  Absorption and emission of pristine-like semiconducting monolayers of BN,
AlN, GaN, and InN are here systematically studied by ab-initio methods. We
calculate the absorption spectra for in-plane and out-of-plane light
polarization including quasiparticle and excitonic effects. Chemical trends
with the cation of the absorption edge and the exciton binding are discussed in
terms of the band structures. Exciton binding energies and localization radii
are explained within the Keldysh model for excitons in two dimensions. The
strong excitonic effects are due to the interplay of low dimensionality,
confinement effects, and reduced screening. We find exciton radiative lifetimes
ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room
temperature, thus making 2D nitrides, especially InN, promising materials for
light-emitting diodes and high-performance solar cells.
