Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown two-dimensional (2D) layer, on a polar substrate, cannot adequately distinguish what are often near-degenerate $0^{\circ }$ and ${180}^{\circ }$ orientations. We employ first-principles calculations to identify a method to lift this near degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer ${\mathrm{MoS}}_2$ grown on hexagonal boron nitride, the predicted defect complex can be more stable than common ${\mathrm{MoS}}_2$ point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in experiments that achieve $\sim 90\%$ consistency in the orientation of as-grown triangular ${\mathrm{MoS}}_2$ flakes on hBN, as confirmed by aberration-corrected scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of $0/{180}^{\circ }$ orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.

• Received 21 October 2018
• Revised 4 April 2019

DOI:https://doi.org/10.1103/PhysRevB.99.155430