# Infrared FEL-Induced Alteration of Zeta Potential in Electrochemically Grown Quantum Dots: Insights into Ion Modification

## TL;DR

This study shows that using infrared lasers can change the surface properties of quantum dots, improving their performance in solar cells and other optoelectronic devices.

## Contribution

The study introduces a novel non-contact method using MIR FEL irradiation to modify quantum dot surfaces for enhanced optoelectronic properties.

## Key findings

- MIR FEL irradiation caused a 40–50 mV inversion in zeta potential and a 10% reduction in hydrodynamic size of quantum dots.
- Photoluminescence and Tauc analysis showed improved emission and bandgap shifts, indicating defect state suppression and better optoelectronic quality.
- TEM confirmed that FEL-induced changes affected only surface chemistry, not the bulk structure of the quantum dots.

## Abstract

This study explores the use of mid-infrared (MIR) free-electron laser (FEL) irradiation as a tool for tailoring the surface properties of electrochemically synthesized TiO2—graphene quantum dots (QDs). The QDs, prepared in colloidal form via a cost-effective electrochemical method in a KCl—citric acid medium, were exposed to MIR wavelengths (5.76, 8.02, and 9.10 µm) at the Kyoto University FEL facility. Post-irradiation measurements revealed a pronounced inversion of zeta potential by 40–50 mV and approximately 10% reduction in hydrodynamic size, indicating double-layer contraction and ionic redistribution at the QD—solvent interface. Photoluminescence spectra showed enhanced emission for GQDs and TiO2/GQD composites, while Tauc analysis revealed modest bandgap blue shifts (0.04–0.08 eV), both consistent with trap-state passivation and sharper band edges. TEM confirmed intact crystalline structures, verifying that FEL-induced modifications were confined to surface chemistry rather than bulk lattice damage. Taken together, these results demonstrate that MIR FEL irradiation provides a resonance-driven, non-contact method to reorganize ions, suppress defect states, and improve the optoelectronic quality of QDs. This approach offers a scalable post-synthetic pathway for enhancing electron transport layers in perovskite solar cells and highlights the broader potential of photonic infrastructure for advanced nanomaterial processing and interface engineering in optoelectronic and energy applications.