Excitonic effects and optical properties of passivated CdSe clusters

TL;DR

This study investigates the optical properties of passivated CdSe clusters using first-principles methods, revealing exciton effects and dark states that influence their optical behavior and radiative lifetimes.

Contribution

It compares time-dependent DFT and many-body approaches to elucidate excitonic effects and dark states in CdSe clusters, providing new insights into their optical excitations.

Findings

Many-body methods predict strong exciton effects with mixed transitions.

Dark states precede the first bright transition, affecting radiative lifetimes.

Time-dependent DFT shows mostly single-level to single-level excitations.

Abstract

We calculate the optical properties of a series of passivated non-stoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local density approximation, and by solving the Bethe-Salpeter equation for optical excitations with the GW approximation for the self-energy. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross-section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level transition. In contrast, many-body methods predict a strong mixture of several different transitions, which is a signature of exciton effects. We also find that the majority of the clusters have a series of dark transitions before the first bright transition. This may explain the long radiative lifetimes observed experimentally