Pioneering nucleation for stable ultraviolet-to-deep-blue illuminating two-dimensional perovskite nanoplates by using saturated salt solution

Two-dimensional (2D) metal halide perovskites are promising materials for efficient ultraviolet-to-blue light emission. However, synthesizing stable, phase-pure, and highly fluorescent 2D perovskite nanomaterials remains challenging. Here, a unique nucleation strategy is used for stabilizing the nucleation of the 2D phase in perovskite stock solution, increasing the luminance and stability of the resulting nanoplates. To kinetically control the formation of 2D perovskite nanoplates with specific <i>n</i> values, this strategy is mainly executed by using a saturated solution of PbX₂ (X = Br or Cl), an optimized amount of CsX and PbX₂ solution and small amounts of oleylamine (OLM) for adjusting the ratio of [Pb_<i>x</i>Br_<i>y</i>]^{2<i>x</i>-<i>y</i>}/Cs⁺ in the stock solution. The saturated PbX₂ solution plays a role in the formation of the [Pb_<i>x</i>Br_<i>y</i>]^{2<i>x</i>-<i>y</i>} complex, resulting in the nucleation of 2D (OLM)₂Cs_<i>n</i>-1Pb_<i>n</i>X_3<i>n</i>+1 perovskite nanoplates and inhibition of 3D nanocrystal formation. The resulting 2D perovskite nanoplates with specific <i>n</i> values exhibited high photoluminescence (PL) from the ultraviolet to deep-blue region. Most importantly, this is the first report to demonstrate 2D (OLM)₂PbCl₄ perovskite nanoplates with <i>n</i> = 1, yielding unique ultraviolet emission. The 2D (OLM)₂Cs_<i>n</i>-1Pb_<i>n</i>Br_3<i>n</i>+1 perovskite nanoplates with <i>n</i> = 1, 2, and 3 demonstrated high emission from violet to deep-blue color. X-ray diffraction (XRD) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) patterns indicate complexation of PbBr₂ and halide in the DMF solution, forming species of the Pb-halide complex ([Pb_<i>x</i>Br_<i>y</i>]^{2<i>x</i>-<i>y</i>}, such as PbBr₃^- and PbBr₄^2-). The Pb-halide complex facilitates the 2D arrangement of PbBr₆^4- octahedra, resulting in phase-pure and stable 2D perovskite nanoplates. Improved luminance and stability are attributed to reduced defects as calculated with the help of Urbach energy. This study provides insights into the formation of 2D perovskite nanoplates controlled by manipulating nucleation in the saturated salt solution, offering a simple method for controlling layer thickness and phase purity.