First evidence of a square-like Sn lattice on the Au2Sn surface alloy on Au(111)

We report on the structural and chemical evolution of submonolayer Sn deposited on Au(111) at room temperature, which leads to the formation of a novel square-like Sn phase at a coverage of approximately 2 / 3 ML . Low-Energy Electron Diffraction (LEED) and atomically resolved Scanning Tunneling Microscopy (STM) reveal a locally square Sn lattice with slight distortions, forming a Rec ( 7 . 7 × 3 . 85 ) superstructure. X-ray Photoelectron Spectroscopy (XPS) and valence band analysis show that this phase does not directly form on the pristine Au(111) surface but instead grows on an intermediate Au 2 Sn surface alloy. While a previous report proposed a honeycomb-like structure for this phase, its atomic arrangement remained unresolved. Our atomically resolved STM data reveal a square-like Sn lattice in the topmost layer and establish the Au 2 Sn surface alloy as a crucial intermediate layer for its formation. Additionally, our results offer insights on the temperature-dependent structural evolution of Sn on Au(111) at ≈ 2 / 3 ML coverage, highlighting the importance of the Au 2 Sn-alloy as an intermediate layer and offering pathways towards the growth of freestanding stanene atop. Our observation of a well-defined square-like Sn lattice beyond the honeycomb arrangement highlights the structural versatility of 2D Sn phases. It provides an appealing approach for exploring elemental monolayers with unconventional atomic arrangements and novel physical properties. • Epitaxial growth of Sn on Au(111) atop an Au 2 Sn-alloy layer. • First evidence of a novel square-like Sn lattice beyond the honeycomb phase. • Analysis reveals a Rec ( 7 . 7 × 3 . 85 ) superstructure of local square-like Sn. • Au 2 Sn-alloy identified as a candidate for supporting freestanding Sn growth.