Optical phonons in layer crystal of As₂S₃: effect of weak interlayer interaction.

As₂S₃ is known as layer crystal with structure which is very similar to the metal chalcogenides as MoS₂ and some other. In such crystalline structure, the molecular unit is indefinitely extended in two dimensions. Center-zone optical phonons in 2D layer crystal As₂S₃ have been investigated in Raman scattering in temperature range of 4K-300K for both polarizations in the layer plane. Analysis of the symmetry of the crystal and 2D symmetry of the individual layer reveals that two symmetries predict different selection rules for optical phonons. Because crystal unit cell contains two layers, there are two times more (60) normal mode for the crystal compare to the layer (30). In the vanishing interaction between the layers, 60 modes collapse to 30 degenerated doublets. Weak interlayer interaction lifts these degeneracies with results of closely spaced doublets. Our experimental data show strong polarization dependence of Raman bands in ac plane at 134.4, 152.3, 185.5, 290.0, 309.4 and triplet 351.8, 354.1, 356.9 cm^-1. The important evidence for the dominance of layer symmetry, along with very weak interaction between the layers, provides understanding of structural motives of As₂S₃ and may predict optical/electronic properties of similar 2D materials.