dissociation into polaron pairs is significantly reduced which may lead to enhanced charge separation.