2 research outputs found
Zone-Folded Phonons and the Commensurate–Incommensurate Charge-Density-Wave Transition in 1<i>T</i>‑TaSe<sub>2</sub> Thin Films
Bulk 1<i>T</i>-TaSe<sub>2</sub> 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)<sup>1/2</sup> × (13)<sup>1/2</sup> 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 <i>ab initio</i> 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<sup>−1</sup> 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 1<i>T</i>-TaSe<sub>2</sub>
Thermal Conductivity of Graphene Laminate
We have investigated
thermal conductivity of graphene laminate films deposited on polyethylene
terephthalate substrates. Two types of graphene laminate were studied,
as deposited and compressed, in order to determine the physical parameters
affecting the heat conduction the most. The measurements were performed
using the optothermal Raman technique and a set of suspended samples
with the graphene laminate thickness from 9 to 44 μm. The thermal
conductivity of graphene laminate was found to be in the range from
40 to 90 W/mK at room temperature. It was found unexpectedly that
the average size and the alignment of graphene flakes are more important
parameters defining the heat conduction than the mass density of the
graphene laminate. The thermal conductivity scales up linearly with
the average graphene flake size in both uncompressed and compressed
laminates. The compressed laminates have higher thermal conductivity
for the same average flake size owing to better flake alignment. Coating
plastic materials with thin graphene laminate films that have up to
600× higher thermal conductivity than plastics may have important
practical implications