Substrate-Assisted Flux Method for Robust Synthesis of Quantum-Grade Transition Metal Dichalcogenides

High-quality two-dimensional transition metal dichalcogenides (TMDs) are crucial for exploring intrinsic electronic, optoelectronic, and quantum transport phenomena. Although flux-based growth produces ultrapure TMD crystals with low defect densities, its broader adoption is limited by long growth cycles, low yield, and extensive post-growth purification. Here, we report a substrate-assisted flux method (SAFM) that enables rapid synthesis of quantum-grade TMD single crystals with enhanced yield and no post-growth purification. By introducing sapphire into a Se-rich flux environment, SAFM enables vapor-mediated precursor delivery and vertical crystal growth under isothermal conditions, as supported by density functional theory calculations. Consequently, millimeter-scale WSe₂ single crystals are obtained with a >75% reduction in growth time compared to conventional flux methods, while simultaneously increasing the yield of sizeable single crystals by suppressing excessive nucleation. SAFM also eliminates extensive post-growth processing and preserves ultralow defect densities, as confirmed by scanning tunneling microscopy. Monolayer WSe₂ devices exhibit robust Shubnikov-de Haas oscillations and quantum mobilities up to 6.6  ×  10³ cm² · V^-1 · s^-1. Furthermore, SAFM is successfully extended to the synthesis of high-quality MoSe₂ single crystals, demonstrating its generality. These results establish SAFM as a robust and scalable route for producing quantum-grade TMD crystals.