Evidence for a New Family of 2-D Honeycomb Surface Reconstructions on Si(111)
A new silicene-like family of reconstructed surfaces on Si111 are discussed which appears to be a polymorph to the well know family of surface reconstructions best epitomized by the 7x7 surface. Several experimental features are discussed which lead to this new conclusion, as are several recently established limitations of density functional theory that may currently limit its ability to predict such 2-D surface structures. The atomic locations of the surface state charge densities from several Scanning Tunneling Spectroscopy studies provide the basis for this new structure which is supported by several previous measurements. The new structure appears to favor a faulted supercell honeycomb motif having a delocalized 2-D state that enables greater sp2 character. This top layer has an unusual periodic pi-orbital structure that also interdigitates with the terminal bulk 'dangling bonds' to create a 2-D pi-bonded structure. This new polymorphic structure resolves many long standing paradoxes of the 7x7 surface while its unusual bonding and structure may help better understand some silicenes as well as the behavior of other 2-D phases on 111 surfaces.