Excitons in layered BiI3: Effects of dimensionality and crystal anisotropy

We carry out a detailed theoretical study of the electronic and optical properties of bulk and monolayer bismuth triiodide (BiI3), a layered metal halide, using the ab initio GW+BSE scheme with a full spinorial formulation. We discuss in detail the effects due to the change of dimensionality and the role of spin-orbit coupling. Moreover, we compute the exciton dispersion by solving the BSE at finite momentum, also analyzing transverse and longitudinal excitons, and the L-T splitting at q≈Γ. The results provide a reference for future experimental measurements. In addition, the interplay between spin-orbit coupling and large binding energy, together with the role of quantum confinement, confirm that BiI3 is an interesting material for optoelectronic applications and show that it is a good candidate for the study of exciton dynamics.