Double role of thallium in growth of Tl-Ag superstructures on Si(111): surfactant and 2D metal layer

• Thallium is used as a surfactant promoting room temperature growth of silver nanocrystals on Si(111) surface. • Shape of the silver islands can be controlled by step density of the silicon substrate. • New lamellar Moiré phase of Tl-Ag system is observed on top of the silver nanocrystals. • Thallium-promoted growth of silver crystals provides platform for study of symmetry-driven topological properties. We present a reproducible method for growing Tl-covered Ag(111) nanocrystals on Si(111) at room temperature. In our approach, thallium plays a double role as surfactant and as 2D metal layer covering the Ag nanocrystals. A pre-deposited Tl monolayer suppresses the native Si(111)-(7 × 7) reconstruction and creates a (1 × 1) surface, allowing subsequent Ag deposition to produce well-shaped, atomically flat nanocrystals with lateral dimensions up to ∼ 100 × 100 nm 2 . Scanning tunneling microscopy reveals that Tl encourages three-dimensional island growth of Ag while preserving crystalline order on Si substrates. In addition to previously known hexagonal structures of Tl-Ag surface, we identify a unique lamellar Moiré phase, probably related to anisotropic strain relaxation. These findings demonstrate that Tl-assisted growth offers a stable and accessible route for creating well-ordered Tl-Ag superstructures on Si(111), which can serve as a platform for exploration of spin-related and symmetry-driven quantum effects.