Piezoelectric materials, which convert mechanical stress into electricity and vice versa, are essential components in sensors, actuators, and energy-harvesting devices. However, the best piezoelectric materials, such as lead zirconate titanate (PZT), are toxic because they contain lead—prompting a search for lead-free alternatives.

Unfortunately, the performance of the leading lead-free candidate, bismuth ferrite (BiFeO₃), plummets when its films are thinner than about 30 nm—a critical barrier for miniaturizing devices like those found in smartphones and medical implants.

Now, in a new advance for next-generation microelectronics, researchers from the Institute of Metal Research of the Chinese Academy of Sciences, together with their collaborators, have broken this "thickness limit" by engineering ultrathin films of BiFeO₃ that exhibit a piezoelectric response more than four times greater than that of conventional forms of the material.

The study was published in the journal Science Advances on March 13.

The researchers discovered and stabilized a transitional "S-phase" within the ultrathin BiFeO₃ films by constructing specially engineered multilayer heterostructures. This metastable phase facilitates a rotation of the material's polarization, unlocking its latent piezoelectric potential even at thicknesses of just a few nanometers.

"This work demonstrates that electric dipoles in BiFeO₃ could strongly couple with interfacial strain and their local atomic environment, giving rise to novel polarization configurations that are critically important for tuning the piezoelectric response," said Prof. TANG Yunlong, corresponding author of the study. "It's like finding a new gear in a tiny engine, allowing it to do powerful work despite being just a few nanometers thick."

Using atomic-scale imaging and quantitative electromechanical microscopy, the researchers directly observed this S-phase and measured a piezoelectric coefficient (d₃₃ ≈ 30 pm/V) in 16-unit-cell-thick films—a value four times higher than that of standard rhombohedral BiFeO₃.

These findings provide a new design strategy for high-performance, eco-friendly piezoelectrics and pave the way for their integration into ultra-miniaturized sensors, actuators, and microelectromechanical systems.

Polarization analysis, thickness-dependent piezoelectric response and d₃₃ statistics of the (BiFeO₃/Ca_0.96Ce_0.04MnO₃)₄ multilayers grown on LaAlO₃ substrates (Image by IMR)