Ultraflat excitonic dispersion in single layer g-C3N4

Single-layer graphitic carbon nitride (g-C3N4) is widely regarded as one of the most promising two-dimensional photocatalysts for hydrogen generation via water splitting. Despite its extensive study, limited information is available on its excitonic dispersion and velocity, critical parameters for achieving high charge mobility and efficient photogeneration. In this work, we employ many-body perturbation theory and the Bethe–Salpeter equation to provide a comprehensive description of the optical absorption and finite-momentum energy loss function for both s-triazine and tri-s-triazine structures. Our findings reveal the exciton dispersion and velocity, emphasizing the significant role of localized nitrogen lone pairs in producing remarkably flat excitonic bands with velocities that are two orders of magnitudes smaller than the typical one in two-dimensional materials and of the same order or smaller than the optical phonon frequencies in single layer g-C3N4. As the time-scale for inter-site exciton hopping is longer or similar to a phonon period, our results point to a highly non conventional exciton propagation.