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 L _c of the A _1g ¹ and A _1g ² phonons of monolayer and few-layer gallium sulfide (GaS). The differences in L _c values are responsible for different enhancements of the A _1g modes, with A _1g ¹ always enhancing more than the A _1g ² , independently of the number of GaS layers. For five layers, the results show an L _c of 64 and 47 nm for A _1g ¹ and A _1g ² , respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.