  The effect of a solid-vacuum interface on the properties of a strongly
coupled electron-phonon system is analyzed using dynamical mean-field theory to
solve the Holstein model in a semi-infinite cubic lattice. Polaron formation is
found to occur more easily (i.e., for a weaker electron-phonon coupling) on the
surface than in the bulk. On the other hand, the metal-insulator transition
associated to the binding of polarons takes place at a unique critical strength
in the bulk and at the surface.
