Symmetry induced phonon renormalization in few layers of 2H-MoTe transistors: Raman and first-principles studies.

Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe channel based field-effect transistors. While the [Formula: see text] and B phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 10/cm, A shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B modes as compared with the A mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS and those observed here in a few-layered MoTe demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.