Probing Dynamics of Lead(II) Sulfide Quantum Dots in Solution Ligand Exchange by Voltammetry

The ligand/quantum dots (QDs) ratio is crucial for the liquid state ligand exchange process to ensure a high-quality surface passivation and stable QDs ink. Herein we report an electrochemical method to investigate the ligand exchanged PbS-PbI₂ QDs. It is found that the shell and core Pb(II) are distinguished by their reduction peak position in the cyclic voltammogram and the peak charge ratio gives the shell/core composition of the QDs. Combined with XPS analysis and UV-vis spectroscopy, it is further indicated that the shell/core ratio of PbS-PbI₂ QDs varies as the ligand PbI₂ concentration changes. Specifically, below a certain concentration, more PbI₂ binds to the QD surface, leading to better passivation when the PbI₂ concentration increases; however, beyond that concentration, decomposition of QDs likely occurs via an anion exchange process. The presented electrochemical method provides a new and powerful tool to investigate and optimize QD surface chemistry for boosting the scale up applications of QD devices.