Wafer-scale single-crystalline MoSe₂ and WSe₂ monolayers grown by molecular-beam epitaxy at low-temperature - the role of island-substrate interaction and surface steps

Ultrathin two-dimensional transition-metal dichalcogenides (TMDs) have been pursued extensively in recent years for interesting physics and application potentials. For the latter, it is essential to synthesize crystalline TMD monolayers at wafer-scale. Here, we report growth of single-crystalline MSe2 (M = Mo, W) monolayers at wafer-scale by molecular-beam epitaxy at low temperatures (200-400degree celsius) on nominally flat Au(1 1 1) substrates. The epifilms have low intrinsic defect densities of low 10(12) cm(-2). The grown films have then been exfoliated and transferred onto SiO2/Si by a wet chemical process, on which some optical measurements are performed, revealing high spatial uniformity of the samples. We also establish that MSe2 grows on Au via the van der Waals epitaxy mechanism, where a continuous film extends across the whole surface, overhangs atomic-layer steps on substrate. We identify that the growth of highly crystalline MSe2 is promoted by an enhanced interaction between Au substrate and MSe2 islands rather than by the guidance of surface steps on substrate. The latter only arrests MSe2 lateral growth if they are multilayer high.Key points:center dot MBE growth of wafer-scale highly crystalline TMD monolayers at low-temperature is achieved.center dot Island-substrate interaction is found to play a critical role in vdW epitaxy of single-crystalline TMDs on on-axis substates.center dot The TMD monolayers are of high uniformity and low intrinsic defect density.