Intense Circular Dichroism and Spin Selectivity in AgBiS₂ Nanocrystals by Chiral Ligand Exchange

Chiral semiconducting nanomaterials offer many potential applications in photodetection, light emission, quantum information, and so on. However, it is difficult to achieve a strong circular dichroism (CD) signal in semiconducting nanocrystals (NCs) due to the complexity of chiral ligand surface engineering and multiple, uncertain mechanisms of chiroptical behavior. Here, a chiral ligand exchange strategy with cysteine on the ternary metal chalcogenide AgBiS₂ NCs is developed, and a strong, long-lasting CD signal in the near-UV region is achieved. By carefully optimizing the ligand concentration, the CD peaks are observed at 260 and 320 nm, respectively, giving insight into the different ligand binding mechanisms influencing the CD signal of AgBiS₂ NCs. Using density-functional theory, a large degree of crystal distortion by the bidentate mode of ligand chelation, and efficient ligand-NC electron transfer, synergistically resulting in the strongest CD signal (g-factor over 10^-2) observed in chiral ligand-exchanged semiconductor NCs to date, is demonstrated. To demonstrate the effective chiral properties of these AgBiS₂ NCs, a spin-filter device with over 86% efficiency is fabricated. This work represents a considerable leap in the field of chiral semiconductor NCs and points toward their future applications.