Silicene Nano-Ribbons: Strong Resistance Towards Oxidation due to sp$^{2}$ Hybridization of the Si Valence Orbitals

We have synthesized for the first time silicene, that is, a new silicon allotrope analogous to graphene recently theoretically predicted [1], in the form of a massively parallel array of quantized zigzag nano-ribbons with a common ``magic'' width of 1.6 nm. They display characteristic linear band dispersions similar to the Dirac cones of graphene, in correspondence with their hexagonal arrangement seen in STM imaging [2]. Here we show, through the angle-dependence of REEL spectra taken at the Si L$_{2,3}$ edge, the typical signatures of 2p $\to \quad \pi $* and 2p $\to \quad \sigma $* transitions associated with sp$^{2}$ hybridization of the Si valence orbitals. We further show through high-resolution synchrotron radiation Si 2p core-level spectroscopy measurements that the afore mentioned silicene grating is very resistant toward oxidation. Typically, the oxygen uptake starts at about 10$^{4}$ higher doses than on the clean Si(111)7x7 surface. Indeed, this striking behavior is directly related to the sp$^{2}$ bonding, an additional confirmation of the silicene (i.e., graphene-like) nature of the nano-ribbons. \\[4pt] [1] S. Cahangorov et al., Phys. Rev. Lett., \textbf{102}, 236804 (2009). \\[0pt] [2] P. De Padova et al., Appl. Phys. Lett. \textbf{96}, 261905 (2010).