Structural analysis of Sn on Au(111) at low coverages: towards the Au2Sn surface alloy with alternating fcc and hcp domains

We report on the structural and chemical evolution of submonolayer Sn on Au(111) up to the formation of the striped Au2Sn surface alloy. Using Low-Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM), we identify a previously unobserved hexagonal (2 × 2) -reconstruction at a Sn film thickness of ˜ 0.28 monolayers (ML). X-ray Photoelectron Spectroscopy (XPS) analysis reveals that the (2 × 2)-structure is not chemically bonded to the Au(111) substrate. With increasing Sn coverage, the (2 × 2)-reconstruction performs a structural transition into a mixed phase before forming a local (v3 × v3)R30.-reconstruction at a Sn film thickness of 0.33ML. This reconstruction is superimposed by a larger periodicity resembling the herringbone reconstruction of clean Au(111). Our XPS analysis identifies this phase as an Au2Sn-alloy. By combining high-resolution x-ray photoelectron diffraction (XPD) measurements of Au 4f and Sn 4d 4d core levels with simulations based on a genetic algorithm, we propose a structural model for the Au2Sn-supercell, revealing an unusually large unit cell with Rec(26 × v3)-periodicity. This study advances the understanding of the structural evolution of Sn surface reconstructions on Au(111) up to the formation of the Au2Sn surface alloy. Furthermore, it provides insights into the structural arrangements emerging at higher submonolayer Sn coverages on Au(111), offering potential pathways towards realizing freestanding stanene.