Ligand and Stoichiometry Engineering Unlock Narrowband Red Emission in Colloidal PEA₂SnI₄ Nanoplatelets

Tin halide perovskites offer a lead-free alternative for optoelectronics, yet their strong tendency toward self-trapped exciton (STE) formation has hindered the development of color-pure emission. Here, we report a strategy to control the emission behavior of two-dimensional PEA₂SnI₄ nanoplatelets through a synergistic approach combining Sn²⁺ vacancy suppression and surface oxidation control. By introducing excess stannous oleate during synthesis, we suppress Sn vacancies, shifting the emission from broadband yellow (STE-dominated) to narrowband red (band-edge excitonic emission). Further narrowing is achieved by replacing conventional ligands with antioxidant surface ligands, stabilizing Sn²⁺ and eliminating residual traps. This approach yields highly luminescent PEA₂SnI₄ nanoplatelet colloidal solution with sharp red emission and a record-high quantum yield of 17.2%. Structural and spectroscopic analyses reveal a clear correlation between lattice distortion and electron-phonon coupling. Our findings offer a rational design strategy for defect-tolerant, lead-free quantum emitters with tunable excitonic properties.