  Starting from a single layer of NbS$_2$ grown on graphene by molecular beam
epitaxy, the single unit cell thick 2D materials Nb$_{5/3}$S$_3$-2D and
Nb$_2$S$_3$-2D are created using two different pathways. Either annealing under
sulfur-deficient conditions at progressively higher temperatures or deposition
of increasing amounts of Nb at elevated temperature result in phase-pure
Nb$_{5/3}$S$_3$-2D followed by Nb$_2$S$_3$-2D. The materials are characterized
by scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray
photoemission spectroscopy. The experimental assessment combined with
systematic density functional theory calculations reveals their structure. The
2D materials are covalently bound without any van der Waals gap. Their stacking
sequence and structure are at variance with expectations based on corresponding
bulk materials highlighting the importance of surface and interface effects in
structure formation.
