  Recent works have revealed that van der Waals (vdW) epitaxial growth of 2D
materials on crystalline substrates, such as hexagonal boron nitride (hBN),
leads to formation of self-aligned grains, which results in defect-free
stitching between the grains. However, how the weak vdW interaction causes
strong limitation on orientation of grains is still not understood yet. In this
work, we have focused on investigation of mechanism of self-alignment of MoS2
grains in vdW epitaxial growth on hBN. Through calculation based on density
functional theory and the Lennard-Jones potential, we found that interaction
energy between MoS2 and hBN strongly depends both on size and orientation of
MoS2. We also found that, when size of MoS2 is ca. 40 nm, rotational energy
barrier can exceed ~ 1 eV, which should suppress rotation to limit orientation
of MoS2 even at growth temperature.
