Crossover between Silicene and Ultra-Thin Si Atomic Layers on Ag(111) Surfaces

TL;DR

This study uses density-functional theory to explore the structural and electronic properties of ultrathin silicon layers on Ag(111), revealing multiple stable silicene structures, their transitions, and the crossover to silicon surface reconstructions.

Contribution

It provides a detailed theoretical analysis of multilayer silicene structures on Ag(111), including stability, transition barriers, and electronic states, highlighting the crossover to silicon surface structures.

Findings

Multiple stable silicene structures identified

Transition barriers suggest flip-flop motion at low temperature

Surface $ m ilde{3} imes ilde{3}$ silicene exhibits a new surface state

Abstract

We report on total-energy electronic structure calculations in the density-functional theory performed for the ultra-thin atomic layers of Si on Ag(111) surfaces. We find several distinct stable silicene structures: $\sqrt{3}\times\sqrt{3}$, $3\times3$, $\sqrt{7}\times\sqrt{7}$ with the thickness of Si increasing from monolayer to quad-layer. The structural bistability and tristability of the multilayer silicene structures on Ag surfaces are obtained, where the calculated transition barriers infer the occurrence of the flip-flop motion at low temperature. The calculated STM images agree well with the experimental observations. We also find the stable existence of $2\times1$ $\pi$-bonded chain and $7\times7$ dimer-adatom-stacking fault Si(111)-surface structures on Ag(111), which clearly shows the crossover of silicene-silicon structures for the multilayer Si on Ag surfaces. We further find the absence of the Dirac states for multilayer silicene on Ag(111) due to the covalent interactions of silicene-Ag interface and Si-Si interlayer. Instead, we find a new state near Fermi level composed of $\pi$ orbitals locating on the surface layer of $\sqrt{3}\times\sqrt{3}$ multilayer silicene, which satisfies the hexagonal symmetry and exhibits the linear energy dispersion. By examining the electronic properties of $2\times1$ $\pi$-bonded chain structures, we find that the surface-related $\pi$ states of multilayer Si structures are robust on Ag surfaces.