Double resonance Raman modes in monolayer and few-layer MoTe2

We study the second-order Raman process of mono- and few-layer MoTe2, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.