Site-Selective Nitrogen Doping Modulates the Electronic and Optical Properties of Bilayer γ-Graphyne: A First Principles Study

Using first-principles DFT calculations, we investigate how nitrogen doping at two distinct sites modulates the electronic and optical properties of AB1-stacked bilayer γ-graphyne: sp-N1 at the alkyne chain and sp²-N2 at the benzene ring. Both doped structures are metastable (formation energy +0.04 meV/atom), indicating experimental feasibility. sp²-N2 doping introduces dispersive defect states near the Fermi level, narrowing the bandgap from 0.3608 eV to 0.0135 eV while retaining semiconducting character, in contrast to the near-metallic 0.0058 eV gap from sp-N1 doping. Moreover, sp²-N2 perturbation is localized to the benzene ring, preserving high electronic polarizability (ε₁(0)=19.53). Optically, sp²-N2 shifts the absorption peak from 2.65 eV to 2.43 eV, reduces reflectivity, increases transmittance, and enhances the stability of the dielectric response - leading to superior optical transparency and broadband photoresponse. Collectively, sp²-N2 offers small structural perturbation, preserved polarizability , and excellent optical stability, providing a promising doping strategy for bilayer γ-graphyne-based optoelectronic devices.