**3.3 Adlayer defect**

The *adlayer defect* is another major defect observed in low-dimensional 2D h-BN. During growth, after a few layers on the metal substrate, Cu islands form structures that are observed along the grain boundaries of h-BN films. Close observation shows that grain boundaries facilitate this island's growth. From the scanning electron microscopy observation, it was found that angular islands with truncated edges were observed. The edges resemble pyramids [51]. Some other 2D materials like MoS2 and WS2 were observed [52, 53]. A similar observation was found during the growth of III-V nanostructures on metal substrates. It was revealed by an X-ray photoelectron spectroscopy (XPS) study that a metal grain boundary acts as the favorable energy site for the nucleation of III-V pyramid islands. Correspondingly, during the growth of h-BN monolayers, it was observed that the orientation of adlayers is strongly affected by the crystalline substrate facets [54–56].
