Mixed electrochemical–ferroelectric states in nanoscale ferroelectrics

• Published in: Nature physics Vol. 13; no. 8; pp. 812 - 818
• Main Authors: Yang, Sang Mo, Morozovska, Anna N., Kumar, Rajeev, Eliseev, Eugene A., Cao, Ye, Mazet, Lucie, Balke, Nina, Jesse, Stephen, Vasudevan, Rama K., Dubourdieu, Catherine, Kalinin, Sergei V.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2017; Nature Publishing Group; Nature Publishing Group (NPG)
• Subjects: 639/301/357; 639/766/119/544; 639/766/119/995; 639/766/119/996; Atomic; Claiming; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Electrochemistry; electronic properties and materials; Ferroelectric materials; Ferroelectrics; ferroelectronics and multiferroics; Hysteresis loops; MATERIALS SCIENCE; Mathematical and Computational Physics; Molecular; Nanomaterials; nanoscale materials; Nanostructure; Optical and Plasma Physics; Physics; surfaces, interfaces and thin films; Switching; Theoretical; Thermodynamics
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys4103

Ferroelectricity on the nanoscale has been the subject of much fascination in condensed-matter physics for over half a century. In recent years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on the formation of remnant polarization states, local electromechanical hysteresis loops, and pressure-induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating a significant level of uncertainty in the field. Here we show that in nanoscale systems the ferroelectric state is fundamentally inseparable from the electrochemical state of the surface, leading to the emergence of a mixed electrochemical–ferroelectric state. We explore the nature, thermodynamics, and thickness evolution of such states, and demonstrate the experimental pathway to establish its presence. This analysis reconciles multiple prior studies, provides guidelines for studies of ferroelectric materials on the nanoscale, and establishes the design paradigm for new generations of ferroelectric-based devices. Nanoscale ferroelectricity is hard to characterize. Studies of BaTiO 3 thin films now reveal a close coupling between the ferroelectric and the surface electrochemical states — a notion important for future applications of ferroelectric nanomaterials.