C 3 N 2 : 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-C 3 N 2 and H-C 3 N 2 , including possible pathways for their experimental synthesis. Based on first-principles calculations, the mechanical, electronic, and optical properties of T-C 3 N 2 and H-C 3 N 2 have been systematically investigated. These two architectural frameworks exhibit contrasting mechanical characteristics owing to their structural differences. Both T-C 3 N 2 and H-C 3 N 2 monolayers are predicted to be intrinsic semiconductors. Exceptionally, the stacking bilayers of T-C 3 N 2 can transform into a rare 2D nodal-line semimetal structure. The narrow bandgap (0.35 eV) of the T-C 3 N 2 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. C 3 N 2 structures possess ultrahigh anisotropic carrier mobilities (×10 4 cm 2 V -1 s -1 ) and exceptional absorption coefficients (×10 5 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.