Excitonic ground states in phosphorene nanoflakes

By using a configuration-interaction approach beyond the framework of independent multiexcitons, we predict that an excitonic ground state may exist in phosphorene nanoflakes when an in-plain electric field is applied. The ground state of the system is shown to undergo a transition from purely electronic to almost fully biexcitonic with the increasing strength of the electric field. As the field exceeds 0.25 V nm^-1, a biexcitonic ground state is revealed to be energetically more favorable by a few hundred meV than the system without excitons. A similar transformation of the ground state is also found as the screening effect varies from strong to weak. The enhanced electron-hole correlation, mostly caused by the applied electric field as well as the lack of strong screening in low-dimensional nanostructures, is believed to account for such an extraordinary transition. Furthermore, the system with a biexcitonic ground state is found to exhibit an absorption spectrum where many transitions are polarized along the zigzag direction, which breaks the optical anisotropy well-known for bulk phosphorene.