Epitaxial growth and collective electronic states of monolayer 1T-NbSe₂

At the single layer limit, transition metal dichalcogenides (TMDs) can adopt two different structural variants, i.e., polymorphs, depending on the anionic environment around the metal ions: the anions arrange in trigonal prismatic fashion in the 1H polymorph, whereas in 1T the arrangement is octahedral. While bulk 1T NbSe₂ doesn’t exist in nature, here we show that single layer 1T NbSe₂ polymorph can be grown by molecular beam epitaxy on epitaxial graphene/SiC(0001) substrates. A (Ö13xÖ13) Star-of-David charge density waves is observed by in situ scanning tunnelling microscopy, which persists above room temperature. A gap of 0.50 eV are further observed by tunnelling spectroscopy and angle resolved photoemission spectroscopy, indicating that this monolayer 1T phase of NbSe₂ is also a Mott insulator, similar to that of bulk 1T TaS₂. Our findings indicate that the presence of epitaxial constraints can generate structural configurations that are prohibited in fully-bonded TMD crystals. These findings and their implication on the collective electronic states of single layer 1T-NbSe₂ will be discussed at the meeting.