Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN<sub>2</sub>: High-Pressure Route to Pentagonal 2D Materials
Maxim Bykov, Elena Bykova, Alena V. Ponomareva, Ferenc Tasnádi, Stella Chariton, Vitali B. Prakapenka, Konstantin Glazyrin, Jesse S. Smith, Mohammad F. Mahmood, Igor A. Abrikosov, Alexander F. Goncharov
Abstract
Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m2, which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap.