Anomalous Photoluminescence in Undoped and Mn2+-Doped 2D Hybrid Lead Halide Systems

Two-dimensional (2D) hybrid lead halide systems have emerged as promising materials for optoelectronic applications due to their remarkable optical properties and enhanced moisture stability. However, photoluminescence (PL) from such 2D samples has been reported to vary substantially across different sample forms and experimental geometries, and the associated recombination dynamics are often inconsistently interpreted in the literature. The incorporation of Mn2+ dopant ions into these 2D systems further complicates the picture by introducing host-to-dopant energy-transfer pathways, highlighting the need for a mechanistic understanding of PL in these materials. Here, we use 2D BA2PbBr4 (BA = butylammonium) as a model system to address these questions by systematically comparing the undoped and Mn2+-doped samples in diverse forms, namely crystals, films, and pellets, through comprehensive spatially resolved, time-resolved, and temperature-dependent PL analyses, together with DFT calculations. Spatial resolution allows us to confirm that the high-energy emission corresponds to interior excitonic recombination, while the secondary low-energy emission arises from edge-related states amplified by waveguiding. The variation in edge-related emission intensity across different samples in a given geometry reflects differences in the amount of edge-related sites present in the volume sampled by the experiment; on the other hand, the dependence on PL collection geometries in a given sample arises from the variations in the fraction of excitonic emission that is waveguided in-plane to the edges prior to detection, thereby preferentially favoring the excitation of edge-related states. These observations resolve the long-standing ambiguity regarding the wide variations in emission spectra reported across different sample forms and measurement geometries. We further show that both interior excitonic and edge-related states transfer energy to the Mn2+ ion, and that variations in host emission across different sample forms do not influence the intrinsic Mn2+ energy-transfer pathways. Finally, we explore the potential of these materials in light-emitting applications.