Recent Progress in the Nanoscale Evaluation of Piezoelectric and Ferroelectric Properties via Scanning Probe Microscopy

Differentiating the origins of local charge transfer in oxides and hybrid halides by accumulating charge

Unveiling the origin of local charge transfer is crucial for advancing electronic devices such as ferroelectric and memristive memories and perovskite solar cells. Exploring charge transfer mechanisms requires sensitive probing of local charge transfer, as electric charges in many materials arise from multiple mechanisms. However, the limited sensitivity of current techniques makes it challenging to unveil the origins of such nanoscale charge behavior. To address this challenge, we propose highly sensitive accumulative charge transfer spectroscopy (ACTS) for probing dynamic charge behaviors at the nanoscale in oxides and hybrid halides, including Pb(Zr0.2Ti0.8)O3, Hf0.5Zr0.5O2, TiO2 and FAPbI3. In ferroelectrics, clear polarization switching charges were detected through accumulative charges generated from a series of relatively low-voltage waveforms, achieving a high sensitivity of 6.66 MV m-1. In contrast, distinctive charge behaviors, potentially associated with oxygen vacancy migration and trap states, respectively, were identified in memristive and hybrid halides. This work demonstrates the potential of ACTS for direct, localized discrimination of charge transfer behaviors at the nanoscale.

Piezoelectric Nanoarrays with Mechanical-Electrical Coupling Microenvironment for Innervated Bone Regeneration

The involvement of neurons in the peripheral nervous system is crucial for bone regeneration. Mimicking extracellular matrix cues provides a more direct and effective strategy to regulate neuronal activity and enhance bone regeneration. However, the simultaneous coupling of the intrinsic mechanical-electrical microenvironment of implants to regulate innervated bone regeneration has been largely neglected. Inspired by the mechanical and bioelectric properties of the bone microenvironment, this study constructed a mechanical-electrical coupling microenvironment (M-E) model based on barium titanate piezoelectric nanoarrays, which could effectively promote innervated bone regeneration. The study found that the mechanical microenvironment provided by the nanostructure, coupled with the electrical microenvironment provided by the piezoelectric properties, created a controllable M-E. In vitro cell experiments demonstrated that this coupled microenvironment activated Piezo2 and VGCC ion channels, promoted calcium influx in DRG neurons, and activated downstream PI3K-AKT and RAS pathways. This cascade of events led to the synthesis and release of CGRP in sensory nerves, ultimately enhancing the osteogenic differentiation of BMSCs. This work not only broadens the current understanding of biomaterials that mimic the bone extracellular matrix but also provides new insights into innervated bone regeneration.

Fabrication of Ultrathin Ferroelectric Al0.7Sc0.3N Films under Complementary-Metal-Oxide-Semiconductor Compatible Conditions by using HfN0.4 Electrode

Aluminum scandium nitride (AlScN) has emerged as a promising candidate for next-generation ferroelectric memories, offering a much higher remanent charge density than other materials with a stable ferroelectric phase. However, the inherently high coercive field requires a substantial decrease in film thickness to lower the operating voltage. Significant leakage currents present a severe challenge during the thickness scaling, especially when maintaining compatibility with complementary-metal-oxide-semiconductor (CMOS) fabrication standards. This study adopts a HfN0.4 bottom electrode, which minimizes lattice mismatch with Al0.7Sc0.3N (ASN), forming a coherent bottom interface that effectively reduces leakage currents even at thickness < 5 nm. CMOS-compatible HfN0.4/ASN/TiN stack, deposited without vacuum break between each layer, demonstrates exceptional scalability, confirming the ferroelectricity of ASN films at thicknesses down to 3 nm. The coercive voltage is decreased to 4.35 V, significantly advancing low-voltage AlScN devices that align with CMOS standards.

Photoconductivity and Photovoltaic Effect Strengthened via Microstructural Cotuning in Ferroelectrics: Intuitively Assessed by Macroscopic Transparency

Photoferroelectrics that involve strong light-matter coupling are regarded as promising candidates for realizing bulk photovoltaic and photoelectric effects via light absorption. Nonetheless, understanding the photoresponse mechanism or modulation of performance from a microscopic point of view is scarcely explored through quantification of macroscopic properties. Herein, we design a model material, Gd3+-doped (K0.5Na0.5)NbO3 ferroelectric-transparent ceramics, and present an advantageous strategy to enhance the optoelectronic coupling through joint modulations of lattice distortion and oxygen vacancies, along with inner defects and ferroelectric domains. Significantly, their microcosmic manipulation can be intuitively and facilely evaluated by the optical transparency of each ceramic. An approximately 104 fold increase in conductivity under ultraviolet irradiation was produced. Under the cocoupling between external physical fields, the synergy of photoelectric stimulation increased the photoconductivity of the ceramics by 13.89 times. Additionally, a significant increase (4.5-fold) in the current output from the photovoltaic effect was achieved via ferroelectric domains of moderate size, whose size could be easily assessed by optical transmittance. In situ microscopic observations confirmed that the configuration of oxygen vacancy-dependent ferroelectric domains contributes to the enhanced optoelectronic response. This research provides a distinct way to develop inexpensive optocoupler devices and meet the requirements for multifunctional integration in single photoferroelectrics.

Laterally gated ferroelectric field effect transistor (LG-FeFET) using α-In2Se3 for stacked in-memory computing array

In-memory computing is an attractive alternative for handling data-intensive tasks as it employs parallel processing without the need for data transfer. Nevertheless, it necessitates a high-density memory array to effectively manage large data volumes. Here, we present a stacked ferroelectric memory array comprised of laterally gated ferroelectric field-effect transistors (LG-FeFETs). The interlocking effect of the α-In2Se3 is utilized to regulate the channel conductance. Our study examined the distinctive characteristics of the LG-FeFET, such as a notably wide memory window, effective ferroelectric switching, long retention time (over 3 × 104 seconds), and high endurance (over 105 cycles). This device is also well-suited for implementing vertically stacked structures because decreasing its height can help mitigate the challenges associated with the integration process. We devised a 3D stacked structure using the LG-FeFET and verified its feasibility by performing multiply-accumulate (MAC) operations in a two-tier stacked memory configuration.

Organic radical ferroelectric crystals with martensitic phase transition

Organic martensitic compounds are an emerging type of smart material with intriguing physical properties including thermosalient effect, ferroelasticity, and shape memory effect. However, due to the high structural symmetry and limited design theories for these materials, the combination of ferroelectricity and martensitic transformation has rarely been found in organic systems. Here, based on the chemical design strategies for molecular ferroelectrics, we show a series of asymmetric 1,4,5,8-naphthalenediimide derivatives with the homochiral amine and 2,2,6,6-tetramethylpiperidine-N-oxyl components, which adopt the low-symmetric polar structure and so allow ferroelectricity. Upon H/F substitution, the fluorinated compounds exhibit reversible ferroelectric and martensitic transitions at 399 K accompanied by a large thermal hysteresis of 132 K. This large thermal hysteresis with two competing (meta)-stable phases is further confirmed by density functional theory calculations. The rare combination of martensitic phase transition and ferroelectricity realizes the bistability with two different ferroelectric phases at room temperature. Our finding provides insight into the exploration of martensitic ferroelectric compounds with potential applications in switchable memory devices, soft robotics, and smart actuators.

Intrinsic Dipole Arrangement to Coordinate Energy Levels for Efficient and Stable Perovskite Solar Cells

Despite great progress in perovskite photovoltaics, it should be noted that the intrinsic disorder dipolar cations in organic-inorganic hybrid perovskites exert negative effects on the energy band structure as well as the carrier separation and transfer dynamics. However, oriented polarization achieved by applying an external electric field may cause irreversible damage to perovskites. Herein, a unique and efficient strategy is developed to modulate the intrinsic dipole arrangement in perovskite films for high-performance and stable perovskite solar cells (PSCs). The spontaneous reorientation of dipolar cation methylamine is triggered by a polar molecule, constructing a vertical polarization during crystallization regulation. The oriented dipole determines a gradient energy-level arrangement in PSCs and more favorable energetics at interfaces, effectively enhancing the built-in electric field and suppressing the nonradiative recombination. Besides, the dipole reorientation induces a local dielectric environment to remarkably reduce exciton binding energy, leading to an ultralong carrier diffusion length of up to 1708 nm. Accordingly, the n-i-p PSCs achieve a significant increase in power conversion efficiency, reaching 24.63% with negligible hysteresis and exhibiting outstanding stabilities. This strategy also provides a facile route to eliminate the mismatched energetics and enhance carrier dynamics for other novel photovoltaic devices.

Piezoelectric Bioactive Glasses Composite Promotes Angiogenesis by the Synergistic Effect of Wireless Electrical Stimulation and Active Ions

Insufficient angiogenesis frequently occurs after the implantation of orthopedic materials, which greatly increases the risk of bone defect reconstruction failure. Therefore, the development of bone implant with improved angiogenic properties is of great importance. Mimicking the extracellular matrix clues provides a more direct and effective strategy to modulate angiogenesis. Herein, inspired by the bioelectrical characteristics of the bone microenvironment, a piezoelectric bioactive glasses composite (P-KNN/BG) based on the incorporation of polarized potassium sodium niobate is constructed, which could effectively promote angiogenesis. It is found that P-KNN/BG has exceptional wireless electrical stimulation performance and sustained active ions release. In vitro cell experiments reveal that P-KNN/BG enhances endothelial cell adhesion, migration, and differentiation via activating the eNOS/NO signaling pathway, which might be contributed to cell membrane hyperpolarization induced by wireless electrical stimulation increase the influx of active ions into the cells. In vivo chick chorioallantoic membrane experiment demonstrates that P-KNN/BG shows excellent pro-angiogenic capacity and biocompatibility. This work broadens the current understanding of bioactive materials with bionic electrical properties, which brings new insights into the clinical treatment of bone defect repair.

One-Dimensional Ferroelectric Nanoarrays with Wireless Switchable Static and Dynamic Electrical Stimulation for Selective Regulating Osteogenesis and Antiosteosarcoma

Preventing local tumor recurrence and simultaneously improving bone-tissue regeneration are in great demand for osteosarcoma therapy. However, the current therapeutic implants fail to selectively suppress tumor growth and enhance osteogenesis, and antitumor therapy may compromise osseointegration of the bone implant. Here, based on the different responses of bone tumor cells and osteoblasts to different electric stimulations, we constructed ferroelectric BaTiO₃ nanorod arrays (NBTO) on the surface of titanium implants with switchable dynamic and static electrical stimulation for selective bone-tumor therapy and bone tissue regeneration. Polarized NBTO (PNBTO) generated a sustained dynamic electrical stimulus in response to wireless ultrasonic irradiation ("switch-on"), which disrupted the orientation of the spindle filaments of the tumor cell, blocked the G2/M phase of mitosis, and ultimately led to tumor cell death, whereas it had almost no cytotoxic effect on normal bone cells. Under the switch-off state, PNBTO with a high surface potential provided static electrical stimulation, accelerating osteogenic differentiation of mesenchymal stem cells and enhancing the quality of bone regeneration both in vitro and in vivo. This study broadens the biomedical potential of electrical stimulation therapy and provides a comprehensive and clinically feasible strategy for the overall treatment and tissue regeneration in osteosarcoma.

Adaptive Memory and In Materia Reinforcement Learning Enabled by Flexoelectric-like Response from Ultrathin HfO2

Reinforcement learning (RL) is a mathematical framework of neural learning by trial and error that revolutionized the field of artificial intelligence. However, until now, RL has been implemented in algorithms with the compatibly of traditional complementary metal-oxide-semiconductor-based von Neumann digital platforms, which thus limits performance in terms of latency, fault tolerance, and robustness. Here, we demonstrate that nanocolumnar (∼12 nm) HfO₂ structures can be used as building blocks to conduct the RL within the material by combining its stress-adjustable charge transport and memory functions. Specifically, HfO₂ nanostructures grown by the sputtering method exhibit self-assembled vertical nanocolumnar structures that generate voltage depending on the impact of stress under self-biased conditions. The observed results are attributed to the flexoelectric-like response of HfO₂. Further, multilevel current (>10^-3 A) modulation with touch and controlled suspension (∼10^-12 A) with a short electric pulse (100 ms) were demonstrated, yielding a proof-of-concept memory with an on/off ratio greater than 10⁹. Utilizing multipattern dynamic memory and tactile sensing, RL was implemented to successfully solve a maze game using an array of 6 × 4. This work could pave the way to do RL within materials for a variety of applications such as memory storage, neuromorphic sensors, smart robots, and human-machine interaction systems.

Ferroelectric Field-Effect-Transistor Integrated with Ferroelectrics Heterostructure

To address the demands of emerging data-centric computing applications, ferroelectric field-effect transistors (Fe-FETs) are considered the forefront of semiconductor electronics owing to their energy and area efficiency and merged logic-memory functionalities. Herein, the fabrication and application of an Fe-FET, which is integrated with a van der Waals ferroelectrics heterostructure (CuInP₂ S₆ /α-In₂ Se₃ ), is reported. Leveraging enhanced polarization originating from the dipole coupling of CIPS and α-In₂ Se₃ , the fabricated Fe-FET exhibits a large memory window of 14.5 V at V_GS = ±10 V, reaching a memory window to sweep range of ≈72%. Piezoelectric force microscopy measurements confirm the enhanced polarization-induced wider hysteresis loop of the double-stacked ferroelectrics compared to single ferroelectric layers. The Landau-Khalatnikov theory is extended to analyze the ferroelectric characteristics of a ferroelectric heterostructure, providing detailed explanations of the hysteresis behaviors and enhanced memory window formation. The fabricated Fe-FET shows nonvolatile memory characteristics, with a high on/off current ratio of over 10⁶ , long retention time (>10⁴ s), and stable cyclic endurance (>10⁴ cycles). Furthermore, the applicability of the ferroelectrics heterostructure is investigated for artificial synapses and for hardware neural networks through training and inference simulation. These results provide a promising pathway for exploring low-dimensional ferroelectronics.

Metal-Free Perovskite Piezoelectric Nanogenerators for Human-Machine Interfaces and Self-Powered Electrical Stimulation Applications

Single crystal metal-free halide perovskites have received great attention in recent years owing to their excellent piezoelectric and ferroelectric properties. However, the nanotoxicity and piezoelectricity within the nanoscale of such materials have yet been reported for the demonstration of practical applications. In this work, the observation of intrinsic piezoelectricity in metal-free perovskite (MDABCO-NH₄ I₃ ) films using piezoresponse force microscopy (PFM) is reported. A cytotoxicity test is also performed on MDABCO-NH₄ I₃ to evaluate its low-toxic nature. The as-synthesized MDABCO-NH₄ I₃ is further integrated into a piezoelectric nanogenerator (PENG). The MDABCO-NH₄ I₃ -based PENG (MN-PENG) exhibits optimal output voltage and current of 15.9 V and 54.5 nA, respectively. In addition, the MN-PENG can serve as a self-powered strain sensor for human-machine interface applications or be adopted in in vitro electrical stimulation devices. This work demonstrates a path of perovskite-based PENG with high performance, low toxicity, and multifunctionality for future advanced wearable sensors and portable therapeutic systems.

Highly enhanced ferroelectricity in HfO2-based ferroelectric thin film by light ion bombardment

Continuous advancement in nonvolatile and morphotropic beyond-Moore electronic devices requires integration of ferroelectric and semiconductor materials. The emergence of hafnium oxide (HfO₂)-based ferroelectrics that are compatible with atomic-layer deposition has opened interesting and promising avenues of research. However, the origins of ferroelectricity and pathways to controlling it in HfO₂ are still mysterious. We demonstrate that local helium (He) implantation can activate ferroelectricity in these materials. The possible competing mechanisms, including He ion-induced molar volume changes, vacancy redistribution, vacancy generation, and activation of vacancy mobility, are analyzed. These findings both reveal the origins of ferroelectricity in this system and open pathways for nanoengineered binary ferroelectrics.

An unprecedented azobenzene-based organic single-component ferroelectric

The first azobenzene-based organic single-component ferroelectric 2-amino-2′,4,4′,6,6′-pentafluoroazobenzene was designed, which shows an exceptionally high Curie temperature (Tc) of 443 K.

Breaking the Fundamental Limitations of Nanoscale Ferroelectric Characterization: Non-Contact Heterodyne Electrostrain Force Microscopy

Perceiving nanoscale ferroelectric phenomena from real space is of great importance for elucidating underlying ferroelectric physics. During the past decades, nanoscale ferroelectric characterization has mainly relied on the Piezoresponse Force Microscopy (PFM) invented in 1992, however, the fundamental limitations of PFM have made the nanoscale ferroelectric studies encounter significant bottlenecks. In this study, a high-resolution non-contact ferroelectric measurement, named Non-Contact Heterodyne Electrostrain Force Microscopy (NC-HEsFM), is introduced. It is demonstrated that NC-HEsFM can operate on multiple eigenmodes to perform ideal high-resolution ferroelectric domain mapping, standard ferroelectric hysteresis loop measurement, and controllable domain manipulation. By using a quartz tuning fork (QTF) sensor, multi-frequency operation, and heterodyne detection schemes, NC-HEsFM achieves a real non-contact yet non-destructive ferroelectric characterization with negligible electrostatic force effect and hence breaks the fundamental limitations of the conventional PFM. It is believed that NC-HEsFM can be extensively used in various ferroelectric or piezoelectric studies with providing substantially improved characterization performance. Meanwhile, the QTF-based force detection makes NC-HEsFM highly compatible for high-vacuum and low-temperature environments, providing ideal conditions for investigating the intrinsic ferroelectric phenomena with the possibility of achieving an atomically resolved ferroelectric characterization.

Quantitative Local Probing of Polarization with Application on HfO2 -Based Thin Films

Owing to their switchable spontaneous polarization, ferroelectric materials have been applied in various fields, such as information technologies, actuators, and sensors. In the last decade, as the characteristic sizes of both devices and materials have decreased significantly below the nanoscale, the development of appropriate characterization tools became essential. Recently, a technique based on conductive atomic force microscopy (AFM), called AFM-positive-up-negative-down (PUND), is employed for the direct measurement of ferroelectric polarization under the AFM tip. However, the main limitation of AFM-PUND is the low frequency (i.e., on the order of a few hertz) that is used to initiate ferroelectric hysteresis. A significantly higher frequency is required to increase the signal-to-noise ratio and the measurement efficiency. In this study, a novel method based on high-frequency AFM-PUND using continuous waveform and simultaneous signal acquisition of the switching current is presented, in which polarization-voltage hysteresis loops are obtained on a high-polarization BiFeO₃ nanocapacitor at frequencies up to 100 kHz. The proposed method is comprehensively evaluated by measuring nanoscale polarization values of the emerging ferroelectric Hf_0.5 Zr_0.5 O₂ under the AFM tip.

"May the Force Be with You!" Force-Volume Mapping with Atomic Force Microscopy

Information of the chemical, mechanical, and electrical properties of materials can be obtained using force volume mapping (FVM), a measurement mode of scanning probe microscopy (SPM). Protocols have been developed with FVM for a broad range of materials, including polymers, organic films, inorganic materials, and biological samples. Multiple force measurements are acquired with the FVM mode within a defined 3D volume of the sample to map interactions (i.e., chemical, electrical, or physical) between the probe and the sample. Forces of adhesion, elasticity, stiffness, deformation, chemical binding interactions, viscoelasticity, and electrical properties have all been mapped at the nanoscale with FVM. Subsequently, force maps can be correlated with features of topographic images for identifying certain chemical groups presented at a sample interface. The SPM tip can be coated to investigate-specific reactions; for example, biological interactions can be probed when the tip is coated with biomolecules such as for recognition of ligand-receptor pairs or antigen-antibody interactions. This review highlights the versatility and diverse measurement protocols that have emerged for studies applying FVM for the analysis of material properties at the nanoscale.

Phase Transition Effect on Ferroelectric Domain Surface Charge Dynamics in BaTiO3 Single Crystal

The ferroelectric domain surface charge dynamics after a cubic-to-tetragonal phase transition on the BaTiO₃ single crystal (001) surface was directly measured through scanning probe microscopy. The captured surface potential distribution shows significant changes: the domain structures formed rapidly, but the surface potential on polarized c domain was unstable and reversed its sign after lengthy lapse; the high broad potential barrier burst at the corrugated a-c domain wall and continued to dissipate thereafter. The generation of polarization charges and the migration of surface screening charges in the surrounding environment take the main responsibility in the experiment. Furthermore, the a-c domain wall suffers large topological defects and polarity variation, resulting in domain wall broadening and stress changes. Thus, the a-c domain wall has excess energy and polarization change is inclined to assemble on it. The potential barrier decay with time after exposing to the surrounding environment also gave proof of the surface screening charge migration at surface. Thus, both domain and domain wall characteristics should be taken into account in ferroelectric application.

Coexistence of magnetic and electric orderings in a divalent Cr2+-based multiaxial molecular ferroelectric

Multiferroic materials have attracted great interest because of their underlying new science and promising applications in data storage and mutual control devices. However, they are still very rare and highly imperative to be developed. Here, we report an organic-inorganic hybrid perovskite trimethylchloromethylammonium chromium chloride (TMCM-CrCl3), showing the coexistence of magnetic and electric orderings. It displays a paraelectric-ferroelectric phase transition at 397 K with an Aizu notation of 6/mFm, and spin-canted antiferromagnetic ordering with a Néel temperature of 4.8 K. The ferroelectricity originates from the orientational ordering of TMCM cations, and the magnetism is from the [CrCl3]- framework. Remarkably, TMCM-CrCl3 is the first experimentally confirmed divalent Cr2+-based multiferroic material as far as we know. A new category of hybrid multiferroic materials is pointed out in this work, and more Cr2+-based multiferroic materials will be expectedly developed in the future.

PFM (piezoresponse force microscopy)-aided design for molecular ferroelectrics

With prosperity, decay, and another spring, molecular ferroelectrics have passed a hundred years since Valasek first discovered ferroelectricity in the molecular compound Rochelle salt. Recently, the proposal of ferroelectrochemistry has injected new vigor into this century-old research field. It should be highlighted that piezoresponse force microscopy (PFM) technique, as a non-destructive imaging and manipulation method for ferroelectric domains at the nanoscale, can significantly speed up the design rate of molecular ferroelectrics as well as enhance the ferroelectric and piezoelectric performances relying on domain engineering. Herein, we provide a brief review of the contribution of the PFM technique toward assisting the design and performance optimization of molecular ferroelectrics. Relying on the relationship between ferroelectric domains and crystallography, together with other physical characteristics such as domain switching and piezoelectricity, we believe that the PFM technique can be effectively applied to assist the design of high-performance molecular ferroelectrics equipped with multifunctionality, and thereby facilitate their practical utilization in optics, electronics, magnetics, thermotics, and mechanics among others.

Nanoscale Ferroelectric Characterization with Heterodyne Megasonic Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM), as a powerful nanoscale characterization technique, has been extensively utilized to elucidate diverse underlying physics of ferroelectricity. However, intensive studies of conventional PFM have revealed a growing number of concerns and limitations which are largely challenging its validity and applications. In this study, an advanced PFM technique is reported, namely heterodyne megasonic piezoresponse force microscopy (HM-PFM), which uses 106 to 108 Hz high-frequency excitation and heterodyne method to measure the piezoelectric strain at nanoscale. It is found that HM-PFM can unambiguously provide standard ferroelectric domain and hysteresis loop measurements, and an effective domain characterization with excitation frequency up to ≈110 MHz is demonstrated. Most importantly, owing to the high-frequency and heterodyne scheme, the contributions from both electrostatic force and electrochemical strain can be significantly minimized in HM-PFM. Furthermore, a special measurement of difference-frequency piezoresponse frequency spectrum (DFPFS) is developed on HM-PFM and a distinct DFPFS characteristic is observed on the materials with piezoelectricity. By performing DFPFS measurement, a truly existed but very weak electromechanical coupling in CH3NH3PbI3 perovskite is revealed. It is believed that HM-PFM can be an excellent candidate for the ferroelectric or piezoelectric studies where conventional PFM results are highly controversial.