Full orientation control of epitaxial MoS2 on hBN assisted by substrate defects

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 ${0}^{\ensuremath{\circ}}$ and ${180}^{\ensuremath{\circ}}$ 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 $\ensuremath{\sim}90%$ 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 $0/{180}^{\ensuremath{\circ}}$ orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.