Photon Correlations in Colloidal Quantum Dot Molecules Controlled by the Neck Barrier

TL;DR

This study explores how the optical properties of colloidal quantum dot molecules can be controlled by tuning the neck barrier, revealing distinct weak and strong coupling regimes with different photon emission behaviors at room temperature.

Contribution

It demonstrates how adjusting the neck girth in CQDMs modulates coupling strength, affecting exciton localization, Auger decay, and photon emission statistics, providing insights into artificial molecular bonding.

Findings

Weak coupling results in highly bunched photon emission.

Strong coupling restores photon antibunching.

Hybridization energy determines exciton localization and decay processes.

Abstract

We investigate the charge re-distribution upon optical excitation of various necked homodimer CQDMs using single particle emission spectroscopy. By tuning the hybridization of the electron wavefunction at a fixed center-to-center distance through controlling the neck girth, we reveal two coupling limits. On one hand a connected-but-confined situation where neighbouring CQDs are weakly fused to each other manifesting a weak coupling regime, and on the other hand, a connected-and-delocalized situation, where the neck is filled beyond the facet size leading to a rod-like architecture manifesting strong-coupling. Either coupling regimes entrust distinct optical signatures clearly resolved at room temperature in terms of photoluminescence quantum yield, intensity time traces, lifetimes, and spectra of the neutral-exciton, charged-exciton, and biexciton states. The interplay between the radiative and non-radiative Auger decays of these states, turns emitted photons from the CQDMs in the weak-coupling regime highly bunched unlike CQD monomers, while the antibunching is regained at the strong-coupling regime. This behavior correlates with the hybridization energy being smaller than the thermal energy (kT approx. 25meV) at the weak-coupling limit (delta E approax.5-10meV), leading to exciton localization suppressing Auger decay. In the neck-filled architectures, the larger hybridization energy (delta E approx.20-30meV) leads to exciton delocalization while activating the fast charged and multi-exciton Auger decay processes. This work sets an analogy for the artificial molecule CQDMs with regular molecules, where the two distinct regimes of weak- and strong-coupling correspond to ionic- or covalent- type bonding, respectively.