  We report results of micro-Raman spectroscopy investigation of the
"graphene-like" mechanically exfoliated single-crystal bismuth telluride films
with the thickness ranging from a few-nm-range to bulk limit. It is found that
the optical phonon mode A1u, which is not-Raman active in bulk bismuth
telluride crystals, appears in the atomically-thin films due to
crystal-symmetry breaking. The intensity ratios of the out-of-plane A1u and A1g
modes to the in-plane Eg mode grow with decreasing film thickness. The
evolution of Raman signatures with the film thickness can be used for
identification of bismuth telluride crystals with the thickness of
few-quintuple layers, which are important for topological insulator and
thermoelectric applications.
