Publication date: 15th May 2025
Manipulating the surface chemistry of colloidal semiconductor quantum dots (QDs) by replacing native ligands with different binding characteristics, such as Z-type metal halide ligands, overcomes fundamental challenges in QDs by passivating surface trap states. However, contradictory results where the trend of photoluminescence (PL) change for ligand exchanged CdSe have been observed under the same metal halide treatment, revealing complexity in the QD surface. Here, we employ oleate-capped CdSe spheres as a rigorously defined model system, executing metal halide ligand exchange in single-phase tetrahydrofuran (THF) and systematically vary the cation or anion identity. By correlating quantitative 1H NMR and optical spectroscopies, we discovered a two-stage mechanism in the ligand exchange reaction, addition followed by native ligand replacement, correlated with the increase and decrease of the PL intensities, respectively. The initial addition stage arises by the passivation of undercoordinated surface sites in as-synthesized QDs, and the replacement stage is associated with dynamic electronic traps by metal halides, which could be further tuned by the anion electronegativity. Our findings bridge previous controversy and demonstrate metal halide exchange as a platform for defect-mediated property design, where targeted trap-state engineering unlocks new opportunities in QD optoelectronics.
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