Temperature-Dependent Optical and Structural Properties of Chiral Two-Dimensional Hybrid Lead-Iodide Perovskites

Layered hybrid halide perovskites gain chirality viathe incorporationof chiral organic cations in the spacer layer. These so-called chiraltwo-dimensional (2D) halide perovskites have attracted considerableinterest recently for chiral optoelectronic, spintronic, and ferroelectricapplications. However, the effect of temperature on these materials,especially how the structure changes with temperature and its impacton the chiral optoelectronic properties, remains an open question.Here, we study the effect of temperature change on chiral optoelectronicand structural properties through temperature-dependent circularlypolarized photoluminescence (CPPL) microscopy and synchrotron powderX-ray diffraction as well as pair distribution function analysis toelucidate the intrinsic chiral optoelectronic and structural variationsfor R-, S-, and racemic-methylbenzylaminelead iodide. Our results show that the temperature-induced band gapchanges indicate a strong electron-phonon coupling comparedto the thermal-induced lattice expansion in chiral 2D perovskites.From powder diffraction measurements, a monotonic lattice expansionis observed on heating with no structural phase transitions over 90-360K detected for the three samples studied herein. Locally, a stronglyanisotropic and even negative expansion in some components of theinstantaneous Pb-I pair-distance distribution is suggestiveof coupling between dynamical intralayer distortions and lattice expansion.This work provides insights into the fundamental understanding ofthe temperature effect on chiral optoelectronic and structural propertiesof chiral 2D perovskites, which can lead to new chiral materials designstrategies for future chiral optoelectronic applications.