Interfacial Atomic Mechanisms of Single-Crystalline MoS₂ Epitaxy on Sapphire

The epitaxial growth of molybdenum disulfide (MoS₂) on sapphire substrates enables the formation of single-crystalline monolayer MoS₂ with exceptional material properties on a wafer scale. Despite this achievement, the underlying growth mechanisms remain a subject of debate. The epitaxial interface is critical for understanding these mechanisms, yet its exact atomic configuration has previously been unclear. In this study, a monolayer single-crystalline MoS₂ grown on a sapphire substrate is analyzed, decisively visualizing the atomic structure of the epitaxial interface and elucidating its role in epitaxial growth from an atomic perspective. The findings reveal that the interface consists of a periodic molecular MoO₃ interlayer, van der Waals epitaxially grown on a single Al-terminated sapphire surface. Additionally, it is discovered that MoO₃ coverage enhances surface interactions and introduces a unique atomic arrangement with 1-fold symmetry at the sapphire surface, thereby facilitating the unidirectional alignment of MoS₂. This discovery provides valuable insights into the growth mechanisms leading to single-crystalline MoS₂ formation, and suggests pathways for quantitatively monitoring and controlling growth dynamics, for the improvement of material quality and process repeatability, applicable for single-crystalline MoS₂ or potentially other transition metal dichalcogenides epitaxially grown on sapphire.