Thermoelectric and Optical Properties of HfSi2N4 and HfGe2N4: A First-Principles Investigation

This study explores the thermoelectric and optoelectronic properties of HfSi2N4 and HfGe2N4 monolayers (ML) through first-principles calculations. Both materials exhibit excellent structural stability, as confirmed by phonon dispersion and ab initio molecular dynamics simulations. HfSi2N4 demonstrates superior power factors and higher thermal conductivity, while HfGe2N4 achieves a remarkable thermoelectric figure of merit (ZT) of 0.92 at 900 K under p-type doping, surpassing many 2D materials. The inclusion of spin-orbit coupling further enhances the thermoelectric performance, especially for HfGe2N4. The electronic properties reveal indirect bandgaps of 2.89 eV for HfSi2N4 and 2.75 eV for HfGe2N4, with strong optical absorption peaks in the visible range, making them suitable for optoelectronic applications. The materials exhibit high carrier mobility, with HfSi2N4 reaching 582 cm(2)V(-1)s(-1) and HfGe2N4 achieving an impressive 1870 cm(2)V(-1)s(-1) for holes. Thermal conductivity analysis reveals that HfGe2N4 has significantly lower values than HfSi2N4, favoring thermoelectric efficiency. The synergy of high Seebeck coefficients (S), tunable thermal conductivity, and optical properties makes these monolayers promising candidates for advanced thermoelectric devices and visible-light optoelectronics. This study provides a comprehensive comparison, offering valuable insights into their applicability in next-generation energy conversion and optoelectronic technologies.