Exciton self-trapping in twisted hexagonal boron nitride homostructures

TL;DR

This study investigates how twisted interfaces in hexagonal boron nitride affect exciton behavior, revealing efficient exciton capture, interface localization, and self-trapping phenomena that influence optical emissions in 2D materials.

Contribution

It provides experimental evidence of exciton self-trapping at twisted hBN interfaces, linking structural twist angles to excitonic properties and optical emissions.

Findings

Efficient capture of free excitons by the interface.

Long-lived, interface-localized excitons observed.

Self-trapping of excitons occurs at high twist angles.

Abstract

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons.