  Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition
temperatures of 600 K and 473 K below which the material exists in the
incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases,
respectively. The C-CDW reconstruction of the lattice coincides with new Raman
peaks resulting from zone-folding of phonon modes from middle regions of the
original Brillouin zone back to the Gamma point. The C-CDW transition
temperatures as a function of film thickness are determined from the evolution
of these new Raman peaks and they are found to decrease from 473K to 413K as
the film thicknesses decrease from 150 nm to 35 nm. A comparison of the Raman
data with ab initio calculations of both the normal and C-CDW phases gives a
consistent picture of the zone-folding of the phonon modes following lattice
reconstruction. In the I-CDW phase, the loss of translational symmetry
coincides with a strong suppression and broadening of the Raman peaks. The
observed change in the C-CDW transition temperature is consistent with total
energy calculations of bulk and monolayer 1T-TaSe2.
