Visualizing the atomic defects by scanning tunneling microscopy in the type-II Dirac semimetal NiTe2

Nickel ditelluride (NiTe2), a newly discovered type-II Dirac semimetal whose Dirac node lies in proximity to the Fermi level, is expected to exhibit exotic phenomena including the pressure-driven Lifshitz transition and novel superconductivity in the monolayer limit. It is known that defects are commonly seen in transition metal dichalcogenides and have remarkable impacts on their optical and electronic properties. However, the systematic study on defects in NiTe2 remains to be explored. Here, by using high-resolution scanning tunneling microscopy combined with the first-principles calculations, the structure and electronic properties of atomic defects in NiTe2 have been systematically investigated. Specifically, we identified five distinct types of atomic defects, involving the vacancy and the intercalation. Our results indicate that the metal intercalation defect is the predominant one due to its lowest formation energy, which differs substantially from the case in MoS2 and PtSe2. It is further revealed that the topological surface states are surprisingly robust against these atomic defects. Our results demonstrate that the electronic properties of NiTe2, especially its topological surface states, are very robust, which may be important for its future applications in microelectronics.