Possible Dual Bandgap in (C₄H₉NH₃)₂PbI₄ 2D Layered Perovskite: Single-Crystal and Exfoliated Few-Layer

Optoelectronic devices of 2D layered Pb-halide perovskites depend on light absorption/emission, exciton dissociation/transfer, and charge transfer processes, which in turn depend on bandgap and band edges. Here, we report that the surface/subsurface region of 2D (C4H9NH3)2PbI4 perovskite single crystals have a wider bandgap compared to the interior of the crystal. Consequently, single crystals exhibit dual excitonic emission peaks at 2.38 and 2.20 eV arising from the surface and interior, respectively. In contrast, exfoliated layers of (C4H9NH3)2PbI4 exhibit single photoluminescence peak at 2.38 eV, similar to the surface/subsurface of single crystals. Temperature-dependent (300–10 K) photoluminescence and single-crystal diffraction suggest that the overall structure–bandgap relationships are similar for both single-crystal and few-layer samples, but with some difference in phase transition hysteresis. Similar minor structural differences between the bulk (interior) and surface/subsurface of (C4H9NH3)2PbI4 single crystals are the probable cause of the dual bandgap. Single crystals of other layered perovskite systems, namely, (C6H13NH3)2PbI4 and (C4H9NH3)2PbBr4, also exhibit a similar dual bandgap.