Electronic and Excitonic Properties of MSi₂Z₄ Monolayers

Abstract MA₂Z₄ monolayers form a new class of hexagonal non‐centrosymmetric materials hosting extraordinary spin‐valley physics. While only two compounds (MoSi₂N₄ and WSi₂N₄) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two‐dimensional (2D) materials. Here, the chemical trends of band gaps and spin‐orbit splittings of bands in selected MSi₂Z₄ (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first‐principles. Effective Bethe–Salpeter‐equation‐based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g‐factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g‐factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi₂Z₄ monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin‐orbit induced bands inversion is observed in the heaviest studied compound, WSi₂Sb₄, a hallmark of its topological nature.