Strongly bound excitons in monolayer MoSi${}_2$Z${}_4$ (Z = pnictogen)

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response even at room temperature. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic MoSi${}_2$Z${}_4$ (Z = N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. On increasing the pump fluence, the exciton binding energies get renormalized, leading to a redshift-blueshift crossover. Our study shows that the MoSi${}_2$Z${}_4$ series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics.