Orientation-Controlled van der Waals Epitaxy of MoSe₂ Monolayers on Graphene by MOCVD

Large-scale growth of high-crystalline-quality two-dimensional (2D) semiconductor films is a prerequisite for next generation of electronics and optoelectronics. As a representative case, 2D transition metal dichalcogenides (TMDCs) are usually grown by a chemical vapor deposition (CVD) method based on a van der Waals (vdW) epitaxy mechanism. Commonly used silicon or Al₂O₃ (sapphire) substrates have either nanoscale roughness or step bunches, and the grown TMDC domains usually show diverse shapes, random orientation alignments with the substrates, and uncontrollable crystalline quality. These problems call for ideal substrates for the vdW epitaxial growth of 2D semiconductors. In this work, we employed graphene monolayers transferred on sapphire wafer as the substrate to investigate the vdW epitaxy of monolayer MoSe₂ using a MOCVD technique. We found that the graphene is essentially the substrate, as it shields the influence of the underlying sapphire. Compared with bare sapphire substrates, the MoSe₂ domains grown on graphene not only show an equilateral triangular shape and mirror symmetry with respect to the graphene lattice in a wide range of parameter windows, but also the domain size, nucleation density, and growth rate can be well controlled by the processing temperatures, pressures, and precursor flows. These results suggest that graphene film is a proper substrate for vdW epitaxy of 2D semiconductors. We further quantitatively investigated the interfacial forces between MoSe₂ domains and graphene using scanning probe microscopy to reveal the weak vdW interaction. Our work highlights the importance of substrates for the vdW epitaxial growth of 2D semiconductors and paves the way for substrate optimization in the 2D film growth and device integrations.