We prepare monolayers of tantalum sulfide on Au(111) evaporation of Ta in a reactive background of H2S. Under sulfur-rich conditions monolayers of 2H-TaS2 develop, whereas under sulfur-poor conditions TaS forms a structure that can be derived from 2H-TaS2 by removal of the bottom 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-TaS2 on Au(111). 2H-TaS2 and TaS have a remarkably similar plane lattice structure and we observe the formation of lateral 2H-TaS2-TaS heterostructures with atomically well-defined and defect-free boundaries. observe mirror twin boundaries within 2H-TaS2 along the S- and Ta-edge.

We study the nucleation stage in the epitaxial growth of monolayer TaS 2 as a model system for monolayer transition-metal sulfides. The growth was done under ultrahighvacuum conditions with Au(111) as a substrate on which the metal atoms are evaporated, and the sulfur is provided from a background of H 2 S. Using scanning tunneling microscopy, we find atomic-scale protrusions with a well-defined triangular shape that act as nuclei for the further growth of extended tantalum sulfide monolayers. We identify these protrusions as TaS 3 using density functional theory. We propose that their unique orientation is the cause of the well-defined orientation of a complete TaS 2 layer found under favorable growth conditions.