Hydrogen Bonding Denticity Governs Surface Modulation in Inorganic Perovskite Nanocrystals

All-inorganic CsPbI₃ perovskite nanocrystals (PNCs) have emerged as promising emitters for optoelectronic applications owing to their high color purity, tunable bandgap, and high photoluminescence quantum yield (PLQY). However, dynamic surface ligands, abundant trap states, and rapid halide ion migration limit their emission efficiency and operational stability. Here, methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GA⁺) were systematically investigated as surface modulators for PNCs via an anti-solvent-assisted post-treatment strategy. Spectroscopic analyses and density functional theory calculations reveal that the passivation efficacy is determined by the strength and denticity of hydrogen-bonding interactions between the organic cations and the PNC surface. Among the three candidates, GA⁺ exhibits the highest efficacy owing to its trifunctional N─H groups, which enable robust multidentate surface interactions that suppress trap formation and halide ion migration. As a result, GA-treated PNCs achieve a PLQY of 99.3% and retain ∼80% of their initial PLQY after 9 days under ambient conditions. The resulting perovskite light-emitting diodes deliver a maximum external quantum efficiency of 10.2% and a maximum luminance of 473 cd m^-2, representing a significant improvement over pristine devices. These findings identify molecular hydrogen-bonding geometry as a key design parameter for surface engineering of efficient and stable PNC-based optoelectronic devices.