Symmetry induced phonon renormalization in few layers of 2H-MoTe$_2$ transistors: Raman and first-principles studies

Abstract

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 2 channel based field-effect transistors (FETs). While the E 1 2g and B 2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼ 2.3 ×10 13 /cm 2 , A 1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the E 1 2g 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 (DFT) analysis of bilayer MoTe 2 that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with E 1 2g or B 2g modes as compared with the A 1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum (VBM). The contrast between the manifestation of EPC in monolayer MoS 2 and those observed here in a few-layered MoTe 2 demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.