Switchable tribology of ferroelectrics

Abstract

Ferroelectrics, where the switchable electric polarization is coupled with mechanical deformation, have been studied extensively in view of their technological applications as sensors, actuators, energy harvesters, memory and electro-optic devices. Fundamentally, these materials show electromechanically coupled properties including piezoelectricity, flexoelectricity, and electrostriction, whose interplay can enrich critical opportunities in the field of condensed matter physics and functional materials engineering, but is only now beginning to be understood and controlled. We started this study with the motivation that the tribological properties of ferroelectrics could be controlled electrically, focusing on that ferroelectric materials have different surface physics and chemistry depending on the polarization orientation. The purpose of the study is to observe the flexoelectrically-coupled polarization-derived tribological asymmetry of ferroelectrics such as nanoscale friction and wear, elucidate the mechanism of the asymmetry, and find practical applications. First, to prove our idea, we start from uniaxial ferroelectric LiNbO$_3$ single crystals as a simple and accessible model system, already widely used in electro-optic applications. We discover polarization-dependent asymmetric friction and surface wear in these materials by applying a sufficiently high mechanical force using a diamond atomic force microscopy (AFM) probe at the nanoscale. We also studied the effect of loading force, degradation and scan rate on the tribological asymmetry. Furthermore, we confirm that this asymmetry does not originate from either electrostatic effects, or inhomogeneous defect distribution, but is linked to the competing vs. synergistic interplay of flexoelectric and ferroelectric polarization in oppositely oriented domains, which moreover leads to an anomalous, positive correlation between the hardness of the materials and its wear rate. This means that increasing the flexoelectric contribution under highly inhomogeneous stress also has emergent consequences for coupled tribological properties—in particular, friction and wear coefficients. Switching the ferroelectric domains by local electric field application should thus allow simultaneous and reversible control of the tribological responses (friction and wear) of the material. Moreover, we show that the tribological asymmetry is universal across the size and the type of ferroelectric materials. Second, we demonstrate polarization-derived lithography as a top-down, chemical-free and resist/mask-less lithography technique, which can be potentially applied to the fabrication of three-dimensional (3D) and monolithic nanostructures when multi-pass switching and milling of the ferroelectric surface is implemented. Combining the switchable nature and the wear asymmetry of ferroelectrics, we fabricated arrays of nano-pillars in LiNbO$_3$ single crystal, and 3D complex nanostructures with text by repetitive switching and milling scans in LiNbO$_3$ thin film. Moreover, in the tetragonal PbTiO$_3$ ferroelectric system, we achieved unit-cell-level lithography where the height difference of etched domains is only few angstrom-scale. Beyond micro/nano-scale wear of ferroelectric single crystals using an AFM probe, we also demonstrate scalability of this process to technologically relevant large-scale patterning by simply polishing the whole crystal using silica nanoparticles that effectively act as millions of mobile AFM tips. Third, we demonstrate polarization-derived friction microscopy, which enables direct visualization of ferroelectric domains without any electrostatic artifact during the imaging at high speed. Friction asymmetry allows purely mechanical visualization of ferroelectric domains under high strain gradient application. In a low strain gradient regime, environmental effects and screening condition dominate the friction response. Furthermore, this low-cost, faster, and voltage-free visualization of polarization domains using scanning probe microscopy has been demonstrated in ferroelectric thin films and single crystals without any signal amplifier. In addition, this methodology doesn’t require any conductive probe or back electrode for visualization of ferroelectric domains. We achieved 14.2 FPS of visualization of ferroelectric domains in PbTiO3 thin film using Cypher VRS commercial AFM set-up. We envision that this high-speed imaging can be useful for the investigation of polarization dynamics and big data acquisition. Our research reports flexoelectrically tunable tribology of ferroelectrics for the first time, elucidates the mechanism behind the asymmetric tribology, and demonstrates high-speed visualization of ferroelectric domains and three dimensional nanostructruing of ferroelectrics. We envision that this top-down, chemical/resist-free and maskless lithography technique and high-speed visualization of ferroelectric open fundamental studies and industrial applications of ferroelectrics.