Quantum and light absorption properties of bilayer g-C₃N₄ with large commensurate twist angles

Twisted bilayer graphitic carbon nitride ( tb - g CN) has emerged as a promising platform for electronic and energy applications, yet its fundamental properties remain underexplored. Using first-principles density functional theory, we systematically investigate the structural, electronic, and optical responses of tb-gCN at large commensurate twist angles 21.8°, 27.8°, and 38.2°. The results reveal that twisting reduces the interlayer distance and slightly strengthen interlayer binding, while simultaneously widening the band gap compared to the AA-stacked bilayer (0°). The electronic structure exhibits suppressed band dispersion and flat-band formation, accompanied by charge localization around the moiré regions, features that point to the possibility of correlated electron phenomena. Orbital-resolved analysis identifies N-2 p _x , N-2- p _y states as the dominant contributors to the valence band maximum and C-2 p _z orbitals to the conduction band minimum, underscoring orbital-selective charge transport. Furthermore, the stacking introduces a red shift of the absorption edge and broadens the optical absorption into the visible range, thereby enhancing light-harvesting capability. The established twist engineering shows to be an effective strategy to tune both the semiconducting and optoelectronic properties of g -C₃N₄, offering new opportunities for next-generation photocatalysis, photovoltaics, and correlated quantum phases in two-dimensional materials.