Thickness-Driven Phase Transition and Electromechanical Optimization in 2D Ferroelectric CuInP ₂ S ₆

The piezoelectric response of CuInP2S6 (CIPS) exhibits a pronounced thickness dependence, yet the microscopic mechanism behind this behavior remains unclear. Here, we uncover the structural-mechanical origin of the thickness-dependent phase transition in CIPS through combined out-of-plane mechanical characterization and nanoscale friction measurements. Contact resonance atomic force microscopy reveals a distinct softening of the out-of-plane modulus near 80–100 nm, coinciding with a triclinic-to-monoclinic structural transition, while friction force microscopy shows reduced shear resistance, evidencing Cu+-ion-mediated lattice rearrangement across layers. These mechanical signatures uncover the coupling between ionic dynamics and the interlayer mechanics governing the phase transition. By correlating the thickness-dependent mechanical compliance with the piezoelectric coefficient (d33 ≈ 27 pm/V), we identify an optimal electromechanical conversion efficiency near 100 nm, exceeding that of conventional piezoelectric materials. This work establishes a mechanical perspective for understanding phase transitions in ionic van der Waals ferroelectrics and offers strategies for designing thickness-engineered, high-performance piezoelectric devices.