Structural, electronic and optical properties of PdSe<sub>2</sub> in bulk and monolayer phases: A G<sub>0</sub>W<sub>0</sub> calculation

This study presents a comprehensive first-principles investigation of the structural, electronic, and optical properties of palladium selenide (PdSe₂) in both bulk and monolayer forms using density functional theory (DFT) and many-body perturbation theory. Structural optimizations using various exchange–correlation (XC) functionals—including LDA, PBE, PBEsol, WC, and vdW-corrected DFC09x—reveal significant sensitivity of the out-of-plane lattice constant c to van der Waals (vdW) interactions. The lattice parameters obtained using vdW-DFC09x (a = 5.7416 Å, b = 5.8661 Å, c = 7.6920 Å) show excellent agreement with experimental values (a = 5.7457 Å, b = 5.8659 Å, c = 7.6946 Å). Electronic band structures calculated along high-symmetry paths confirm the indirect nature of the band gap in both phases. For bulk PdSe₂, the band gap increases from 0.03 eV (PBE+vdW) to 0.22 eV (HSE), and further to 0.41 eV when corrected with the G₀W₀ approximation, aligning well with experimental data (0.40 eV). In the monolayer phase, quantum confinement leads to larger gaps: 1.30 eV (PBE), 1.60 eV (HSE), and 2.27 eV (G₀W₀), indicating suitability for optoelectronic applications. Optical properties were assessed via dielectric function analysis using DFT+RPA, G₀W₀+RPA, and G₀W₀+BSE approaches. The first major absorption peaks appear at approximately 0.4 eV for bulk and 2.2 eV for monolayer, corresponding to their band gaps. The monolayer exhibits stronger absorption in the visible region, making it a promising candidate for photovoltaic and thermoelectric applications. Overall, the inclusion of vdW and many-body effects is essential for accurate predictions in layered materials like PdSe₂