Photoluminescence Line Shapes of Nanocrystals: Contributions from First- and Second-Order Vibronic Couplings

TL;DR

This paper introduces a parameter-free microscopic method to compute photoluminescence spectra of semiconductor nanocrystals, incorporating first- and second-order vibronic couplings, and successfully reproduces experimental results for CdSe/CdS nanocrystals.

Contribution

The work presents a novel, systematic approach that includes both diagonal and off-diagonal exciton-phonon interactions up to second order, enhancing the understanding of spectral line shapes.

Findings

Quadratic phonon couplings significantly contribute to linewidth at high temperatures.

Off-diagonal couplings have a minor role in exciton thermalization below 300K.

The method accurately reproduces experimental spectra across a wide temperature range.

Abstract

We present a microscopic, parameter-free approach for computing the photoluminescence spectra of a single semiconductor nanocrystal. The method derives exciton-phonon coupling directly from the semi-empirical pseudopotential framework and systematically incorporates both diagonal and off-diagonal interactions, expanded to second-order in the phonon modes. The dipole-dipole correlation function was calculated using a Dyson expansion within the Kubo-Toyozawa formalism, enabling a consistent description of the role of pure dephasing and population-transfer on the photoluminescence spectral features. Applied to CdSe/CdS core-shell nanocrystals, the approach quantitatively reproduces experimental photoluminescence spectra over a wide temperature range, revealing that quadratic phonon couplings account for nearly half of the homogeneous linewidth above 100-150 K, while off-diagonal couplings leading to exciton thermalization play only a minor role and only as T approaches 300K.