Scanning tunneling microscopy study of monolayer NbSe₂

Two-dimensional transition metal dichalcogenides (TMD) have received significant interest due to their unique structural and electrical properties. Within the family of TMD materials, NbSe₂ has a 2H phase which is metallic in nature and interestingly exhibits a charge density wave and superconducting phase transition. Naturally occurring bulk NbSe₂ is a 2H phase, however, recently a 1T phase of NbSe₂ was grown by MBE. Intriguingly, this 1T phase exhibits an energy gap and insulating behavior at low temperatures. In this work, we study MBE grown monolayer NbSe₂ using scanning tunneling microscopy and spectroscopy (STM/S). We report both 2H and 1T phases are present in the growth, and whose relative abundance suggests a growth temperature dependence. We further report comparative studies of the electronic properties of both 2H and 1T phases using STM/S.