Study on Enhanced Electromechanical Response of Antiferroelectric Thin Films Based on Clamping Effect

Background

Antiferroelectric thin film materials have garnered significant attention for their potential applications in micro/nano electromechanical systems. These systems require materials with high electromechanical responses, capable of generating significant electromechanical strain when an electric field is applied. However, traditional electromechanical materials (such as ferroelectric and relaxor ferroelectric materials) exhibit significantly reduced feedback responses when their thickness is reduced to the submicron level, primarily due to the mechanical clamping effect of the substrate which limits polarization rotation and lattice deformation.

To overcome this limitation, researchers have proposed a non-traditional method. By coupling the electric field-induced antiferroelectric to ferroelectric phase transition with the substrate constraint, significant electromechanical responses were achieved in antiferroelectric thin films. Observations indicate that the detroration of the oxygen octahedra coincides with the expansion of the lattice volume in all dimensions, with in-plane clamping further enhancing out-of-plane expansion.

Research Origin

This paper was co-authored by researchers from several renowned institutions, including the University of California, Berkeley, Massachusetts Institute of Technology, Dartmouth College, DECVOM Army Research Laboratory, among others. The article was published online by “Nature Materials” on April 24, 2024.

Research Process

Experimental Design and Sample Preparation

The research team synthesized 100-nanometer thick antiferroelectric thin films (primarily PbZrO3 and PbHfO3) using pulsed laser deposition, along with ferroelectric thin films (PbZr0.52Ti0.48O3) and relaxor ferroelectric thin films (0.67PbMg_1/3Nb_2/3O3–0.33PbTiO3) as control groups. These films were fabricated into symmetric capacitor structures and their crystalline structures were characterized using X-ray diffraction and reciprocal space mapping methods.

Performance Testing

The phenomenon of oxygen octahedra detilting was studied using field-induced scanning transmission electron microscopy (STEM) to reveal the microscopic mechanisms of electric field-induced antiferroelectric to ferroelectric phase transitions. First-principles calculations were used to explain the physical principles of the enhanced electromechanical response. The electric field dependence of polarization and out-of-plane electromechanical responses of the films were measured using a laser Doppler vibrometer under different electric field conditions.

Main Results

Experimental Observations

1. Polarization Hysteresis Loops and Electromechanical Strain Measurements: In PbZr0.52Ti0.48O3 and 0.67PbMg_1/3Nb_2/3O3–0.33PbTiO3 films, pronounced polarization hysteresis loops and electromechanical strains, which monotonically increased with the electric field, were observed, reaching 0.27% and 0.33% respectively. However, the polarization response of these thin films was significantly lower than that of their bulk counterparts.
2. Bipolar Hysteresis Loops in Antiferroelectric Thin Films: PbZrO3 and PbHfO3 films exhibited characteristic bipolar hysteresis loops, showing abrupt changes under electric fields. The phase transitions resulted in significant strains, reaching ~1.0% and ~0.85% respectively.
3. In-situ STEM Studies: By increasing the applied voltage, the detroration of the lattice during the antiferroelectric to ferroelectric phase transition was observed. Analysis of the detroration of the oxygen octahedra confirmed that the ferroelectric phase has r3m symmetry.

Theoretical Support

First-principles calculations based on density functional theory (DFT) showed that during the electric field-induced phase transition from the antiferroelectric Pbam phase (orthorhombic system) to the ferroelectric r3m phase (rhombohedral), there was an isotropic expansion of the lattice. However, due to the substrate clamping effect, this expansion was primarily manifested in the significant out-of-plane expansion of the lattice.

Conclusions and Application Prospects

This paper demonstrates through experimental and theoretical studies how the mechanical clamping of the substrate can be utilized to achieve high electromechanical responses in antiferroelectric thin films. This method effectively overcomes the challenge of reduced responses in thin films of traditional ferroelectric and relaxor ferroelectric materials. 100-nanometer oriented engineered PbZrO3 films exhibited stable high strain values (~1.7%) under frequency and long cycle conditions, which is significant for the development of high-performance micro/nano electromechanical systems.

Research Highlights

1. Novel Phase Transition and Clamping Coupling Mechanism: The study reveals how antiferroelectric to ferroelectric phase transitions behave under substrate clamping effects. Unlike traditional ferroelectric materials, this coupling leads to significant enhancements in electromechanical responses.
2. Broad Application Prospects: The research not only provides a profound theoretical understanding of antiferroelectric materials but also demonstrates their practical application potential, especially in micro/nano electromechanical systems requiring high-frequency stability and low power consumption.

Outlook

Future research will focus on further optimizing the composition, orientation, and microstructure of antiferroelectric thin films to enhance their breakdown strength and reduce phase transition field strength, thereby improving their application reliability and energy efficiency in integrated micro/nano electromechanical systems.