Fabrication and Surface Engineering of Two-Dimensional SnS Toward Piezoelectric Nanogenerator Application

Abstract

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 SnO_x, 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 SnO_x works as a passivation layer for SnS. Self-passivated SnS monolayer can be obtained by a controlled post-oxidation.