  Probing phonons, quasi-particle excitations and their coupling has enriched
our understanding of these 2D materials and proved to be crucial for developing
their potential applications. Here, we report comprehensive temperature, 4-330
K, and polarization-dependent Raman measurements on mono and bilayer MoSe2.
Phonon's modes up to fourth-order are observed including forbidden Raman and IR
modes, understood considering Frohlich mechanism of exciton-phonon coupling.
Most notably, anomalous variations in the phonon linewidths with temperature
pointed at the significant role of electron-phonon coupling in these systems,
especially for the out-of-plane (A1g) and shear mode (E22g), which is found to
be more prominent in the narrow-gaped bilayer than the large gapped monolayer.
Via polarization-dependent measurements, we deciphered the ambiguity in
symmetry assignments, especially to the peaks around ~ 170 cm-1 and ~ 350 cm-1.
Temperature-dependent thermal expansion coefficient, an important parameter for
the device performance, is carefully extracted for both mono and bilayer by
monitoring the temperature-dependence of the real-part of the phonon
self-energy parameter. Our temperature-dependent in-depth Raman studies provide
a pave for uncovering the deeper role of phonons in these 2D layered materials
from a fundamental as well as application point of view.
