First-Principles Study of Fe Adsorption and Its Effects on the Mechanical and Electrical Properties of Monolayer and Bilayer Biphenylene Networks

Biphenylene network (BPN) is a two-dimensional carbon allotrope that exhibits promising potential for applications across a wide range of fields. In this work, we systematically investigated the adsorption characteristics of Fe atoms on monolayer and bilayer BPN using first-principles calculations. Structural optimization and adsorption energy analysis reveal that the average Fe adsorption energy on monolayer BPN generally becomes more negative with increasing Fe loading, although nonmonotonic variations occur at higher coverages. The optimized structures reveal increasingly pronounced Fe aggregation and structural rearrangements at higher coverages, indicating a transition toward three-dimensional Fe growth on the BPN surface. For bilayer BPN, the energetically preferred adsorption site for Fe atom is located at the center of the interlayer four-membered ring, with an average adsorption energy of −4.3 eV for a single Fe atom. Mechanical properties are further evaluated for pristine and Fe-decorated BPN. The results demonstrate that monolayer and bilayer BPN possess relatively high in-plane Young’s and shear moduli, indicative of excellent in-plane mechanical stability. For monolayer BPN, Fe adsorption induces nonmonotonic variations in the in-plane Young's moduli, with no systematic enhancement in stiffness as the Fe coverage increases. At several Fe coverages, the in-plane Young's moduli are reduced relative to pristine BPN. In contrast, the out-of-plane mechanical response of bilayer BPN is significantly affected by Fe incorporation. The effective out-of-plane elastic constant C 33 of pristine bilayer BPN is calculated to be 24.59 GPa, indicating relatively weak interlayer interactions and facile deformation along the out-of-plane direction. Notably, this property can be substantially enhanced by interlayer Fe adsorption, with C 33 increasing dramatically to 515.63 GPa upon an Fe/C ratio of 25 %. The transport calculations reveal pronounced anisotropy in the conductivity normalized by the relaxation time σ/τ, with markedly different values along the two in-plane crystallographic directions