Single- and few-layer 2H-SnS₂ and 4H-SnS₂ nanosheets for high-performance photodetection

The properties of two-dimensional (2D) materials are highly dependent on their phase and thickness. Various phases exist in tin disulfide (SnS₂), resulting in promising electronic and optical properties. Hence, accurately identifying the phase and thickness of SnS₂ nanosheets is prior to their optoelectronic applications. Herein, layered 2H-SnS₂ and 4H-SnS₂ crystals were grown by chemical vapor transportation and the crystalline phase of SnS₂ was characterized by X-ray diffraction, ultralow frequency (ULF) Raman spectroscopy and high-resolution transmission electron microscope. As-grown crystals were mechanically exfoliated to single- and few-layer nanosheets, which were investigated by optical microscopy, atomic force microscopy and ULF Raman spectroscopy. Although the 2H-SnS₂ and 4H-SnS₂ nanosheets have similar optical contrast on SiO₂/Si substrates, their ULF Raman spectra obviously show different shear and breathing modes, which are highly dependent on their phases and thicknesses. Interestingly, the SnS₂ nanosheets have shown phase-dependent electrical properties. The 4H-SnS₂ nanosheet shows a current on/off ratio of 2.58 × 10⁵ and excellent photosensitivity, which are much higher than those of the 2H-SnS₂ nanosheet. Our work not only offers an accurate method for identifying single- and few-layer SnS₂ nanosheets with different phases, but also paves the way for the application of SnS₂ nanosheets in high-performance optoelectronic devices.