Research Projects

Self-trapped excitons (STEs) are a subset of excitons that become localized due to strong interactions with the lattice. Unlike free excitons, which propagate freely through the crystal, STEs are confined to specific lattice sites, resulting in unique photophysical properties such as broad emission spectra, long radiative lifetimes, and sensitivity to lattice dynamics. STE formation is particularly pronounced in materials with soft crystal lattices and significant lattice anharmonicity, where atomic displacements and nonlinear vibrational modes enhance electron-phonon coupling. While STEs have been well-studied in ionic crystals and molecular solids, the phenomenon has gained renewed attention in low-dimensional systems such as 0D metal halides. The reduced dimensionality, enhanced electron-phonon coupling, and soft lattice structures in these materials create favourable conditions for exciton localization, making them ideal platforms for the investigation of STE. In low dimensional materials, the lattice can be easily distorted or polarized with external stimuli due to lesser polyhedral connectivity. This structural distortion results a soft lattice which often exhibits strong anharmonicity.1–3 This STE which originates from strong electron–phonon coupling results in broadband photoluminescence (PL) with long carrier lifetime accompanied by large Stokes shifts.4 Recent studies using local structural analysis on materials like MPb2Br5 (M = Cs, Rb) and Cs5Cu3Cl6I2 clearly evidenced the role of lattice anharmonicity on the formation of STE.1–3 Although, significant amount of study has been made regarding the photo physical process related to STE; the structural origin of STE is still limited to few literatures.1–3,5 Hence, understanding and controlling STEs in low-dimensional materials could unlock new paradigms in light-emitting devices like applications. Some of the key challenges include: • Quantifying the contributions of anharmonic phonons to STE formation. • Distinguishing STEs from defect-trapped excitons or other localized states. • Developing strategies to control STE formation for practical applications. Hence, the local structural origin of STE is a necessary research subject for designing new materials with tuneable emission and related applications. The present proposal aims to investigate how lattice anharmonicity and the inherent softness of crystal lattices in low-dimensional metal halides facilitate the formation of STEs and explore the interplay between lattice anharmonicity, soft phonon modes, and exciton localization. Reference: 1 J. Pradhan, and K. Biswas, Chem. Mater., 2024, 36, 3405–3416. 2 J. Pradhan, and K. Biswas, Chem. Sci., 2022, 13, 9952–9959. 3 A. Das, and K. Biswas, Angew. Chemie, , DOI:10.1002/ange.202506404. 4 Y. Han, and B.-B. Cui, Mater. Adv., 2023, 4, 355–373. 5 M. D. Smith and H. I. Karunadasa, Acc. Chem. Res., 2018, 51, 619–627.

Self Trapped Exciton, Lattice Anharmonicity, Soft Lattice, Transient Distortion, SCXRD