Electron-phonon coupling, thermal expansion coefficient, resonance effect, and phonon dynamics in high-quality CVD-grown monolayer and bilayer MoSe2

Probing phonons, quasiparticle excitations, and their coupling has enriched our understanding of two-dimensional (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 monolayer (1L) and bilayer (2L) MoSe2. Phonon modes up to the fourth order are observed, including forbidden Raman and infrared modes, and understood considering the Fröhlich mechanism of exciton-phonon coupling. Most notably, anomalous variations in the phonon linewidths with temperature point at the significant role of electron-phonon coupling in these systems, especially for the out-of-plane (A1g) and shear modes (E2g2), which is found to be more prominent in the narrow-gapped 2L than the large gapped 1L. We deciphered the ambiguity in symmetry assignments, especially to the peaks ∼170 and ∼350cm-1 via polarization-dependent measurements. Temperature-dependent thermal expansion coefficient, an important parameter for device performance, is carefully extracted for both 1L and 2L by monitoring the temperature dependence of the real part of the phonon self-energy parameter. Our temperature-dependent in-depth Raman studies pave a way for uncovering the deeper role of phonons in these 2D layered materials from a fundamental as well as application point of view.