on hBN assisted by substrate defects: towards full orientation control*

Progress in growing exceptionally high-quality single crystals have long been impeded in polar 2D materials by the ubiquitous presence of inversion domain boundaries caused by what are often near-degenerate 0° and 180° orientations with respect to their substrate. For transition metal dichalcogenides (TMD), it has not yet proven possible to lift this degeneracy, even when grown on lattice-matched polar substrates. We perform a systematic structural search for a TMD/hBN heterostack system using density functional theory and hybrid functional calculations to identify a new mechanism to lift this near-degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. Orientation control is then verified in experiments that achieve ~90% consistency in the orientation of as-grown MoS₂ flakes on hBN, as confirmed by aberration-corrected scanning/transmission electron microscopy. [arXiv:1801.00487]