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 WSe 2 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 WSe 2 devices exhibit robust Shubnikov‐de Haas oscillations and quantum mobilities up to 6.6 × 10 3 cm 2 · V −1 · s −1 . Furthermore, SAFM is successfully extended to the synthesis of high‐quality MoSe 2 single crystals, demonstrating its generality. These results establish SAFM as a robust and scalable route for producing quantum‐grade TMD crystals.