  Silicene monolayers grown on Ag(111) surfaces demonstrate a band gap that is
tunable by oxygen adatoms from semimetallic to semiconducting type. By using
low-temperature scanning tunneling microscopy, it is found that the adsorption
configurations and amounts of oxygen adatoms on the silicene surface are
critical for band-gap engineering, which is dominated by different buckled
structures in R13xR13, 4x4, and 2R3x2R3 silicene layers. The Si-O-Si bonds are
the most energy-favored species formed on R13xR13, 4x4, and 2R3x2R3 structures
under oxidation, which is verified by in-situ Raman spectroscopy as well as
first-principles calculations. The silicene monolayers retain their structures
when fully covered by oxygen adatoms. Our work demonstrates the feasibility of
tuning the band gap of silicene with oxygen adatoms, which, in turn, expands
the base of available two-dimensional electronic materials for devices with
properties that is hardly achieved with graphene oxide.
