Ultrasound-responsive microfibers promoted infected wound healing with neuro-vascularization by segmented sonodynamic therapy and electrical stimulation.

Bacteria-infected wounds pose challenges to healing due to persistent infection and associated damage to nerves and vessels. Although sonodynamic therapy can help kill bacteria, it is limited by the residual oxidative stress, resulting in prolonged inflammation. To tackle these barriers, novel 4 octyl itaconate-coated Li-doped ZnO/PLLA piezoelectric composite microfibers are developed, offering a whole-course "targeted" treatment under ultrasound therapy. The inclusion of Li atoms causes the ZnO lattice distortion and increases the band gap, enhancing the piezoelectric and sonocatalytic properties of the composite microfibers, collaborated by an aligned PLLA conformation design. During the infection and inflammation stages, the piezoelectric microfibers exhibit spatiotemporal-dependent therapeutic effects, swiftly eliminating over 94.2 % of S. aureus within 15 min under sonodynamic therapy. Following this phase, the microfibers capture reactive oxygen species and aid macrophage reprogramming, restoring mitochondrial function, achieving homeostasis, and shortening inflammation cycles. As the wound progresses through the healing stages, bioactive Zn2+ and Li + ions are continuously released, improving cell recruitment, and the piezoelectrical stimulation enhances wound recovery with neuro-vascularization. Compared to commercially available dressings, our microfibers accelerate the closure of rat wounds (Φ = 15 mm) without scarring in 12 days. Overall, this "one stone, four birds" wound management strategy presents a promising avenue for infected wound therapy.

One-step synthesis of a piezoelectric hybrid BNNT/BaTiO3 composite and its application in bone tissue engineering.

Nanomaterials with piezoelectric properties can significantly improve the applicability of polymers used in tissue engineering applications. In this study, we report the one-step synthesis of a novel hybrid piezoelectric composite comprising barium titanates and boron nitride nanotubes. This composite is distinguished by its unique microstructures, including nanoflakes, triangular boron nitride structures, and fiber-like boron nitride nanotube configurations, which contribute to its enhanced piezoelectric properties. The composite was incorporated into a chitosan-based tissue scaffold and evaluated in vitro. Electric-responsive Human Osteoblast cells cultured on the scaffolds are exposed to low-frequency ultrasound stimulation during cell growth. The biocompatibility, cell adhesion, alkaline phosphatase activities, and mineralization of osteoblast cells on the piezo-composite scaffolds were evaluated. The results show that the hybrid piezoelectric composite significantly enhances the properties of chitosan-based scaffold.

Osteogenic Potential and Long-Term Enzymatic Biodegradation of PHB-based Scaffolds with Composite Magnetic Nanofillers in a Magnetic Field.

Millions of people worldwide suffer from musculoskeletal damage, thus using the largest proportion of rehabilitation services. The limited self-regenerative capacity of bone and cartilage tissues necessitates the development of functional biomaterials. Magnetoactive materials are a promising solution due to clinical safety and deep tissue penetration of magnetic fields (MFs) without attenuation and tissue heating. Herein, electrospun microfibrous scaffolds were developed based on piezoelectric poly(3-hydroxybutyrate) (PHB) and composite magnetic nanofillers [magnetite with graphene oxide (GO) or reduced GO]. The scaffolds' morphology, structure, mechanical properties, surface potential, and piezoelectric response were systematically investigated. Furthermore, a complex mechanism of enzymatic biodegradation of these scaffolds is proposed that involves (i) a release of polymer crystallites, (ii) crystallization of the amorphous phase, and (iii) dissolution of the amorphous phase. Incorporation of Fe3O4, Fe3O4-GO, or Fe3O4-rGO accelerated the biodegradation of PHB scaffolds owing to pores on the surface of composite fibers and the enlarged content of polymer amorphous phase in the composite scaffolds. Six-month biodegradation caused a reduction in surface potential (1.5-fold) and in a vertical piezoresponse (3.5-fold) of the Fe3O4-GO scaffold because of a decrease in the PHB ß-phase content. In vitro assays in the absence of an MF showed a significantly more pronounced mesenchymal stem cell proliferation on composite magnetic scaffolds compared to the neat scaffold, whereas in an MF (68 mT, 0.67 Hz), cell proliferation was not statistically significantly different when all the studied scaffolds were compared. The PHB/Fe3O4-GO scaffold was implanted into femur bone defects in rats, resulting in successful bone repair after nonperiodic magnetic stimulation (200 mT, 0.04 Hz) owing to a synergetic influence of increased surface roughness, the presence of hydrophilic groups near the surface, and magnetoelectric and magnetomechanical effects of the material.

Biodegradable Piezoelectric-Conductive Integrated Hydrogel Scaffold for Repair of Osteochondral Defects.

Osteochondral injury is a prevalent condition for which no specific treatment is currently available. This study presents a piezoelectric-conductive scaffold composed of a piezoelectric cartilage-decellularized extracellular matrix (dECM) and piezoelectric-conductive modified gelatin (Gel-PC). The piezoelectricity of the scaffold is achieved through the modification of diphenylalanine (FF) assembly on the pore surface, while the conductive properties of scaffold are achieved by the incorporating poly(3,4-ethylenedioxythiophene). In vitro experiments demonstrate that bone marrow mesenchymal stem cells (BMSCs) undergo biphasic division during differentiation. In vivo studies using a Parma pig model of osteochondral defects demonstrate that the piezoelectric-conductive scaffold exhibits superior reparative efficacy. Notably, the generation of electrical stimulation is linked to joint movement. During joint activity, mechanical forces compress the scaffold, leading to deformation and the subsequent generation of an electric potential difference. The positive charges accumulated on the upper layer of the scaffold attract BMSCs, promoting their migration to the upper layer and chondrogenic differentiation. Meanwhile, the negative charges in the lower layer induce the osteogenic differentiation of BMSCs. Overall, this piezoelectric-conducive scaffold provides a promising platform for the effective repair of osteochondral defects.

Piezoelectric Properties of As-Spun Poly(vinylidene Fluoride)/Multi-Walled Carbon Nanotube/Zinc Oxide Nanoparticle (PVDF/MWCNT/ZnO) Nanofibrous Films.

Conductive multi-walled carbon nanotubes (MWCNTs) as well as piezoelectric zinc oxide (ZnO) nanoparticles are frequently used as a single additive and dispersed in polyvinylidene fluoride (PVDF) solutions for the fabrication of piezoelectric composite films. In this study, MWCNT/ZnO binary dispersions are used as spinning liquids to fabricate composite nanofibrous films by electrospinning. Binary additives are conducive to increasing the crystallinity, piezoelectric voltage coefficient, and consequent piezoelectricity of as-spun films owing to the stretch-enhanced polarization of the electrospinning process under an applied electric field. PCZ-1.5 film (10 wt. % PVDF/0.1 wt. % MWCNTs/1.5 wt. % ZnO nanoparticles) contains the maximum ß-phase content of 79.0% and the highest crystallinity of 87.9% in nanofibers. A sensor using a PCZ-1.5 film as a functional layer generates an open-circuit voltage of 10 V as it is subjected to impact loads with an amplitude of 6 mm at 10 Hz. The piezoelectric sensor reaches a power density of 0.33 µW/cm2 and a force sensitivity of 582 mV/N. In addition, the sensor is successfully applied to test irregular motions of a bending finger and stepping foot. The result indicates that electrospun PVDF/MWCNT/ZnO nanofibrous films are suitable for wearable devices.

Nanomechanical Crystalline AlN Resonators with High Quality Factors for Quantum Optoelectromechanics.

High-quality factor (Qm) mechanical resonators are crucial for applications where low noise and long coherence time are required, as mirror suspensions, quantum cavity optomechanical devices, or nanomechanical sensors. Tensile strain in the material enables the use of dissipation dilution and strain engineering techniques, which increase the mechanical quality factor. These techniques have been employed for high-Qm mechanical resonators made from amorphous materials and, recently, from crystalline materials such as InGaP, SiC, and Si. A strained crystalline film exhibiting substantial piezoelectricity expands the capability of high-Qm nanomechanical resonators to directly utilize electronic degrees of freedom. In this work, nanomechanical resonators with Qm up to 2.9 × 107 made from tensile-strained 290 nm-thick AlN are realized. AlN is an epitaxially-grown crystalline material offering strong piezoelectricity. Nanomechanical resonators that exploit dissipation dilution and strain engineering to reach a Qm × fm-product approaching 1013 Hz at room temperature are demonstrated. A novel resonator geometry is realized, triangline, whose shape follows the Al-N bonds and offers a central pad patterned with a photonic crystal. This allows to reach an optical reflectivity above 80% for efficient coupling to out-of-plane light. The presented results pave the way for quantum optoelectromechanical devices at room temperature based on tensile-strained AlN.

Thermoplastic PHB-Reinforced Chitosan Piezoelectric Films for Biodegradable Pressure Sensors.

Flexible and wearable pressure sensors have attracted significant attention in the fields of smart medicine and human health monitoring. Nevertheless, the design and fabrication of degradable disposable pressure sensors still face urgent challenges. Herein, we fabricated poly(3-hydroxybutyrate) (PHB)-reinforced chitosan (CS) piezoelectric films for intelligent sensors through a simple, low-cost, and environmentally friendly roll-forming method. The results show that PHB doping successfully increased the effective piezoelectric coefficient of the chitosan-based film from 40.12 to 49.38 pm/V (a 23% increase). Simultaneously, the pressure sensor based on the CS/PHB film exhibited excellent response sensitivity (484 mV/kPa) and a wide linear response range (0-130 kPa), which could be used as haptic sensors and motion monitoring sensors for the fast response to human motion signals. Additionally, the CS/PHB film could be completely degraded within 18 days in a natural soil environment, demonstrating outstanding degradability. Therefore, chitosan-based piezoelectric films with excellent biodegradability and piezoelectric characteristics have been successfully fabricated in this work, which will promote the innovative development of green chitosan-based electronic devices and disposable pressure sensors.

Simultaneous low-frequency vibration isolation and energy harvesting via attachable metamaterials.

In this study, we achieved energy localization and amplification of flexural vibrations by utilizing the defect mode of plate-attachable locally resonant metamaterials, thereby realizing compact and low-frequency vibration energy suppression and energy harvesting with enhanced output performance. We designed a cantilever-based metamaterial unit cell to induce local resonance inside a periodic supercell structure and form a bandgap within the targeted low-frequency range of 300-450 Hz. Subsequently, a defect area was created by removing some unit cells to break the periodicity inside the metamaterial, which led to the isolation and localization of the vibration energy. This localized vibration energy was simultaneously converted into electrical energy by a piezoelectric energy harvester coupled with a metamaterial inside the defect area. Consequently, a substantially enhanced energy harvesting output power was achieved at 360 Hz, which was 43-times higher than that of a bare plate without metamaterials. The proposed local resonant metamaterial offers a useful and multifunctional platform with the capability of vibration energy isolation and harvesting, while exhibiting easy handling via attachable designs that can be tailored in the low-frequency regime.

Advanced Materials for Energy Harvesting and Soft Robotics: Emerging Frontiers to Enhance Piezoelectric Performance and Functionality.

Piezoelectric energy harvesting captures mechanical energy from a number of sources, such as vibrations, the movement of objects and bodies, impact events, and fluid flow to generate electric power. Such power can be employed to support wireless communication, electronic components, ocean monitoring, tissue engineering, and biomedical devices. A variety of self-powered piezoelectric sensors, transducers, and actuators have been produced for these applications, however approaches to enhance the piezoelectric properties of materials to increase device performance remain a challenging frontier of materials research. In this regard, the intrinsic polarization and properties of materials can be designed or deliberately engineered to enhance the piezo-generated power. This review provides insights into the mechanisms of piezoelectricity in advanced materials, including perovskites, active polymers, and natural biomaterials, with a focus on the chemical and physical strategies employed to enhance the piezo-response and facilitate their integration into complex electronic systems. Applications in energy harvesting and soft robotics are overviewed by highlighting the primary performance figures of merits, the actuation mechanisms, and relevant applications. Key breakthroughs and valuable strategies to further improve both materials and device performance are discussed, together with a critical assessment of the requirements of next-generation piezoelectric systems, and future scientific and technological solutions.

Application of a Modified First-Order Plate Theory to Structural Analysis of Sensitive Elements in a Pyroelectric Detector.

Pyroelectric materials, with piezoelectricity and pyroelectricity, have been widely used in infrared thermal detectors. In this paper, a modified first-order plate theory is extended to analyze a pyroelectric sensitive element structure. The displacement, temperature, and electric potential expand along the thickness direction. The governing equation of the pyroelectric plate is built up. The potential distributions with upper and lower electrodes are obtained under different supported boundary conditions. The corresponding numerical results of electric potential are consistent with those obtained by the three-dimensional finite element method. Meanwhile, the theoretical results of electric potential are close to that of experiments. The influence of supported boundary conditions, piezoelectric effect, and plate thickness are analyzed. Numerical results show that the piezoelectric effect reduces the electric potential. The thickness of the pyroelectric plate enhances the electric potential but reduces the response speed of the detector. It is anticipated that the pyroelectric plate theory can provide a theoretical approach for the structural design of pyroelectric sensitive elements.

Integrated Piezoelectric/Pyroelectric Sensing from Organic-Inorganic Perovskite Nanocomposites.

Flexible ferroelectric materials are in high demand in emerging energy harvesting and self-powered sensing electronics. However, current flexible ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF) and P(VDF-co-trifluoroethylene) [P(VDF-TrFE)], cannot fulfill the requirement of emerging applications because of their low piezoelectric/pyroelectric performance. In this work, using organic-inorganic hybrid perovskite [(4-aminotetrahydropyran)2PbBr2Cl2] ferroelectric nanorods as reinforcement and P(VDF-TrFE) as the matrix, we prepared flexible core-sheath piezoelectric nanofibers and pyroelectric nanocomposite films. The core-sheath nanofibers possess a record-high piezoelectric coefficient of 78.1 pC·N-1, and the output voltage reaches to 192 V, with the maximum power density of 1.04 W·m-2. On the other hand, the nanocomposite film exhibits a high pyroelectric coefficient of 58.2 µC·m-2·K-1 at 333 K, which yields a voltage of 6.1 V under 6.6 K temperature fluctuation. An integrated flexible sensing device was prepared by combining piezoelectric nanofibers and pyroelectric films, which can wirelessly detect vibration and temperature fluctuation simultaneously. The integrated device is suitable for pipelines, power equipment, and other scenarios, where vibration and temperature need to be monitored at the same time.

Significant Enhancement in THz Emission and Piezoelectricity in Atomically Thin Nb-Doped MoS2.

A significantly enhanced THz radiation generation from femtosecond photoexcited MoS2 layers due to Nb-doping is reported here. Different microscopic mechanisms involved in the THz photocurrent generation vary in their relative contributions in the two cases of photoexcitation, i.e., above and below the electronic bandgap of the layers. For a moderate Nb-doping level of just ∼0.05%, we have observed a multifold enhancement in the THz emission for the case of the above bandgap excitation, which is, though, nearly 1.5 times for the case of the below bandgap excitation of the monolayer MoS2. Alongside the difference in THz generation efficiency, the THz pulse polarity is also reversed at the above bandgap excitation of the Nb-doped layers, consequent to the reversed surface depletion field. Except for a slightly smaller difference in the THz enhancement factor, all the observations are reproducible in the bilayers as well to imply a weaker inversion symmetry and reduced screening of the surface depletion field due to Nb-doping. Furthermore, we employed pristine MoS2 and Nb-doped MoS2 monolayers to fabricate piezoelectric nanogenerator devices. Like enhancement in the ultrafast THz emission, the piezoelectric performance of the nanogenerator, fabricated with the Nb-doped MoS2 monolayer is also increased by a similar factor.

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.

MXene-coated piezoelectric poly-L-lactic acid membrane accelerates wound healing by biomimicking low-voltage electrical pulses.

Electrical stimulation therapy is effective in promoting wound healing by rescuing the decreased endogenous electrical field, where self-powered and miniaturized devices such as nanogenerators become the emerging trends. While high-voltage and unidirectional electric field may pose thermal effect and damage to the skin, nanogenerators with lower voltages, pulsed or bidirectional currents, and less invasive electrodes are preferred. Herein, we construct a polydopamine (PDA)-modified poly-L-lactic acid (PLLA) /MXene (PDMP/MXene) nanofibrous composite membrane that generates piezoelectric voltages matching the transepithelial potential (TEP) to accelerate wound healing. PDA coating not only enhances the piezoelectricity of PLLA by dipole attraction and alignment, but also increases its hydrophilicity and facilitates subsequent MXene adhesion for electrical conductivity and stability in physiological environment. When applied as wound dressings in mice, the PDMP/MXene membranes act as a nanogenerators with reduced internal resistances and satisfactory piezoelectric performances that resemble bioelectric potentials (~10 mV) responding to physical activities. The membrane significantly accelerates wound closure by facilitating fibroblast migration, collagen deposition and angiogenesis, and suppressing the expression of inflammatory responses. This piezoelectric fibrous membrane therefore provides a convenient solution for speeding up wound healing by sustained low voltage mimicking bioelectricity, better cell affinity.

High Quality Epitaxial Piezoelectric and Ferroelectric Wurtzite Al1- xScxN Thin Films.

Piezoelectric and ferroelectric wurtzite are promising to reshape modern microelectronics because they can be easily integrated with mainstream semiconductor technology. Sc doped AlN (Al1- xScxN) has attracted much attention for its enhanced piezoelectric and emerging ferroelectric properties, yet the commonly used sputtering results in polycrystalline Al1- xScxN films with high leakage current. Here, the pulsed laser deposition of single crystalline epitaxial Al1- xScxN thin films on sapphire and 4H-SiC substrates is reported. Pure wurtzite phase is maintained up to x = 0.3 with ≤0.1 at% oxygen contamination. Polarization is estimated to be 140 µC cm-2 via atomic scale microscopy imaging and found to be switchable via a scanning probe. The piezoelectric coefficient is found to be five times of the undoped one when x = 0.3, making it desirable for high-frequency radiofrequency (RF) filters and 3D nonvolatile memories.

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.

Composite Nanoarchitectonics of Electrospun Piezoelectric PVDF/AgNPs for Biomedical Applications, Including Breast Cancer Treatment.

This study focused on preparing composite nanomats by incorporating silver nanoparticles (AgNPs) in polyvinylidene fluoride (PVDF) nanofibers through the electrospinning process. A short review of piezoelectric PVDF-related research is presented. PVDF is known for its biocompatibility and piezoelectric properties. Since electrical signals in biological tissues have been shown to be relevant for therapeutic applications, the influence of the addition of AgNPs to PVDF on its piezoelectricity is studied, due to the ability of AgNPs to increase the piezoelectric signal, along with providing antibacterial properties. The prepared samples were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. In addition, the biological activity of composites was examined using a cytotoxicity assay and an assessment of the antibacterial activity. The obtained results show that the incorporation of AgNPs into PVDF nanofibers further enhances the piezoelectricity (crystalline ß-phase fraction), already improved by the electrospinning process, compared to solution-casted samples, but only with a AgNPs/PVDF concentration of up to 0.3%; a further increase in the nanoparticles led to a ß-phase reduction. The cytotoxicity assay showed a promising effect of PVDF/AgNPs nanofibers on the MDA-MB-231 breast cancer cell line, following the non-toxicity displayed in regard to the healthy MRC-5 cell line. The antibacterial effect of PVDF/AgNPs nanofibers showed promising antibacterial activity against Pseudomonas aeruginosa and Staphylococcus aureus, as a result of the Ag content. The anticancer activity, combined with the electrical properties of nanofibers, presents new possibilities for smart, multifunctional materials for cancer treatment development.

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.

Lateral Piezoelectricity of Alzheimer's Aß Aggregates.

Alzheimer's disease (AD) is the most frequent neurodegenerative disorder in the elderly aged over 65. The extracellular accumulation of beta-amyloid (Aß) aggregates in the brain is considered as the major event worsening the AD symptoms, but its underlying reason has remained unclear. Here the piezoelectric characteristics of Aß aggregates are revealed. The vector piezoresponse force microscopy (PFM) analysis results exhibit that Aß fibrils have spiraling piezoelectric domains along the length and a lateral piezoelectric constant of 44.1 pC N-1. Also, the continuous sideband Kelvin probe force microscopy (KPFM) images display that the increment of charge-induced surface potential on a single Aß fibril is allowed to reach above +1700 mV in response to applied forces. These findings shed light on the peculiar mechano-electrical surface properties of pathological Aß fibrils that exceed those of normal body components.

Excellent Hole Mobility and Out-of-Plane Piezoelectricity in X-Penta-Graphene (X = Si or Ge) with Poisson's Ratio Inversion.

Recently, the application of two-dimensional (2D) piezoelectric materials has been seriously hindered because most of them possess only in-plane piezoelectricity but lack out-of-plane piezoelectricity. In this work, using first-principles calculation, by atomic substitution of penta-graphene (PG) with tiny out-of-plane piezoelectricity, we design and predict stable 2D X-PG (X = Si or Ge) semiconductors with excellent in-plane and out-of-plane piezoelectricity and extremely high in-plane hole mobility. Among them, Ge-PG exhibits better performance in all aspects with an in-plane strain piezoelectric coefficient d11 = 8.43 pm/V, an out-of-plane strain piezoelectric coefficient d33 = -3.63 pm/V, and in-plane hole mobility µh = 57.33 × 103 cm2 V-1 s-1. By doping Si and Ge atoms, the negative Poisson's ratio of PG approaches zero and reaches a positive value, which is due to the gradual weakening of the structure's mechanical strength. The bandgaps of Si-PG (0.78 eV) and Ge-PG (0.89 eV) are much smaller than that of PG (2.20 eV), by 2.82 and 2.47 times, respectively. This indicates that the substitution of X atoms can regulate the bandgap of PG. Importantly, the physical mechanism of the out-of-plane piezoelectricity of these monolayers is revealed. The super-dipole-moment effect proposed in the previous work is proved to exist in PG and X-PG, i.e., it is proved that their out-of-plane piezoelectric stress coefficient e33 increases with the super-dipole-moment. The e33-induced polarization direction is also consistent with the super-dipole-moment direction. X-PG is predicted to have prominent potential for nanodevices applied as electromechanical coupling systems: wearable, ultra-thin devices; high-speed electronic transmission devices; and so on.