  On the basis of many-body {\it ab-initio} calculations, using single-shot
G$_0$W$_0$ method and Bethe-Salpeter equation, we study phosphorene nanoribbons
(PNRs) in the two typical zigzag and armchair directions. The electronic
structure, optical absorption, electron-hole (exciton) binding energy, exciton
exchange splitting, and exciton wave functions are calculated for different
size of PNRs. The typically strong splitting between singlet and triplet
excitonic states make PNRs favorable systems for application in optoelectronic.
Quantum confinement occurs in both kinds of PNRs, and it is stronger in the
zPNRs, as behave like quasi-zero-dimensional systems. Scaling laws are
investigated for the size-dependent behaviors of PNRs. The first bright
excitonic state in PNRs is explored in detail.

Mesoscale and Nanoscale Physics;Materials Science