The growth of atomically thin transition metal dichalcogenide (TMDC) films via van der Waals (vdW) epitaxy offers a promising route to overcome the stringent lattice-matching constraints of conventional heteroepitaxy. However, the substrate effect remains critical and complex, influencing the crystallinity, orientation, as well as device performance of the TMDC films. Sapphire is widely used for TMDC epitaxy due to its atomic flatness and chemical stability; a universal understanding of how its crystallographic planes determine epitaxial behavior—separating the inherent substrate effect from other processing variables—is still lacking. Here, we investigate the epitaxial growth kinetics of the MoS₂ on a-plane, m-plane, and c-plane sapphire substrates under identical metal-organic chemical vapor deposition (MOCVD) growth conditions and substrate pretreatment. Our results reveal a strong surface-guided epitaxy on c-plane sapphire, leading to highly aligned MoS₂ domains with high coverage. In contrast, weaker interfacial interactions on a-plane and m-plane sapphire result in smaller, randomly oriented domains with lower density. Importantly, first-principles calculations indicate that the c-plane sapphire substrate has the highest surface adsorption energy and interlayer charge density, demonstrating strong coupling characteristics. Furthermore, electrical characterizations further demonstrate that the MoS₂ films on c-plane sapphire exhibit outstanding electronic properties, including an average mobility of 25.8 cm²·V⁻¹·s⁻¹ and an on/off ratio exceeding 10⁵. This study elucidates the inherent influence of the sapphire’s crystallographic planes, providing general insights into substrate-guided vdW epitaxy and a reliable strategy for wafer-scale single-crystal TMDC growth.

Atomic-layer-thick two-dimensional transition metal dichalcogenides (2D TMDs) exhibit unique electronic structures with versatile phase-dependent tunability. The structural transition between trigonal (T) and hexagonal (H) phases critically modulates their properties, yet the underlying microscopic mechanisms demand rigorous elucidation through integrated experimental and theoretical investigations. Herein, we demonstrate that domain coalescence angles during growth govern the phase transition efficiency from monolayer 1T-TaS₂ to 1H-TaS₂; when adjacent domains merge at 60°, atoms at the grain boundary naturally rearrange to form 1H-phase nuclei. Density functional theory (DFT) calculations reveal that charge asymmetry at domain boundaries drives the preferential unidirectional growth of the 1H phase from these nucleation sites. By employing rapid cooling (~550 K·min^-1) to shorten the time window for phase conversion, we successfully suppressed the 1T-to-1H phase transition and synthesized large-area monolayer 1T-TaS₂ (≥180 nm × 100 nm), reducing the post-coalescence H-phase formation ratio from 75% to 24%. This study comprehensively deciphers the microscopic mechanisms of the 1T-to-1H phase transition via coupled experiments and theory, which possesses generalizability to TMDC materials, provides a reliable phase modulation strategy, and expands the methodology for precise microscopic-scale phase engineering.