Terrace-Mediated Growth of WS2 Using Molecular Beam Epitaxy: Impact on Lattice Orientation and Grain Boundary Formation

Abstract

Sulfur-based transition metal dichalcogenides (TMDs) such as WS2 exhibit a lattice mismatch with graphene, in contrast to the more commensurate behavior of Se-based compounds like MoSe2 and WSe2. This registry makes epitaxial WS2 on graphene, without transition metal substrates, an ideal platform for exploring the underlying interfacial phenomena. However, its controlled synthesis with molecular beam epitaxy (MBE) remains a formidable challenge. Here, we demonstrate the epitaxial growth of WS2 on highly oriented pyrolytic graphite (HOPG) and epitaxial bilayer graphene on SiC using a two-step method under ultra-high vacuum (UHV). Scanning tunneling microscopy and spectroscopy (STM/S) reveal pronounced moiré periodicities—in this work at 3 and 19.1° twist angles—along with terrace-mediated grain boundary formation at small angles. On epitaxial graphene, we demonstrate the growth of highly oriented WS2 films. These findings establish a robust strategy for epitaxial WS2 growth, opening avenues for investigating the formation mechanisms and emergent properties of lattice-mismatched two-dimensional heterostructures.