Unusual role of epilayer-substrate interactions in determining orientational relations in van der Waals epitaxy

van der Waals (vdW) heterostructures of dissimilar 2D material sheets held together by vdW interactions promise new physics and applications not necessarily associated with the constituent 2D materials. Only epitaxial growth can achieve the orientational alignment in vdW heterostructures needed to obtain certain novel phenomena. As a case study of vdW epitaxy, we experimentally find that hexagonal boron nitride strictly aligns to Cu(100), whereas crystallographically similar graphene is known to exhibit a wide spread of in-plane rotations. Theoretical investigation reveals that this stark difference occurs because the C-Cu interactions are stronger than the B-Cu and N-Cu interactions. This counterintuitive discovery sheds light on orientational relationships in vdW epitaxy and their case specificity.