Atomic-Step-Induced Screw-Dislocation-Driven Spiral Growth of SnS

The in-plane piezoelectricity or ferroelectricity of two-dimensional (2D) materials can vanish due to the appearance of inversion symmetry with increasing flake thickness, which drastically limits the development of their energy-harvesting application. Although the inversion symmetry breaking in spiral structure of 2D material may solve this problem, the control of spiral growth remains immature. Here, a novel technique to achieve high percentage of spiral SnS flakes with superior control of nucleation position is demonstrated. By introducing atomic steps on substrates, the screw dislocation can be easily formed when SnS partially grows across these steps and leads to over 90% of spiral SnS flakes grown by physical vapor deposition (PVD). Furthermore, the preference for SnS to nucleate at steps can introduce remarkable nucleation site control of spiral growth even on substrates with artificially transferred graphene atomic steps. Interestingly, it turns out that the spiral SnS structure exhibits centrosymmetric characteristic, indicating that single-spiral 2D materials with monolayer step height do not guarantee an inversion symmetry breaking structure. The high spiral flake percentage and precise control of nucleation sites in this study will facilitate future development of spiral 2D materials.