Giant Narrow-Band Optical Absorption and Distinctive Excitonic Structures of Monolayer C3 N and C3 B

Low-dimensional materials provide a unique platform for exploring exotic properties that are otherwise unachievable in bulk solids. C3N and C3B are two graphene-derived two-dimensional (2D) ordered alloys that have attracted increasing research attention. These materials are best known for their remarkable stability and moderate band gaps, and thus, are suitable for a range of applications. Perhaps the most interesting feature of the electronic structures of C3N and C3B is the existence of nearly parallel valence and conduction bands across a large region of the Brillouin zone. In this work, using many-body perturbation theory within the GW and Bethe-Salpeter-equation approach, we predict that the primarily pz-orbital-derived nearly parallel valence and conduction bands in monolayer C3N and C3B give rise to a giant narrow-band absorption peak in their optical absorption spectra. More surprisingly, two degenerate excitonic states contribute to over 90% and 80% of the dipole absorption below 5 eV for C3N and C3B, respectively. Detailed examinations of the exciton-binding energies unveil a unique shell-like distribution of the excitonic states, with each shell (series) converging to a different excitation edge. Such distinctive absorption properties are not observed in any other 2D materials. We further investigate the internal structure of the excitonic states using a multifaceted approach and reveal several important characteristics of the excitonic states in these 2D materials.