Strain tunable electronic, optical, and photovoltaic properties of monolayer β2-SrX2Y4 (X = Al, Ga, In, Y = S, Se)

In recent years, the experimentally synthesized two-dimensional material MoSi₂N₄, known for its excellent mechanical strength and environmental stability, has attracted significant attention as a representative of the MA₂Z₄ family. However, people's understanding of some MA₂Z₄ materials is still limited. In this study, we systematically investigated the properties of monolayer β₂-SrX₂Y₄ (X=Al, Ga, In; Y=S, Se) with a seven-layer atomic configuration using first-principles calculations, focusing on their response to external strain engineering. Our results demonstrate that monolayer β₂-SrX₂Y₄ exhibits characteristics of a direct bandgap semiconductor, and the majority of these materials retain these characteristics under applied strain. However, an unexpected transition from a direct bandgap to an indirect bandgap was observed in monolayer β₂-SrAl₂S₄ under strain. Moreover, as strain changes from compressive to tensile, the imaginary part peak of the dielectric function for most β₂-SrX₂Y₄ materials shifted towards lower energies (redshifted). Notably, in their pristine structures, only β₂-SrAl₂S₄ and β₂-SrAl₂Se₄ can facilitate water splitting by crossing the oxidation and reduction potentials of water. By applying strain, we successfully enabled β₂-SrGa₂S₄ and β₂-SrIn₂S₄ to possess the capability to drive water oxidation–reduction reactions. Furthermore, we calculated the second-order elastic constants and analyzed the Grüneisen parameter and thermal expansion coefficient based on the fitted energy density and strain relationships. This study highlights the potential applications of β₂-SrX₂Y₄ in photocatalysis and optoelectronic devices and provides valuable insights for implementing biaxial strain in two-dimensional materials.