Compositional Engineering in CVD-Grown Bismuth Halide Perovskites for Photonics and Spin Couplings

Emerging metal halide perovskites have established themselves as a class of preeminent semiconductors for photonic studies and optoelectronic devices. The diverse chemical and structural properties in the metal halide perovskites enable the compositional tuning, facilitating the modulation of their optoelectronic performance and the emergence of exotic photonic and spin properties. A deeper understanding of the many-body interactions among the quasiparticles and the matters, modifiable through the compositional engineering strategies, benefits the performance optimization and function versatilities in metal halide perovskites. In this series of studies, we focus on the bismuth halide perovskites synthesized via chemical vapor deposition, which have exhibited intriguing and distinctive photonic features, as the templates for compositional engineering. We performed (i) the anion exchange processes in Cs₃Bi₂I₉ perovskite to produce the mixed anion variants, as well as (ii) the cation insertion reactions in Cs₃BiBr₆ perovskites to achieve the cation doped perovskites. Post-synthetic vapor phase treatments yielded the gradient distributions of the mixed anions or cation dopants within perovskite crystals. Applying the time-of-flight secondary ion mass spectrometry technic, we determined the anion and cation diffusivities during the respective ion diffusion processes, in corresponding bismuth halide perovskite crystals. (i) Through the flake thickness control and anion composition engineering, we demonstrated the optical waveguide modulation in bismuth halide perovskites. The mixed anion Cs₃Bi₂(Br/I)₉ perovskites showed more confined propagation wavelengths than Cs₃Bi₂I₉ perovskite counterparts. (ii) Through the cation insertion processes, we successfully doped iron (III) (Fe³⁺) and antimony (III) (Sb³⁺) in Cs₃BiBr₆ perovskites. The doped Fe³⁺ ions introduce the spin interactions in the nonmagnetic bismuth halide perovskites. We observed the spin-phonon couplings and magnetostriction effects by investigating the phonon behaviors in polarized temperature-dependent and magnetic field-dependent Raman spectroscopy. Furthermore, we employed cathodoluminescence to investigate electron beam-induced optical emission in the compositional engineered wide band gap bismuth bromide perovskites. High spatial-resolution hyperspectral imaging cathodoluminescence allows us to distinguish the excitonic features in Cs₃BiBr₆ and Cs₃Bi₂Br₉ perovskites, as well as to observe the self-trapping emission activated by Sb³⁺ doping. Our studies pave the way for tuning the photonic properties and spin interactions in metal halide perovskites, highlighting their potential for microscale photonic and spintronic metal halide perovskite devices through compositional optimization.