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${}_{2g}^1$ and B${}_{2g}$ phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (\textit{p}) up to $\sim$ 2.3 $\times$10${}^{13}$/cm${}^2$, A${}_{1g}$ shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the E${}_{2g}^1$ 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${}_{2g}^1$ 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.