  Using full potential density functional calculations within local density
approximation (LDA), we report our investigation of the structural electronic
properties of silicene (the graphene analogue of silicon), the strips of which
has been synthesized recently on Ag(110) and Ag(100) surfaces. An assumed
planar and an optimized buckled two dimensional (2D) hexagonal structures have
been considered for comparisons of their electronic properties. Planar silicene
shows a gapless band structure analogous to the band structure of graphene with
charge carriers behaving like mass-less Dirac fermions, while the structurally
optimized buckled silicene shows a small direct energy band gap of about 25 meV
(at the K point of the hexagonal Brillouin zone) in its electronic structure
and the charge carriers in this case behave like massive Dirac fermions. The
actual band gap would be larger than this as LDA is known to underestimate the
gap. The average Fermi velocity of the Dirac fermions in silicene was estimated
at about half the value experimentally measured in graphene. These properties
of silicene are attractive for some of the applications one envisages for
graphene. Our finding of a direct band gap in silicene is something new. The
results, if verified by experiments, are expected to have huge industrial
impact in the silicon-based nano-electronics and nano-optics because of the
possible compatibility silicene with current silicon-based micro-/nano
technology.
