Research on Thermoelectric and Optoelectronic Properties of Two-Dimensional VSi₂N₄, VGe₂N₄, and Janus VSiGeN₄

Two-dimensional magnetic semiconductors have recently attracted considerable attention owing to their potential for integrating charge, spin, and optical functionalities in next-generation nanoelectronics and spintronic devices. However, achieving efficient and controllable spin-polarized transport, particularly beyond conventional thermoelectric mechanisms, remains a major challenge, and the role of symmetry breaking and spin-selective optical transitions is still not fully understood. This work researches the structural stability, electronic properties, magnetic characteristics, thermoelectric, and optoelectronic performance of VSi₂N₄, VGe₂N₄, and Janus VSiGeN₄ through first-principles calculations combined with the nonequilibrium Green's functional. Calculations indicate that all three are dynamically and thermodynamically stable narrow-bandgap semiconductors and exhibit spin-orbit coupling-induced splitting. Magnetic calculations reveal that VSi₂N₄ exhibits strong ferromagnetic coupling with an X-direction easy magnetization axis, while Ge substitution shifts the easy magnetization axis of VGe₂N₄ to the Z-direction and weakens the exchange interaction. The Janus VSiGeN₄ maintains a Z-direction easy magnetization axis while partially retaining strong ferromagnetic coupling. Thermoelectric analysis indicates that VGe₂N₄ exhibits the strongest dimensionless figure of merit. Furthermore, optoelectronic analyses reveal strong spin-selective photoresponses in all three systems, enabling the generation of purely spin-polarized photocurrents, a feature that is unattainable in conventional thermoelectric effects. Specifically, VSi₂N₄ and VGe₂N₄ exhibit photocurrent peaks at 1.3 eV photon energy under 180° linearly polarized light, while VSiGeN₄ shows the highest photocurrent response (18.7 mA/W) at 1.7 eV under 90° polarization due to symmetry breaking. This work provides a theoretical foundation for designing multifunctional two-dimensional spintronic and optoelectronic devices.