Mirror twin boundaries in WSe₂ induced by vanadium doping

Mirror twin boundaries (MTBs) observed in MoSe₂ are formed due to incorporation of excess Mo into the lattice. In contrast, MTBs in WSe₂ have a high formation energy and consequently are not present in this system. Here we show that V-doping of WSe₂, achieved by co-deposition of V and W during molecular beam epitaxy (MBE) growth of WSe₂, can also induce MTB formation in WSe₂, as revealed by scanning tunneling microscopy. Our experimental results are supported by density functional theory calculations that show that V-doped WSe₂ is susceptible to the incorporation of more V-atoms at interstitial sites. This increases the transition metal atom concentration in the lattice, and these excess atoms rearrange into MTBs, which is associated with energy lowering of the excess metal atoms. While formation of MTBs gives rise to the pinning of the Fermi-level and thus prevents V-induced electronic doping, MTBs do not appear to affect the magnetic properties, and a diluted ferromagnetic material is observed for low V-doping levels, as reported previously for V-doped WSe₂.