Stacked functionalized silicene: a powerful system to adjust the electronic structure of silicene
Herein, we employed first principle density functional periodic calculations to characterize the silicon counterpart of graphene:silicene. We found that silicene is far more reactive than graphene, very stable and strong Si–X bonds can be formed, where X = H, CH3, OH and F. The Si–F bond is the strongest one, with a binding energy of 114.9 kcal mol−1. When radicals are agglomerated, the binding energy per functional grows up to 17 kcal mol−1. The functionalization with OH radicals produces the largest alterations of the structure of silicene, due to the presence of intralayer hydrogen bonds. The covalent addition of H, CH3, OH and F to silicene enables the adjustment of its electronic structure. In effect, functionalized silicene can be a semiconductor or even exhibit metallic properties when the type and concentration of radicals are varied. The most interesting results were obtained when two layers of functionalized silicene were stacked, given that the band gaps experienced a significant reduction with respect to those computed for symmetrically and asymmetrically (Janus) functionalized monolayer silicenes. In the case of fluorine, the largest changes in the electronic structure of bilayer silicene were appreciated when at least one side of silicene was completely fluorinated. In general, the fluorinated side induces metallic properties in a large number of functionalized silicenes. In some cases which presented band gaps as large as 3.2 eV when isolated, the deposition over fluorinated silicene was able to close that gap and induce a metallic character. In addition to this, in four cases small gaps in the range of 0.1–0.6 eV were obtained for bilayer silicenes. Therefore, functionalization of silicene is a powerful method to produce stable two-dimensional silicon based nanomaterials with tunable optical band gaps.