Molecular Construction from AgGaS₂ to CuZnPS₄: Defect-Induced Second Harmonic Generation Enhancement and Cosubstitution-Driven Band Gap Enlargement

Herein, we offer a simple but crucial case for rational design and syntheses of infrared nonlinear optical (NLO) materials by employing state-of-the-art AgGaS₂ as a parent model. On the basis of inheriting structural advantages of AgGaS₂, an as-grown CuZnPS₄ crystal exhibits a sharply enlarged energy gap (3.0 eV) benefiting from the cosubstitution of Ag with lighter Cu/Zn. Remarkably, the valence electron distribution of CuZnPS₄ is optimized by bringing in cation vacancy defects, which significantly reinforce the second harmonic generation (SHG) (3 × AgGaS₂) and compensate for the adverse effect of band gap enlargement. Careful experimental and theoretical investigations illustrate that CuZnPS₄ strikes a desirable balance among a strong SHG response, good phase matchability, large band gap, high laser damage threshold, outstanding physicochemical stability, low melting point, and cost-effective but non-toxic composition, which might shed light on follow-up design and exploratory synthesis of NLO materials.