Stress-activated friction in sheared suspensions probed with piezoelectric nanoparticles.

A hallmark of concentrated suspensions is non-Newtonian behavior, whereby the viscosity increases dramatically once a characteristic shear rate or stress is exceeded. Such strong shear thickening is thought to originate from a network of frictional particle-particle contact forces, which forms under sufficiently large stress, evolves dynamically, and adapts to changing loads. While there is much evidence from simulations for the emergence of this network during shear thickening, experimental confirmation has been difficult. Here, we use suspensions of piezoelectric nanoparticles and exploit the strong local stress focusing within the network to activate charge generation. This charging can then be detected in the measured ac conductance and serve as a signature of frictional contact formation. The direct link between stress-activated frictional particle interactions and piezoelectric suspension response is further demonstrated by tracking the emergence of structural memory in the contact network under oscillatory shear and by showing how stress-activated friction can drive mechano-transduction of chemical reactions with nonlinear reaction kinetics. Taken together, this makes the ac conductance of piezoelectric suspensions a sensitive in-situ reporter of the micromechanics associated with frictional interactions.

Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer.

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

Facile Coordination Transitions in AgCrX2 (X = S, Se): Unprecedented Electrostrain, Negative Piezoelectricity and Thermal Expansion.

The coefficients of piezoelectricity and thermal expansion are generally positive due to the bond anharmonicity. For converse piezoelectricity, the electrostrain obtained in prevalent ceramics is only around 1%. Here we propose that the coordination transition of metal cations may make a paradigm shift. Through first-principles calculations, we predict a series of low-energy phases with distinct coordinations for Ag ions in superionic conductor AgCrX2 (X = S, Se), including ferroelectric and nonpolar phases with distinct interlayer distances. The mobile feature of Ag ions, which can be attributed to its complex coordination chemistry, can facilitate transformation between various coordination phases. Such facile transitions with ultralow barriers can be driven by applying either pressure, an electric field, or a change in temperature, giving rise to various exotic effects, including electrostrain, negative piezoelectricity, and negative thermal expansion. All with unprecedented giant constants, those mechanisms stem from the coordination transitions, distinct from the weak linear effects in previous reports.

Unconventional Piezoelectricity of Two-Dimensional Materials Driven by the Hall Effect.

Piezoelectricity has been widely explored for nanoelectromechanical applications, yet its working modes are mainly limited in polar directions. Here we discover the intrinsic electro-mechanical response in crystal materials that is transverse to the conventional polarized direction, which is named unconventional piezoelectricity. A Hall-like mechanism is proposed to interpret unconventional piezoelectricity as charge polarization driven by a built-in electric field for systems with asymmetric Berry curvature distributions. Density functional theory simulations and statistical analyses justify such a mechanism and confirm that unconventional piezoelectricity is a general property for various two-dimensional materials with spin splitting or valley splitting. An empirical formula is derived to connect the conventional and unconventional piezoelectricity. The extended understanding of the piezoelectric tensor in quantum materials opens an opportunity for applications in multidirectional energy conversion, broadband operation, and multifunctional sensing.

Core/Shell-Like Localized Emission at Atomically Thin Semiconductor-Au Interface.

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Elucidating Piezoelectricity and Strain in Monolayer MoS2 at the Nanoscale Using Kelvin Probe Force Microscopy.

Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.

Organic-Inorganic Rare-Earth Double Perovskite Ferroelectric with Large Piezoelectric Response and Ferroelasticity for Flexible Composite Energy Harvesters.

Hybrid organic-inorganic perovskite (HOIP) ferroelectric materials have great potential for developing self-powered electronic transducers owing to their impressive piezoelectric performance, structural tunability and low processing temperatures. Nevertheless, their inherent brittle and low elastic moduli limit their application in electromechanical conversion. Integration of HOIP ferroelectrics and soft polymers is a promising solution. In this work, a hybrid organic-inorganic rare-earth double perovskite ferroelectric, [RM3HQ]2RbPr(NO3)6 (RM3HQ = (R)-N-methyl-3-hydroxylquinuclidinium) is presented, which possesses multiaxial nature, ferroelasticity and satisfactory piezoelectric properties, including piezoelectric charge coefficient (d33) of 102.3 pC N-1 and piezoelectric voltage coefficient (g33) of 680 × 10-3 V m N-1. The piezoelectric generators (PEG) based on composite films of [RM3HQ]2RbPr(NO3)6@polyurethane (PU) can generate an open-circuit voltage (Voc) of 30 V and short-circuit current (Isc) of 18 µA, representing one of the state-of-the-art PEGs to date. This work has promoted the exploration of new HOIP ferroelectrics and their development of applications in electromechanical conversion devices.

Supramolecular Organic Ferroelectric Materials from Donor-Acceptor Systems.

Organic ferroelectric (FE) materials, though known for more than a century, are yet to reach close to the benchmark of inorganic or hybrid materials in terms of the magnitude of polarization. Amongst the different classes of organic systems, donor (D)-acceptor (A) charge-transfer (CT) complexes are recognized as promising for ferroelectricity owing to their neutral-to-ionic phase transition at low temperature. This review presents an overview of different supramolecular D-A systems that have been explored for FE phase transitions. The discussion begins with a general introduction of ferroelectricity and its different associated parameters. Then it moves on to show early examples of CT cocrystals that have shown FE properties at sub-ambient temperature. Subsequently, recent developments in the field of room temperature (RT) ferroelectricity, exhibited by H-bond-stabilized lock-arm supramolecular-ordering (LASO) in D-A co-crystals or other FE CT-crystals devoid of neutral-ionic phase transition are discussed. Then the discussion moves on to emerging reports on other D-A soft materials such as gel and foldable polymers; finally it shows very recent developments in ferroelectricity in supramolecular assemblies of single-component dipolar or ambipolar π-systems, exhibiting intra-molecular charge transfer. The effects of structural nuances such as H-bonding, balanced charge transfer and chirality on the observed ferroelectricity is described with the available examples. Finally, piezoelectricity in recently reported ambipolar ADA-type systems are discussed to highlight the future potential of these soft materials in micropower energy harvesting.

Ultrasound-assisted water oxidation: unveiling the role of piezoelectric metal-oxide sonocatalysts for cancer treatment.

Ultrasound radiation has been widely used in biomedical application for both diagnosis and therapy. Metal oxides nanoparticles (NPs), like ZnO or TiO2 NPs, have been widely demonstrated to act as excellent sonocatalysts and significantly enhance cavitation at their surface, making them optimal for sonodynamic cancer therapy. These NPs often possess semiconductive and piezoelectric properties that contribute to the complex phenomena occurring at the water-oxide interface during sonostimulation. Despite the great potential in applied sonocatalysis and water splitting, the complex mechanism that governs the phenomenon is still a research subject. This work investigates the role of piezoelectric ZnO micro- and nano-particles in ultrasound-assisted water oxidation. Three metal oxides presenting fundamental electronic and mechanical differences are evaluated in terms of ultrasound-triggered reactive oxygen species generation in aqueous media: electromechanically inert SiO2 NPs, semiconducting TiO2 NPs, piezoelectric and semiconducting ZnO micro- and nanoparticles with different surface areas and sizes. The presence of silver ions in the aqueous solution was further considered to impart a potential electron scavenging effects and better evaluate the oxygen generation performances of the different structures. Following sonoirradiation, the particles are optically and chemically analyzed to study the effect of sonostimulation at their surface. The production of gaseous molecular oxygen is measured, revealing the potential of piezoelectric particles to generate oxygen under hypoxic conditions typical of some cancer environments. Finally, the best candidates, i.e. ZnO nano and micro particles, were tested on osteosarcoma and glioblastoma cell lines to demonstrate their potential for cancer treatment.

Flexible piezoelectric sensor based on polyvinylidene fluoride/polyacrylonitrile/carboxy-terminated multi-walled carbon nanotube composite films for human motion monitoring.

Flexible piezoelectric devices have attracted much attention in the fields of intelligent devices and biomedicine because of their high sensitivity, stability, and flexibility. In this paper, a multifunctional flexible pressure sensor was prepared by adding polyacrylonitrile (PAN) and carboxylic-terminated multi-walled carbon nanotubes (c-MWCNTs) with polyvinylidene difluoride (PVDF) as the substrate. Theß-phase content of PVDF/PAN blended fibers compounded with c-MWCNT was up to 95%. At the same time, when PAN was added, the mechanical properties of the composite fibers were constantly improved. The results show that the polymer blending method can improve the comprehensive properties of PVDF composite. The flexible sensor prepared from the PVDF/PAN/c-MWCNT composite film has an output voltage of 2.1 V and a current of 7µA. The addition of c-MWCNT can largely improve the sensitivity of the sensor (4.19 V N-1). The sensor is attached to the finger and shows good output performance under different degrees of bending of the finger. The maximum output voltage of the sensor is 0.4 V, 0.56 V and 1.15 V when the finger bending angle is 30°, 60°, and 90°, respectively. Moreover, the developed piezoelectric sensor can monitor large-scale movements of various parts of the human body. Therefore, this composite material shows potential in areas such as motion monitoring and energy storage devices.

Density-controlled electrochemical synthesis of ZnO nanowire arrays using nanotextured cathode.

Zinc oxide (ZnO) nanowires fabricated via wet chemical synthesis on flexible polymer substrates are inherently unstable against mechanical bending stress because of their high density and weak adhesion to the substrate. We introduce a novel method for controlling the density of such ZnO nanowire arrays using a three-dimensional corrugated metal substrate. These metal substrates, featuring extruded and recessed patterns fabricated via nanoimprint lithography, were employed as cathodes during the electrochemical deposition of ZnO nanowire arrays. The ZnO nanowire arrays synthesized on the patterned metal thin film exhibited smaller diameters and lower densities compared to those on non-patterned metal films. This reduction in density can be attributed to aligned nucleation and limited growth on the patterned metal surface. Crucially, ZnO nanowires synthesized on patterned metal substrates displayed remarkable mechanical robustness against external forces, a direct consequence of their reduced density. In contrast, nanowires synthesized on non-patterned metal substrates were broken under mechanical bending. Detailed morphological analyses performed after mechanical bending tests confirm that ZnO nanowires synthesized on nanoimprinted metal electrodes exhibited enhanced mechanical characteristics compared to those on non-patterned metal electrodes. These findings clearly demonstrate the promise of utilizing density-controlled ZnO nanowires in piezoelectric devices.

Ultrasound stimulated piezoelectric barium titanate and boron nitride nanotubes in nonconductive poly-ε-caprolactone nanofibrous scaffold for bone tissue engineering.

Nanomaterials can provide unique solutions for the problems experienced in tissue engineering by improving a scaffold's physico-bio-chemical properties. With its piezoelectric property, bone is an active tissue with easy adaptation and remodeling through complicated mechanisms of electromechanical operations. Although poly(ε-caprolactone) (PCL) is an excellent polymer for bone tissue engineering, it is lack of conductivity. In this study, piezoelectric barium titanates (BaTiO3) and boron nitride nanotubes (BNNTs) are used as ultrasound (US) stimulated piezoelectric components in PCL to mimic piezoelectric nature of bone tissue. Electric-responsive Human Osteoblast cells on the scaffolds were stimulated by applying low-frequency US during cell growth. Biocompatibility, cell adhesion, alkaline phosphatase activities and mineralization of osteoblast cells on piezo-composite scaffolds were investigated. BaTiO3or BNNTs as reinforcement agents improved physical and mechanical properties of PCL scaffolds.In vitrostudies show that the use of BaTiO3or BNNTs as additives in non-conductive scaffolds significantly induces and increases the osteogenic activities even without US stimulation. Although BaTiO3is one of the best piezoelectric materials, the improvement is more dramatic in the case of BNNTs with the increased mineralization, and excellent chemical and mechanical properties.

Enhancing piezoelectric performance of CNTs through B and N substitution under combined mechanical loads: insights from MD simulations.

The piezoelectric properties of carbon nanotubes (CNTs) doped with boron (B) and nitrogen (N) were studied using the classical molecular dynamics (MD) simulation software package large scale atomic/molecular massively parallel simulator. The interactions among the nanotube atoms C, N, and B were calculated using the Tersoff potential. MD simulations were performed to observe the changes in the piezoelectric coefficient of the doped CNTs under loading conditions like tension, torsion, and a combination of both. We considered a wide range of chirality to determine the influence of structural variation on the piezoelectric effect. The study revealed that B-CNTs exhibit superior piezoelectric coefficients compared to N-CNTs, indicating the significant role of dopant type. Moreover, under tensile loading, zigzag-oriented B-CNTs showed higher piezoelectric coefficients with a maximume33= 0.2441 C m-2, whereas under torsional loading, armchair-oriented B-CNTs showed enhanced response with a maximume36= 0.0564 C m-2. A notable observation was that under combined loading conditions (tensile and torsional), the piezoelectric behavior of the B-CNTs was dependent on the nanotube's chirality and did not yield a linear additive response. The polarization induced under combined loading in most of the doped CNTs is significantly higher than the sum of polarization generated under tensile and torsional loading conditions. This behavior suggests that the overall piezoelectric effect under combined loading can be enhanced, which emphasizes the need for an approach to optimize the mechanical loading condition. The results showcase the potential of B-/N-CNTs to be engineered for efficient performance by demonstrating that tailored mechanical loading can enhance the piezoelectric responses in doped CNTs, opening a pathway for highly functional and efficient nanoscale piezoelectric devices.

An ultra-low-cost electroporator with microneedle electrodes (ePatch) for SARS-CoV-2 vaccination.

Vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other pathogens with pandemic potential requires safe, protective, inexpensive, and easily accessible vaccines that can be developed and manufactured rapidly at a large scale. DNA vaccines can achieve these criteria, but induction of strong immune responses has often required bulky, expensive electroporation devices. Here, we report an ultra-low-cost (<1 USD), handheld (<50 g) electroporation system utilizing a microneedle electrode array ("ePatch") for DNA vaccination against SARS-CoV-2. The low cost and small size are achieved by combining a thumb-operated piezoelectric pulser derived from a common household stove lighter that emits microsecond, bipolar, oscillatory electric pulses and a microneedle electrode array that targets delivery of high electric field strength pulses to the skin's epidermis. Antibody responses against SARS-CoV-2 induced by this electroporation system in mice were strong and enabled at least 10-fold dose sparing compared to conventional intramuscular or intradermal injection of the DNA vaccine. Vaccination was well tolerated with mild, transient effects on the skin. This ePatch system is easily portable, without any battery or other power source supply, offering an attractive, inexpensive approach for rapid and accessible DNA vaccination to combat COVID-19, as well as other epidemics.

Flexural-Mode Piezoelectric Resonators: Structure, Performance, and Emerging Applications in Physical Sensing Technology, Micropower Systems, and Biomedicine.

Piezoelectric material-based devices have garnered considerable attention from scientists and engineers due to their unique physical characteristics, resulting in numerous intriguing and practical applications. Among these, flexural-mode piezoelectric resonators (FMPRs) are progressively gaining prominence due to their compact, precise, and efficient performance in diverse applications. FMPRs, resonators that utilize one- or two-dimensional piezoelectric materials as their resonant structure, vibrate in a flexural mode. The resonant properties of the resonator directly influence its performance, making in-depth research into the resonant characteristics of FMPRs practically significant for optimizing their design and enhancing their performance. With the swift advancement of micro-nano electronic technology, the application range of FMPRs continues to broaden. These resonators, representing a domain of piezoelectric material application in micro-nanoelectromechanical systems, have found extensive use in the field of physical sensing and are starting to be used in micropower systems and biomedicine. This paper reviews the structure, working principle, resonance characteristics, applications, and future prospects of FMPRs.

Piezoelectric Behaviour in Biodegradable Carrageenan and Iron (III) Oxide Based Sensor.

This paper is dedicated to the research of phenomena noticed during tests of biodegradable carrageenan-based force and pressure sensors. Peculiar voltage characteristics were noticed during the impact tests. Therefore, the sensors' responses to impact were researched more thoroughly, defining time-dependent sensor output signals from calibrated energy impact. The research was performed using experimental methods when a free-falling steel ball impacted the sensor material to create relatively definable impact energy. The sensor's output signal, which is analogue voltage, was registered using an oscilloscope and transmitted to the PC for further analysis. The obtained results showed a very interesting outcome, where the sensor, which was intended to be piezoresistive, demonstrated a combination of behaviour typical for galvanic cells and piezoelectric material. It provides a stable DC output that is sensitive to the applied statical pressure, and in case of a sudden impact, like a hit, it demonstrates piezoelectric behaviour with some particular effects, which are described in the paper as proton transfer in the sensor-sensitive material. Such phenomena and sensor design are a matter of further development and research.

Pressure-Driven Piezoelectric Sensors and Energy Harvesting in Biaxially Oriented Polyethylene Terephthalate Film.

This study reports the possibility of using biaxially oriented polyethylene terephthalate (BOPET) plastic packaging to convert mechanical energy into electrical energy. Electricity is generated due to the piezoelectricity of BOPET. Electricity generation depends on the mechanical deformation of the processing aids (inorganic crystals), which were found and identified by SEM and EDAX analyses as SiO2. BOPET, as an electron source, was assembled and tested as an energy conversion and self-powered mechanical stimuli sensor using potential applications in wearable electronics. When a pressure pulse after pendulum impact with a maximum stress of 926 kPa and an impact velocity of 2.1 m/s was applied, a voltage of 60 V was generated with short-circuit current and charge densities of 15 µAcm-2 and 138 nCm-2, respectively. Due to the orientation and stress-induced crystallization of polymer chains, BOPET films acquire very good mechanical properties, which are not lost during their primary usage as packaging materials and are beneficial for the durability of the sensors. The signals detected using BOPET sensors derived from pendulum impact and sieve analyses were also harvested for up to 80 cycles and up to 40 s with short-circuit voltages of 107 V and 95 V, respectively. In addition to their low price, the advantage of sensors made from BOPET plastic packaging waste lies in their chemical resistance and stability under exposure to oxygen, ultraviolet light, and moisture.

Performance Correction and Parameters Identification Considering Non-Uniform Electric Field in Cantilevered Piezoelectric Energy Harvesters.

In the current electromechanical model of cantilevered piezoelectric energy harvesters, the assumption of uniform electric field strength within the piezoelectric layer is commonly made. This uniform electric field assumption seems reasonable since the piezoelectric layer looks like a parallel-plate capacitor. However, for a piezoelectric bender, the strain distribution along the thickness direction is not uniform, which means the internal electric field generated by the spontaneous polarization cannot be uniform. In the present study, a non-uniform electric field in the piezoelectric layer is resolved using electrostatic equilibrium equations. Based on these, the traditional distributed parameter electromechanical model is corrected and simplified to a practical single mode one. Compared with a traditional model adopting a uniform electric field, the bending stiffness term involved in the electromechanical governing equations is explicitly corrected. Through comparisons of predicted power output with two-dimensional finite element analysis, the results show that the present model can better predict the power output performance compared with the traditional model. It is found that the relative corrections to traditional model have nothing to do with the absolute dimensions of the harvesters, but only relate to three dimensionless parameters, i.e., the ratio of the elastic layer's to the piezoelectric layer's thickness; the ratio of the elastic modulus of the elastic layer to the piezoelectric layer; and the piezoelectric materials' electromechanical coupling coefficient squared, k312. It is also found that the upper-limit relative corrections are only related to k312, i.e., the higher k312 is, the larger the upper-limit relative corrections will be. For a PZT-5 unimorph harvester, the relative corrections of bending stiffness and corresponding resonant frequency are up to 17.8% and 8.5%, respectively. An inverse problem to identify the material parameters based on experimentally obtained power output performance is also investigated. The results show that the accuracy of material parameters identification is improved when considering a non-uniform electric field.

Different Scenarios of Autonomous Operation of an Environmental Sensor Node Using a Piezoelectric-Vibration-Based Energy Harvester.

This paper delves into the application of vibration-based energy harvesting to power environmental sensor nodes, a critical component of modern data collection systems. These sensor nodes play a crucial role in structural health monitoring, providing essential data on external conditions that can affect the health and performance of structures. We investigate the feasibility and efficiency of utilizing piezoelectric vibration energy harvesters to sustainably power environmental wireless sensor nodes on the one hand. On the other hand, we exploit different approaches to minimize the sensor node's power consumption and maximize its efficiency. The investigations consider various sensor node platforms and assess their performance under different voltage levels and broadcast frequencies. The findings reveal that optimized harvester designs enable real-time data broadcasting with short intervals, ranging from 1 to 3 s, expanding the horizons of environmental monitoring, and show that in case the system includes a battery as a backup plan, the battery's lifetime can be extended up to 9 times. This work underscores the potential of vibration energy harvesting as a viable solution for powering sensor nodes, enhancing their autonomy, and reducing maintenance costs in remote and challenging environments. It opens doors to broader applications of sustainable energy sources in environmental monitoring and data collection systems.

Flexo-/Piezoelectric Polarization Boosting Exciton Dissociation in Curved Two-Dimensional Carbon Nitride Photocatalyst.

Two-dimensional (2D) carbon nitride (CN) materials have received tremendous attention as photocatalysts for clean energy and environmental treatment. However, the photocatalytic efficiency of CN is constrained by the high exciton binding energy and sluggish charge kinetics due to weak dielectric screening, impeding the overall process. Herein, localized flexo-/piezoelectric polarization is introduced via strain engineering, boosting exciton dissociation and promoting charge separation to enhance the multielectron photocatalytic process. Consequently, the exciton binding energy of polarized CN is reduced from 52 to 34 meV, and the hydrogen evolution yield increased by 2.9 times compared to that of the pristine CN. For other photocatalytic reactions (e.g., H2O2 production), the polarized CN also maintained a 2.1-fold increase compared to the pristine CN. This strategy of inducing localized polarization via strain engineering provides new insights for boosting photocatalytic reactions involving electrons.