Tunable Optoelectronic and Thermoelectric Properties of Ag/Ga-Doped PbS Surfaces: A DFT Study on Doping and Surface Engineering

Lead sulfide (PbS) is a narrow bandgap IV–VI semiconductor with important applications in infrared optoelectronics and thermoelectric energy conversion. Surface engineering and controlled doping provide effective strategies for tuning its electronic and optical properties. In this work, the structural, electronic, optical, and thermoelectric properties of bulk PbS, pristine PbS (110) surfaces, and Ga- and Ag-doped PbS (110) surfaces are systematically investigated using density functional theory within the full-potential linearized augmented plane wave framework. The calculated lattice constant of bulk PbS is 5.88 Å, which agrees well with experimental data. Electronic structure calculations show that bulk PbS exhibits a direct bandgap of 0.75 eV at the L point. The pristine PbS (110) surface shows an enlarged bandgap of 1.07 eV due to surface quantum confinement. Surface doping strongly modifies the electronic structure. Ag doping increases the bandgap to 1.24 eV through donor-like states, whereas Ga doping slightly reduces it to 1.01 eV by introducing acceptor states near the valence band. These electronic modifications lead to significant changes in optical behavior, including enhanced absorption in the visible and near-infrared regions and tunable dielectric response. Thermoelectric analysis reveals that the pristine surface exhibits a high power factor of approximately 4.0 × 10¹¹ W/m·K² at 300 K, while bulk PbS reaches a maximum value of 4.7 × 10¹¹ W/m·K² at 800 K. These results demonstrate that dopant-induced band modulation and surface engineering provide an effective approach for tuning the optoelectronic and thermoelectric performance of PbS-based nanostructures for advanced energy and photonic applications.