Complete mapping of interacting charging states in single coupled colloidal quantum dot molecules

TL;DR

This study provides a comprehensive mapping of how charge states influence emission spectra in coupled colloidal quantum dot molecules, revealing complex interactions and charge effects crucial for quantum technology applications.

Contribution

It introduces a detailed spectral mapping of charging states in coupled CQDs, combining experimental spectroscopy with theoretical calculations to understand charge interactions.

Findings

Complex emission spectra in CQDMs due to coupling.

Correlation between spectral diffusion and charge states.

Identification of simultaneous negative charges on both centers.

Abstract

Colloidal Quantum Dots (CQDs), major building blocks in modern opto-electronic devices, have so far been synthesized with only one emission center where the exciton resides. Recent development of coupled Colloidal Quantum Dots Molecules (CQDM), where two core-shell CQDs are fused to form two emission centers in close proximity, allows to explore how charge carriers in one CQD affect the charge carriers in the other CQD. Cryogenic single particle spectroscopy reveals that while CQD monomers manifest a simple emission spectrum comprising a main emission peak with well-defined phonon sidebands, CQDMs exhibit a complex spectrum with multiple peaks that are not all spaced according to the known phonon frequencies. Based on complementary emission polarization and time-resolved analysis, this is assigned to fluorescence of the two coupled emission centers. Moreover, the complex peak structure shows correlated spectral diffusion indicative of the coupling between the two emission centers. Utilizing Schrodinger-Poisson self-consistent calculations, we directly map the spectral behavior, alternating between neutral and charged states of the CQDM. Spectral shifts related to electrostatic interaction between a charged emission center and the second emission center are thus fully mapped. Furthermore, effects of moving surface charges are identified, whereby the emission center proximal to the charge shows larger shifts. Instances where the two emission centers are negatively charged simultaneously are also identified. Such detailed mapping of charging states is enabled by the coupling within the CQDM and its anisotropic structure. This understanding of the coupling interactions is a progress towards quantum technology and sensing applications based on CQDMs.