Two-dimensional (2D) honeycomb lattices
beyond graphene,
such as
germanene, appear very promising due to their outstanding electronic
properties, such as the quantum spin Hall effects. While there have
been many claims of germanene monolayers up to now, no experimental
evidence of a honeycomb structure has been provided up to now for
these grown monolayers. Using scanning tunneling microscopy (STM),
surface X-ray diffraction (SXRD), and density functional theory, we
have elucidated the Ge-induced
reconstruction on Ag(111). We demonstrate
that a powerful algorithm combining SXRD with STM allows us to solve
a giant surface reconstruction with more than a hundred atoms per
unit cell. Its extensive unit cell indeed consists of 98 2-fold or
3-fold coordinated Ge atoms, forming a periodic arrangement of pentagons,
hexagons, and heptagons, with the inclusion of six dispersed Ag atoms.
By analogy, we show that the
reconstruction obtained
by segregation
of Ge through an epitaxial Ag/Ge(111) film possesses a similar structure,
i.e., Ge pentagons/hexagons/heptagons with a few Ag atoms. Such an
organization is more stable than that of pure Ge monolayers and can
be assigned to the ground state of epitaxial germanene.