Nucleation Stage for the Oriented Growth of Tantalum Sulfide Monolayers on Au(111)

We study the nucleation stage in the epitaxial growth of monolayer TaS2 as a model system for monolayer transition-metal sulfides. The growth was done under ultrahigh vacuum conditions with Au(111) as a substrate on which the metal atoms are evaporated, and the sulfur is provided from a background of H2S. 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 TaS3 using density functional theory. We propose that their unique orientation is the cause of the well-defined orientation of a complete TaS2 layer found under favorable growth conditions.