Controlled growth and photoconductive properties of hexagonal SnS₂ nanoflakes with mesa-shaped atomic steps

We demonstrated the controlled growth of two-dimensional (2D) hexagonal tin disulfide (SnS₂) nanoflakes with stacked monolayer atomic steps. The morphology was similar to flat-topped and step-sided mesa plateaus or step pyramids. The SnS₂ nanoflakes were grown on mica substrates via an atmospheric-pressure chemical vapor deposition process using tin monosulfide and sulfur powder as precursors. Atomic force microscopy (AFM), electron microscopy, and Raman characterizations were performed to investigate the structural features, and a sequential layer-wise epitaxial growth mechanism was revealed. In addition, systematic Raman characterizations were performed on individual SnS₂ nanoflakes with a wide range of thicknesses (1–100 nm), indicating that the A_1g peak intensity and Raman shifts were closely related to the thickness of the SnS₂ nanoflakes. Moreover, photoconductive AFM was performed on the monolayer-stepped SnS₂ nanoflakes, revealing that the flat surface and the edges of the SnS₂ atomic steps had different electrical conductive properties and photoconductive behaviors. This is ascribed to the dangling bonds and defects at the atomic step edges, which caused a height difference of the Schottky barriers formed at the interfaces between the PtIr-coated AFM tip and the step edges or the flat surface of the SnS₂ nanoflakes. The 2D SnS₂ crystals with regular monolayer atomic steps and fast photoresponsivity are promising for novel applications in photodetectors and integrated optoelectronic circuits.