Atomic-Scale Insights into Mastering Epitaxial Growth of 2D Transition Metal Dichalcogenides

The epitaxial growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as MoS₂, on sapphire substrates is a crucial method for producing large-area single-crystal films. While both step-edges and surface symmetry of the substrates have been proposed as controlling factors, the underlying fundamentals remained unclear. This paper presents our recent efforts to control the epitaxial growth of MoS₂ monolayers by manipulating the sapphire surface at the atomic scale. By growing MoS₂ on C/M-plane sapphire, we discovered that the sulfur evaporation rate dictates whether atomic-edge guided epitaxy or van der Waals epitaxy occurs. Specifically, we found that a high sulfur evaporation rate leads to the formation of S-terminated atomic edges, which inhibits edge nucleation, whereas a low sulfur evaporation rate results in O/Al-terminated atomic edges, thereby promoting edge nucleation. Specifically, a high sulfur evaporation rate creates S-terminated atomic edges that inhibit edge nucleation, while a low sulfur evaporation rate results in O/Al-terminated atomic edges that promote edge nucleation. By precisely controlling the step height to ensure a single-exposed atomic surface, we successfully synthesized 2-inch scale single-crystal MoS₂ thin films using van der Waals epitaxy. Our experiments reveal that over a 2-inch wafer, the van der Waals epitaxy mechanism allows better control of MoS₂ alignment (~99%) compared to the step-edge mechanism (<85%). Our findings highlight how atomic-level thermodynamics govern the nucleation modes of TMDCs, providing a pathway for precisely fabricating wafer-scale single-crystal 2D materials.