Anomalous lattice vibration in monolayer MoS₂ induced by DUV laser: A first-principles investigation

MoS₂ monolayer, as a highly promising two-dimensional semiconducting material for electronic and optoelectronic applications, exhibits deep-ultraviolet (DUV) laser-induced anomalous lattice dynamics as revealed by Raman spectroscopy. Remarkably, not only the Raman intensity of many second-order Raman peaks but also the intensity ratio between the first-order modes E′ and $A_1^{\prime }$ exhibits a non-monotonic behavior that depends on laser energy. Moreover, there are significant inconsistencies in the literature regarding the assignments of these second-order Raman modes. In this work, we perform a thorough exploration of the anomalous lattice dynamics and conduct a renewed assignment of the numerous double resonant Raman modes of MoS₂ monolayer. At three laser energies (E_L = 2.33, 3.50, and 4.66 eV) spanning from the visible to the ultraviolet and further into the DUV region, the calculated double-resonance Raman spectra correlate reasonably well with the experimental ones in terms of both peak positions and relative intensities. We confirm that the P₁ peak at ∼ 450 cm⁻¹ represents the second-order longitudinal acoustic (2LA) overtone mode. Each of the P_i (i = 1, 2, …, 7) peaks has multiple contributions from two phonons with distinct q wavevectors. Our calculations further reveal that the DUV laser-induced anomalous lattice dynamics stems from the quantum interference effect among different Raman scattering channels.