Interface Control of Ferroelectricity in an SrRuO3 /BaTiO3 /SrRuO3 Capacitor and its Critical Thickness

Proton-mediated reversible switching of metastable ferroelectric phases with low operation voltages

The exploration of ferroelectric phase transitions enables an in-depth understanding of ferroelectric switching and promising applications in information storage. However, controllably tuning the dynamics of ferroelectric phase transitions remains challenging owing to inaccessible hidden phases. Here, using protonic gating technology, we create a series of metastable ferroelectric phases and demonstrate their reversible transitions in layered ferroelectric α-In₂Se₃ transistors. By varying the gate bias, protons can be incrementally injected or extracted, achieving controllable tuning of the ferroelectric α-In₂Se₃ protonic dynamics across the channel and obtaining numerous intermediate phases. We unexpectedly discover that the gate tuning of α-In₂Se₃ protonation is volatile and the created phases remain polar. Their origin, revealed by first-principles calculations, is related to the formation of metastable hydrogen-stabilized α-In₂Se₃ phases. Furthermore, our approach enables ultralow gate voltage switching of different phases (below 0.4 volts). This work provides a possible avenue for accessing hidden phases in ferroelectric switching.

Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes

Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high-energy-density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO₃ membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single-phase below 40 nm, as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform.

Interface-engineered ferroelectricity of epitaxial Hf0.5Zr0.5O2 thin films

Ferroelectric hafnia-based thin films have attracted intense attention due to their compatibility with complementary metal-oxide-semiconductor technology. However, the ferroelectric orthorhombic phase is thermodynamically metastable. Various efforts have been made to stabilize the ferroelectric orthorhombic phase of hafnia-based films such as controlling the growth kinetics and mechanical confinement. Here, we demonstrate a key interface engineering strategy to stabilize and enhance the ferroelectric orthorhombic phase of the Hf_0.5Zr_0.5O₂ thin film by deliberately controlling the termination of the bottom La_0.67Sr_0.33MnO₃ layer. We find that the Hf_0.5Zr_0.5O₂ films on the MnO₂-terminated La_0.67Sr_0.33MnO₃ have more ferroelectric orthorhombic phase than those on the LaSrO-terminated La_0.67Sr_0.33MnO₃, while with no wake-up effect. Even though the Hf_0.5Zr_0.5O₂ thickness is as thin as 1.5 nm, the clear ferroelectric orthorhombic (111) orientation is observed on the MnO₂ termination. Our transmission electron microscopy characterization and theoretical modelling reveal that reconstruction at the Hf_0.5Zr_0.5O₂/ La_0.67Sr_0.33MnO₃ interface and hole doping of the Hf_0.5Zr_0.5O₂ layer resulting from the MnO₂ interface termination are responsible for the stabilization of the metastable ferroelectric phase of Hf_0.5Zr_0.5O₂. We anticipate that these results will inspire further studies of interface-engineered hafnia-based systems.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Neuromorphic Devices for Bionic Sensing and Perception

Neuromorphic devices that can emulate the bionic sensory and perceptual functions of neural systems have great applications in personal healthcare monitoring, neuro-prosthetics, and human-machine interfaces. In order to realize bionic sensing and perception, it's crucial to prepare neuromorphic devices with the function of perceiving environment in real-time. Up to now, lots of efforts have been made in the incorporation of the bio-inspired sensing and neuromorphic engineering in the booming artificial intelligence industry. In this review, we first introduce neuromorphic devices based on diverse materials and mechanisms. Then we summarize the progress made in the emulation of biological sensing and perception systems. Finally, the challenges and opportunities in these fields are also discussed.

Emergent Ferroelectricity in Otherwise Nonferroelectric Oxides by Oxygen Vacancy Design at Heterointerfaces

Introducing point defects in complex metal oxides is a very effective route to engineer crystal symmetry and therefore control physical properties. However, the inversion symmetry breaking, which is vital for many tantalizing properties, such as ferroelectricity and chiral spin structure, is usually hard to be induced in the bulk crystal by point defects. By designing the oxygen vacancy formation energy profile and migration path across the oxide heterostructure, our first-principles density functional theory (DFT) calculations demonstrate that the point defects can effectively break the inversion symmetry and hence create novel ferroelectricity in superlattices consisting of otherwise nonferroelectric materials SrTiO₃ and SrRuO₃. This induced ferroelectricity can be significantly enhanced by reducing the SrTiO₃ thickness. Inspired by theory calculation, SrTiO₃/SrRuO₃ superlattices were experimentally fabricated and are found to exhibit surprising strong ferroelectric properties. Our finding paves a simple and effective pathway to engineer the inversion symmetry and thus properties by point defect control in oxide heterostructures.

Ferroelectric Tunnel Junctions: Modulations on the Potential Barrier

Recently, ferroelectric tunnel junctions (FTJs) have attracted considerable attention for potential applications in next-generation memories, owing to attractive advantages such as high-density of data storage, nondestructive readout, fast write/read access, and low energy consumption. Herein, recent progress regarding FTJ devices is reviewed with an emphasis on the modulation of the potential barrier. Electronic and ionic approaches that modulate the ferroelectric barriers themselves and/or induce extra barriers in electrodes or at ferroelectric/electrode interfaces are discussed with the enhancement of memory performance. Emerging physics, such as nanoscale ferroelectricity, resonant tunneling, and interfacial metallization, and the applications of FTJs in nonvolatile data storage, neuromorphic synapse emulation, and electromagnetic multistate memory are summarized. Finally, challenges and perspectives of FTJ devices are underlined.

Flexible Neuromorphic Electronics for Computing, Soft Robotics, and Neuroprosthetics

Flexible neuromorphic electronics that emulate biological neuronal systems constitute a promising candidate for next-generation wearable computing, soft robotics, and neuroprosthetics. For realization, with the achievement of simple synaptic behaviors in a single device, the construction of artificial synapses with various functions of sensing and responding and integrated systems to mimic complicated computing, sensing, and responding in biological systems is a prerequisite. Artificial synapses that have learning ability can perceive and react to events in the real world; these abilities expand the neuromorphic applications toward health monitoring and cybernetic devices in the future Internet of Things. To demonstrate the flexible neuromorphic systems successfully, it is essential to develop artificial synapses and nerves replicating the functionalities of the biological counterparts and satisfying the requirements for constructing the elements and the integrated systems such as flexibility, low power consumption, high-density integration, and biocompatibility. Here, the progress of flexible neuromorphic electronics is addressed, from basic backgrounds including synaptic characteristics, device structures, and mechanisms of artificial synapses and nerves, to applications for computing, soft robotics, and neuroprosthetics. Finally, future research directions toward wearable artificial neuromorphic systems are suggested for this emerging area.

Atomic-Scale Metal-Insulator Transition in SrRuO3 Ultrathin Films Triggered by Surface Termination Conversion

The metal-insulator transition (MIT) in transition-metal-oxide is fertile ground for exploring intriguing physics and potential device applications. Here, an atomic-scale MIT triggered by surface termination conversion in SrRuO₃ ultrathin films is reported. Uniform and effective termination engineering at the SrRuO₃ (001) surface can be realized via a self-limiting water-leaching process. As the surface termination converts from SrO to RuO₂ , a highly insulating and nonferromagnetic phase emerges within the topmost SrRuO₃ monolayer. Such a spatially confined MIT is corroborated by systematic characterizations on electrical transport, magnetism, and scanning tunneling spectroscopy. Density functional theory calculations and X-ray linear dichroism further suggest that the surface termination conversion breaks the local octahedral symmetry of the crystal field. The resultant modulation in 4d orbital occupancy stabilizes a nonferromagnetic insulating surface state. This work introduces a new paradigm to stimulate and tune exotic functionalities of oxide heterostructures with atomic precision.

Origin of robust out-of-plane ferroelectricity in d1T-MoS2 monolayer

The d1T-MoS₂, distorted-1T group-VIB transition metal dichalcogenides monolayer, is considered as promising atomically thin out-of-plane ferroelectric materials. We study the origin of the ferroelectricity in d1T-MoS₂ monolayer using first-principles calculations and the Landau theory of phase transition. In contrast to conventional improper ferroelectrics, we find that the polarization has dependence on both primary and secondary modes. It turns out that the charge imbalance between chalcogen atoms at different symmetry sites is the source for the out-of-plane polarization without any out-of-plane displacement in the primary mode. The secondary mode following the primary mode cancels partially the polarization and secures it against the depolarization field. We show that the polarization of d1T-MoS₂ is robust to external electric fields and can be manipulated by the uniaxial strain.

Depolarizing-Field Effects in Epitaxial Capacitor Heterostructures

We identify a transient enhancement of the depolarizing field, leading to an unexpected quench of net polarization, during the growth of a prototypical metal-ferroelectric-metal epitaxial system made of BaTiO_{3} and SrRuO_{3}. Reduced conductivity and, hence, charge screening efficiency in the early growth stage of the SrRuO_{3} top electrode promotes a breakdown of ferroelectric BaTiO_{3} into domains. We demonstrate how a thermal annealing procedure can recover the single-domain state. By tracking the polarization state in situ, using optical second harmonic generation, we bring new understanding to interface-related electrostatic effects in ferroelectric capacitors.

Ferroelectrically tunable magnetic skyrmions in ultrathin oxide heterostructures

Magnetic skyrmions are topologically protected whirling spin texture. Their nanoscale dimensions, topologically protected stability and solitonic nature, together are promising for future spintronics applications. To translate these compelling features into practical spintronic devices, a key challenge lies in achieving effective control of skyrmion properties, such as size, density and thermodynamic stability. Here, we report the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO₃/SrRuO₃ bilayer heterostructures. The ferroelectric proximity effect at the BaTiO₃/SrRuO₃ heterointerface triggers a sizeable Dzyaloshinskii-Moriya interaction, thus stabilizing robust skyrmions with diameters less than a hundred nanometres. Moreover, by manipulating the ferroelectric polarization of the BaTiO₃ layer, we achieve local, switchable and nonvolatile control of both skyrmion density and thermodynamic stability. This ferroelectrically tunable skyrmion system can simultaneously enhance the integratability and addressability of skyrmion-based functional devices.

Wireless piezoelectric devices based on electrospun PVDF/BaTiO3 NW nanocomposite fibers for human motion monitoring

Real-time personalized motion monitoring and analysis are important for human health. Thus, to satisfy the needs in this area and the ever-increasing demand for wearable electronics, we design and develop a wireless piezoelectric device consisting of a piezoelectric pressure sensor based on electrospun PVDF/BaTiO3 nanowire (NW) nanocomposite fibers and a wireless circuit system integrated with a data conversion control module, a signal acquisition and amplification module, and a Bluetooth module. Finally, real-time piezoelectric signals of human motion can be displayed by an App on an Android mobile phone for wireless monitoring and analysis. This wireless piezoelectric device is proven to be sensitive to human motion such as squatting up and down, walking, and running. The results indicate that our wireless piezoelectric device has potential applications in wearable medical electronics, particularly in the fields of rehabilitation and sports medicine.

Electronic-Reconstruction-Enhanced Tunneling Conductance at Terrace Edges of Ultrathin Oxide Films

Quantum mechanical tunneling of electrons across ultrathin insulating oxide barriers has been studied extensively for decades due to its great potential in electronic-device applications. In the few-nanometers-thick epitaxial oxide films, atomic-scale structural imperfections, such as the ubiquitously existed one-unit-cell-high terrace edges, can dramatically affect the tunneling probability and device performance. However, the underlying physics has not been investigated adequately. Here, taking ultrathin BaTiO₃ films as a model system, an intrinsic tunneling-conductance enhancement is reported near the terrace edges. Scanning-probe-microscopy results demonstrate the existence of highly conductive regions (tens of nanometers wide) near the terrace edges. First-principles calculations suggest that the terrace-edge geometry can trigger an electronic reconstruction, which reduces the effective tunneling barrier width locally. Furthermore, such tunneling-conductance enhancement can be discovered in other transition metal oxides and controlled by surface-termination engineering. The controllable electronic reconstruction can facilitate the implementation of oxide electronic devices and discovery of exotic low-dimensional quantum phases.

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO₃/BaTiO₃/SrRuO₃ (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO₂ or TiO₂-SrO) with oxygen partial pressure (P_O2) during PLD. We found that a uniform SrO-TiO₂ termination sequence at the SRO/BTO interface can be achieved by lowering the P_O2 to 5 mTorr, regardless of the total background gas pressure (P_total), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.