Dimension engineering of single-layer PtN2 with the Cairo tessellation

Abstract

Single-layer PtN2 exhibits an intriguing structure consisting of a tessellation pattern called the Cairo tessellation of type 2 pentagons, which belongs to one of the existing 15 types of convex pentagons discovered so far that can monohedrally tile a plane. Single-layer PtN2 has also been predicted to show semiconducting behavior with direct bandgaps. Full exploration of the structure–property relationship awaits the successful exfoliation or synthesis of this novel single-layer material, which depends on the structure of its bulk counterpart with the same stoichiometry to some extent. Bulk PtN2 with the pyrite structure is commonly regarded as the most stable structure in the literature. But comparing the energies of single-layer PtN2 and bulk PtN2 leads to a dilemma that a single-layer material is more stable than its bulk counterpart. To solve this dilemma, we propose stacking single-layer PtN2 sheets infinitely to form a new bulk structure of PtN2. The resulting tetragonal layered structure is energetically more stable than the pyrite structure and single-layer PtN2. We also find that the predicted bulk structure is metallic, in contrast to the semiconducting pyrite structure. In addition to predicting the 3D structure, we explore the possibility of rolling single-layer PtN2 sheets into nanotubes. The required energies are comparable to those needed to form carbon or boron nitride nanotubes from their single-layer sheets, implying the feasibility of obtaining PtN2 nanotubes. We finally study the electronic structures of PtN2 nanotubes and find that the bandgaps of PtN2 nanotubes are tunable by changing the number of unit cells of single-layer PtN2 used to construct the nanotubes. Our work shows that dimension engineering of PtN2 not only leads to a more stable 3D structure but also to 1D materials with novel properties.