Oxygen-defective electrostrictors for soft electromechanics

Abstract

Electromechanical metal oxides, such as piezoceramics, are often incompatible with soft polymers due to their crystallinity requirements, leading to high processing temperatures. This study explores the potential of ceria-based thin films as electromechanical actuators for flexible electronics. Oxygen-deficient fluorites, like cerium oxide, are centrosymmetric nonpiezoelectric crystalline metal oxides that demonstrate giant electrostriction. These films, deposited at low temperatures, integrate seamlessly with various soft substrates like polyimide and PET. Ceria thin films exhibit remarkable electrostriction (M₃₃ > 10^-16 m² V^-2) and inverse pseudo-piezo coefficients (e₃₃ > 500 pmV^-1), enabling large displacements in soft electromechanical systems. Our study explores resonant and off-resonant configurations in the low-frequency regime (<1 kHz), demonstrating versatility for three-dimensional and transparent electronics. This work advances the understanding of oxygen-defective metal oxide electromechanical properties and paves the way for developing versatile and efficient electromechanical systems for applications in biomedical devices, optical devices, and beyond.

Fig. 1.. Flat cantilevers.

(A) Cross-section view by scanning electron microscopy (SEM) and TEM of the columnar microstructure of the CGO and TiN thin films. (B) Experimental and simulated electrostrictive responses of cantilevers based on ceria thin films (ca. 500 nm) on polyimide (PI) and fused silica (FS) substrates. The bias (U_DC, 60 kV/cm) that induced first-harmonic response (pseudo-piezoelectric) was measured at 10 Hz under a 6-kV alternating field. (C) ioT-XRD of the as-prepared and under-bias films. The sample was kept under a 0.4 MV/cm bias for 12 hours for the “ON” state. The right graph shows the (111), (200), and (311) peak shifts with increasing bias from 15 V at a rate of 0.1 V/s.

Fig. 2.. Tubular film coating.

(A) Illustration of the coating process on the tubular substrates. Cross-section SEM view of the thin films deposited in (B) in-plane and (C) cross-plane geometries.

Fig. 3.. Tubular devices performance.

Electromechanical response over field and frequency for the (A) in-plane and (B) cross-plane tubular devices. The top illustrations are finite element simulations of the geometries at 10 Hz, assuming an M₃₃ of 6.10⁻¹⁶ m²/V². The u_s and u_z are relative to the radial and axis displacement.

Fig. 4.. Depositions on transparent PET.

(A) Photo and schematic representation of the optical device with a centered optical path. (B) Electromechanical performance over field and frequency for the sample deposited on ITO/PET. (C) Finite element simulations of the proposed geometry under a 3 V.

Fig. 5.. Proof-of-concept lens.

Light scattering schematic and 3D graph of the light intensity fluctuations over time at different positions with an excitation at 170 Hz (resonance frequency). Intensity fluctuations are normalized from minimum (0, blue) to maximum (1, red).