  We report first-principles results on the electronic structure of various
silicene structures. For planar and simply buckled silicenes, we confirm their
zero-gap nature and show a significant renormalization of their Fermi velocity
by including many-electron effects. However, the other two recently proposed
silicene structures exhibit a finite band gap, indicating that they are gapped
semiconductors instead of previously expected Dirac-fermion semimetals.
Moreover, our calculated quasiparticle gap quantitatively explains the recent
angle-resolved photoemission spectroscopy measurements. In particular, the band
gap of the latter two structures can be tuned in a wide range by applying
strain, giving hope to bipolar-devices applications.
