Probing the optical characteristics of MoS₂ under external electrical fields using polarized Raman spectroscopy

Raman spectroscopy effectively revealed a strain-induced electronic structure change during polarization-dependent studies on molybdenum disulfide (MoS₂). Initially, we used a polarized Raman system to obtain the polarization-dependent Raman spectra of single-, bi-, and tri-layer MoS₂ and analyzed their vibration modes. We determined that the in-plane () mode of vibration was double-degenerate whereas the out-of-plane (A _1g) mode of vibration was not. We also observed the mode to be independent of polarization, and the A _1g mode to be quadratically dependent on the cosine of the polarization angle. The experimental data agreed strongly with the theoretical models. We present the Raman spectra collected with applied top gate voltages and operational frequencies for an MoS₂ transistor. The Raman signal intensities, peak positions, and linewidths of two active modes were calculated under different applied top gate voltages and operational frequencies. The broadening of linewidth for the out-of-plane mode was a result of the strengthening of electron-phonon coupling with increasing gate voltage. Owing to the electric potential across the metal-MoS₂ interface, a dramatic frequency shift can be observed. For the AC operational mode, the Raman signals can be regarded as constant in the proper operational frequency region.