Quenching of low-energy optical absorption in bilayer C3 N polytypes

In this work we provide a first-principles description of the electronic and optical properties of bilayers C3N, with different stacking motifs AB, AB′, and AA′. Starting from quasiparticle electronic band structures, we solve the Bethe-Salpeter equation (BSE) to access the excitonic properties of these bilayers. For all stacking sequences, we see strong optical absorption at energies lower than but close to that of the monolayer. Most relevant, we predict a strong quenching of the low-energy optical absorption, with negligible oscillator strengths of low-lying bound excitons. This is a unique phenomenology that does not arise in the monolayer case, nor in other common homobilayers. We explain these findings in terms of the small interband dipole matrix elements associated to the valence-conduction transitions involved in these excitons, and we discuss them in view of the different stacking motifs.