Atomic structure and optical signature of silicene-like Si/Ag(110) and Si/Ag(111) from first principles

The prediction of silicene [1], a 2D honeycomb crystal of silicon analogous to graphene, lead to intensive theoretical study [2]. Experimental realization has, on the one hand, been successful: although synthesis of free-standing silicene has remained elusive, various reports of silicene growth on Ag(110), Ag(111), ZrB2, and Ir(111) have appeared, culminating in the recent demonstration of a field-effect silicene-based transistor [3]. On the other hand, many of these reports have since been questioned as the strongly hybridized nature of the Si-substrate bonding apparently destroys the key electronic property (Dirac cone) [4]. In this presentation we report first principles calculations of the structure and optical properties of silicene-like nanoribbons reported to form on Ag(110) and as 2D overlayers on Ag(111). In the case of Si/Ag(110), several structural models have been proposed that suggest a Si-stabilized missing row reconstruction based on zigzag [5] or pentamer [6] chains. Refined experimental studies using grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and surface optical spectra (reflectance anisotropy spectroscopy RAS and surface differential reflectance spectroscopy SDRS), are interpreted using density-functional theory (DFT) and confirm the correct structure as being a pentamer chain reconstruction [7], definitively ruling out silicene nanoribbon structures [8]. The case of Si/Ag(111) is complicated by the abundance of single and multilayer phases [9], and we attempt to cast light on the atomic geometry by interpreting recent experimental SDRS measurements [9,10] using ab initio DFT simulations of the full Si/Ag overlayer phase. The importance of including many-body effects in the optical spectra calculations is examined [11]. [1] G.G Guzman-Verri and L.C. Lew Van Yoon, Phys. Rev. B. 76, 075131 (2007) [2] See e.g. C. Grazianetti, E. Cinquanta and A. Molle, 2D Mater. 3, 012001 (2016) [3] L. Tao et al, Nature Nanotechnology 10 227 (2015) [4] R. Quhe et al, Sci. Rep. 4, 5476 (2014) [5] C. Hogan et al Phys. Rev. B 92, 115439 (2015). [6] J. I. Cerdá, et al, Nat. Commun. 7, 13076 (2016). [7] G. Prévot, C. Hogan, et al, Phys. Rev. Lett. 117, 276102 (2016); [8] P. De Padova, et al, APL 96, 261905 (2010); B. Aufray et al, APL 96, 183102 (2010); C. Lian and J. Ni, Physica B: Condensed Matter 407, 4695 (2012). [9] Y. Borensztein et al, Phys. Rev. B. 92, 155407 (2015) [10] Y. Borensztein, G. Prevot, L. Masson, Physical Review B 89, 245410 (2014); [11] C. Hogan, O. Pulci, D. Martin, A. Curcella, Y. Borensztein, in preparation.