Growth of rhombohedral-stacked single-crystal WS₂/MoS₂ vertical heterostructures

Vertical van der Waals heterostructures (vdWHs), formed through layer-by-layer stacking of two-dimensional materials, offer appealing opportunities for exploring novel physics and potential applications. However, the conventional approach of mechanical exfoliation followed by manual stacking faces significant challenges in achieving atomically clean interfaces and large size, severely limiting practical applications. Here, we proposed a comprehensive mechanism governing nucleation, orientation and stack control of vertical vdWHs during chemical vapour deposition (CVD) and demonstrated the layer-by-layer epitaxial growth of 1 cm × 1 cm sized single-crystal, rhombohedral-stacked WS₂/MoS₂ films. First-principles calculations reveal that sulphur (S) vacancies in the underlying MoS₂ layer preferentially form at step edges. These S vacancies serve as nucleation sites for the upper WS₂ layer and enhance symmetry breaking between rhombohedral- and hexagonal-stacked WS₂/MoS₂. Leveraging this mechanism, we achieved unidirectionally aligned WS₂ islands on MoS₂ substrate that seamlessly coalesce into continuous single-crystal WS₂/MoS₂ films. Systematic characterizations confirmed the single-crystal nature and rhombohedral-stacked configuration, while revealing the ferroelectric properties and self-driven photoelectric response in the resulting heterostructures. Our work establishes a fundamental mechanism for controlled growth of single-crystal vertical heterostructures, providing a robust foundation for scalable manufacturing and future industrial applications.