Density functional theory evaluation of the strain-induced optoelectronic and thermoelectric attributes of monolayer XO2 (X = Pd, Pt)

Abstract

Two-dimensional (2D) metal oxide (MO) monolayers (MLs) have attracted considerable research attention due to their promising thermoelectric (TE) and optoelectronic (OE) characteristics. In this study, density functional theory (DFT) calculations are performed to investigate pristine 2D XO₂ (X = Pd, Pt) MLs and their behaviour under uniform biaxial tensile and compressive strains ranging from − 10 to + 10%. Their TE performance is evaluated using the BoltzTraP code based on Boltzmann transport theory. Our findings reveal that both MLs exhibit a direct-band-gap (E_g) semiconducting nature in their pristine state. Notably, the semiconducting behaviour is preserved across the entire strain range, with no closure of E_g observed. However, at − 10% compressive strain, the nature of E_g for both MLs transitions from direct to indirect. Furthermore, the TE properties of both MLs exhibit strong strain dependence. For PdO₂, the power factor (PF) increases significantly from 709.9 in the pristine state to 5.52 × 10⁶ under 10% tensile strain. Similarly, PtO₂ shows enhanced TE performance, with a PF of 4070.59 at 1% tensile strain compared to 1430.55 in the unstrained condition. The optical analysis further reveals strong light-matter interaction, with both MLs exhibiting a high refractive index n(ω) in the low-energy region (~ 2–3 eV). These results demonstrate that strain engineering can effectively tune and enhance the optical spectra and TE response of XO₂ MLs.