Studying the effect of dimensions and spacer ligands on the optical properties of 2D lead iodide perovskites

In recent years, metal-halide perovskites (MHPs) have emerged as highly promising optoelectronic materials based on their exceptional properties and versatility in applications such as solar cells, light-emitting devices, and radiation detectors. This study investigates the optical properties of two-dimensional (2D) MHPs, with the Ruddlesden-Popper structure, comparing three morphologies-bulk poly-crystals, colloidal nanoplatelets (NPs), and thin films, aiming to bridge between the bulk and nano dimensionalities. By synthesizing bulk 2D MHPs using long alkyl ammonium spacers, typically found in colloidal systems, and NPs using shorter ligands suitable for bulk growth, we elucidate the relationship between these materials' structural modifications and optical characteristics. We propose the existence of two regions in these 2D MHPs, which differ in their optoelectronic properties and are associated with "bulk" and "surface" regions. Specifically, for poly-crystals, we observe the appearance of a lower energy "bulk" phase associated with the stacking of many 2D sheets, apparent both in absorption and photoluminescence. For NPs, this stacking is hindered, and hence, only the "surface" phase exists. With the elongation of the spacer chain, the poly-crystal becomes more similar to the NPs. For thin films, an interesting phenomenon is observed - the rapid film formation mechanism forces a more colloid-like structure for the shorter ligands and a more poly-crystal-like structure for the longer ones. Overall, this study bridging the different dimensions of 2D MHPs may support new possibilities for future research and development in this innovative field.