Excitons in bent black phosphorus nanoribbons: multiple excitonic funnels

Abstract Quasi-one-dimensional nanoribbons (NRs) are promising candidate materials for flexible optoelectronics with high device density. The optical properties of nanomaterials are strongly influenced by excitons, and it is important for flexible devices to explore how excitons behave under strain. In this work, we investigate the electronic and excitonic properties of bent black phosphorus (BP) NRs via density-functional theory and many-body Green's function methods. We find that inhomogeneous strain fields affect not only the first excitonic energy and thus the optical gap, but also the spatial distribution of the excitons. In particular, excitonic funnels can occur in bent black phosphorus nanoribbons (BPNRs), which drive higher-energy excitons toward lower energy locations. The funnels can be effectively tuned by the intensity and sign (compressed to tensile) of the strain field. Another critical factor is anisotropy: the funnel effect is much more pronounced in armchair BPNRs than in zigzag BPNRs. In addition, we find that the excitonic funnel effect in two-dimensional monolayer BP differs from that in multilayer BP films.