Non-linear temperature dependent Raman parametric changes: An identification of Fano intervened systems

Raman spectroscopy, since its discovery in 1928, left millions of footprints touching almost all researchers coming from multidisciplinary research areas and has established itself as an extremely important analytical tool. In recent times, it also has exhibited capabilities to get information about non-traditional physical processes in a material at microscopic levels. For example, the manifestation of temperature/thermal effect on a Raman spectrum. Conventionally termed anharmonic effect has been widely explored in various materials using Raman spectroscopy in elemental semiconductors (Si, Ge), binary materials (GaAs, Si-Ge), two-dimensional layered materials (Graphene, MoS 2, WS2), and transition metal oxides (TiO 2, Fe2O3). Anharmonic effects manifest themselves in terms of shift in Raman peak position and broadening in the Raman spectra as a consequence of change in phonon energy and lifetime respectively. A lot of studies are available for temperature dependent Raman spectra which followed the phonon annihilation theory of Balkanski, but there are some materials which do not follow the traditional anharmonic trend only, also show some nonlinear trend with temperature. Deviation from the anharmonic theory in various materials like graphene, heavily doped silicon, thin films and some complex materials raised due to various reasons such as band structure, doping concentration, thickness of the film, etc. which causes the electron–phonon interaction or inherent phase transition in the material. Temperature dependent nonlinear behavior of Raman spectra has been given a very less attention and requires a wide study. Although the materials which show divergence from Balkanski's anharmonic theory, show the predominance of electron–phonon interaction but at certain temperature anharmonic effect also take part which also needs to be explored and summarized in a perspective framework. A detailed review of available work in this less touched area has been presented here so as to give a different approach to analyze the effect of thermal perturbations on Raman line-shape. A compilation of temperature dependent Raman study from different range of materials has been presented and any observed deviation from the well-known anharmonic theory has been highlighted and possible reason for such deviation has been provided.