Circularly Polarized Raman Spectroscopy of Interlayer Modes in Few-layer SnSe₂

The interlayer modes in few-layer SnSe₂ were investigated by using circularly polarized micro-Raman spectroscopy. Six different excitation energies of 1.96, 2.33, 2.41, 2.54, 2.71 and 2.81-eV were used, with the laser power below 100 μW. The mono- and few-layer SnSe₂ samples were kept inside a vacuum chamber (~10^–6 Torr) during the Raman measurements to minimize sample damage due to local heating and photo-oxidation. The E_g, E_g' and A_1g intralayer phonon modes were resolved at ~110 cm^–1, ~147 cm^–1 and ~185 cm^–1, respectively. The 2.41-eV excitation laser provides a strong enhancement of both the inter- and intralayer modes in few-layer SnSe₂ samples, and the interlayer force constants are estimated to be 1.71×10¹⁹ N.m^-3 (shear) and 5.1×10¹⁹ N.m^-3 (breathing). The characteristics for the sample thickness determination by using polarized micro-Raman spectroscopy are established. Finally, different polytypes in SnSe₂ were observed to have different interlayer modes in the low-frequency region.