Excitons in nanoribbons derived from a monolayer biphenylene network.

The biphenylene monolayer (BPN), a recently synthesized nonbenzenoid 2D carbon allotrope, exhibits metallic electronic transport in its quasi-one-dimensional nanoribbon form. In contrast, its boron nitride analogue (BPN-BN) is a wide-band-gap insulator, offering a natural platform for in-plane heterostructure engineering. Here, we theoretically design atomically sharp in-plane BPN/BPN-BN heterojunction nanoribbons with both armchair (AC) and zigzag (ZZ) edge orientations and systematically investigate their structural stability, electronic structure, and excitonic optical response using first-principles calculations. We demonstrate that edge topology critically governs the electronic behavior of the heterojunctions. Armchair configurations exhibit moderate and tunable band gaps in the range of 1.3 eV to 1.5 eV, whereas zigzag heterojunctions display narrow-gap characteristics. Many-body effects reveal substantial exciton binding energies exceeding 0.20 eV, indicative of reduced dielectric screening in these low-dimensional systems. Remarkably, zigzag heterojunctions exhibit signatures of excitonic instability, suggesting excitonic-insulator-like behavior driven by enhanced Coulomb interactions and quasi-one-dimensional confinement. Excitonic effects meaningfully reshape the optical absorption onset, leading to strong red shifts relative to the independent-particle scenario. These findings establish BPN/BPN-BN in-plane heterojunctions as a versatile platform for edge-controlled band-gap engineering and excitonic physics in nonbenzenoid 2D nanosystems, with promising applications in nanoscale optoelectronic and quantum devices.