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

Abstract

We report a combined experimental and theoretical study of the optical response of epitaxial silicene on silver. The silicene/Ag(111) ultraviolet-visible absorption spectra, which turn out to be strongly nonadditive, are analyzed in the framework of ab initio calculations. Electronic transitions involving silver states are found to provide huge contributions to the optical absorption of silicene, compatible with a strong Si-Ag hybridization. The results are independent of the specific silicene configuration and are also worked out for thin amorphous silicon. This points 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 metalliclike relaxation time, hence demonstrating the effects of the strong hybridization arising in silicene/Ag (111) Recently, the integration of silicene in field-effect transistors (FET) [1] opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional (2D) 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 [2,3]. Although recent experiments report on the linear dispersive bands [4], strong hybridization effects have been invoked as responsible for the disruption of π and π * bands in silicene superstructures on silver [5][6][7][8][9]. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility when Ag is withdrawn, points to a complex physics at the silicene-silver interface, demanding a deeper comprehension of its details on the atomic scale.The present paper aims at elucidating the role of the Ag(111) metallic support in determining the physical properties of the Si/Ag (2D) interface, by means of optical techniques combined with theoretical calculations. In particular we show that interface states built up by mixed silicon and silver wave functions have a striking impact on the optical response of the low-dimensional Si/Ag system.

Formation of silicene on silver: Strong interaction between Ag and Si

“…Optical measurements have been also performed by use of DRS during the growth of Si on Ag(111) at 479 K. In the case of 1 ML, the spectrum appears to be in reasonable agreement with recent calculated absorption performed for supported layers of silicene in strong electronic interaction with the Ag substrate, which have a metallic‐like character, but not with the one of a silicene film with no interaction with the substrate . These results indicate that, even if some of the Si‐induced phases are actually silicene, the interaction with the Ag substrate modifies strongly the expected electronic properties for free standing silicene.…”

“…A first identification in 2007 of a ‘graphitic silicon layer’ was proposed on Ag(100) , followed by the observation of ordered Si nanoribbons on Ag(110), and from 2010, several structures of Si on Ag(111) were described as silicene layers with different epitaxial orientations on the substrate . However, it has been shown recently that large interaction occurs between the silicene layer and the Ag substrate, whether it is on the (110) or on the (111) surface , leading to an hybridisation of the Ag and of the silicene states .…”

Ab initio study of the structural, electronic, magnetic, and optical properties of silicene nanoribbons

“…The most frequently investigated phase is a 4 × 4 silicene reconstruction on the Ag(111) substrate [7]. However, a strong hybridization between the silicene states and the silver ones affects both the electronic [11,12] and the optical properties [13].…”

Optical Properties of Free and Si(001)‐Adsorbed Pyrimidinic Nucleobases

“…To better understand this seeming contradiction, we have taken THY as an example and compared the anisotropy in the absorbance spectra, , for a model constituted by “free standing” molecular rows, frozen at their absorbed geometry with oxygens artificially saturated in order to simulate the presence of the Si substrate, and for adsorbed molecules on the Si(001) slab . We remark that the latter also include light absorption with excitation channels due to the Si substrate: In order to disentangle the different effects, and to recognize possible molecular contributions to the optical spectra, the spectra for adsorbed molecules have been further decomposed according to a procedure developed by some of us and consisting in separating the contributions to the absorption spectrum according to the wavefunction amplitude of the valence and conduction states integrated over Si or THY atoms. By this method we could demonstrate that peak (i) in the RAS stems from electronic states located in the Si slab rather than in the molecular region, therefore explaining the insensitivity of such feature to molecular details.…”