First-Principles Investigation of Janus Penta-PdXY (X = S, Se, Te; Y = O, Se, Te; X ≠ Y) Monolayers with High Piezoelectricity and Tunable Electronic Properties for Flexible Piezoelectric Sensors

Janus pentagonal two-dimensional (2D) materials combine intrinsic polarization with a low-symmetry geometry, offering unique opportunities for high-performance piezoelectric and flexible nanoelectronic devices. Here, we propose a family of 2D Janus penta-PdXY monolayers (X = S, Se, Te; Y = O, Se, Te; X ≠ Y) based on first-principles calculations. The pentagonal configuration imparts pronounced anisotropy and flexibility, with reduced Young’s modulus (down to 27.97 N/m) and bending rigidity (15.53 N/m). Janus-induced charge asymmetry generates strong spontaneous polarization, leading to outstanding in-plane piezoelectric coefficients (up to 141.81 pm/V) and excellent out-of-plane responses (22.83 pm/V), surpassing those of most known 2D and bulk piezoelectrics. The interplay between interatomic distances and electronegativity differences governs these exceptional properties. Combined with high carrier mobility (up to 3480.10 cm 2 /V·s), Janus penta-PdXY monolayers emerge as promising candidates for next-generation flexible and high-performance piezoelectric devices.