  We prepare monolayers of tantalum sulfide on Au(111) by evaporation of Ta in
a reactive background of H$_2$S. Under sulfur-rich conditions, monolayers of
2H-TaS$_2$ develop, whereas under sulfur-poor conditions TaS forms, a structure
that can be derived from 2H-TaS$_2$ by removal of the bottom S layer. We
analyse the alignment of the layers with respect to the substrate and the
relation with the domains in the Au(111) herringbone reconstruction using
scanning tunneling microscopy (STM). With the help of density functional theory
(DFT) calculations we can determine the registry of the two phases with the
substrate. We develop a growth process that allows preparation of uniquely
oriented 2H-TaS$_2$ on Au(111). 2H-TaS$_2$ and TaS have a remarkably similar
in-plane lattice structure and we observe the formation of lateral
2H-TaS$_2$-TaS heterostructures with atomically well-defined and defect-free
boundaries. We observe mirror twin boundaries within 2H-TaS$_2$ along the S-
and Ta-edge.
