2D van der Waals ferroelectrics have emerged as an attractive building block with immense potential to provide multifunctionality in nanoelectronics. Although several accomplishments have been reported in ferroelectric switching for out-of-plane ferroelectrics down to the monolayer, a purely in-plane ferroelectric has not been experimentally validated at the monolayer thickness. Herein, an in-plane ferroelectricity is demonstrated for micrometersize monolayer SnS at room temperature. SnS has been commonly regarded to exhibit the odd-even effect, where the centrosymmetry breaks only in the odd-number layers to exhibit ferroelectricity. Remarkably, however, a robust room temperature ferroelectricity exists in SnS below a critical thickness of 15 layers with both an odd and even number of layers, suggesting the possibility of controlling the stacking sequence of multilayer SnS beyond the limit of ferroelectricity in the monolayer. This work will pave the way for nanoscale ferroelectric applications based on SnS as a platform for in-plane ferroelectrics.

An SnS layer with a monolayer thickness was realized with a stable SnOx passivation layer via mechanical exfoliation, followed by moderate oxygen annealing.

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.

Mechanical exfoliation is performed to fabricate ultrathin SnS layers, and chemical/thermal stability of SnS layers is discussed in comparison with GeS, toward piezoelectric nanogenerator application. Both SnS and GeS are difficult to be exfoliated under 10 nm using tape exfoliation due to strong interlayer ionic bonding by lone pair electrons in Sn or Ge atoms. Au-mediated exfoliation enables to fabricate larger-scale ultrathin SnS and GeS layers thinner than 10 nm owing to strong semi-covalent bonding between Au and S atoms, but GeS surface immediately degrades during Au etching in an oxidative KI/I2 solution. Although the surface of SnS after the Au-mediated exfoliation reveals several-nm oxide layer of SnOx, the surface morphology retains the flatness unlike the case of GeS. The SnS layers are more robust than GeS against the thermal annealing as well as the chemical treatment, suggesting that SnOx works as a passivation layer for SnS. Self-passivated SnS monolayer can be obtained by a controlled post-oxidation.

We investigate the PVD growth of monolayer SnS from two different feed powders, highly purified SnS and commercial phase-impure SnS.

No abstract

No abstract

No abstract