Fig. 2

Polarization of phonon modes in dependence of the charge carrier density. a Raman active optical phonons in MoS₂: the in-plane, polar E’ mode and the out-of-plane, homopolar A’₁ mode. b Illustration of the movement of the atoms for the LO phonon mode. The resulting electric field is indicated with red arrows. The interaction strength between the macroscopic electric field and an exciton depends on the ratio between the exciton radius and the phonon wave vector. c–e Polarization resolved Raman spectra for circularly polarized light from a 1L-MoS₂ flake in a field effect device with polymer electrolyte gate at T = 300 K. The filled curves are Lorentzian fits to the data. c Non-resonant excitation and low charge carrier density n⁰ (V_TG = −0.5 V, V_BG = −40 V). d Resonant excitation and low charge carrier density n⁰. e Resonant excitation and high charge carrier density n⁺⁺ (V_TG = 0 V, V_BG = 0 V). Asterisks mark additional Raman signatures that are visible under resonant excitation and that are subject to discussion in literature. f–h Polar-plots of the normalized amplitude of the fitted peaks shown in the panel above the respective plot versus the rotation of the quarter wave plate. The black arrows mark 0°; 0° and 90° correspond to the (σ⁺, σ⁺) and (σ⁺, σ⁻) configurations, respectively