  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 2D
layer -- on a polar substrate -- cannot adequately distinguish what are often
near-degenerate 0{\deg} and 180{\deg} 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 MoS2 grown on hexagonal boron
nitride, the predicted defect complex can be more stable than common MoS2 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 ~90% consistency in the orientation of as-grown
triangular MoS2 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{\deg} orientations of polar 2D materials on polar substrates, overcoming
a long-standing impediment to scalable synthesis of single-crystal 2D
semiconductors.

Mesoscale and Nanoscale Physics