Low-frequency Interlayer Modes of Few-layer 2H-SnS₂

We investigated interlayer phonon modes of mechanically exfoliated few-layer 2H-SnS₂ samples obtained from a SnS₂ single crystal by using low-frequency micro-Raman spectroscopy. The atomic force microscope (AFM) was used to measure the sample thickness. Raman investigations were then performed using 632.8 nm, 532 nm, 514.4 nm and 441.6 nm lasers of power lower than 100 μW in a micro-Raman setup, in which the samples were measured in a vacuum chamber to avoid photo-oxidation. The intralayer E_g and A_1g modes were resolved at ~206 cm^-1 and ~314 cm^-1, respectively. Despite the bandgap of ~2.41 eV of mono- and few-layer SnS₂, which is close to the excitation energy of the 514.4 nm laser, among the lasers that we used, the 532 nm laser provided the strongest Raman signals of A_1g mode as well as the interlayer shear (S1, S2) and breathing (B1, B2) modes. These interlayer modes appeared to red-shift as the thickness increased. The frequency dispersion of interlayer modes provides the characteristics for thickness determination of few-layer 2H-SnS₂ by Raman spectroscopy.