Optical response and ultrafast carrier dynamics of the silicene-silver interface

Silicene, a two-dimensional silicon sheet with honeycomb structure analogous to graphene, may also show the presence of massless Dirac fermions similarly to the more famous two-dimensional allotrope of carbon. Silicene can be stabilized in various forms on Ag(111) surface and was recently integrated in field-effect transistors (FET) showing high mobility when Ag is withdrawn. However, strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of its peculiar electronic properties. We investigate the role of the Ag(111) metallic support in determining the physical properties of the Si/Ag interface, by means of theoretical calculations of the optical response of the supported system. Ab initio simulations based on density functional theory show that the silicene/Ag(111) absorption spectra are strongly non-additive, while the presence of the silicene layer still produces a clear signature. Individual contributions to the spectra are singled out, allowing us to quantify the role of electronic transitions involving silver and silicon states. Silver states, in particular, are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The same conclusions are derived for amorphous two-dimensional Si layers. The results point to a dimensionality-driven peculiar dielectric response of the silicon/silver interface, which is confirmed by means of measurements by Transient-Reflectance spectroscopy. These observations show a metallic-like carrier dynamics, both for silicene and ultra-thin amorphous silicon, hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.