  Defect-free epitaxial growth of 2D materials is one of the holy grails for a
successful integration of van der Waals (vdW) materials in the semiconductor
industry. The large-area (quasi-)vdW epitaxy of layered 2D chalcogenides is
consequently carefully being researched since these materials hold very
promising properties for future nanoelectronic applications. The formation of
defects such as stacking faults like 60o twins and consequently 60o grain
boundaries is still of major concern for the defect-free epitaxial growth of 2D
chalcogenides. Although growth strategies to overcome the occurrence of these
defects are currently being considered, more fundamental understanding on the
origin of these defects at the initial stages of the growth is highly
essential. Therefore this work focuses on the understanding of 60o twin
formation in (quasi-)vdW epitaxy of 2D chalcogenides relying on systematic
molecular beam epitaxy (MBE) experiments supported by density functional theory
(DFT) calculations. The MBE experiments reveal the striking difference in 60o
twin formation between WSe2 and Bi2Se3 in both quasi-vdW heteroepitaxy and vdW
homoepitaxy, which from our DFT calculations links to the difference in
interlayer vdW coupling strength. The stronger interlayer vdW coupling in
Bi2Se3 compared to WSe2 results in a striking enhanced control on twin
formation and hence shows significantly more promise for defect-free epitaxial
integration. This interesting aspect of (quasi-)vdW epitaxy reveals that the
strength of interlayer vdW coupling is key for functional 2D materials and
opens perspectives for other vdW materials sharing strong interlayer
interactions.
