Resonant and Non-resonant Raman Spectroscopy

In recent years, group six of the atomically thin two-dimensional transition metal dichalcogenides (VI-TMDCs) like MX2 (M = Mo, W and X = S, Se), have gain a tremendous attention due to their unique electronic and optoelectronic properties, strong spin-orbit coupling, spin and valley degrees of freedom, thickness dependent modulation of the electronic and vibrational properties and the existence of a zoo of excitonic quasiparticle energy states like excitons, trions and bi-excitons. The understanding of these various aspects has become pertinent from the fundamental research as well as potential applications point of view. Raman spectroscopy has been established to be a very effective, powerful, sensitive and non-destructive technique to understand the various aspects of group VI-TMDCs. Furthermore, Resonant Raman spectroscopy, a special form of Raman spectroscopy, has played a vital role in our current understanding of group VI-TMDCs and lead to the observation of conventionally/generally forbidden backscattering/infrared active phonons as well as the phonons from the entire Brillouin zone, Davydov splitting, electron (exciton)-phonon coupling etc. The observation of these various aspects is not possible with the normal Raman/non-resonant Raman spectroscopy. Moreover, due to the existence of various excitonic quasiparticles energy states in these materials provide a better platform to understand the crossover between resonant and non-resonant conditions with different external perturbations such as laser excitation energy, thickness of the sample, temperature, pressure, strain etc. In this chapter we will discuss these various aspects of resonant and non-resonant Raman scattering and their remarkable uses to understand the various intriguing properties of group VI-TMDCs.