  The two-dimensional silicon allotrope, silicene, could spur the development
of new and original concepts in Si-based nanotechnology. Up to now silicene can
only be epitaxially synthesized on a supporting substrate such as Ag(111). Even
though the structural and electronic properties of these epitaxial silicene
layers have been intensively studied, very little is known about its
vibrational characteristics. Here, we present a detailed study of epitaxial
silicene on Ag(111) using \textit{in situ} Raman spectroscopy, which is one of
the most extensively employed experimental techniques to characterize 2D
materials, such as graphene, transition metal dichalcogenides, and black
phosphorous. The vibrational fingerprint of epitaxial silicene, in contrast to
all previous interpretations, is characterized by three distinct phonon modes
with A and E symmetries. The temperature dependent spectral evolution of these
modes demonstrates unique thermal properties of epitaxial silicene and a
significant electron-phonon coupling. These results unambiguously support the
purely two-dimensional character of epitaxial silicene up to about
$300^{\circ}C$, whereupon a 2D-to-3D phase transition takes place.
