In this study, various concentrations of the largesized cation iodide, namely, phenylethylammonium iodide (PEAI), incorporated in lead iodide (PbI 2 ) solid films react with heated methylammonium iodide (MAI) to fabricate 2D/3D mixed perovskite films with vertical phase distribution, which are prepared by the low-pressure vapor-assisted solution process (LP-VASP). The 2D/3D mixed perovskite films exhibit multiple photoluminescence (PL) emissions that correspond to a 2D Ruddlesden−Popper layered perovskite of PEA 2 MA n−1 Pb n X 3n+1 with varying n values and a 3D MAPbI 3 perovskite. Their crystalline structure along with crystal orientation, surface morphology, and surface potential is examined by X-ray diffraction, grazing-incidence wide-angle X-ray scattering, and Kelvin probe force microscopy, respectively. Confocal PL microscopy, two-photon excitation microscopy, and scanning transmission X-ray microscopy are further conducted to map the spatial and energy spectral distribution of 2D layered perovskites in the resultant film. From these results, multiple 2D layered perovskites are presumably formed in the bulk, while a 3D perovskite is mainly produced on the surface. The dimensional variation across the vapor-treated film is attributed to the phenylethylamine vapor (PEA (g) ) evaporating from the PEAI-added PbI 2 film and the methylamine vapor (MA (g) ) reaction with the PEAI-added PbI 2 film initiated from the film surface. A gradient phase distribution from 2D (bottom) to 3D (surface) perovskites is intuitively fabricated.

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