Local Reconstructions of Silicene Induced by Adatoms

The interaction of silicene with Si, C, H, O, and Ti atoms along with H-2, H2O, and O-2 molecules are investigated and the induced functionalities thereof are analyzed using first principles density functional theory. Si adatom initially adsorbed at the top site of silicene pushes down the Si atom underneath to form a dumbbell like structure with 3 + 1 coordination. This prediction is important for silicene research and reveals new physical phenomena related to the formation of multilayer Si, which is apparently the precursor state for the missing layered structure of silicon. We found that dumbbell structure attributes coverage-dependent electronic and magnetic properties to nonmagnetic bare silicene. Even more interesting is that silicene with dumbbells is energetically more favorable than the pristine silicene: The more dense the dumbbell coverage, the stronger is the cohesion. Incidentally, these structures appear to be intermediate between between silicene and silicon. The carbon adatom, which is initially adsorbed to the bridge position, substitutes one Si atom, if it overcomes a small energy barrier. The oxygen molecule can dissociate on the silicene surface, whereby constituent oxygen atoms oxidize silicene by forming strong bonds. By varying the concentration and decoration of carbon, hydrogen and oxygen atoms, one can tune the band gap of silicene. Through the adsorption of hydrogen or titanium adatom, silicene acquires spin-polarized state. A half-metallic ferromagnetic behavior is attained at specific uniform coverage of Ti adatom, which may function as a spin valve.