Is silicene stable in O 2 ? —First-principles study of O 2 dissociation and O 2 -dissociation–induced oxygen atoms adsorption on free-standing silicene

The stability of free-standing silicene in O2 is an open question. In this letter, the O2 dissociation and O2-dissociation–induced O atoms adsorption on free-standing silicene are studied by using first-principles calculations. Our results show that the O2 molecule dissociates on the free-standing silicene surface easily from both the thermodynamic and kinetic points of view, which is different from the case of graphene. The dissociation reaction is an exothermic process, and the dissociated O atoms form strong bonds with Si atoms, which lowers the energy of the system substantially. On the other hand, the dissociation reaction occurs spontaneously on the free-standing silicene without overcoming any energy barrier. Furthermore, the migration and desorption of O atoms are relatively difficult under room temperature due to the strong Si-O bonds in the O-adsorbed silicene, which is in favor of forming silicon oxides. Our results provide convictive evidence to show that free-standing silicene is unstable in O2.