Effect of Doping in Controlling the Structure, Reactivity, and Electronic Properties of Pristine and Ca(II)-Intercalated Layered Silicene

Hybrid two-dimensional (2D) materials composed of carbon- and germanium-doped silicene monolayers, denoted as Si1–xCx, Si1–xGex, and Si1–2xCxGex (0 ≤ x ≤ 1), were investigated by first-principles calculations. Their structural features can be understood on the basis of Vegard's law. The lattice parameters were found to correlate well with the arithmetic mean of the percentage doping of the individual constituents. The electronic band gap showed an interesting bell-shaped behavior for Si1–xCx with respect to doping, wherein Eg increased from 0 (for x = 0) to a maximum (for x = 0.5) and eventually decreased again to 0 (for x = 1). Clearly controlled carbon doping of the silicene monolayer was found to open up the band gaps and also provides a means of harvesting solar energy for semiconductor and photovoltaic applications. In addition to pristine monolayer silicene, we also investigated the effects of Ca(II)-intercalated multilayer silicene/germanene as available in the van der Waals mineral phases of CaSi2 and CaGe2. The stabilities and reaction energies for hydrogenation, fluorination, and mixed hydrogenation–fluorination for these Ca(II)-intercalated structures were compared with those of pristine and doped monolayer silicene. Optical absorption calculations demonstrated that doped monolayers and Ca(II)-intercalated multilayer silicene/germanene have strong photoabsorption in the visible light region. The presence of Ca(II) stabilizes the silicene/germanene layers, and the Ca(II) also affects the electronic structures of these 2D materials.