Investigation on broadband emission of two-dimensional melamine lead iodide perovskite (2D-C₃H₈N₆PbI₄): An experimental and theoretical approach

Novel two-dimensional melamine lead iodide perovskite (2D-C₃H₈N₆PbI₄) is synthesized to investigate its crystallinity, optical band gap and broadband emission properties and to make comparisons with 2D-C₃H₈N₆PbCl₄/2D-C₃H₈N₆PbBr₄ perovskites. Both experimental and density functional theory (DFT) interrogations on 2D-C₃H₈N₆PbX₄ (X = Cl, Br and I) are conducted. The crystal structure, morphology and percentile of Pb and halide elements are confirmed using scanning electron microscope (SEM), and energy dispersive spectrum (EDS), powder/single crystal X-ray diffraction (PXRD/SXRD), DFT and X-ray crystallography simulations. The optical band gaps of 2D-C₃H₈N₆PbX₄ perovskites are determined from the Tauc plot fitting of absorbance and DFT studies. Distinct broadband emission of 2D-C₃H₈N₆PbX₄ perovskites between 300 and 800 nm is observed, which can be fitted with multiple Gaussian distributions. The fittings of broad PL spectra from 2D-C₃H₈N₆PbCl₄/2D-C₃H₈N₆PbBr₄ perovskites confirm the involvement of both Dexter energy transfer from melamine cation and self-trapped excitons (STEs). However, the broadband emission of 2D-C₃H₈N₆PbI₄ is attributed only to the Dexter energy transfer from melamine cation and the absence of STEs is attributed to the larger lattice deformation of 2D-C₃H₈N₆PbI₄. Moreover, the involvement of spin–orbit coupling (SOC) in the energy transfer is clarified to attest that the broadband emission of 2D-C₃H₈N₆PbI₄ is distinct among its halide family.