Corrugation, nanocrystallinity, and midgap states in phosphorus-modified graphitic carbon nitride

Graphitic carbon nitride (g-C₃N₄) is a two-dimensional polymeric semiconductor whose properties are strongly constrained by lattice topology. By combining experiments (electron microscopy, diffraction, spectroscopy) with density functional theory calculations, we investigate the structural and electronic changes induced by phosphorus incorporation into triazine-based g-C₃N₄ (g-CN1). Phosphorus preserves global lattice connectivity but triggers lattice relaxation and corrugation, observed experimentally as sharp structural ripples (6.5–13.4 nm) in P@g-CN1 compared with smooth sheets in pristine g-CN1. DFT calculations semi-quantitatively reproduce the corrugation period and amplitude and further reveal that phosphorus incorporation introduces weakly dispersing, localized midgap states near the Fermi level. A comparison between a planar-constrained model and a fully relaxed structure shows that these midgap states are primarily chemically driven (dominant phosphorus orbital contribution), while lattice corrugation shifts the valence and conduction band edges downward by 1.2 eV and 0.6 eV, respectively and dramatically modulate the peaks shape and intensity. The calculated midgap states act as efficient non-radiative recombination centers, providing a direct explanation for the experimentally observed photoluminescence quenching. Together, the experimental and theoretical results establish corrugation and midgap electronic states as intrinsic features of heteroatom incorporation into polymeric two-dimensional materials, with direct implications for their optoelectronic and photocatalytic functionality.