Chiral Hybrid Lead Halides : Chiroptic, Piezoelectric and Thermosalient Properties

Abstract

Chiral hybrid organic–inorganic perovskites, comprising chiral organic cations and lead halide frameworks, combine chirality with semiconducting properties. The present thesis work explores 1D and 2D hybrid lead halides, that are derived from the perovskite structure.,We show that a combination of chirality, halide ion tuning, and film morphology influences circular dichroism (CD), piezoelectricity, and thermomechanical behaviour in hybrid lead halides with R/S-MBA (MBA: methylbenzylammonium) as the chiral organic A-site cation. Films of (R-MBA)PbBr₃ show textured morphologies causing inconsistencies in their CD spectra. Non-textured (R-MBA)PbI₃ and (R-MBA)₂PbI₄ films show uniform CD spectra, indicating correlation of film morphology with chiroptic properties. The chiral compounds are obviously non-centrosymmetric making them relevant for piezoelectric properties. Building on this property, a mixture of (R-MBA)PbBr₃ and (R-MBA)PbI₃ exhibits an enhanced d₃₃ of 5.2 pC·N⁻¹. Embedding the optimized mixture in a flexible polycaprolactone matrix further amplifies voltage output and power density, suggesting that more unidirectional dipoles are created. On the single-crystal scale, (R-MBA)PbI₃ and (S-MBA)PbI₃ display thermosalient behavior, exhibiting abrupt jumping motion upon heating. The jumping of crystal does not involve a structural phase transition, but occurs due to anisotropic expansion along the crystallographic c-axis. Together, these findings provide insights by connecting the emerging properties of chiral hybrid lead halides, with their chemical composition, structure and film morphology.