A new 2D auxetic CN2 nanostructure with high energy density and mechanical strength

C3N2: the missing part of highly stable porous graphitic carbon nitride semiconductors

Two-dimensional (2D) porous graphitic carbon nitrides (PGCNs) with semiconducting features have attracted wide attention because of built-in pores with various active sites, large surface area, and high physicochemical stability. However, only a few PGCNs have been synthesized, covering a 1.23-3.18 eV band gap. We systematically investigate two new 2D PGCN monolayers, T-C₃N₂ and H-C₃N₂, including possible pathways for their experimental synthesis. Based on first-principles calculations, the mechanical, electronic, and optical properties of T-C₃N₂ and H-C₃N₂ have been systematically investigated. These two architectural frameworks exhibit contrasting mechanical characteristics owing to their structural differences. Both T-C₃N₂ and H-C₃N₂ monolayers are predicted to be intrinsic semiconductors. Exceptionally, the stacking bilayers of T-C₃N₂ can transform into a rare 2D nodal-line semimetal structure. The narrow bandgap (0.35 eV) of the T-C₃N₂ monolayer and its extraordinary transformation in the bilayer electronic structure fill the vacancy of PGCNs as electronic devices in the middle/long wave infrared region. C₃N₂ structures possess ultrahigh anisotropic carrier mobilities (×10⁴ cm² V^-1 s^-1) and exceptional absorption coefficients (×10⁵ cm^-1) in the near-infrared and visible light regions, suggesting its possible optoelectronic applications. The findings expand the scope of 2D PGCNs and offer guides for their experimental realization.

Two dimensional monolayers TetraHex-CX2 (X = N, P, As, and Sb) with superior electronic, mechanical and optical properties

The discovery of graphene in 2004 opened a new world of two dimensional (2D) materials, stimulating the broad explorations of other novel 2D carbon structures and their derivatives in many materials fields. Although many 2D materials have been proposed theoretically, the experimental fabrication of them remains a big challenge, leading to more efforts to explore novel 2D materials with excellent properties. Here, we constructed four 2D monolayers TetraHex-CX₂ (X = N, P, As, and Sb) using first-principles calculations. These thin materials composed of tetragonal and hexagonal rings exhibit good stabilities, extraordinarily flexible mechanical properties, indirect bandgaps (≤2.30 eV except TetraHex-CN₂) with a semiconducting nature and a strong optical absorption up to 10⁵ cm^-1, showing the potential nanomechanical, nanoelectronic and optoelectronic applications. On building the structure-property relationship, we found that the Pauling electronegativity of X has an important influence on the electronic and mechanical properties of CX₂, which provides a significant understanding of the fundamental origin of materials properties and is helpful to design novel 2D materials with special properties.