  By combining experimental techniques with ab-initio density functional theory
calculations, we describe the Si/Ag(111) two-dimensional system in terms of a
sp2-sp3 crystalline form of silicon characterized by a vertically distorted
honeycomb lattice. We show that 2D sp2-sp3 Si NSs are qualified by a prevailing
Raman peak which can be assigned to a graphene-like E2g vibrational mode and
that highly distorted superstructures are semiconductive whereas low distorted
ones behave as semimetals.
