  The boson peak is an excess in the phonon density of states relative to the
Debye Model, which occurs at frequencies below the Debye limit. It is present
in most amorphous materials and, as was recently shown, can sometimes be found
also in crystals. Here we present first experimental evidence of the boson peak
in a 2D material, namely 2D silica (SiO$_2$). The measurements were obtained by
helium atom scattering. A dispersionless boson peak is seen at $6 \pm 0.5$ meV
(1.5$ \pm$ 0.15 THz) and $-6 \pm 1.5$ meV ($-1.5 \pm 0.4$ THz), with reasonable
evidence for a double excitation at $\pm 12 \pm 2.5$ meV ($2.9 \pm 0.6$ THz).
