Dual-Facet Passivation of Lead Sulfide Quantum Dots via an Ion-Pair Ligand toward High-Performance Near-Infrared Photodetectors

A high-performance near-infrared (NIR, λ > 1400 nm) photodetector is developed based on large-sized lead sulfide colloidal quantum dots (PbS CQDs) surface-engineered with the ionic liquid ligand 1-ethyl-3-methylimidazolium thiocyanate (EMIMSCN). Through the synergistic action of thiocyanate (SCN–) anions and EMIM+ cations, a robust dual-facet passivation structure is formed on both the polar (111) and nonpolar (100) facets. The SCN– anion strongly coordinates with under-coordinated Pb2+ sites, while the EMIM+ cation adopts a face-to-face π-conjugated orientation on the (100) surface, leading to effective suppression of surface trap states and halide migration. This cooperative passivation mechanism significantly improves film quality, reduces defect density, and enhances carrier transport. As a result, the optimized devices exhibited a dark current density as low as 4 μA cm–2 at −1 V, a responsivity of 0.75 A W–1, and a specific detectivity of 5.2 × 1011 Jones at 1490 nm, outperforming conventional halide-passivated CQD devices. Furthermore, monolithic NIR-to-visible upconversion photodetectors are successfully demonstrated, achieving a record photon-to-photon conversion efficiency of 3.15%. This ion-pair ligand strategy provides a versatile and stable passivation route for large-sized PbS CQDs, enabling next-generation low-noise and high-stability NIR optoelectronic devices.