Optical response of epitaxial silicene on silver probed by transition reflectance spectroscopy

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of massless Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of π and π* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. The present work aims to elucidate the role of the Ag(111) metallic support in determining the physical properties of the Si/Ag two-dimensional interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out different contributions. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.