Differences in the Vibrational Spectra of MoS2 Nanoparticles and Microcrystallites, the Manifestation of Nonlinear Resonant Wave Processes

A detailed comparison of Raman (excitation of 488 and 632.8 nm) and IR spectra (ATR and diffuse scattering) was made for MoS₂ nanoparticles and microcrystallites. Under resonant excitation of 632.8 nm, a change in the ratio of the intensities of the ν₃(A_1g) and ν₁(E_1g) bands was detected by more than 10² times, as well as a consistent doubling of the size of the MoS₂ unit cell, which leads to the observation of sharp lines of acoustic vibrations TA, LA ZA, inactive vibrations, as well as the anomalous high oxidation rate of MoS₂ to MoO₃. In the IR absorption spectra, for the first time, negative polarity bands for the overtones 3ν₂ and 3ν₃ were observed, and a significantly higher intensity of the bands in the diffuse scattering spectra of nanocrystallites was found in comparison with the spectra of MoS₂ microcrystallites. The observation of the glow of vibrational overtones proves the important role of nonlinear resonant wave processes (NRWP) for the vibrational modes of MoS₂. The role of strong vibrational-electron interactions leading to the induction of new electronic states in the MoS₂ band gap, as well as the NRWP in vibrational spectroscopy of MX₂ dichalcogenides, is analyzed.