Protonation-Driven Polarization Retention Failure in Nano-Columnar Lead-Free Ferroelectric Thin Films

Understanding microscopic mechanisms of polarization retention characteristics in ferroelectric thin films is of great significance for exploring unusual physical phenomena inaccessible in the bulk counterparts and for realizing thin-film-based functional electronic devices. Perovskite (K,Na)NbO₃ is an excellent class of lead-free ferroelectric oxides attracting tremendous interest thanks to its potential applications to non-volatile memory and eco-friendly energy harvester/storage. Nonetheless, in-depth investigation of ferroelectric properties of (K,Na)NbO₃ films and the following developments of nano-devices are limited due to challenging thin-film fabrication associated with non-stoichiometry by volatile K and Na atoms. Herein, we epitaxially grow ferroelectric (K,Na)NbO₃ films of which the atomic-level geometrical structures strongly depend on thickness-dependent strain relaxation. Nanopillar crystal structures are identified in fully relaxed (K,Na)NbO₃ films to the bulk states representing a continuous reduction of switchable polarization under air environments, that is, polarization retention failures. Protonation by water dissociation is responsible for the humidity-induced retention loss in nano-columnar (K,Na)NbO₃ films. The protonation-driven polarization retention failure originates from domain wall pinning by the accumulation of mobile hydrogen ions at charged domain walls for effective screening of polarization-bound charges. Conceptually, the results will be utilized for rational design to advanced energy materials such as photo-catalysts enabling ferroelectric tuning of water splitting.