Dopant–Carrier Exchange Coupling in Mn2+-Doped Metal Halide Perovskite Nanocrystals

Abstract

While early studies of lead halide perovskites (LHPs) have focused on their tunable optoelectronic properties and applications in photovoltaics and optoelectronics, their rich spin-related properties, including strong spin–orbit coupling (SOC), long spin relaxation and coherence times, as well as their strong coupling to the lattice, have more recently attracted attention for spin-based functionalities. These characteristics position LHPs as promising candidates for spintronic and quantum information applications. Incorporation of magnetic ions into the perovskite lattice introduces additional spin degree of freedom, giving rise to hybrid materials that combine semiconducting behavior with magnetic interactions. Such dopant-induced exchange interactions enable coupling between localized spins and band carriers, providing a pathway toward spin-dependent transport in future spintronic devices. However, compared to conventional II–VI chalcogenide semiconductors, magnetic-dopant-induced exchange interactions in halide perovskites remain less well understood. In this thesis, magnetic exchange interactions in Mn2+-doped metal halide perovskite nanocrystals (NCs) are investigated using magnetic circular dichroism (MCD) spectroscopy to probe dopant–exciton coupling. This thesis begins with three-dimensional (3D) CsPbCl3 NCs as a model host lattice for magneto-optical investigation, owing to their high Mn2+ miscibility via the hot-injection method and an efficient energy transfer. At cryogenic temperatures, the excitonic MCD responses are significantly enhanced relative to pristine NCs and exhibit magnetic-field-dependent saturation, consistent with magnetization arising from paramagnetic Mn2+ (d5) spins. Distinct from conventional II–VI chalcogenide diluted magnetic semiconductors (DMSs), the MCD spectra display absorption-like line shapes rather than inverted derivative features, suggesting differences in coupling to band-edge carriers in LHPs. The similar MCD line shapes observed for both the band-edge and higher-lying excitonic transitions further provide insight into the origin of the coupling of higher-energy states to Mn2+. The effect of heterohalide alloying on dopant–exciton interactions is further examined by incorporating Br- into the CsPbCl3 lattice. The resulting excitonic magneto-optical responses show a pronounced enhancement relative to the pure Cl system, which is associated with anomalous kinetic exchange involving Mn2+ d electrons and the s-orbital-derived valence band maximum (VBM). Temperature-dependent MCD measurements additionally provide an alternative approach to estimate the Zeeman splitting of band-edge states. These findings contribute to understanding of spin-induced excitonic Zeeman effects in lead halide perovskite DMSs with inverted electronic structures relative to conventional chalcogenide semiconductors. The investigation of Mn2+-induced magnetic coupling is further extended beyond 3D LHPs to zero-dimensional (0D) Mn2+-doped Cs4PbCl6 NCs. Despite sharing the same PbCl6 motif with 3D CsPbCl3, spin-induced MCD responses in this 0D lattice exhibit a contribution comparable to the intrinsic component, contrasting with the exchange-dominated behavior of the 3D counterpart. The resolved exchange component retains an absorption-like line shape associated with band-edge transitions, indicating selective coupling of Mn2+ spins to the electronic states of the isolated PbCl6 units. Finally, magneto-optical investigations of lead-free, bismuth-based halide double perovskites (HDPs) reveal the presence of magnetic exchange interactions. The relatively weak exchange interaction in HDPs reflects the role of chemical heterogeneity in shaping excitonic properties within the rock-salt lattice. The sign inversion observed in the MCD spectra upon Mn2+ doping suggests an exchange mechanism distinct from that in lead-based halide perovskites, underscoring the influence of band-edge electronic structure on magnetic coupling. Overall, the findings of this thesis demonstrate the tunability of spin-dependent interactions in halide perovskites through Mn2+ doping, establishing a framework for understanding dopant–exciton coupling across diverse perovskite lattices.