Lattice dynamics in monolayer and few-layer SnSe2

Hexagonal tin diselenide (6Hb-SnSe2), a two-dimensional (2D) layered metal dichalcogenide from the IVA and VIA groups, has recently drawn numerous attention in 2D nano-optoelectronics. In this paper, we investigate characteristic lattice dynamics of mechanically exfoliated mono-and few-layer 6Hb-SnSe2 samples by Raman spectroscopy. Bulk SnSe2 has all four Raman active modes of low-frequency shear E-8(2) and layer-breathing A(1g)(2) modes, and high-frequency intralayer vibrational E-g(1) and A(1g)(1) modes observed around 18.9, 33.6, 107.9, and 182.1 cm(-1), respectively. From polarized Raman measurements, we find that E-g(1) mode intensity is independent of polarization configuration and increases linearly with layer number, which provides an effective approach to determine sample thickness. From low-temperature Raman measurements, E-g(1) and A(1g)(1) mode temperature coefficients of one-layer, three-layer, and bulk SnSe2 are around -0.018 and -0.014 cm(-1)/K, whereas they have almost zero values for low-frequency E-g(2) and A(1g)(2) modes of bulk SnSe2 due to different thermal responses of intralayer and interlayer vibrations. Using multiple excitation laser lines of 488, 514.5, 568, 647, and 785 nm, E-g(1) andA(1g)(1) mode intensities of bulk SnSe2 have a similar trend with weak maxima around 2.41 eV. Our work provides valuable information about SnSe2 lattice vibrations for further fundamental research and potential applications in 2D devices such as thermoelectric and infrared light detectors.