Ligand Length Effects on the Structural and Optical Properties of 2D Perovskites

Two-dimensional (2D) perovskites, organic-inorganic heterostructures with confined excitons are ideal for light-emitting applications due to their high oscillator strengths, tunable emission bandwidths, and low non-radiative recombination rates. The valence and conduction band edges, primarily governed by orbital hybridization in the metal halide cage, allow significant band structure tuning by changing the composition of halides and metals. However, introducing mixed halides can cause undesired phase segregation and substituting lead with tin leads to stability issues. An alternative method to tune the bandgap is by using cations of different lengths. Although the organic cation does not significantly affect the electronic band structure, varying the ligand lengths rearranges the crystal structure, inducing tilting in the metal halide octahedra and altering lattice dynamics. Despite being widely used to control the electronic properties of 2D perovskites, the effects of these modifications are not fully understood.<br/><br/>In this study, we explore changes in the interplanar distance in 2D perovskites by synthesizing 2D perovskites with three different chain length phenylalkylammonium cations, ranging from one carbon chain Phenylmethylammonium to three carbon Phenylpropylammonium. We track the subtle changes in the crystallographic structure using x-ray diffraction and grazing incident wide angle x-ray scattering (GIWAXS) and correlate it with the excited state dynamics using transient absorption spectroscopy. Furthermore, we investigate the polaronic effects of these structural modifications using resonant impulsive stimulated Raman spectroscopy (RISRS). By directly probing the phonon dynamics and crystallographic structure as the interplanar distances change, we gain insights into the interplay between changes in excitonic transitions caused by crystallographic distortions and modifications to the phonon environment induced by the longer ligands.