Chemical Functionalization of Silicene: Spontaneous Structural Transition and Exotic Electronic Properties

The use of newly discovered silicene for various optoelectronic applications depends largely on the possibility of controlling its electronic properties by chemical functionalization. To investigate this possibility, we systemically study the structural and electronic properties of chemically functionalized silicene by employing first-principles calculations combined with the cluster expansion approach. Interestingly, we find that chemically functionalized epitaxial silicene is generally accompanied by a spontaneous structural transition, which originates from the preference of $s{p}^{3}$ hybridization of silicon. To realized continuously tunable band gaps, chemical functionalization of freestanding silicene at $\ensuremath{\sim}900\text{ }\text{ }\mathrm{K}$ is proposed. Finally, we predict that metastable silicene can also be used as an important host material to produce novel functional materials via substitutional doping. For example, the discovered ordered ${\mathrm{Si}}_{8}{\mathrm{P}}_{4}$ could be a strong candidate for thin-film solar cell absorbers beyond bulk Si.