Strain Effects on Vibrational Modes of 2D Transition Metal Dichalcogenides in the 2H Structure

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are semiconducting materials that have applications in electronics, optoelectronics, spintronics, and catalysis. Strain can tune their optical and electronic properties, affecting device functionality, and also shifts phonon frequencies, an effect which can be used to characterize strain via Raman spectroscopy. To explore these issues, vibrational properties of the 2H bulk phase of six TMDs (MX₂ where M=Mo, W; X=S, Se, Te) were studied through density functional perturbation theory. We calculated vibrational modes under uniaxial strain, in plane and out of plane, and determined frequency shifts, splittings of degenerate modes, mode Grüneisen parameters, and changes in Raman and IR intensities. We compared the behavior between the six TMDs and analyzed the influence of the M and X atoms. This study helps understand anharmonicity in 2D TMDs, and provides the calibration data needed to infer strain from frequency shifts in experimental measurements such as micro-Raman or nano-infrared spectroscopy.