The role of sulfur supersaturation in the growth of single-crystalline MoS₂ on stepped sapphire

Step-guided growth has emerged as a scalable pathway for synthesizing non-centrosymmetric 2D TMD single crystals. A key challenge arises when nucleation kinetically prefers the terraces over the step edges, thereby disrupting epitaxial alignment. Although the epitaxial interface is a critical factor, it is ultimately the thermodynamic driving forces that are essential for overcoming the kinetic barrier to step-edge nucleation; however, a fundamental understanding of these forces has been lacking. Herein, we demonstrated that the sulfur supersaturation provides a strong driving force for adatom on terrace planes to diffuse toward step edges, completely suppressing terrace nucleation and achieving near-unity unidirectional alignment in wafer-scale molybdenum disulfide (MoS₂). This thermodynamic control enables deterministic morphological evolution from 2D flakes to 1D nanoribbons. The synthesized single-crystalline MoS₂ exhibits exceptional uniformity and room-temperature mobility up to 91 cm²/V·s. This work provides valuable insights into the growth mechanisms underlying CVD-grown MoS₂ single crystals on stepped sapphire and solves the persistent challenge of step-edge decoupling during terrace-dominated nucleation, enhancing material quality and reproducibility for TMD single-crystal electronics.