Metallic-Substrate-Induced Renormalization of Quasiparticle Band Gaps and Excitonic Effects in Biphenylene Nanoribbons

Biphenylene networks, as well as their nanoribbons, have been fabricated successfully by a two-step dehydrofluorination polymerization method on Au(111) [Science 2021, 372, 852–856]. Their band gap measured by differential conductance shows a monotonic decrease with the increasing width of the nanoribbon. However, the metallic substrate (Au(111)) induced band gap renormalization challenges the direct comparison with the theoretical result. In this study, based on density functional theory with the many-body perturbation method, we include the additional screening from the metallic substrate and study its effects on quasiparticle electronic structure and electron–hole excitation in these one-dimensional systems. The obtained quasiparticle band gap is in good agreement with the experimental value. Significantly, compared with the pristine system, the screening effect of the substrate on the attractive interaction of electron–hole (the direct term of the kernel) decreases the exciton binding energy strongly by about 1.2 eV. Moreover, its influence on the repulsive interaction of electron–hole (the exchange term of the kernel) further increases the exciton binding energy by about 0.3 eV, leading to a final variation of 0.9 eV. For biphenylene nanoribbons with six C–C dimers, the exciton with the metallic substrate effect exhibits a transition from a bound exciton state to a resonant one. The exciton profile, on the other hand, also shows obvious renormalization in both real space and reciprocal space when the screening of the substrate is considered to be in the exchange term of the kernel. Our study has provided a deep understanding of environmental dielectric screening and its effects on electronic excited states in low-dimensional systems.