Phase engineering in tantalum sulfide monolayers on Au(111)

Abstract We prepare monolayers of tantalum sulfide on Au(111) by 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 S layer. We analyze 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. With the help of density functional theory 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 in-plane lattice structure and we observe the formation of lateral 2H-TaS₂-TaS heterostructures with atomically well-defined and defect-free boundaries. We observe mirror twin boundaries within 2H-TaS₂ along the S- and Ta-edge.