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

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_1g¹ and A_1g² 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_1g¹ always enhancing more than the A_1g², independently of the number of GaS layers. For five layers, the results show an L_c of 64 and 47 nm for A_1g¹ and A_1g², respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.