Characteristic excitonic absorption of MoSi2N4 and WSi2N4 monolayers

As new members of the two-dimensional materials family, MoSi2N4 and WSi2N4 exhibit unique physical properties. However, their optical properties in consideration of spin-orbit coupling (SOC) have not been discussed. In this work, the excited-state properties of MoSi2N4 and WSi2N4 monolayers are studied by means of many-body perturbation theory in combination with first-principles calculations. We find that the quasiparticle correction leads to a large band gap renormalization of more than 1 eV for MoSi2N4 and WSi2N4 monolayers. Because of the SOC, characteristic A and B excitons form with large binding energies of about 1 eV. The excitation energy difference of A and B excitons can be used to well address the spin-valley splitting. MoSi2N4 shows more abundant excitons (A & PRIME;, B & PRIME; and C excitons), turning out to be a promising candidate to explore intra- and inter-exciton transitions. The exciton wave function indicates that the low-energy excitons in MoSi2N4 and WSi2N4 monolayers are confined in the middle MoN2/WN2 layer, which is unfavorable for excitonic photocatalysis. On the other hand, the valley states based on excitons can be protected by SiN layers from both sides.