Mechanistic Insight into Tunable Spin Relaxation in Two-Dimensional Type-II Ligand-Perovskite Heterostructures

Two-dimensional (2D) metal-halide perovskites with spin-dependent optical properties hold great promise for spintronic and quantum applications. However, their spin lifetimes, especially for n = 1 2D perovskites, are typically limited to subpicosecond time scales due to rapid spin relaxation driven by strong spin-orbit coupling (SOC), electron-hole exchange interactions, and phonon-mediated scattering. Here, we demonstrate that type-II ligand-perovskite heterostructures overcome these constraints by reducing electron-hole wave function overlap and exciton binding energy. Compared to the type-I 2D perovskite (PEA)₂PbI₄ with a spin lifetime of 0.29 ps at room temperature, our engineered type-II systems achieve substantially extended spin lifetimes, ∼6.37 ps for (4Tm)₂PbI₄ and ∼18.47 ps for (4TCNm)₂PbI₄. In both materials, spatial charge separation across the perovskite-ligand interface mitigates the Bir-Aronov-Pikus (BAP) mechanism. Temperature- and fluence-dependent measurements reveal Elliott-Yafet (EY)-dominated spin relaxation in (4Tm)₂PbI₄, consistent with the observation of coherent phonon oscillation, whereas (4TCNm)₂PbI₄ exhibits D'yakonov-Perel (DP)-dominated spin relaxation, with weaker phonon coupling further suppressing the EY relaxation, enabling spin lifetimes up to ∼126.81 ps at 5 K. Our findings establish a structural design framework for tailoring spin dynamics in 2D perovskites, offering a promising strategy to engineering spin and optoelectronic properties via rational ligand engineering.