A Temporal Decoupling Strategy for Controlled Synthesis of 2D TbOBr and Moiré Superlattices

Rare-earth terbium (Tb³⁺) ions are essential building blocks for nuclear-spin-based devices and green-light optoelectronics. Despite recent progress in two-dimensional (2D) rare-earth oxychlorides, the synthesis of 2D terbium oxybromide (TbOBr) and rare-earth-based moiré superlattices remains challenging, hindered by the extreme oxophilicity of rare-earth precursors. Here, we report the first controllable CVD synthesis of high-quality 2D TbOBr crystals via a temporal decoupling strategy. By synchronizing in situ H₂O generation with the vaporization of TbBr₃, we successfully suppressed premature precursor deactivation. The growth mode can be switched between epitaxial "lie-down" and substrate-independent "stand-on" orientations by modulating the precursor mass flux. The "stand-on" TbOBr enables polymer-free dry transfer, ensuring atomically clean interfaces. The 2D TbOBr exhibits a narrow green emission (⁵D₄ → ⁷F₅ transition, 5.0 meV at 543 nm), intrinsic high-κ dielectric behavior (κ ∼ 10.1), and exceptional ambient stability. Notably, we constructed the first rare-earth-based twisted van der Waals structures, revealing uniform square moiré superlattices covering the full range of twist angles (0°-45°). This work provides new insights into rare-earth material synthesis and establishes a robust materials platform for exploring two-dimensional rare-earth moiré physics, extending twistronics into f-electron systems.