Control of chiral optical and spin properties in low-dimensional perovskites

In materials design, the dimensional reduction from three-dimensional (3D) to two-dimensional (2D) metal halide perovskites represents a paradigm shift that expands the chemical space, introducing new variables such as metal species, enantiomers, and ionic compositions for property engineering. While 3D perovskites are constrained by geometric tolerance factors to small monovalent cations and a limited set of simple organic molecules, 2D perovskites can accommodate large organic spacer cations between inorganic layers, enabling diverse organic cations incorporation. This expanded molecular design allows the introduction of chirality through chiral organic cations, an approach unattainable in conventional 3D systems. Consequently, chiral 2D hybrid perovskites combine the favorable optoelectronic properties of the inorganic substructure with chirality-induced symmetry breaking, giving rise to spin-selective and circularly polarized optical phenomena. The introduction of chiral organic cations breaks inversion and mirror symmetries and induces unique structural behavior in the absence of external magnetic fields, e.g., circularly polarized second harmonic generation and Rashba spin splitting. Notably, spin-selective charge transport in such materials differs from chirality-induced spin selectivity (CISS), originating instead from momentum-dependent spin splitting in the inorganic substructure rather than from helical organic pathways. Such features present practical avenues for high-performance quarter-wave plates, spin light-emitting diodes, spin-photovoltaic devices, and quantum information architectures. This dissertation presents a comprehensive investigation into controlling chiral optical and spin properties in low-dimensional perovskites, encompassing rational structure design, advanced synthesis methods, and fundamental mechanism elucidation. The introductory chapter traces the evolution from conventional 3D to low-dimensional architectures and highlights the transition from achiral to chiral organic cationss. Subsequent chapters provide detailed analyses of the structural and functional advantages of 2D perovskites, outlining design principles and specialized synthesis techniques such as water–oil interface growth. Crystallographic analyses reveal that chiral systems exhibit broken inversion symmetry and intrinsic lattice distortions, which underpin their strong chiroptical responses. Chiral optical properties are explored, with emphasis on the coupling of circularly polarized photoluminescence and second harmonic generation (SHG). Chiral perovskites exhibit SHG intensities an order of magnitude higher than those of conventional KDP crystals and unprecedented circularly polarized SHG ratios. Density functional theory calculations elucidate mechanistic insights through the analysis of Bir–Aronov–Pikus and D'yakonov–Perel' spin relaxation pathways. In Chapter 3, we demonstrated that incorporating mixed chiral cations (R-MBA and R-THFA) in low-dimensional lead iodide perovskites effectively enhances spin polarization and prolongs spin lifetime. Systematic variation of the MBA:THFA ratio identified (R-MBA)1.5(R-THFA)0.5PbI4 (3:1) as optimal, achieving a spin polarization of ~0.14 and a spin lifetime three times longer than (R-MBA)2PbI4 and five times longer than (R-THFA)2PbI4 at room temperature. Spin-resolved transient absorption spectroscopy revealed that spin lifetime directly determines spin polarization, with both following identical concentration-dependent trends. Temperature-dependent studies confirmed that both Elliott–Yafet and D'yakonov–Perel' mechanisms contribute to spin relaxation. The superior performance of the 3:1 system arises from defect passivation by THFA's N and O atoms, which suppresses EY relaxation, and a lower excitonic binding energy that weakens DP relaxation. These results establish mixed chiral-cation engineering as an effective approach for controlling spin dynamics in 2D perovskites and offer new routes toward high-performance spintronic devices. In Chapter 4, we uncover a previously unrecognized pathway for spin angular momentum transfer, mediated by chiral phonons through chiral–achiral perovskite heterostructures. The spin-resolved transient absorption spectroscopy and simulated Raman scattering used directly captured periodic oscillations of phonon wave packets whose helicity reverses with the handedness of the optical excitation. These chiral phonons — arising from the octahedral framework of R-MBA2PbI4 — carry angular momentum across the interface to the achiral CsPbI3 layer and promote spin transfer without magnetic fields. The reciprocal variations of spin lifetimes between the 2D and 3D layers verify phonon-mediated spin transfer (linking lattice vibrations with spin dynamics). Research on angle- and thickness-dependent transfer efficiency shows that it is a function of crystal orientation and interface morphology, indicating that lattice matching plays an important role. R- and S-based systems show the inversion of the vibrational helicity, confirming the enantioselective nature of this process, and the absence of oscillations in racemic structures confirms the lack of chiral phonons in achiral systems. Collectively, these observations demonstrate that chiral phonons are not only agents of angular momentum in low-dimensional materials but also a novel design paradigm for spintronic and spin-photonic applications. The final chapter highlights the potential of these findings in practical applications. Chiral perovskites enable cost-effective achromatic quarter-wave plates through intrinsic broadband circular birefringence, replacing complex multilayer designs with single-layer architectures. Spin light-emitting diodes utilizing circularly polarized electroluminescence achieve direct electrical injection and emission of highly polarized light without magnetic fields, with implications for 3D displays, quantum communication, and spin-based information processing. This work establishes chiral low-dimensional perovskites as a versatile platform bridging fundamental spin–photon interactions and next-generation chiroptical and spintronic technologies.