Phonon softening mechanisms in power-dependent Raman spectroscopy of low-dimensional materials: The effects of thermal conductivity, absorption coefficients, and defects

Power-dependent Raman spectroscopy has been regularly used in material science, especially in low-dimensional materials. The origin of Raman mode frequency changes with increased laser power has not yet been generalized. So far, the phonon mode response to increased laser power has been discussed in the context of a rise in the local temperature due to small particle size and reduced thermal conductivity. Here, we established the origin of phonon mode softening (redshift) in power-dependent Raman spectroscopy. We performed a comprehensive Raman analysis on materials with large variations in thermal conductivity, including Si, Ge, GaAs, diamond, sapphire (Al2O3), ZrO2, SnO2, and ZnO. We proved that thermal conductivity had a negligible role in power-dependent Raman spectroscopy. We exclusively observed phonon shifts with increased laser power in particles of micrometer and nanoscale dimensions with above-bandgap excitations. We found that grain boundaries, external surfaces, and particle size (mass) play a key role in the softening of phonon modes with increased laser power. However, these factors were ineffective in the case of below-bandgap excitations. In addition, we found that defects, which have not been considered until now, play a crucial role in power-dependent Raman spectroscopy.