  Silicene, a sheet of silicon atoms in a honeycomb lattice, was proposed to be
a new Dirac-type electron system similar as graphene. We performed scanning
tunneling microscopy and spectroscopy studies on the atomic and electronic
properties of silicene on Ag(111). An unexpected $\sqrt{3}\times \sqrt{3}$
reconstruction was found, which is explained by an extra-buckling model.
Pronounced quasi-particle interferences (QPI) patterns, originating from both
the intervalley and intravalley scattering, were observed. From the QPI
patterns we derived a linear energy-momentum dispersion and a large Fermi
velocity, which prove the existence of Dirac Fermions in silicene.
