Interface and surface stabilization of the polarization in ferroelectric thin films

An Electrode Design Strategy to Minimize Ferroelectric Imprint Effect

The phenomenon of ferroelectric imprint, characterized by an asymmetric polarization switching behavior, poses significant challenges in the reliability and performance of ultra-low-voltage ferroelectric devices, including MagnetoElectric Spin-Orbit devices, Ferroelectric Random-Access Memory, Ferroelectric Field-Effect Transistors, and Ferroelectric Tunnel Junctions. In this study, the influence of electrode configuration in different device architectures are systematically investigated on their imprint effect. By tuning the work function of La0.7Sr0.3MnO3 (LSMO) electrodes through oxygen pressure during deposition, precise control over the built-in voltage offset (Voffset) in ferroelectric capacitors are demonstrated. This results reveal that higher oxygen pressures increase the work function of LSMO, effectively compensating for Voffset and enhancing device stability. Finally, a ferroelectric device with a hybrid bottom electrode of LSMO and SrRuO3 is optimized to mitigate the imprint effect. The optimal device showcases small coercive voltage of 0.3 V, a minimal Voffset of 0.06 V, excellent endurance (electrical cycle up to 109), and robust zero bias applied polarization retention. These findings provide a practical guideline for electrode design in ferroelectric devices, addressing the imprint effect and improving operational reliability. This approach, combining material tuning and in situ diagnostics, offers a pathway to optimize ferroelectric device performance, with implications for advancing ultra-low-power electronics.

Subsurface Oxygen Vacancy Mediated Surface Reconstruction and Depolarization of Ferroelectric BaTiO3 (001) Surface

The interplay between surface reconstruction and depolarization of ferroelectric oxide surfaces is strongly influenced by oxygen vacancies (VO). Using in-situ atomic-resolution electron microscopy imaging and spectroscopy techniques, it is directly observed that a clean BaTiO3 (001) surface stabilizes into (2 × 1) BaO-terminated reconstruction during vacuum annealing. This surface reconstruction is achieved with accommodating BaO deficiency and incorporates TiOx adunits. The cooperative atomic rumpling in both the surface and subsurface layers, arranged in a tail-to-tail configuration, is stabilized by planar accumulation of VO in the subsurface TiO2 layer. This reduces the net polarization of surface unit cells, contributing to overall depolarization. Under this atomic rumpling, the polarization-down (P↓) state is energetically favored over the polarization-up (P↑) state, as the P↓ state requires less atomic relaxation in the bulk layers to achieve dipole inversion at the subsurface. The energetic preference for VO in the subsurface TiO2 layer of the P↓ state is confirmed through calculations of VO formation energy and the energy barrier for surface-to-subsurface migration. These findings reveal that the presence of VO in the subsurface layer lifts the degeneracy in the double-well potential between the P↓ and P↑ states in BaTiO3 (001).

Piezoelectrostatic Catalysis of the Azide-Alkyne Huisgen Cycloaddition

Electric fields are increasingly recognized for their role as 'smart reagents' that can trigger or accelerate chemical reactions. Expanding upon this concept, our research introduces an innovative method that exploits electric fields induced by ultrasound on piezoelectric nanoparticles to facilitate the azide-alkyne Huisgen cycloaddition in nonaqueous environments. The intense electric field generated around the BaTiO3 nanoparticles, as supported by density functional theory calculations, provides the suitable conditions necessary to trigger the cycloaddition of the alkyne-functionalized nanoparticles and the azide present in the solution. To quantitatively assess the occurrence of the click cycloaddition reaction at the nanoparticle surface interface, we tacked the azide with either an electroactive ferrocene moiety or with gold nanoparticles, which act as surface Raman enhancers. These experiments not only provide experimental validation of our approach, but also highlights the potential of piezoelectrostatic catalysts in enhancing the scalability of electrostatic catalysis.

Tuning Self-Polarization of Epitaxial BiFeO3 Thin Films through Interface Effects

Interface effects and strain engineering have emerged as critical strategies for modulating polarization and internal electric fields in ferroelectric materials, playing a vital role in exploring coupling mechanisms and developing ferroelectric diode devices. In this study, we selected BiFeO3 as a representative ferroelectric material and utilized interface engineering to control its polarization. By precisely manipulating the atomic stacking sequence at the interface, we influenced the electrostatic potential step across the interface, resulting in a bias voltage in the ferroelectric hysteresis loops that defined the ferroelectric state. The introduction of strain and strain gradients through a lattice mismatch between the film and substrate generated a flexoelectric field of approximately 3 MV/m, significantly impacting the internal electric field. Additionally, we successfully modified the Schottky barrier height within BiFeO3 films through the synergy and competition between interfacial and flexoelectric effects. This work expands the potential applications of thin-film flexoelectricity in Schottky diodes, sensors, and memory devices.

Optical Excitation of Coherent THz Dynamics of the Rare-Earth Lattice through Resonant Pumping of f-f Electronic Transition in a Complex Perovskite DyFeO_{3}

Resonant pumping of the electronic f-f transitions in the orbital multiplet of dysprosium ions (Dy^{3+}) in a complex perovskite DyFeO_{3} is shown to impulsively launch THz lattice dynamics corresponding to the B_{2g} phonon mode, which is dominanted by the motion of Dy^{3+} ions. The findings, supported by symmetry analysis and density-functional theory calculations, not only provide a novel route for highly selective excitation of the rare-earth crystal lattices but also establish important relationships between the symmetry of the electronic and lattice excitations in complex oxides.

Depth-Resolved X-Ray Photoelectron Spectroscopy Evidence of Intrinsic Polar States in HfO2-Based Ferroelectrics

The discovery of ferroelectricity in nanoscale hafnia-based oxide films has spurred interest in understanding their emergent properties. Investigation focuses on the size-dependent polarization behavior, which is sensitive to content and movement of oxygen vacancies. Though polarization switching and electrochemical reactions is shown to co-occur, their relationship remains unclear. This study employs X-ray photoelectron spectroscopy with depth sensitivity to examine changes in electrochemical states occurring during polarization switching. Contrasting Hf0.5Zr0.5O2 (HZO) with Hf0.88La0.04Ta0.08O2 (HLTO), a composition with an equivalent structure and comparable average ionic radius, electrochemical states are directly observed for specific polarization directions. Lower-polarization films exhibit more significant electrochemical changes upon switching, suggesting an indirect relationship between polarization and electrochemical state. This research illuminates the complex interplay between polarization and electrochemical dynamics, providing evidence for intrinsic polar states in HfO2-based ferroelectrics.

Surface polarization profile of ferroelectric thin films probed by X-ray standing waves and photoelectron spectroscopy

Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, we demonstrate that the X-ray standing wave technique allows the surface polarization profile of a ferroelectric thin film, as opposed to the average film polarity, to be probed directly. The X-ray standing wave technique provides the average Ti and Ba atomic positions, along the out-of-plane direction, near the surface of three differently strained [Formula: see text] thin films. This technique gives direct access to the local ferroelectric polarization at and below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface is obtained. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to different type, amount and spatial distribution of the oxygen-containing adsorbates.

Engineering Sub-Nanometer Hafnia-Based Ferroelectrics to Break the Scaling Relation for High-Efficiency Piezocatalytic Water Splitting

Reversible control of ferroelectric polarization is essential to overcome the heterocatalytic kinetic limitation. This can be achieved by creating a surface with switchable electron density; however, owing to the rigidity of traditional ferroelectric oxides, achieving polarization reversal in piezocatalytic processes remains challenging. Herein, sub-nanometer-sized Hf0.5 Zr0.5 O2 (HZO) nanowires with a polymer-like flexibility are synthesized. Oxygen K-edge X-ray absorption spectroscopy and negative spherical aberration-corrected transmission electron microscopy reveal an orthorhombic (Pca21 ) ferroelectric phase of the HZO sub-nanometer wires (SNWs). The ferroelectric polarization of the flexible HZO SNWs can be easily switched by slight external vibration, resulting in dynamic modulation of the binding energy of adsorbates and thus breaking the "scaling relationship" during piezocatalysis. Consequently, the as-synthesized ultrathin HZO nanowires display superb water-splitting activity, with H2 production rate of 25687 µmol g-1  h-1 under 40 kHz ultrasonic vibration, which is 235 and 41 times higher than those of non-ferroelectric hafnium oxides and rigid BaTiO3 nanoparticles, respectively. More strikingly, the hydrogen production rates can reach 5.2 µmol g-1  h-1 by addition of stirring exclusively.

Experimental Band Structure of Pb(Zr,Ti)O3 : Mechanism of Ferroelectric Stabilization

Pb(Zr,Ti)O3 (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2 Ti0.8 )O3 using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and Lax SrMn1- x O3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Atomic and Electronic Manipulation of Robust Ferroelectric Polymorphs

Polymorphism allows the symmetry of the lattice and spatial charge distributions of atomically thin materials to be designed. While various polymorphs for superconducting, magnetic, and topological states have been extensively studied, polymorphic control is a challenge for robust ferroelectricity in atomically thin geometries. Here, the atomic and electric manipulation of ferroelectric polymorphs in Mo_{1- x} W_x Te₂ is reported. Atomic manipulation for polymorphic control via chemical pressure (substituting tungsten for molybdenum atoms) and charge density modulation can realize tunable polar lattice structures and robust ferroelectricity up to T = 400 K with a constant coercive field in an atomically thin material. Owing to the effective inversion symmetry breaking, the ferroelectric switching withstands a charge carrier density of up to 1.1 × 10¹³ cm^-2 , developing an original diagram for ferroelectric switching in atomically thin materials.

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Multilevel polarization switching in ferroelectric thin films

Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr_0.52Ti_0.48O₃ films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance.

Setting any polarization value in ferroelectric thin films is a key step for their implementation in neuromorphic devices. Here, the authors demonstrate continuous modulation of the remanent polarization at the nanoscale in PbZr_0.52Ti_0.48O₃ films.

Ferroelectric Modulation of Surface Electronic States in BaTiO3 for Enhanced Hydrogen Evolution Activity

Ferroelectric nanomaterials offer the promise of switchable electronic properties at the surface, with implications for photo- and electrocatalysis. Studies to date on the effect of ferroelectric surfaces in electrocatalysis have been primarily limited to nanoparticle systems where complex interfaces arise. Here, we use MBE-grown epitaxial BaTiO₃ thin films with atomically sharp interfaces as model surfaces to demonstrate the effect of ferroelectric polarization on the electronic structure, intermediate binding energy, and electrochemical activity toward the hydrogen evolution reaction (HER). Surface spectroscopy and ab initio DFT+U calculations of the well-defined (001) surfaces indicate that an upward polarized surface reduces the work function relative to downward polarization and leads to a smaller HER barrier, in agreement with the higher activity observed experimentally. Employing ferroelectric polarization to create multiple adsorbate interactions over a single electrocatalytic surface, as demonstrated in this work, may offer new opportunities for nanoscale catalysis design beyond traditional descriptors.

Self-consistently derived sample permittivity in stabilization of ferroelectricity due to charge accumulated at interfaces

Recently, a simple model was proposed for the microscopic energy associated to the ferroelectric phase, to be used in a statistical approach in order to derive the equations of state for a ferroelectric thin film [C. M. Teodorescu, Phys. Chem. Chem. Phys., 2021, 23, 4085-4093]. The stabilization energy for an elemental dipole in a polar thin film is the result of the interaction of this dipole with the field generated by charges accumulated at surfaces or interfaces of the thin film. An essential parameter of this interaction is the permittivity of the film, assumed to be a material constant, together with the maximum value of an elemental dipole and the density of the elemental dipoles. These can be connected to three experimental parameters which are the saturation polarization P_s, the coercive field at zero temperature E(0)c and the Curie temperature T_C. However, for a ferroelectric material both the global and the differential permittivity depend on the temperature and on the polarization. This raises the question whether such a non-constant permittivity should be used in the stabilization energy of the ferroelectric phase, and whether it can be identified self-consistently with the function resulting after applying the statistics based on the microscopic model. In such case, a mutual interdependence should exist between P_s, E(0)c and T_C. A model is built up, able to predict coercitivity, however E(0)c and T_C yield values several orders of magnitude higher than the experimental ones. Therefore, one has to introduce a background dielectric constant of several hundreds to accommodate the result of the model with the experimental data. The poling history of the film has to be taken into account, together with the presence of a small bias field. The model is able to predict self-consistently the equation of state of a ferroelectric, and in particular the linear decrease of the coercive field with temperature. The microscopic parameters, in particular the background dielectric constant and the density of elemental dipoles may be expressed directly from experimental quantities.

Strain and orientation engineering in ABO3perovskite oxide thin films

Perovskite oxides with chemical formula ABO₃are widely studied for their properties including ferroelectricity, magnetism, strongly correlated physics, optical effects, and superconductivity. A thriving research direction using such materials is through their integration as epitaxial thin films, allowing many novel and exotic effects to be discovered. The integration of the thin film on a single crystal substrate, however, can produce unique and powerful effects, and can even induce phases in the thin film that are not stable in bulk. The substrate imposed mechanical boundary conditions such as strain, crystallographic orientation, octahedral rotation patterns, and symmetry can also affect the functional properties of perovskite films. Here, the author reviews the current state of the art in epitaxial strain and orientation engineering in perovskite oxide thin films. The paper begins by introducing the effect of uniform conventional biaxial strain, and then moves to describe how the substrate crystallographic orientation can induce symmetry changes in the film materials. Various material case studies, including ferroelectrics, magnetically ordered materials, and nonlinear optical oxides are covered. The connectivity of the oxygen octahedra between film and substrate depending on the strain level as well as the crystallographic orientation is then discussed. The review concludes with open questions and suggestions worthy of the community's focus in the future.

In situmonitoring of epitaxial ferroelectric thin-film growth

In ferroelectric thin films, the polarization state and the domain configuration define the macroscopic ferroelectric properties such as the switching dynamics. Engineering of the ferroelectric domain configuration during synthesis is in permanent evolution and can be achieved by a range of approaches, extending from epitaxial strain tuning over electrostatic environment control to the influence of interface atomic termination. Exotic polar states are now designed in the technologically relevant ultrathin regime. The promise of energy-efficient devices based on ultrathin ferroelectric films depends on the ability to create, probe, and manipulate polar states in ever more complex epitaxial architectures. Because most ferroelectric oxides exhibit ferroelectricity during the epitaxial deposition process, the direct access to the polarization emergence and its evolution during the growth process, beyond the realm of existing structuralin situdiagnostic tools, is becoming of paramount importance. We review the recent progress in the field of monitoring polar states with an emphasis on the non-invasive probes allowing investigations of polarization during the thin film growth of ferroelectric oxides. A particular importance is given to optical second harmonic generationin situ. The ability to determine the net polarization and domain configuration of ultrathin films and multilayers during the growth of multilayers brings new insights towards a better understanding of the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructures for devices.

Layer and spontaneous polarizations in perovskite oxides and their interplay in multiferroic bismuth ferrite

We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability-the so-called "polar catastrophe"-if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO₃, which is unusual in that its spontaneous ferroelectric polarization and the polarization arising from its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface termination, its surface charge is self-compensating. We use density functional calculations of BiFeO₃ slabs and superlattices, analysis of high-resolution transmission electron micrographs, and examples from the literature to explore the consequences of this peculiarity.

Ferroelectricity in thin films driven by charges accumulated at interfaces

A simple view of ferroelectricity is proposed for a thin film with uniform polarization oriented perpendicular to its surface, starting from the assumption that this situation is always accompanied by charge accumulation in the outer metal electrodes, in the contamination layers or near the surface, in the ferroelectric film itself. Starting with the formula derived for an "elemental" dipole moment in the film, simple statistical mechanics allows one to derive hysteresis cycles, and their dependence on temperature starting with only two parameters: the dielectric constant of the material and the maximum value of the dipole moment of a unit cell. Values obtained for Curie temperatures and coercive fields agree well with experiments. "Exact" energy dependencies on the asymmetry parameter are derived, and their connection with the Landau-Ginsburg-Devonshire is proven. By considering also the dipolar interaction in a continuous model, in addition to the ordering energy in the presence of surface charge accumulation, one may estimate the distribution of the polarization inside the film and the validity of the hypothesis of uniform polarization.

On the happiness of ferroelectric surfaces and its role in water dissociation: The example of bismuth ferrite

We investigate, using density functional theory, how the interaction between the ferroelectric polarization and the chemical structure of the (001) surfaces of bismuth ferrite influences the surface properties and reactivity of this material. A precise understanding of the surface behavior of ferroelectrics is necessary for their use in surface science applications such as catalysis as well as for their incorporation in microelectronic devices. Using the (001) surface of bismuth ferrite as a model system, we show that the most energetically favored surface geometries are combinations of surface termination and polarization direction that lead to uncharged stable surfaces. On the unfavorable charged surfaces, we explore the compensation mechanisms of surface charges provided by the introduction of point defects and adsorbates, such as water. Finally, we propose that the special surface properties of bismuth ferrite (001) could be used to produce an effective water splitting cycle through cyclic polarization switching.

In-situ monitoring of interface proximity effects in ultrathin ferroelectrics

The development of energy-efficient nanoelectronics based on ferroelectrics is hampered by a notorious polarization loss in the ultrathin regime caused by the unscreened polar discontinuity at the interfaces. So far, engineering charge screening at either the bottom or the top interface has been used to optimize the polarization state. Yet, it is expected that the combined effect of both interfaces determines the final polarization state; in fact the more so the thinner a film is. The competition and cooperation between interfaces have, however, remained unexplored so far. Taking PbTiO₃ as a model system, we observe drastic differences between the influence of a single interface and the competition and cooperation of two interfaces. We investigate the impact of these configurations on the PbTiO₃ polarization when the interfaces are in close proximity, during thin-film synthesis in the ultrathin limit. By tailoring the interface chemistry towards a cooperative configuration, we stabilize a robust polarization state with giant polarization enhancement. Interface cooperation hence constitutes a powerful route for engineering the polarization in thin-film ferroelectrics towards improved integrability for oxide electronics in reduced dimension.

How to maintain a robust polarization in ferroelectrics despite its inherent suppression when going to the thin-film limit is a long-standing issue. Here, the authors propose the concept of competitive and cooperative interfaces and establish robust polarization states in the ultrathin regime.