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Colloidal PbS/PbCl2 core/shell nanoplatelets were synthesized via wet-chemical methods using lead oleate and lead chloride as lead precursors. The resulting heterostructures consist of a PbS core, a PbCl2 shell, and an intermediate layer of lead sulfochloride alloy. These nanoplatelets exhibit a photoluminescence quantum yield of approximately 20%, nearly an order of magnitude higher than that of unshelled PbS nanoplatelets. Cyclic voltammetry measurements reveal a type-I band alignment between the core and shell. Despite the presence of strong attractive biexciton Auger recombination, as observed in transient absorption spectroscopy, the nanoplatelets achieve amplified spontaneous emission at a low pump threshold of 76 μJ cm–2. Their large lateral dimensions support the spatial distribution of multiple excitons, including repulsive biexcitons, enabling efficient light amplification. 

Two-dimensional halide perovskites exhibit rich structural diversity and tunable optoelectronic properties, making them promising materials for energy, sensing, and photonic applications. We report the structural mapping of four distinct polymorphs, γ (P21/c), β (P4/mbm), α (P4/mmm), and δ (Cmcm) in the two-dimensional iodoplumbate, iodostannate, and iodogermanate perovskite so-called Dion–Jacobson series (3AMP)MI4 (M = Sn, Pb, Ge), where 3AMP is 3-(aminomethyl)piperidinium. The phases exhibit systematic evolution in octahedral distortion, lattice symmetry, and metal–halide geometry, enabling structural control over optoelectronic properties. Notably, the α-phase of (3AMP)SnI4 represents a rare, ambient-stable, high-symmetry structure for Sn-based perovskites, without a phase transition down to 100 K. Variable-temperature single-crystal diffraction, powder X-ray diffraction (PXRD), and calorimetry reveal metal- and temperature-dependent polymorph interconversions, including the emergence of long-range supercell reflections in Pb-rich compositions at low temperature. Optical spectroscopy and photoelectron yield spectroscopy confirm band gap tunability and band alignment trends, highlighting symmetry-dependent shifts and anomalous band gap bowing in mixed-metal systems, verified by electronic structure calculations. Calculations additionally indicate that the higher symmetry phases have reduced electron and hole effective masses compared to the lower symmetry phases.