  Graphane is graphene fully hydrogenated from both sides, forming a 1x1
structure, where all C atoms are in sp3 configuration. In silicene, the Si
atoms are in a mix-sp2/sp3 configuration, it is therefore natural to imagine
silicane in analogue to graphane. However, monoatomic silicene sheet grown on
substrates generally reconstructs into different phases, and only partially
hydrogenated silicene with reconstructions had been reported before. In this
report we produce half-silicane, where one Si sublattice is fully H-saturated
and the other sublattice is intact, forming a perfect 1x1 structure. By
hydrogenating various silicene phases on Ag(111) substrate, we found that only
the (2r3x2r3)R30{\deg} phase can produce half-silicane. Interestingly, this
phase was previous considered to be a highly defective or incomplete silicene
structure. Our results indicate that the structure of (2r3x2r3)R30{\deg} phase
involves a complete silicene-1x1 lattice instead of defective fragments, and
the formation mechanism of half-silicane was discussed with the help of first
principles calculations.
