Recent Developments in (K, Na)NbO3-Based Lead-Free Piezoceramics.

(K0.5Na0.5)NbO3 (KNN)-based ceramics have been extensively investigated as replacements for Pb(Zr, Ti)O3-based ceramics. KNN-based ceramics exhibit an orthorhombic structure at room temperature and a rhombohedral-orthorhombic (R-O) phase transition temperature (TR-O), orthorhombic-tetragonal (O-T) phase transition temperature (TO-T), and Curie temperature of -110, 190, and 420 °C, respectively. Forming KNN-based ceramics with a multistructure that can assist in domain rotation is one technique for enhancing their piezoelectric properties. This review investigates and introduces KNN-based ceramics with various multistructures. A reactive-templated grain growth method that aligns the grains of piezoceramics in a specific orientation is another approach for improving the piezoelectric properties of KNN-modified ceramics. The piezoelectric properties of the [001]-textured KNN-based ceramics are improved because their microstructures are similar to those of the [001]-oriented single crystals. The improvement in the piezoelectric properties after [001] texturing is largely influenced by the crystal structure of the textured ceramics. In this review, [001]-textured KNN-based ceramics with different crystal structures are investigated and systematically summarized.

Direct Printing of Ultrathin Block Copolymer Film with Nano-in-Micro Pattern Structures.

Nanotransfer printing (nTP) is one of the most promising nanopatterning methods given that it can be used to produce nano-to-micro patterns effectively with functionalities for electronic device applications. However, the nTP process is hindered by several critical obstacles, such as sub-20 nm mold technology, reliable large-area replication, and uniform transfer-printing of functional materials. Here, for the first time, a dual nanopatterning process is demonstrated that creates periodic sub-20 nm structures on the eight-inch wafer by the transfer-printing of patterned ultra-thin (<50 nm) block copolymer (BCP) film onto desired substrates. This study shows how to transfer self-assembled BCP patterns from the Si mold onto rigid and/or flexible substrates through a nanopatterning method of thermally assisted nTP (T-nTP) and directed self-assembly (DSA) of Si-containing BCPs. In particular, the successful microscale patternization of well-ordered sub-20 nm SiOx patterns is systematically presented by controlling the self-assembly conditions of BCP and printing temperature. In addition, various complex pattern geometries of nano-in-micro structures are displayed over a large patterning area by T-nTP, such as angular line, wave line, ring, dot-in-hole, and dot-in-honeycomb structures. This advanced BCP-replicated nanopatterning technology is expected to be widely applicable to nanofabrication of nano-to-micro electronic devices with complex circuits.

Layer-Controlled Perovskite 2D Nanosheet Interlayer for the Energy Storage Performance of Nanocomposites.

Polymer-based nanocomposites are desirable materials for next-generation dielectric capacitors. 2D dielectric nanosheets have received significant attention as a filler. However, randomly spreading the 2D filler causes residual stresses and agglomerated defect sites in the polymer matrix, which leads to the growth of an electric tree, resulting in a more premature breakdown than expected. Therefore, realizing a well-aligned 2D nanosheet layer with a small amount is a key challenge; it can inhibit the growth of conduction paths without degrading the performance of the material. Here, an ultrathin Sr1.8 Bi0.2 Nb3 O10 (SBNO) nanosheet filler is added as a layer into poly(vinylidene fluoride) (PVDF) films via the Langmuir-Blodgett method. The structural properties, breakdown strength, and energy storage capacity of a PVDF and multilayer PVDF/SBNO/PVDF composites as a function of the thickness-controlled SBNO layer are examined. The seven-layered (only 14 nm) SBNO nanosheets thin film can sufficiently prevent the electrical path in the PVDF/SBNO/PVDF composite and shows a high energy density of 12.8 J cm-3 at 508 MV m-1 , which is significantly higher than that of the bare PVDF film (9.2 J cm-3 at 439 MV m-1 ). At present, this composite has the highest energy density among the polymer-based nanocomposites under the filler of thin thickness.

Highly Reliable Threshold Switching Characteristics of Surface-Modulated Diffusive Memristors Immune to Atmospheric Changes.

Active cation-based diffusive memristors featuring essentially volatile threshold switching have been proposed for novel applications, such as a selector in a one-selector-and-one-resistor structure and signal generators in neuromorphic computing. However, the high variability of the switching behavior, which results from the high electroforming voltage, external environmental conditions, and transition to the non-volatile switching mode in a high-current range, is considered a major impediment to such applications. Herein, for the first time, we developed a highly reliable threshold switching device immune to atmospheric changes based on an ultraviolet-ozone (UVO)-treated diffusive memristor consisting of Ag and SiO2 nanorods (NRs). UVO treatment forms a stable water reservoir on the surface of SiO2 NRs, facilitating the redox reaction and ion migration of Ag. Consequently, diffusive memristors possess reliable switching characteristics, including electroforming-free, repeatable, and consistent switching with resistance to changes in ambient conditions and compliance levels during operation. We demonstrated that our approach is suitable for various metal oxides and can be used in numerous applications.

Autonomous Resonance-Tuning Mechanism for Environmental Adaptive Energy Harvesting.

An innovative autonomous resonance-tuning (ART) energy harvester is reported that utilizes adaptive clamping systems driven by intrinsic mechanical mechanisms without outsourcing additional energy. The adaptive clamping system modulates the natural frequency of the harvester's main beam (MB) by adjusting the clamping position of the MB. The pulling force induced by the resonance vibration of the tuning beam (TB) provides the driving force for operating the adaptive clamp. The ART mechanism is possible by matching the natural frequencies of the TB and clamped MB. Detailed evaluations are conducted on the optimization of the adaptive clamp tolerance and TB design to increase the pulling force. The energy harvester exhibits an ultrawide resonance bandwidth of over 30 Hz in the commonly accessible low vibration frequency range (<100 Hz) owing to the ART function. The practical feasibility is demonstrated by evaluating the ART performance under both frequency and acceleration-variant conditions and powering a location tracking sensor.

Correlating multimode strain and electrode configurations for high-performance gradient-index phononic crystal-based piezoelectric energy harvesting.

A gradient-index phononic crystal (GRIN PnC) capable of manipulating wave propagation can serve as an excellent input wave energy focusing platform for amplifying energy harvesting power generation. However, despite its remarkable focusing capability, the finite wavelength of the propagating elastic waves in the focal area causes voltage cancellation inside a piezoelectric element under multimode strains having opposite directions; this limits the capacity of the GRIN PnC-based energy harvesting system. This study demonstrates a rational electrode configuration for a piezoelectric energy harvesting (PEH) device that can maximize the performance of a given GRIN PnC platform. The multimode strain analysis experimentally performed on the PEHs distributed over the focusing area confirms that the patterned electrode PEH configuration is the most effective in alleviating strain and voltage cancellation while efficiently transferring the focused elastic wave energy. Furthermore, a proper combination of electrical connections between the patterned electrodes substantially increases the piezoelectric potential across the ceramic by maximizing the strain difference. The simultaneous tailoring of the piezoelectric ceramic composition and the electrode configuration leads to a maximum power generation of 7.06 mW even under off-resonance conditions, the largest ever reported in elastic wave energy harvesting.

Multiscale landscaping of droplet wettability on fibrous layers of facial masks.

Liquid mobility is ubiquitous in nature, with droplets emerging at all size scales, and artificial surfaces have been designed to mimic such mobility over the past few decades. Meanwhile, millimeter-sized droplets are frequently used for wettability characterization, even with facial mask applications, although these applications have a droplet-size target range that spans from millimeters to aerosols measuring less than a few micrometers. Unlike large droplets, microdroplets can interact sensitively with the fibers they contact with and are prone to evaporation. However, wetting behaviors at the single-microfiber level remain poorly understood. Herein, we characterized the wettability of fibrous layers, which revealed that a multiscale landscape of droplets ranged from the millimeter to the micrometer scale. The contact angle (CA) values of small droplets on pristine fibrous media showed sudden decrements, especially on a single microfiber, owing to the lack of air cushions for the tiny droplets. Moreover, droplets easily adhered to the pristine layer during droplet impact tests and then yielding widespread areas of contamination on the microfibers. To resolve this, we carved nanowalls on the pristine fibers by plasma etching, which effectively suppressed such wetting phenomena. Significantly, the resulting topographies of the microfibers managed the dynamic wettability of droplets at the multiscale, which reduced the probability of contamination with impact droplets and suppressed the wetting transition upon evaporation. These findings for the dynamic wettability of fibrous media will be useful in the fight against infectious droplets.

Heterointerface Effect on Two-Step Nucleation Mechanism of Bi Particles.

The nucleation and crystallization of Bi particles on two matrices, crystalline bismuth sulfide (c-Bi2S3) and amorphized bismuth titanium oxide (a-Bi12TiO20), were studied by using in situ transmission electron microscopy (TEM) analysis. The atomic structures of the Bi particles were monitored by acquiring high-resolution TEM images in real time. The Bi particles were grown on c-Bi2S3 and a-Bi12TiO20 via a two-step nucleation mechanism; dense liquid clusters were clearly observed at the initial stage of nucleation, and the coalescence of clusters was frequently observed during the growth. However, the nucleation and crystallization behaviors of Bi particles were governed by the matrix; in particular, the evolution of their morphology and atomic structure was confined on c-Bi2S3 but free from matrix effects on a-Bi12TiO20. The matrix effect on the two-step nucleation mechanism was demonstrated from a thermodynamic point of view.

Effect of Oxygen Vacancy on the Conduction Modulation Linearity and Classification Accuracy of Pr0.7Ca0.3MnO3 Memristor.

An amorphous Pr0.7Ca0.3MnO3 (PCMO) film was grown on a TiN/SiO2/Si (TiN-Si) substrate at 300 °C and at an oxygen pressure (OP) of 100 mTorr. This PCMO memristor showed typical bipolar switching characteristics, which were attributed to the generation and disruption of oxygen vacancy (OV) filaments. Fabrication of the PCMO memristor at a high OP resulted in nonlinear conduction modulation with the application of equivalent pulses. However, the memristor fabricated at a low OP of 100 mTorr exhibited linear conduction modulation. The linearity of this memristor improved because the growth and disruption of the OV filaments were mostly determined by the redox reaction of OV owing to the presence of numerous OVs in this PCMO film. Furthermore, simulation using a convolutional neural network revealed that this PCMO memristor has enhanced classification performance owing to its linear conduction modulation. This memristor also exhibited several biological synaptic characteristics, indicating that an amorphous PCMO thin film fabricated at a low OP would be a suitable candidate for artificial synapses.

Optimization of Hybrid Ink Formulation and IPL Sintering Process for Ink-Jet 3D Printing.

Ink-jet 3D printing technology facilitates the use of various materials of ink on each ink-jet head and simultaneous printing of multiple materials. It is suitable for manufacturing to process a complex multifunctional structure such as sensors and printed circuit boards. In this study, a complex structure of a SiO2 insulation layer and a conductive Cu layer was fabricated with photo-curable nano SiO2 ink and Intense Pulsed Light (IPL)-sinterable Cu nano ink using multi-material ink-jet 3D printing technology. A precise photo-cured SiO2 insulation layer was designed by optimizing the operating conditions and the ink rheological properties, and the resistance of the insulation layer was 2.43 × 1013 Ω·cm. On the photo-cured SiO2 insulation layer, a Cu conductive layer was printed by controlling droplet distance. The sintering of the IPL-sinterable nano Cu ink was performed using an IPL sintering process, and electrical and mechanical properties were confirmed according to the annealing temperature and applied voltage. Then, Cu conductive layer was annealed at 100 °C to remove the solvent, and IPL sintered at 700 V. The Cu conductive layer of the complex structure had an electrical property of 29 µΩ·cm and an adhesive property with SiO2 insulation layer of 5B.

Piezoelectric Energy Harvesting Design Principles for Materials and Structures: Material Figure-of-Merit and Self-Resonance Tuning.

Piezoelectric energy harvesters (PEHs) aim to generate sufficient power to operate targeting device from the limited ambient energy. PEH includes mechanical-to-mechanical, mechanical-to-electrical, and electrical-to-electrical energy conversions, which are related to PEH structures, materials, and circuits, respectively; these should be efficient for increasing the total power. This critical review focuses on PEH structures and materials associated with the two major energy conversions to improve PEH performance. First, the resonance tuning mechanisms for PEH structures maintaining continuous resonance, regardless of a change in the vibration frequency, are presented. Based on the manual tuning technique, the electrically- and mechanically-driven self-resonance tuning (SRT) techniques are introduced in detail. The representative SRT harvesters are summarized in terms of tunability, power consumption, and net power. Second, the figure-of-merits of the piezoelectric materials for output power are summarized based on the operating conditions, and optimal piezoelectric materials are suggested. Piezoelectric materials with large kij , dij , and gij values are suitable for most PEHs, whereas those with large kij and Qm values should be used for on-resonance conditions, wherein the mechanical energy is directly supplied to the piezoelectric material. This comprehensive review provides insights for designing efficient structures and selection of proper piezoelectric materials for PEHs.

Single-step plasma-induced hierarchical structures for tunable water adhesion.

Smart surfaces in nature have been extensively studied to identify their hierarchical structures in micro-/nanoscale to elucidate their superhydrophobicity with varying water adhesion. However, mimicking hybrid features in multiscale requires complex, multi-step processes. Here, we proposed a one-step process for the fabrication of hierarchical structures composed in micro-/nanoscales for superhydrophobic surfaces with tunable water adhesion. Hierarchical patterns were fabricated using a plasma-based selective etching process assisted by a dual scale etching mask. As the metallic mesh is placed above the substrate, it serves the role of dual scale etching masks on the substrate: microscale masks to form the micro-wall network and nanoscale masks to form high-aspect-ratio nanostructures. The micro-walls and nanostructures can be selectively hybridized by adjusting the gap distance between the mesh and the target surface: single nanostructures on a large area for a larger gap distance and hybrid/hierarchical structures with nanostructures nested on micro-walls for a shorter gap distance. The hierarchically nanostructured surface shows superhydrophobicity with low water adhesion, while the hybrid structured surface becomes become superhydrophobic with high adhesion. These water adhesion tunable surfaces were explored for water transport and evaporation. Additionally, we demonstrated a robust superhydrophobic surface with anti-reflectance over a large area.

Subwavelength Hollow-Nanopillared Glass with Gradient Refractive Index for Ultralow Diffuse Reflectance and Antifogging.

Nanostructured glass with subwavelength hollow nanopillars of diameters of sub-65 nm was fabricated, showing high optical transmittance and ultralow diffuse reflectance. A simple process involving single-step plasma etching was used on a glass slide coated with a SiO2 sacrificial film. First, SiO2 nanodot structures were formed using plasma-induced anisotropic etching with CF4 plasma. The SiO2 nanodot array then became a secondary etching mask to form hollow nanopillars on the glass. The hollow structures formed at the upper part reaching up to the apex of the nanopillar had a lower solid fraction, while the lower part had a higher fraction. The refractive index (RI) gradually increased from 1.09 (near the value for air) to 1.42 (near the value for glass). Geometry-induced RI gradient enhanced light transmi, while it significantly reduced diffuse reflectance, particularly in the shorter wavelengths, thus suppressing the haziness or milky appearance of the nanostructured glass. Superhydrophilic and antifogging properties of nanostructured glasses and dental mirrored glasses were also demonstrated with water spraying and exhaled breath tests. Results showed that the wettability was enhanced in hydrophilicity and antifogging property by both the hydrophilic nature of the glass and the newly formed nanostructures. The nanostructured, superhydrophilic glass was also found to have easy cleaning nature against fine sand dust adhesion by simply blowing air or spraying water. Results of this study showed that such a hollow-pillared glass surface with gradient RI and special wettability could be applied in a variety of optical and optoelectronic applications requiring superwetting, such as optical windows for solar cell panels, display panels, light-emitting diodes, and medical devices even with curved surfaces.

Improvement of Conductance Modulation Linearity in a Cu2+-Doped KNbO3 Memristor through the Increase of the Number of Oxygen Vacancies.

The Pt/KNbO3/TiN/Si (KN) memristor exhibits various biological synaptic properties. However, it also displays nonlinear conductance modulation with the application of identical pulses, indicating that it should be improved for neuromorphic applications. The abrupt change of the conductance originates from the inhomogeneous growth/dissolution of oxygen vacancy filaments in the KN film. The change of the filaments in a KN film is controlled by two mechanisms with different growth/dissolution rates: a redox process with a fast rate and an oxygen vacancy diffusion process with a slow rate. Therefore, the conductance modulation linearity can be improved if the growth/dissolution of the filaments is controlled by only one mechanism. When the number of oxygen vacancies in the KN film was increased through doping of Cu2+ ions, the growth/dissolution of the filaments in the Cu2+-doped KN (CKN) film was mainly influenced by the redox process of oxygen vacancies. Therefore, the CKN film exhibited improved conductance modulation linearity, confirming that the linearity of conductance modulation can be improved by increasing the number of oxygen vacancies in the memristor. This method can be applied to other memristors to improve the linearity of conductance modulation. The CKN memristor also provides excellent biological synaptic characteristics for neuromorphic computing systems.

Highly Sensitive and Selective PbTiO3 Gas Sensors with Negligible Humidity Interference in Ambient Atmosphere.

Three PbTiO3 nanostructures were synthesized using a one-step hydrothermal reaction with different TiO2 powders as Ti sources, and their gas-sensing properties were investigated. The sensor comprising PbTiO3 nanoplates (NPs) exhibited a high response (resistance ratio = 80.4) to 5 ppm ethanol at 300 °C and could detect trace concentrations of ethanol down to 100 ppb. Moreover, the sensor showed high ethanol selectivity and nearly the same sensing characteristics despite the wide range of humidity variation from 20 to 80% RH. The mechanism for humidity-independent gas sensing was elucidated using diffuse reflectance infrared Fourier transform spectra. PbTiO3 NPs are new and promising sensing materials that can be used for detecting ethanol in a highly sensitive and selective manner with negligible interference from ambient humidity.

Synaptic Plasticity and Metaplasticity of Biological Synapse Realized in a KNbO3 Memristor for Application to Artificial Synapse.

Amorphous KNbO3 (KN) films were grown on a TiN/SiO2/Si substrate to synthesize a KN memristor as a potential artificial synapse. The Pt/KN/TiN memristor exhibited typical and reliable bipolar switching behavior with multiple resistance levels. It also showed the transmission properties of a biological synapse, with a good conductance modulation linearity. Moreover, the KN memristor can emulate various biological synaptic plasticity characteristics including short-term plasticity, long-term plasticity, spike-rate dependent plasticity, paired-pulse facilitation, and post-tetanic potentiation by controlling the number and rate of the potentiation spike. Spike-timing-dependent plasticity (STDP), which is an essential property of biological synapses, is also realized in the KN memristor. The synaptic plasticity of the KN memristor can be explained by oxygen vacancy movement and oxygen vacancy filaments. The metaplasticity of biological synapses was also implemented in the KN memristor, including the metaplasticity of long-term potentiation and depression, and of STDP. Therefore, the KN memristor could be used as an artificial synapse in neuromorphic computing systems.

Physical Properties of (Na1- xK x)NbO3 Thin Film Grown at Low Temperature Using Two-Dimensional Ca2Nb3O10 Nanosheet Seed Layer.

A monolayer Ca2Nb3O10 (CNO) nanosheet was deposited on a Pt/Ti/SiO2/Si substrate using the Langmuir-Blodgett method. This monolayer CNO nanosheet with a (001) surface termination was used as a seed layer to reduce the growth temperature of the crystalline (Na1- xK x)NbO3 (NKN) film. The crystalline NKN film was preferentially grown along the [001] direction at 400 °C. The ferroelectric and piezoelectric properties of this NKN film were influenced by the postannealing atmosphere due to the variations in the amounts of oxygen vacancies in the NKN film. The crystalline NKN film annealed at 300 °C under 50 Torr O2 atmosphere showed promising ferroelectric and piezoelectric properties; εr of 303 and tan δ of 2.0% at 100 kHz, Ps of 15.3 µC/cm2, Pr of 11.7 µC/cm2, and Ec of 78 kV/cm, and d33 of 139 pm/V. This NKN film showed the lowest leakage current, which can be explained by the Schottky emission mechanism. The Schottky barrier heights of the Pt/NKN and NKN/CNO/Pt interfaces were calculated to be 0.97 and 0.28 eV, respectively. The results of this work suggest a new method to grow crystalline thin films at low temperatures by using metal-oxide nanosheets as the seed layer.

Low-Temperature-Grown KNbO3 Thin Films and Their Application to Piezoelectric Nanogenerators and Self-Powered ReRAM Device.

Amorphous KNbO3 (KN) film containing KN nanocrystals was grown on TiN/SiO2/Si substrate at 350 °C. This KN film showed a dielectric constant (εr) and a piezoelectric strain constant (d33) of 43 and 80 pm/V at 10 V, respectively, owing to the existence of KN nanocrystals. Piezoelectric nanogenerators (PNGs) were fabricated using KN films grown on the TiN/polyimide/poly(ethylene terephthalate) substrates. The PNG fabricated with the KN film grown at 350 °C showed an open-circuit output voltage of 2.5 V and a short-circuit current of 70 nA. The KN film grown at 350 °C exhibited a bipolar resistive switching behavior with good reliability characteristics that can be explained by the formation and rupture of the oxygen vacancy filaments. The KN resistive random access memory device powered by the KN PNG also showed promising resistive switching behavior. Moreover, the KN film shows good biocompatibility. Therefore, the KN film can be used for self-powered biomedical devices.

Flexible Indium-Tin Oxide Crystal on Plastic Substrates Supported by Graphene Monolayer.

Flexible and crystallized indium-tin oxide (ITO) thin films were successfully obtained on plastic polyethylene terephthalate (PET) films with monolayered graphene as a platform. The highly crystalline ITO (c-ITO) was first fabricated on a rigid substrate of graphene on copper foil and it was subsequently transferred onto a PET substrate by a well-established technique. Despite the plasma damage during ITO deposition, the graphene layer effectively acted as a Cu-diffusion barrier. The c-ITO/graphene/PET electrode with the 60-nm-thick ITO exhibited a reasonable sheet resistance of ~45 Ω sq-1 and a transmittance of ~92% at a wavelength of 550 nm. The c-ITO on the monolayered graphene support showed significant enhancement in flexibility compared with the ITO/PET film without graphene because the atomically controlled monolayered graphene acted as a mechanically robust support. The prepared flexible transparent c-ITO/graphene/PET electrode was applied as the anode in a bulk heterojunction polymer solar cell (PSC) to evaluate its performance, which was comparable with that of the commonly used c-ITO/glass electrode. These results represent important progress in the fabrication of flexible transparent electrodes for future optoelectronics applications.

Sintering and Microstructure of BaTiO3 Nano Particles Synthesized by Molten Salt Method.

In order to establish thinner dielectric layers in thick film electronic components such as MLCC (Multilayer ceramic capacitor), BaTiO3 nanoparticles have been utilized. However, studies on the synthesis of nanoparticles smaller than 20 nm, the characteristics of the BaTiO3 powder, and the powder's sintering are lacking. Therefore, this paper aims to synthesize BaTiO3 particles smaller than 20 nm by using the molten salt method and evaluate the microstructure and dielectric properties by varying the sintering temperature from 750 degrees C to 1200 degrees C. Through the molten salt method and by using KOH-KCl mixed salt, 20 nm BaTiO3 powder was synthesized at a low temperature of 150 degrees C. Sintering the pellets formed from the synthesized 20 nm BaTiO3 nano powder led to the observation of an unusual phenomenon where the particles grew to approximate sizes below 850 degrees C where densification progressed. At sintering temperatures above 950 degrees C, particles that expanded into rod shapes were observed and these particles were identified to be unreacted TiO2 based on the results of the EDX (Energy Dispersive X-ray Spectroscopy) analysis and phase analysis results.