X-ray-induced piezoresponse during X-ray photon correlation spectroscopy of PbMg1/3Nb2/3O3.

X-ray photon correlation spectroscopy (XPCS) holds strong promise for observing atomic-scale dynamics in materials, both at equilibrium and during non-equilibrium transitions. Here an in situ XPCS study of the relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) is reported. A weak applied AC electric field generates strong response in the speckle of the diffuse scattering from the polar nanodomains, which is captured using the two-time correlation function. Correlated motions of the Bragg peak are also observed, which indicate dynamic tilting of the illuminated volume. This tilting quantitatively accounts for the observed two-time speckle correlations. The magnitude of the tilting would not be expected solely from the modest applied field, since PMN is an electrostrictive material with no linear strain response to the field. A model is developed based on non-uniform static charging of the illuminated surface spot by the incident micrometre-scale X-ray beam and the electrostrictive material response to the combination of static and dynamic fields. The model qualitatively explains the direction and magnitude of the observed tilting, and predicts that X-ray-induced piezoresponse could be an important factor in correctly interpreting results from XPCS and nanodiffraction studies of other insulating materials under applied AC field or varying X-ray illumination.

Ferroelectric domain wall dynamics characterized with X-ray photon correlation spectroscopy.

Technologically important properties of ferroic materials are determined by their intricate response to external stimuli. This response is driven by distortions of the crystal structure and/or by domain wall motion. Experimental separation of these two mechanisms is a challenging problem which has not been solved so far. Here, we apply X-ray photon correlation spectroscopy (XPCS) to extract the contribution of domain wall dynamics to the overall response. Furthermore, we show how to distinguish the dynamics related to the passing of domain walls through the periodic (Peierls) potential of the crystal lattice and through the random potential caused by lattice defects (pinning centers). The approach involves the statistical analysis of correlations between X-ray speckle patterns produced by the interference of coherent synchrotron X-rays scattered from different nanosize volumes of the crystal and identification of Poisson-type contribution to the statistics. We find such a contribution in the thermally driven response of the monoclinic phase of a ferroelectric PbZr0.55Ti0.45O3 crystal and calculate the number of domain wall jumps in the studied microvolume.

Reliable resistive switching of epitaxial single crystalline cubic Y-HfO2 RRAMs with Si as bottom electrodes.

Previous studies have mainly focused on the resistive switching (RS) of amorphous or polycrystalline HfO2-RRAM. The RS of single crystalline HfO2 films has been rarely reported. Yttrium doped HfO2 (YDH) thin films were fabricated and successful Y incorporation into HfO2 was confirmed by x-ray photoemission spectroscopy. A pure cubic phase of YDH and an abrupt YDH/Si interface were obtained and verified by x-ray diffraction, Raman spectroscopy and transmission electron microscopy. A Pt/YDH/n++-Si heterostructure using Si as the bottom electrode was fabricated, which shows stable RS with an ON/OFF ratio of 100 and a reliable data retention (104 s). The electron transport mechanism was investigated in detail. It indicates that hopping conduction is dominating when the device is at a high resistance state, while space charge limited conduction acts as the dominant factor at a low resistance state. Such behavior, which is different from devices using TiN or Ti as electrodes, was attributed to the Y doping and specific YDH/Si interface. Our results demonstrate a proof of concept study to use highly doped Si as bottom electrodes along with single crystalline YDH as insulator layer for such RRAM applications as wireless sensors and synaptic simulation.

A Highly Thermostable In2O3/ITO Thin Film Thermocouple Prepared via Screen Printing for High Temperature Measurements.

An In2O3/ITO thin film thermocouple was prepared via screen printing. Glass additives were added to improve the sintering process and to increase the density of the In2O3/ITO films. The surface and cross-sectional images indicate that both the grain size and densification of the ITO and In2O3 films increased with the increase in annealing time. The thermoelectric voltage of the In2O3/ITO thermocouple was 53.5 mV at 1270 °C at the hot junction. The average Seebeck coefficient of the thermocouple was calculated as 44.5 µV/°C. The drift rate of the In2O3/ITO thermocouple was 5.44 °C/h at a measuring time of 10 h at 1270 °C.

Role of random electric fields in relaxors.

PbZr(1-x)Ti(x)O3 (PZT) and Pb(Mg1/3Nb2/3)(1-x)Ti(x)O3 (PMN-xPT) are complex lead-oxide perovskites that display exceptional piezoelectric properties for pseudorhombohedral compositions near a tetragonal phase boundary. In PZT these compositions are ferroelectrics, but in PMN-xPT they are relaxors because the dielectric permittivity is frequency dependent and exhibits non-Arrhenius behavior. We show that the nanoscale structure unique to PMN-xPT and other lead-oxide perovskite relaxors is absent in PZT and correlates with a greater than 100% enhancement of the longitudinal piezoelectric coefficient in PMN-xPT relative to that in PZT. By comparing dielectric, structural, lattice dynamical, and piezoelectric measurements on PZT and PMN-xPT, two nearly identical compounds that represent weak and strong random electric field limits, we show that quenched (static) random fields establish the relaxor phase and identify the order parameter.

Lead-Free Piezoelectric Diaphragm Biosensors Based on Micro-Machining Technology and Chemical Solution Deposition.

In this paper, we present a new approach to the fabrication of integrated silicon-based piezoelectric diaphragm-type biosensors by using sodium potassium niobate-silver niobate (0.82KNN-0.18AN) composite lead-free thin film as the piezoelectric layer. The piezoelectric diaphragms were designed and fabricated by micro-machining technology and chemical solution deposition. The fabricated device was very sensitive to the mass changes caused by various targets attached on the surface of diaphragm. The measured mass sensitivity value was about 931 Hz/µg. Its good performance shows that the piezoelectric diaphragm biosensor can be used as a cost-effective platform for nucleic acid testing.

The role of local non-tetragonal polar displacements in the temperature- and pressure-induced phase transitions in PbTiO3-BiMeO3 ferroelectrics.

In situ high-pressure/high-temperature Raman-scattering analyses on PbTiO 3 , 0.92PbTiO 3 - 0.08Bi(Zn 0.5 Ti 0.5 )O 3 and 0.83PbTiO 3 - 0.17Bi(Mg 0.5 Ti 0.5 )O 3 single crystals reveal an intensity transfer between the fine-structure components of the A 1 (TO) soft mode. The enhancement of the lowest-energy subpeak, which stems from intrinsic local non-tetragonal polar distortions, along with the suppression of the tetragonal A 1 (1TO) fundamental mode with increasing pressure and temperature indicates the key role of the local polarization fluctuations in transformation processes and emphasizes the significance of the order-disorder phenomena in both the pressure- and temperature-induced phase transitions of pure PbTiO 3 and its solid solutions with complex perovskites. Moreover, the temperature and pressure evolution of the fraction of the local non-tetragonal polar distortions is highly sensitive to the type of B-site substituent.

Structure, Antiferroelectricity and Energy-Storage Performance of Lead Hafnate in a Wide Temperature Range.

Lead hafnate (PbHfO3) has attracted a lot of renewed interest due to its potential as antiferroelectric (AFE) material for energy storage. However, its room temperature (RT) energy-storage performance has not been well established and no reports on the energy-storage feature of its high-temperature intermediate phase (IM) are available. In this work, high-quality PbHfO3 ceramics were prepared via the solid-state synthesis route. Based on high-temperature X-ray diffraction data, the IM of PbHfO3 was found to be orthorhombic, Imma space group, with antiparallel alignment of Pb2+ ions along the [001]cubic directions. The polarization-electric field (P-E) relation of PbHfO3 is displayed at RT as well as in the temperature range of the IM. A typical AFE loop revealed an optimal recoverable energy-storage density (Wrec) of 2.7 J/cm3, which is 286% higher than the reported data with an efficiency (η) of 65% at 235 kV/cm at RT. A relatively high Wrec value of 0.7 J/cm3 was found at 190 °C with an η of 89% at 65 kV/cm. These results demonstrate that PbHfO3 is a prototypical AFE from RT up to 200 °C, making it a suitable material for energy-storage applications in a wide temperature range.

Tuning of Polar Domain Boundaries in Nonpolar Perovskite.

Domain boundaries in ferroic materials are found to have various physical properties not observed in the surrounding domains. Such differences can be enhanced and bring promising functionalities when centrosymmetric nonpolar materials encounter polar domain boundaries. In this work, a tunable polar domain boundary is discovered in an antiferroelectric single crystal. Under a small stress or electric field, the density, volume, and polarity of the boundaries are successfully controlled.

Effects of Composition Segregation in PMN-PT Crystals on Ultrasound Transducer Performance.

This study investigates the relationship between the composition segregation in lead magnesium niobate-lead titanate (PMN-PT; PMN-29%PT, PMN-29.5%PT, PMN-30%PT, PMN-30.5%PT, and PMN-31%PT) single crystals within morphotropic phase boundary (MPB) and the corresponding ultrasonic transducer performance through PiezoCAD modeling and real transducer testing. For five crystals with compositions distributed across the main body of a crystal ingot, the piezoelectric coefficient and free relative permittivity values were measured to vary by over 30%, whereas the transducer bandwidth and center frequency values were modeled to change by less than 10%. For the single-element ultrasonic transducers fabricated using those crystals without matching layers, the variations of -6-dB bandwidth, insertion loss, receiver-free field voltage response, and center frequency were measured to be 9.61%, -15.23%, 9.76%, and 1.41%, respectively, confirming the modeling results. Using the Mason and Krimholtz, Leedom, and Matthaei (KLM) models, it is found that the relatively stable transducer performance can be attributed to the relatively consistent electromechanical coupling coefficient, acoustic impedance, and clamped relative permittivity originated from the stable elastic compliance properties among the crystals of various compositions. It is expected that the relatively stable performance could be extended to multielement transducers with matching layers for the same contributing mechanisms. Our results suggest that it is possible to use crystal plates of different compositions within the MPB region, obtained from one and the same ingot, to fabricate a batch of ultrasonic transducers that will exhibit a similar performance, significantly reducing the cost of materials.

Strain-Induced Magnetoelectric Coupling in Fe3O4/BaTiO3 Nanopillar Composites.

Magnetoelectric coupling properties are limited to the substrate clamping effect in traditional ferroelectric/ferromagnetic heterostructures. Here, Fe3O4/BaTiO3 nanopillar composites are successfully constructed. The well-ordered BaTiO3 nanopillar arrays are prepared through template-assisted pulsed laser deposition. The Fe3O4 layer is coated on BaTiO3 nanopillar arrays by atomic layer deposition. The nanopillar arrays and heterostructure are confirmed by scanning electron microscopy and transmission electron microscopy. A large thermally driven magnetoelectric coupling coefficient of 395 Oe °C-1 near the phase transition of BaTiO3 (orthorhombic to rhombohedral) is obtained, indicating a strong strain-induced magnetoelectric coupling effect. The enhanced magnetoelectric coupling effect originated from the reduced substrate clamping effect and increased the interface area in nanopillar structures. This work opens a door toward cutting-edge potential applications in spintronic devices.

Highly heterogeneous epitaxy of flexoelectric BaTiO3-δ membrane on Ge.

The integration of complex oxides with a wide spectrum of functionalities on Si, Ge and flexible substrates is highly demanded for functional devices in information technology. We demonstrate the remote epitaxy of BaTiO3 (BTO) on Ge using a graphene intermediate layer, which forms a prototype of highly heterogeneous epitaxial systems. The Ge surface orientation dictates the outcome of remote epitaxy. Single crystalline epitaxial BTO3-δ films were grown on graphene/Ge (011), whereas graphene/Ge (001) led to textured films. The graphene plays an important role in surface passivation. The remote epitaxial deposition of BTO3-δ follows the Volmer-Weber growth mode, with the strain being partially relaxed at the very beginning of the growth. Such BTO3-δ films can be easily exfoliated and transferred to arbitrary substrates like Si and flexible polyimide. The transferred BTO3-δ films possess enhanced flexoelectric properties with a gauge factor of as high as 1127. These results not only expand the understanding of heteroepitaxy, but also open a pathway for the applications of devices based on complex oxides.

Self-Assembled Epitaxial Ferroelectric Oxide Nanospring with Super-Scalability.

Oxide nanosprings have attracted many research interests because of their anticorrosion, high-temperature tolerance, oxidation resistance, and enhanced-mechanic-response from unique helix structures, enabling various applications like nanomanipulators, nanomotors, nanoswitches, sensors, and energy harvesters. However, preparing oxide nanosprings is a challenge for their intrinsic lack of elasticity. Here, an approach for preparing self-assembled, epitaxial, ferroelectric nanosprings with built-in strain due to the lattice mismatch in freestanding La0.7 Sr0.3 MnO3 /BaTiO3 (LSMO/BTO) bilayer heterostructures is developed. It is found that these LSMO/BTO nanosprings can be extensively pulled or pushed up to their geometrical limits back and forth without breaking, exhibiting super-scalability with full recovery capability. The phase-field simulations reveal that the excellent scalability originates from the continuous ferroelastic domain structures, resulting from twisting under co-existing axial and shear strains. In addition, the oxide heterostructural springs exhibit strong resilience due to the limited plastic deformation nature and the built-in strain between the bilayers. This discovery provides an alternative way for preparing and operating functional oxide nanosprings that can be applied to various technologies.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection.

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Lead-Free KNN-Based Textured Ceramics for High-Frequency Ultrasonic Transducer Application.

Environment-friendly lead-free piezoelectric materials with excellent piezoelectric properties are needed for high-frequency ultrasonic transducer applications. Recently, lead-free 0.915(K0.45Na0.5Li0.05)NbO3-0.075BaZrO 3-0.01(Bi0.5Na0.5)TiO3 (KNLN-BZ-BNT) textured piezo- electric ceramics have high piezoelectric response, superior thermal stability, and excellent fatigue resistance, which are promising for devices applications. In this work, the KNLN-BZ-BNT textured ceramics were prepared by the tape-casting method. Microstructural morphology, phase transition, and electrical properties of KNLN-BZ-BNT textured ceramics were investigated. High-frequency needle-type ultrasonic transducers were designed and fabricated with these textured ceramics. The tightly focused transducers have a center frequency higher than 80 MHz and a -6-dB fractional bandwidth of 52%. Such transducers were built for an f -number close to 1, and the desired focal depth was achieved by press-focusing technology associated with a set of customer design fixture. Its lateral resolution was better than [Formula: see text] by scanning a 15- [Formula: see text] tungsten wire target. These promising results demonstrate that the lead-free KNLN-BZ-BNT textured ceramic is a good candidate for high-frequency ultrasonic transducer applications.

Ferroelectric phase-transition frustration near a tricritical composition point.

Phase transition describes a mutational behavior of matter states at a critical transition temperature or external field. Despite the phase-transition orders are well sorted by classic thermodynamic theory, ambiguous situations interposed between the first- and second-order transitions were exposed one after another. Here, we report discovery of phase-transition frustration near a tricritical composition point in ferroelectric Pb(Zr1-xTix)O3. Our multi-scale transmission electron microscopy characterization reveals a number of geometrically frustrated microstructure features such as self-assembled hierarchical domain structure, degeneracy of mesoscale domain tetragonality and decoupled polarization-strain relationship. Associated with deviation from the classic mean-field theory, dielectric critical exponent anomalies and temperature dependent birefringence data unveil that the frustrated transition order stems from intricate competition of short-range polar orders and their decoupling to long-range lattice deformation. With supports from effective Hamiltonian Monte Carlo simulations, our findings point out a potentially universal mechanism to comprehend the abnormal critical phenomena occurring in phase-transition materials.

The use of polarizable [AuX2(CN)2]- (X = Br, I) building blocks toward the formation of birefringent coordination polymers.

The [(n)Bu(4)N][AuX(2)(CN)(2)] (X = Br, I) salts were synthesized and structurally characterized. Both feature square-planar [AuX(2)(CN)(2)](-) anions, with trans cyano and halo ligands, which aggregate via halogen-halogen interactions. The aggregation of [AuX(2)(CN)(2)](-) units results in the parallel alignment of all of the Br-Au-Br moieties in the anions along the [110] and [110] directions. Two crystal habits of [(n)Bu(4)N][AuBr(2)(CN)(2)] were grown: with (110) and (001) as the primary faces. The birefringence in the (110) plane was found to be Δn = 0.051(4) and was <0.03 in the (001) plane. Using the [AuBr(2)(CN)(2)](-) unit, [M(phen)(2)][AuBr(2)(CN)(2)](2) (M = Fe, Ni), [Ni(terpy)(2)][AuBr(2)(CN)(2)](2), [Fe(terpy)(2)][AuBr(2)(CN)(2)][ClO(4)], and [Cu(phen)(2)(NO(3))][AuBr(2)(CN)(2)] (phen = 1,10-phenanthroline, terpy = 2,2';6',2''-terpyridine) were synthesized and structurally characterized: they formed ionic structures with coordinatively saturated metal cations and structurally aligning Br···Br interactions between the [AuBr(2)(CN)(2)](-) anions. A molecular complex, Cu(terpy)[AuBr(2)(CN)(2)](2), was prepared, as well as the coordination polymer, [Ni(en)(2)(AuBr(2)(CN)(2))][AuBr(2)(CN)(2)]·MeOH (en = ethylenediamine). The structure consists of layers of chains of Ni(en)(2)(AuBr(2)(CN)(2))(+) units and chains of unbound [AuBr(2)(CN)(2)](-) units formed via Br···Br interactions; a Δn = 0.131(3) was measured. The Δn values were related to the supramolecular structures in terms of the relative intermolecular alignment of Br-Au-Br and NC-Au-CN bonds. These measurements both demonstrate the utility of the Au-Br bonds in enhancing birefringence and show that the contribution of the M-CN units to the overall birefringence of cyanometallate coordinations polymers is non-negligible.

Raman Scattering Studies of the Structural Phase Transitions in Single-Crystalline CH3NH3PbCl3.

Methylammonium lead trihalide perovskites CH3NH3PbX3 (X = Cl, Br, and I) have recently attracted huge attention as a promising candidate for highly efficient solar cell absorber materials. To understand the physical properties of halide perovskites, we investigated the CH3NH3PbCl3 single crystal by Raman scattering spectroscopy from 80 K to room temperature. Benchmarking the phonon modes and their Raman activities obtained by density functional calculations, we successfully assign the molecular vibrations of methylammonium in the frequency range from 400 to 3300 cm-1. In the temperature-dependent Raman scattering spectra, the internal vibrational modes of the CH3NH3+ cation are observed in the frequency range above 400 cm-1 and a number of peaks among them show characteristic changes that reflect the phase transition occurring at about 160 K in CH3NH3PbCl3. A noticeable Raman peak at 2900 cm-1 persists in a wide range of temperature, suggesting the existence of microcrystalline methylammonium chloride possibly left on the crystal surface from the growth process or spontaneously formed after synthesis.

Polymer Matrix Nanocomposites with 1D Ceramic Nanofillers for Energy Storage Capacitor Applications.

Recent developments in various technologies, such as hybrid electric vehicles and pulsed power systems, have challenged researchers to discover affordable, compact, and super-functioning electric energy storage devices. Among the existing energy storage devices, polymer nanocomposite film capacitors are a preferred choice due to their high power density, fast charge and discharge speed, high operation voltage, and long service lifetime. In the past several years, they have been extensively researched worldwide, with 0D, 1D, and 2D nanofillers being incorporated into various polymer matrixes. However, 1D nanofillers appeared to be the most effective in producing large dipole moments, which leads to a considerably enhanced dielectric permittivity and energy density of the nanocomposite. As such, this Review focuses on recent advances in polymer matrix nanocomposites using various types of 1D nanofillers, i.e., linear, ferroelectric, paraelectric, and relaxor-ferroelectric for energy storage applications. Correspondingly, the latest developments in the nanocomposite dielectrics with highly oriented, surface-coated, and surface-decorated 1D nanofillers are presented. Special attention has been paid to identifying the underlying mechanisms of maximizing dielectric displacement, increasing dielectric breakdown strength, and enhancing the energy density. This Review also presents some suggestions for future research in low-loss, high energy storage devices.

Toward a Reliable Synaptic Simulation Using Al-Doped HfO2 RRAM.

The potential in a synaptic simulation for neuromorphic computation has revived the research interest of resistive random access memory (RRAM). However, novel applications require reliable multilevel resistive switching (RS), which still represents a challenge. We demonstrate in this work the achievement of reliable HfO2-based RRAM devices for synaptic simulation by performing the Al doping and the postdeposition annealing (PDA). Transmission electron microscopy and operando hard X-ray photoelectron spectroscopy results reveal the positive impact of Al doping on the formation of oxygen vacancies. Detailed I-V characterizations demonstrate that the 16.5% Al doping concentration leads to better RS properties of the device. In comparison with the other reported results based on HfO2 RRAM, our devices with 16.5% Al-doping and PDA at 450 °C show better reliable multilevel RS (∼20 levels) performance and an increased on/off ratio. The 16.5% Al:HfO2 sample with PDA at 450 °C shows good potentiation/depression characteristics with low pulse width (10 µs) along with a good On/Off ratio (>1000), good data retention at room temperature, and high temperature and good program/erase endurance characteristics with a pulse width of 50 ns. The synapse features including potentiation, depression, and spike time-dependent plasticity were successfully achieved using optimized Al-HfO2 RRAM devices. Our results demonstrate the beneficial effects of Al doping and PDA on the enhancement of the performances of RRAM devices for the synaptic simulation in neuromorphic computing applications.