Forbidden and Second-Order Phonons in Raman Spectra of Single and Few-Layer MoS2 Close to C Exciton Resonance

The complex Raman spectra of single- to few-layer MoS2 obtained by different methods and substrates are analyzed and compared to bulk. Depending on the Raman process that originates the phonons, these are sensitive to different variables such as the number of layers (N), the growth method, or the substrate. The behavior of forbidden 150 and 190 cm(-1) peaks is explained by a double resonant mechanism close to C exciton assisted by intrinsic defects, and their strong dependence on N is due to electronic structure modifications. The band around 450 cm(-1), in off-resonance conditions, is found to be little sensitive to the number of layers or to the obtaining method but strongly influenced by the coupling to the substrate and by temperature; it is thus conjectured to be an excellent indicator of strain. Several peaks are found to occur solely for vapor deposition methods, indicating the presence of specific defects.