Theoretical Investigation of Strain-Driven Phase Transition of Pentagonal PdTe ₂ Monolayers for Second-Harmonic Generation

The metastable pentagonal PdTe 2 monolayer on a Pd(100) substrate was recently synthesized via symmetry-driven epitaxy in experiment and opened the possibilities for expandable optical applications. In this study, taking the penta-PdTe 2 monolayer as a model system, we theoretically investigate the strain-driven phase transitions between α- and γ-phases and introduce the β-phase as an intermediate by DFT calculations. The strain-induced α-β-α′ (the vertically oriented α-phase) and α-β-γ transition pathways are clearly demonstrated. The β-phase intermediation reduces the transition barriers by 33% and 25% compared to direct α-α′ and α-γ transitions, respectively. As an optical signature, the anisotropic light absorption in α-phase contrasts with isotropic absorption in β- and γ-phases. Notably, pronounced second-harmonic generation emerges in noncentrosymmetric polymorphs. Second-order nonlinear susceptibility tensor of β-phase penta-PdTe 2 monolayer exhibits 27 nonzero components and 10 independent coefficients with dominant |χ xxx (2) |, |χ xyx (2) |, and |χ xyy (2) |. For the γ-phase monolayer, six nonzero components exist with two independent coefficients |χ xyz (2) | and |χ zxy (2) |. Due to several features, such as low phase transition barrier, broadband spectral response, and enhanced second-harmonic generation, phase engineering is shown as a robust strategy for tuning the unique optical properties in pentagonal palladium-based dichalcogenide monolayers.