Direct observation of the exciton polaron by serial femtosecond crystallography on single CsPbBr$_3$ quantum dots

TL;DR

This study uses serial femtosecond crystallography to directly observe the atomistic lattice deformation caused by exciton polarons in single CsPbBr$_3$ quantum dots, providing new insights into their opto-electronic properties.

Contribution

It presents the first direct observation of exciton polaron-induced lattice deformation in perovskite quantum dots using SFX, supported by DFT calculations.

Findings

Detection of crystal-wide deformation fields via Bragg peak shifts

Confirmation of lattice distortion consistent with exciton polaron formation

Establishment of SFX as a sensitive tool for lattice studies in nanocrystals

Abstract

The outstanding opto-electronic properties of lead halide perovskites have been related to the formation of polarons. Nevertheless, the observation of the atomistic deformation brought about by one electron-hole pair in these materials has remained elusive. Here, we measure the diffraction patterns of single CsPbBr$_3$ quantum dots (QDs) with and without resonant excitation in the single exciton limit using serial femtosecond crystallography (SFX). By reconstructing the 3D differential diffraction pattern, we observe small shifts of the Bragg peaks indicative of a crystal-wide deformation field. Building on DFT calculations, we show that these shifts are consistent with the lattice distortion induced by a delocalized electron and a localized hole, forming a mixed large/small exciton polaron. This result creates a clear picture of the polaronic deformation in CsPbBr$_3$ QDs, highlights the exceptional sensitivity of SFX to lattice distortions in few-nanometer crystallites, and establishes an experimental platform for future studies of electron-lattice interactions.