Probing Spatial Phonon Correlation Length in Post-Transition Metal Monochalcogenide GaS Using Tip-Enhanced Raman Spectroscopy.

The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length of the A and A phonons of monolayer and few-layer gallium sulfide (GaS). The differences in values are responsible for different enhancements of the A modes, with A always enhancing more than the A, independently of the number of GaS layers. For five layers, the results show an of 64 and 47 nm for A and A, respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.