  Silicene, the silicon equivalent of graphene, is attracting increasing
scientific and technological attention in view of the exploitation of its
exotic electronic properties. This novel material has been theoretically
predicted to exist as a free-standing layer in a low-buckled, stable form, and
can be synthesized by the deposition of Si on appropriate crystalline
substrates. By employing low-energy electron diffraction and microscopy, we
have studied the growth of Si on Ag(111) and observed a rich variety of
rotationally non-equivalent silicene structures. Our results highlight a very
complex formation diagram, reflecting the coexistence of different and nearly
degenerate silicene phases, whose relative abundance can be controlled by
varying the Si coverage and growth temperature. At variance with other studies,
we find that the formation of single-phase silicene monolayers cannot be
achieved on Ag(111).
