Single‐Particle FRET Probes Heterogeneity in the Ligand Shell of Colloidal Perovskite Quantum Dots

The functionality of colloidal quantum dots (QDs), whether in fundamental studies, optoelectronics, or emerging quantum technologies, critically depends on their surface chemistry, with organic ligands influencing nearly every aspect of their behavior. Whereas the inorganic core can be comprehensively characterized at atomic resolution, ligands remain difficult to probe owing to their low atomic contrast and soft, disordered structure. Here, we empower single-particle photoluminescence (PL) spectroscopy to characterize the organic ligand shell. Tagging ligand tails with organic dyes enables measuring their distance from the QD via Förster resonance energy transfer (FRET) at sub-nanometer resolution. Single-particle, single-molecule FRET experiments performed on lead halide perovskite QDs reveal nanometer-scale variations in dye-surface distances, both within individual particles (0.81 ± 0.22 nm on average) and between QDs with varied ligand chemistries (from 0.25 to 1.02 nm). Supported by atomistic modelling, our experiments tie diverse local ligand environments to these variations, validating a powerful yet accessible spectroscopic method for interrogating the complex surface chemistry of QDs, highly relevant to QD-based optoelectronics, quantum applications, and photocatalysis.