  Ab initio calculations within the density-functional theory formalism are
performed to investigate the chemical functionalization of a graphene-like
monolayer of silicon - silicene - with B, N, Al or P atoms. The structural,
electronic, magnetic and vibrational properties are reported. The most
preferable adsorption sites are found to be valley, bridge, valley and hill
site for B, N, Al and P adatoms, respectively. All the relaxed systems with
adsorbed/substituted atoms exhibit metallic behaviour with strongly bonded B,
N, Al, and P atoms accompanied by an appreciable electron transfer from
silicene to the B, N and P adatom/substituent. The Al atoms exhibit opposite
charge transfer, with n-type doping of silicene and weaker bonding. The
adatoms/substituents induce characteristic branches in the phonon spectrum of
silicene, which can be probed by Raman measurements. Using molecular dynamics
we found that the systems under study are stable up to at least T = 500 K. Our
results demonstrate that silicene has a very reactive and functionalizable
surface.
