Growth of singly orientated MoSe2 monolayer on Al2O3(112¯0)

Singly oriented MoSe2 monolayer has been epitaxially grown on the Al2O3(112¯0) substrate using molecular beam epitaxy (MBE). Based on the evolution of the optical anisotropy of growing MoSe2 layer monitored in-situ and in real-time, the growth can be divided into two steps: small island growth and subsequent coalescence. Manipulated by the two-fold lattice symmetry of the substrate surface, MoSe2 nuclei and domains orientate themselves preferentially by aligning their zig-zag direction along the Al2O3[0001] direction. This preferential orientation maintains during coalescence leading to the formation of singly oriented monolayer. The increase of the optical anisotropy of MoSe2 throughout the island growth indicates the dominating interfacial interplay with substrate surface, whereas its decrease actuated by coalescence signifies the overtaking of the lateral interaction within MoSe2 monolayer. Our results reveal the detailed growth mechanism of the van der Waals epitaxial of single crystal 2D layer guided by the surfaces with reduced lattice symmetry.