A-site cation substitution in CsPbI₃ perovskite nanoplatelets enables their tunable emission and efficient nonlinear absorption

Metal halide perovskite nanoplatelets (NPls) exhibit attractive optical properties as a result of quantum and dielectric confinement effects. Their strong and narrow photoluminescence (PL), tunable over the entire visible spectral range, makes them highly promising for light-emission and lasing applications. In metal halide perovskites, partial cation substitution serves as an additional tool for fine-tuning their structure and optical properties. While A-site cation substitution has been successfully used for perovskite nanocrystals, its application to their NPl counterparts has not yet been widely addressed. In this study, we demonstrate that sufficient surface passivation is required to be performed prior to A-site cation exchange in CsPbI₃ perovskite NPls by treatment with ZnI₂ and oleylamine, enabling efficient A-site cation substitution with both smaller (rubidium, Rb⁺) and larger (formamidinium, FA⁺) cations. This allowed us to tune the PL peak position of NPls over a broad spectral range of 590-630 nm. In particular, mixed-cation Cs_1-<i>x</i>FA_<i>x</i>PbI₃ (<i>x</i> = 0.68-0.87) NPls exhibited narrow (full width at half maximum ∼ 25 nm) and efficient (PL quantum yield ∼ 90%) pure-red emission at 620-626 nm, making them promising candidates for light emission applications. We have furthermore demonstrated that the partial A-site substitution of (Cs⁺) with organic (FA⁺) cations enhanced the nonlinear optical responses of Cs_1-<i>x</i>FA_<i>x</i>PbI₃ NPls, whose TPA cross-section increased up to 3.85 × 10⁶ GM, positioning them as promising materials for nonlinear optical applications.