Microscopic Mechanism of Van der Waals Heteroepitaxy in the Formation of MoS/hBN Vertical Heterostructures.

Recent studies have revealed that van der Waals (vdW) heteroepitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to the formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes a strong limitation on the crystal orientation of grains is still not understood yet. In this work, we have focused on investigating the microscopic mechanism of the self-alignment of MoS grains in vdW epitaxial growth on hBN. Using the density functional theory and the Lennard-Jones potential, we found that the interlayer energy between MoS and hBN strongly depends on the size and crystal orientation of MoS. We also found that, when the size of MoS is several tens of nanometers, the rotational energy barrier can exceed ∼1 eV, which should suppress rotation to align the crystal orientation of MoS even at the growth temperature.