Thermal stability and decomposition kinetics of mixed-cation halide perovskites.

Organic-inorganic halide perovskites (OIHPs) have emerged as one of the most efficient photovoltaic materials due to their superior properties. However, improving their stability remains a key challenge. Herein, we investigate the thermal decomposition properties of OIHP FAxMA1-xPbI3 with mixed cations of formamidinium (FA) and methylammonium (MA). Using thermogravimetric analysis together with Fourier transform infrared spectroscopy, we identify and monitor the gaseous decomposition products as a function of temperature and cation composition. Thermal decomposition products of both MA and FA cations were observed at all stages of the thermal decomposition process, contrary to previous expectations. The yield, release sequence and kinetics of the organic gaseous products were found to depend strongly on the ratio between FA and MA cations. Furthermore, cesium ion doping was investigated as a potential strategy to improve the thermal stability of mixed cation perovskites. These results provide new insights into the effect of cation mixing on perovskite stability, suggesting that optimizing the cation ratios and decomposition pathways can guide approaches to boost the stability and performance of mixed cation perovskites.

A Highly Flexible Piezoelectric Ultrasonic Sensor for Wearable Bone Density Testing.

Driven by the loss of bone calcium, the elderly are prone to osteoporosis, and regular routine checks on bone status are necessary, which mainly rely on bone testing equipment. Therefore, wearable real-time healthcare devices have become a research hotspot. Herein, we designed a high-performance flexible ultrasonic bone testing system using axial transmission technology based on quantitative ultrasound theory. First, a new rare-earth-element-doped PMN-PZT piezoelectric ceramic was synthesized using a solid-state reaction, and characterized by X-ray diffraction and SEM. Both a high piezoelectric coefficient d33 = 525 pC/N and electromechanical coupling factors of k33 = 0.77, kt = 0.58 and kp = 0.63 were achieved in 1%La/Sm-doped 0.17 PMN-0.47 PZ-0.36 PT ceramics. Combining a flexible PDMS substrate with an ultrasonic array, a flexible hardware circuit was designed which includes a pulse excitation module, ultrasound array module, amplification module, filter module, digital-to-analog conversion module and wireless transmission module, showing high power transfer efficiency and power intensity with values of 35% and 55.4 mW/cm2, respectively. Finally, the humerus, femur and fibula were examined by the flexible device attached to the skin, and the bone condition was displayed in real time on the mobile client, which indicates the potential clinical application of this device in the field of wearable healthcare.

Conformable ultrasound breast patch for deep tissue scanning and imaging.

Ultrasound is widely used for tissue imaging such as breast cancer diagnosis; however, fundamental challenges limit its integration with wearable technologies, namely, imaging over large-area curvilinear organs. We introduced a wearable, conformable ultrasound breast patch (cUSBr-Patch) that enables standardized and reproducible image acquisition over the entire breast with less reliance on operator training and applied transducer compression. A nature-inspired honeycomb-shaped patch combined with a phased array is guided by an easy-to-operate tracker that provides for large-area, deep scanning, and multiangle breast imaging capability. The in vitro studies and clinical trials reveal that the array using a piezoelectric crystal [Yb/Bi-Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3] (Yb/Bi-PIN-PMN-PT) exhibits a sufficient contrast resolution (~3 dB) and axial/lateral resolutions of 0.25/1.0 mm at 30 mm depth, allowing the observation of small cysts (~0.3 cm) in the breast. This research develops a first-of-its-kind ultrasound technology for breast tissue scanning and imaging that offers a noninvasive method for tracking real-time dynamic changes of soft tissue.

A Review of Fingerprint Sensors: Mechanism, Characteristics, and Applications.

Identification technology based on biometrics is a branch of research that employs the unique individual traits of humans to authenticate identity, which is the most secure method of identification based on its exceptional high dependability and stability of human biometrics. Common biometric identifiers include fingerprints, irises, and facial sounds, among others. In the realm of biometric recognition, fingerprint recognition has gained success with its convenient operation and fast identif ication speed. Different fingerprint collecting techniques, which supply fingerprint information for fingerprint identification systems, have attracted a significant deal of interest in authentication technology regarding fingerprint identification systems. This work presents several fingerprint acquisition techniques, such as optical capacitive and ultrasonic, and analyzes acquisition types and structures. In addition, the pros and drawbacks of various sensor types, as well as the limits and benefits of optical, capacitive, and ultrasonic kinds, are discussed. It is the necessary stage for the application of the Internet of Things (IoT).

Recent Progress on Hydrogel-Based Piezoelectric Devices for Biomedical Applications.

Flexible electronics have great potential in the application of wearable and implantable devices. Through suitable chemical alteration, hydrogels, which are three-dimensional polymeric networks, demonstrate amazing stretchability and flexibility. Hydrogel-based electronics have been widely used in wearable sensing devices because of their biomimetic structure, biocompatibility, and stimuli-responsive electrical properties. Recently, hydrogel-based piezoelectric devices have attracted intensive attention because of the combination of their unique piezoelectric performance and conductive hydrogel configuration. This mini review is to give a summary of this exciting topic with a new insight into the design and strategy of hydrogel-based piezoelectric devices. We first briefly review the representative synthesis methods and strategies of hydrogels. Subsequently, this review provides several promising biomedical applications, such as bio-signal sensing, energy harvesting, wound healing, and ultrasonic stimulation. In the end, we also provide a personal perspective on the future strategies and address the remaining challenges on hydrogel-based piezoelectric electronics.

Ultrahigh Piezoelectric Performance through Synergistic Compositional and Microstructural Engineering.

Piezoelectric materials enable the conversion of mechanical energy into electrical energy and vice-versa. Ultrahigh piezoelectricity has been only observed in single crystals. Realization of piezoelectric ceramics with longitudinal piezoelectric constant (d33 ) close to 2000 pC N-1 , which combines single crystal-like high properties and ceramic-like cost effectiveness, large-scale manufacturing, and machinability will be a milestone in advancement of piezoelectric ceramic materials. Here, guided by phenomenological models and phase-field simulations that provide conditions for flattening the energy landscape of polarization, a synergistic design strategy is demonstrated that exploits compositionally driven local structural heterogeneity and microstructural grain orientation/texturing to provide record piezoelectricity in ceramics. This strategy is demonstrated on [001]PC -textured and Eu3+ -doped Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 (PMN-PT) ceramics that exhibit the highest piezoelectric coefficient (small-signal d33 of up to 1950 pC N-1 and large-signal d33 * of ≈2100 pm V-1 ) among all the reported piezoelectric ceramics. Extensive characterization conducted using high-resolution microscopy and diffraction techniques in conjunction with the computational models reveals the underlying mechanisms governing the piezoelectric performance. Further, the impact of losses on the electromechanical coupling is identified, which plays major role in suppressing the percentage of piezoelectricity enhancement, and the fundamental understanding of loss in this study sheds light on further enhancement of piezoelectricity. These results on cost-effective and record performance piezoelectric ceramics will launch a new generation of piezoelectric applications.

Fabrication and Optical Properties of Transparent P(VDF-TrFE) Ultrathin Films.

The films of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) are widely used in piezoelectric tactile sensors, vibration energy harvesters, optical frequency conversion materials and organic photo-voltaic devices because of high electroactive, good optical and nonlinear optical properties, respectively. In this work, the multilayer structured ultrathin films were fabricated by the Langmuir-Blodgett technique, and the thickness per layer can be controlled accurately. It was found that as the collapse pressure of P(VDF-TrFE) (25:75) and the optimal dipping value are 60~70 mN/m and 15 mN/m, respectively, a high-density film can be obtained due to the compression of molecules. The surface topography and optical properties of the LB films were characterized by X-ray diffraction, white light interferometer and variable-angle spectrum ellipsometer. It was observed that the films are transparent in the visible region and IR-band, but show a high absorption in the UV band. Besides, the transmittance of the films ranges from 50% to 85% in the visible region, and it linearly decreases with the number of monolayers. The average thickness of per deposition layer is 2.447 nm, 2.688 nm and 2.072 nm, respectively, under three measurement methods. The calculated refractive index ranged from 1.443 to 1.598 (600~650 nm) by the Cauchy-model.

FEM Simulation of a High-Performance 128°Y-X LiNbO3/SiO2/Si Functional Substrate for Surface Acoustic Wave Gyroscopes.

To obtain a high-performance surface acoustic wave (SAW) gyroscope substrate, the propagation characteristics and gyroscopic effect of Rayleigh waves in a 128°Y-X LiNbO3/SiO2/Si (LNOI) functional substrate were investigated with a three-dimensional finite element method. The influence of LNOI structural parameters on Rayleigh wave characteristics, including the phase velocity (vp), electromechanical coupling coefficient (K2) and temperature coefficient of frequency (TCF), were analyzed. The results demonstrate that the SiO2 layer compensates for the negative TCF of 128°Y-X LiNbO3 and enhances the K2 of the LNOI substrate. The Rayleigh wave velocity change of the LNOI substrate after rotations in different directions was studied. The gyroscope gain factor (η) represents the strength of the gyroscopic effect in the differential traveling wave SAW gyroscope and is defined. The ηy and ηz of the LNOI substrate with different structural parameters were investigated. Finally, an LNOI substrate with an hLN/λ of 0.2 and an hSiO2/λ of 0.05 was obtained by balancing the characteristic parameters, with a K2 of 3.96%, TCF of -18.75 ppm/°C and ηy of 0.26. The LNOI substrate has a better gyroscopic effect and temperature stability than the 128°Y-X LiNbO3 crystal. The LNOI substrate meets device miniaturization and integration needs.

Trap-free exciton dynamics in monolayer WS2via oleic acid passivation.

Two-dimensional transition metal dichalcogenides have attracted a great deal of attention in the past few decades owing to their attractive optoelectronic properties. However, their widespread utility in photonic devices and components is still limited owing to their weak photoluminescence. While various treating methods are in place to improve the photoluminescence yield, the impact of these treatments on the excited state (especially exciton) dynamics in these two-dimensional materials remains ill defined. In this work, exciton dynamics in pristine and oleic acid-treated monolayer WS2 were comprehensively studied through various ultrafast experimental techniques. We demonstrate that oleic acid effectively passivates the defect states in as-fabricated WS2, resulting in trap-free exciton dynamics and exciton annihilation rate reduction, which leads to stronger steady-state photoluminescence and longer photoluminescence lifetime. These results provide valuable information on the intrinsic exciton dynamics in monolayer WS2, which could also be applicable in other two-dimensional transition metal dichalcogenides and help improve optoelectronic device performance.

Recent Advances on Conducting Polymers Based Nanogenerators for Energy Harvesting.

With the rapid growth of numerous portable electronics, it is critical to develop high-performance, lightweight, and environmentally sustainable energy generation and power supply systems. The flexible nanogenerators, including piezoelectric nanogenerators (PENG) and triboelectric nanogenerators (TENG), are currently viable candidates for combination with personal devices and wireless sensors to achieve sustained energy for long-term working circumstances due to their great mechanical qualities, superior environmental adaptability, and outstanding energy-harvesting performance. Conductive materials for electrode as the critical component in nanogenerators, have been intensively investigated to optimize their performance and avoid high-cost and time-consuming manufacture processing. Recently, because of their low cost, large-scale production, simple synthesis procedures, and controlled electrical conductivity, conducting polymers (CPs) have been utilized in a wide range of scientific domains. CPs have also become increasingly significant in nanogenerators. In this review, we summarize the recent advances on CP-based PENG and TENG for biomechanical energy harvesting. A thorough overview of recent advancements and development of CP-based nanogenerators with various configurations are presented and prospects of scientific and technological challenges from performance to potential applications are discussed.

Novel Bilayer Micropyramid Structure Photonic Nanojet for Enhancing a Focused Optical Field.

In this paper, synthetically using refraction, diffraction, and interference effects to achieve free manipulation of the focused optical field, we firstly present a photonic nanojet (PNJ) generated by a micropyramid, which is combined with multilayer thin films. The theory of total internal reflection (TIR) was creatively used to design the base angle of the micropyramid, and the size parameters and material properties of the microstructure were deduced via the expected optical field distribution. The as-designed bilayer micropyramid array was fabricated by using the single-point diamond turning (SPDT) technique, nanoimprint lithography (NIL), and proportional inductively coupled plasma (ICP) etching. After the investigation, the results of optical field measurement were highly consistent with those of the numerical simulation, and they were both within the theoretical calculation range. The bilayer micropyramid array PNJ enhanced the interference effect of incident and scattered fields; thus, the intensity of the focused light field reached 33.8-times that of the initial light, and the range of the focused light field was extended to 10.08λ. Moreover, the full width at half maximum (FWHM) of the focal spot achieved was 0.6λ, which was close to the diffraction limit.

Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide.

Femtosecond transient absorption spectroscopy has been employed to unravel separate initial nonequilibrium dynamic processes of photo-injected electrons and holes during the formation process of the lowest excitons at the K-valley in few-layer tungsten disulfide. Charge carrier thermalization and cooling, as well as concomitant many-body effects on the exciton resonances, are distinguished. The thermalization of holes is observed to be faster than that of electrons. Both of them proceed predominantly via carrier-carrier scattering, as evidenced by the observed dependence of the thermalization time on pump fluences. The fluence dependent time constants also suggest that the subsequent cooling for electrons is probably dominated by acoustic phonons, whereas for holes it is mostly controlled by LO phonons. An extremely fast red- and blue-shift crossover followed by a slow blue-shift of exciton resonance was observed in the temporal evolution of exciton resonances by resonant exciton A excitation. The rapid red-shift could be due to the strong screening of the Coulomb interaction between quasi-free charge carriers in electron-hole plasma. The subsequent slow blue-shift is the net result of the competition among many-body effects in the hot-exciton cooling process. Our findings elucidate the carrier-selective ultrafast dynamics and their many-body effects, underpinning new possibilities for developing optoelectronic devices based on transport properties of a single type of carrier.

Synthesis, Giant Dielectric, and Pyroelectric Response of [001]-Oriented Pr3+ Doped Pb(Mg1/3Nb2/3)O3-PbTiO3 Ferroelectric Nano-Films Grown on Si Substrates.

The [001]-oriented Pr3+ doped Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 (Pr-PMN-PT) thin films with a composition near the morphotropic phase boundary (MPB) were synthesized by a sol⁻gel method. The crystal structure was characterized using X-ray diffraction. It was found that a single perovskite phase was achieved in Pr-PMN-PT thin films annealed at 650 °C for 3 min. The dielectric constant (εr) value was 2400 in 2.5% Pr-PMN-PT thin films at room temperature, 110% higher than that of pure PMN-PT samples. Through 2.5% Pr3+ doping, remanent polarization (Pr) and coercive field (Ec) values increased from 11.5 µC/cm² and 35 kV/cm to 17.3 µC/cm² and 63.5 kV/cm, respectively, in PMN-PT thin films. The leakage current densities of pure and 2.5% Pr-PMN-PT thin films were on the order of 1.24 × 10-4 A/cm² and 5.8 × 10-5 A/cm², respectively, at 100 kV/cm. A high pyroelectric coefficient (py) with a value of 167 µC/m²K was obtained in 2.5% Pr-PMN-PT thin films on Si substrate, which makes this material suitable for application in infrared detectors.

Ultrahigh piezoelectricity in ferroelectric ceramics by design.

Piezoelectric materials, which respond mechanically to applied electric field and vice versa, are essential for electromechanical transducers. Previous theoretical analyses have shown that high piezoelectricity in perovskite oxides is associated with a flat thermodynamic energy landscape connecting two or more ferroelectric phases. Here, guided by phenomenological theories and phase-field simulations, we propose an alternative design strategy to commonly used morphotropic phase boundaries to further flatten the energy landscape, by judiciously introducing local structural heterogeneity to manipulate interfacial energies (that is, extra interaction energies, such as electrostatic and elastic energies associated with the interfaces). To validate this, we synthesize rare-earth-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), as rare-earth dopants tend to change the local structure of Pb-based perovskite ferroelectrics. We achieve ultrahigh piezoelectric coefficients d33 of up to 1,500 pC N-1 and dielectric permittivity ε33/ε0 above 13,000 in a Sm-doped PMN-PT ceramic with a Curie temperature of 89 °C. Our research provides a new paradigm for designing material properties through engineering local structural heterogeneity, expected to benefit a wide range of functional materials.

Surface Acoustic Wave Gyroscopic Effect in an Interdigital Transducer.

The surface acoustic wave (SAW) gyroscopic effect in an interdigital transducer (IDT) deposited on a piezoelectric substrate is different from that in the piezoelectric substrate due to a reflection induced by IDT. In this work, an extended coupling-of-mode (COM) model including the gyroscopic effect and the reflection was developed to analyze the SAW gyroscopic effect. First, dispersion characteristics parameters of SAW were fitted according to the data derived using the finite element method (FEM). Then, variations of stop band edge frequency were calculated using the extended COM theory by integrating dispersion characteristics parameters into the COM model. We compared its results with those obtained via FEM analysis to confirm the proposed model's validity. We found that the variation in stop band edge frequency related to gyroscope effect reached the maximum value with a zero reflectivity value. For split IDT, the sensitivity of gyroscope effect is 0.036 Hz/rad/s with a lower than 1% normalized thickness. Conversely, the value of sensitivity was almost zero for bidirectional IDT and electrode width controlled single-phase unidirectional transducer (EWC/SPUDT).

Piezoelectric activity in Perovskite ferroelectric crystals.

Perovskite ferroelectrics (PFs) have been the dominant piezoelectric materials for various electromechanical applications, such as ultrasonic transducers, sensors, and actuators, to name a few. In this review article, the development of PF crystals is introduced, focusing on the crystal growth and piezoelectric activity. The critical factors responsible for the high piezoelectric activity of PFs (i.e., phase transition, monoclinic phase, domain size, relaxor component, dopants, and piezoelectric anisotropy) are surveyed and discussed. A general picture of the present understanding on the high piezoelectricity of PFs is described. At the end of this review, potential approaches to further improve the piezoelectricity of PFs are proposed.

Domain size engineering in tetragonal Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) crystals.

The effect of domain size on the dielectric and piezoelectric properties of [111]-oriented tetragonal Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) crystals was investigated. The dielectric permittivity (ɛ(33 ) (T)∕ɛ(0)) and piezoelectric coefficient (d(33)) were found to be on the order of 13 800 and 1630 pC∕N, respectively, for samples with domain size of ∼500 nm, a 3-fold increase to crystals with domain size of ∼50 µm. Rayleigh analysis revealed that the extrinsic contribution to the piezoelectric response increased from ∼8% to 30% with decreasing domain size, due to the increased domain wall density and associated irreversible domain wall motion. The enhanced properties were thought to relate to the fine domain structures, however, showing a poor electric field and temperature stabilities with domain size of 500 nm. Of particular significance is that samples with domain size being on the order of 5 µm exhibit field and temperature stabilities, with yet high piezoelectric properties, make it potential for transducer applications.

Influence of Domain Size on the Scaling Effects in Pb(Mg1/3Nb2/3)O3-PbTiO3 Ferroelectric Crystals.

The property degradation observed in thin Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PMN-PT) crystals is believed to relate to large domains and subsequent clamping induced by surface-boundary. In this work, the properties were investigated as function of domain size, using controlled poling. The degraded piezoelectric and dielectric properties of thin PMN-PT were found to increase significantly, by decreasing domain size. Furthermore, the fine domain structure was found to be stable at 3kV/cm after 7.0×10(5) negative-pulse cycles, hence, enabling PMN-PT crystals for high-frequency (>20 MHz) ultrasound-transducers.

Electromechanical properties of Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) single crystals.

The Pb(In(1∕2)Nb(1∕2))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) (PIN-PMN-PT) crystals were studied as function of phase and orientation. The properties, including the Curie temperature T(C), ferroelectric-ferroelectric phase transition temperature T(R∕O-T), coercive field, and piezoelectric∕dielectric responses, were systematically investigated with respect to the composition of PIN-PMN-PT crystals. The Curie temperature T(C) was found to increase from 160 to 220 °C with ferroelectric-ferroelectric phase transition temperature T(R-T) and T(O-T) being in the range of 120-105 °C and105-50 °C, respectively. The piezoelectric activity of PIN-PMN-PT crystals was analyzed by Rayleigh approach. The ultrahigh piezoelectric response for domain engineered [001] (1600-2200 pC∕N) and [011] (830-1550 pC∕N) crystals was believed to be mainly from the intrinsic contribution, whereas the enhanced level of piezoelectric and dielectric losses at the compositions around morphotropic phase boundaries (MPBs) was attributed to the phase boundaries motion.