Manipulating ferroelectricity without electrical bias: A perspective

Ferroelectric materials are established candidates for beyond complementary metal-oxide-semiconductor technology, owing to their non-volatile spontaneous electrical polarization. The recent boom in electric dipole texture engineering and manipulation in such materials has revealed exciting routes for controlling ferroelectric polarization, offering alternatives to the classical, sometimes challenging, application of electrical fields. In this short perspective, we shed light on electrode-free external stimuli enabling control over polar states in thin films. We bring awareness to the polarizing role of chemically-engineered surface contributions and provide insights into the combination of chemical substitution and mechanical pressure, complementing the polar state tuning capabilities readily enabled by flexoelectricity. Finally, we describe recent developments in the optical modulation of polarization. Thus, our perspective aims to stimulate the advancement of alternative means to act on polarization states and facilitate the development of ferroelectric-based applications.

💡 Research Summary

This perspective reviews recent advances in manipulating ferroelectric polarization in thin films without the use of external electrical bias. The authors organize the discussion into three electrode‑free routes: (i) polarizing surfaces and crystal‑chemical engineering, (ii) mechanical and chemical pressure, and (iii) optical excitation.

In the first section, the paper revisits the problem of the depolarizing field that suppresses net polarization in ultrathin ferroelectrics. While traditional approaches rely on dielectric buffer layers to screen bound charge, the authors highlight a newer strategy that exploits charged atomic planes or deliberately engineered surface terminations. Charged interfaces—such as Bi₂O₂ sheets in Aurivillius compounds or deliberately terminated perovskite layers—create built‑in electrostatic potential steps that bias the direction of bound charge accumulation, thereby fixing the out‑of‑plane polarization during growth. The authors cite examples of polar vortex and skyrmion textures in PbTiO₃/SrTiO₃ superlattices, synthetic antiferroelectric states achieved by strain‑induced in‑plane polarization, and ferrielectric ordering in BiFeO₃‑incorporated Aurivillius composites. Moreover, surface chemistry (adsorbates, pH‑controlled reactions) and ordered point‑defect gradients (oxygen‑vacancy profiles) can be tuned post‑growth to reverse or re‑orient the polarization, offering a reversible, electrode‑free poling method suitable for catalysis or environmental sensing.

The second section focuses on mechanical and chemical pressure as direct means to alter the lattice distortions that underlie ferroelectricity. Hydrostatic or uniaxial pressure in bulk crystals has long been known to shift phase boundaries; in thin films, localized forces delivered by scanning probe tips or nano‑indenters generate strain gradients that drive domain wall motion, pressure‑induced phase transitions, and flexoelectric polarization. The authors discuss experimental demonstrations where forces of 10–100 nN produce antipolar‑to‑ferroelectric conversion in Bi‑substituted LaFeO₃ layers, and where pressure gradients couple with epitaxial strain to stabilize novel textures. Chemical pressure, achieved by isovalent or aliovalent substitution (e.g., La³⁺ for Bi³⁺), introduces internal strain via ionic‑radius mismatch, enabling systematic tuning of transition temperatures, coercive fields, and anisotropy without any external field.

The third section examines optical routes. Light can generate internal electric fields through the bulk photovoltaic effect, built‑in Schottky barriers, or photovoltage at interfaces. These fields act as an effective bias, allowing light‑induced switching of polarization, creation of photogenerated flexoelectric fields, and even light‑driven polar‑antipolar phase conversion. The authors reference recent work where specific photon energies (hν) produce a measurable photovoltage that aligns dipoles, and where illumination modulates flexoelectric coefficients, enabling remote, contact‑free control of ferroelectric states.

Finally, the authors stress the synergistic potential of combining these three modalities. For instance, a chemically engineered charged surface can be illuminated while a localized mechanical load is applied, producing additive or even emergent effects that could be harnessed for multi‑state memory, neuromorphic devices, or adaptive sensors. They conclude that mastering electrode‑free manipulation expands the design space for ferroelectric‑based technologies, especially where conventional electrodes are impractical, and they call for systematic studies of stability, scalability, and integration pathways toward functional devices.