  The interaction of silicene with Si, C, H, O, Ti atoms along with H$_2$,
H$_2$O 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 reveal new physical phenomena
related with the formation of multilayer Si, which is apparently the precursor
state for 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.
Carbon adatom, which is initially adsorbed to the bridge position, substitutes
one Si atom, if it overcomes a small energy barrier. Oxygen molecule can
dissociate on 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.
