Geometrical and Electronic structures of Planar and Buckled Silicene on Ag(111)

Silicene grown on the Ag(111) surface was investigated by scanning tunneling microscopy / spectroscopy (STM / STS). Two atomic arrangements of honeycomb lattice, planar and buckled, were found. The planar silicene is consisted of Si atoms in the same height while the buckled one shows slightly twisted structure in the vertical direction. Compared to the planar silicene, the lateral distance between two neighboring atoms in the buckled silicene is reduced with about 7{\%}. Assuming that there is no change in the length of Si-Si bond, the angle between the Si-Si bond and the axis normal to the surface is 110 degrees, which is close to the bond angle of 109 degrees in the sp$^{3}$ hybridization. This might suggest that the Si-Si bonding in the buckled silicene is formed mainly by sp$^{3}$ hybridization rather than sp$^{2}$ However, our STS observations certainly showed a Dirac cone feature at the Fermi level for both types of silicene. Thus, we conclude that the electronic configuration of buckled silicene partially remains in sp$^{2}$ hybridization and the charge carriers still behave as the massless Dirac fermions.