First-principles study of structural, electronic, thermodynamic, thermoelectric, and phonon properties of NbX2 (X=S, Se, and Te) nanosheets

A first-principles study of the structural, electronic, thermodynamic, thermoelectric, and phonon properties of NbX₂ (X = S, Se, and Te) nanosheets is presented. Density functional theory (DFT) calculations using the Quantum ESPRESSO package are employed. Structural analysis reveals a hexagonal lattice. The calculated bond lengths are 2.43 Å (NbS₂), 2.55 Å (NbSe₂), and 2.73 Å (NbTe₂), increasing with chalcogen atomic radius. All compounds exhibit intrinsic magnetism and metallic behavior. Phonon dispersion calculations reveal the presence of phonon gaps. The phonon gap values are 28.93 cm⁻¹ (NbS₂), 2.56 cm⁻¹ (NbSe₂), and 31.06 cm⁻¹ (NbTe₂). Specific heat capacities, computed within the quasi-harmonic approximation, show a decrease with increasing atomic mass and approach the Dulong-Petit limit at high temperatures. Thermoelectric analysis demonstrates high Seebeck coefficients, with maximum values around 2385 μV/K at 300 K, and promising figures of merit (ZT) approaching 1.0 for all nanosheets at various doping levels and temperatures. These results highlight the potential of NbX₂ nanosheets for applications in thermoelectric devices, energy harvesting, and nanoelectronics.