posted on 2015-05-13, 00:00authored byR. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero, A. A. Balandin
Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition
temperatures of 600 and 473 K below which the material exists in the
incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave
phases, respectively. The (13)1/2 × (13)1/2 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 Γ. 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 473 to 413 K as the film thicknesses decrease from 150 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. The Raman peak at ∼154 cm−1 originates from the zone-folded phonons in the C-CDW phase. 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.