Seed Crystal Homogeneity Controls Lateral and Vertical Heteroepitaxy of Monolayer MoS₂ and WS₂

Heteroepitaxy between transition-metal dichalcogenide (TMDC) monolayers can fabricate atomically thin semiconductor heterojunctions without interfacial contamination, which are essential for next-generation electronics and optoelectronics. Here we report a controllable two-step chemical vapor deposition (CVD) process for lateral and vertical heteroepitaxy between monolayer WS₂ and MoS₂ on a c-cut sapphire substrate. Lateral and vertical heteroepitaxy can be selectively achieved by carefully controlling the growth of MoS₂ monolayers that are used as two-dimensional (2D) seed crystals. Using hydrogen as a carrier gas, we synthesize ultraclean MoS₂ monolayers, which enable lateral heteroepitaxial growth of monolayer WS₂ from the MoS₂ edges to create atomically coherent and sharp in-plane WS₂/MoS₂ heterojunctions. When no hydrogen is used, we obtain MoS₂ monolayers decorated with small particles along the edges, inducing vertical heteroepitaxial growth of monolayer WS₂ on top of the MoS₂ to form vertical WS₂/MoS₂ heterojunctions. Our lateral and vertical atomic layer heteroepitaxy steered by seed defect engineering opens up a new route toward atomically controlled fabrication of 2D heterojunction architectures.