Nanoconfinement induced crystal orientation and large piezoelectric coefficient in vertically aligned P(VDF-TrFE) nanotube array

Review of Piezoelectric Properties and Power Output of PVDF and Copolymer-Based Piezoelectric Nanogenerators

The highest energy conversion efficiencies are typically shown by lead-containing piezoelectric materials, but the harmful environmental impacts of lead and its toxicity limit future use. At the bulk scale, lead-based piezoelectric materials have significantly higher piezoelectric properties when compared to lead-free piezoelectric materials. However, at the nanoscale, the piezoelectric properties of lead-free piezoelectric material can be significantly larger than the bulk scale. The piezoelectric properties of Poly(vinylidene fluoride) (PVDF) and Poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) lead-free piezoelectric nanomaterials are reviewed and their suitability for use in piezoelectric nanogenerators (PENGs) is determined. The impact of different PVDF/PVDF-TrFE composite structures on power output is explained. Strategies to improve the power output are given. Overall, this review finds that PVDF/PVDF-TrFE can have significantly increased piezoelectric properties at the nanoscale. However, these values are still lower than lead-free ceramics at the nanoscale. If the sole goal in developing a lead-free PENG is to maximize output power, lead-free ceramics at the nanoscale should be considered. However, lead-free ceramics are brittle, and thus encapsulation of lead-free ceramics in PVDF is a way to increase the flexibility of these PENGs. PVDF/PVDF-TrFE offers the advantage of being nontoxic and biocompatible, which is useful for many applications.

Scaffold-Guided Crystallization of Oriented α-FAPbI3 Nanowire Arrays for Solar Cells

Perovskite nanowire arrays with large surface areas for efficient charge transfer and continuous highly crystalline domains for efficient charge transport exhibit ideal morphologies for solar-cell active layers. Here, we introduce a room temperature two-step method to grow dense, vertical nanowire arrays of formamidinium lead iodide (FAPbI3). PbI2 nanocrystals embedded in the cylindrical nanopores of anodized titanium dioxide scaffolds were converted to FAPbI3 by immersion in a FAI solution for a period of 0.5-30 min. During immersion, FAPbI3 crystals grew vertically from the scaffold surface as nanowires with diameters and densities determined by the underlying scaffold. The presence of butylammonium cations during nanowire growth stabilized the active α polymorph of FAPbI3, precluding the need for a thermal annealing step. Solar cells comprising α-FAPbI3 nanowire arrays exhibited maximum solar conversion efficiencies of >14%. Short-circuit current densities of 22-23 mA cm-2 were achieved, on par with those recorded for the best-performing FAPbI3 solar cells reported to date. Such large photocurrents are attributed to the single-crystalline, low-defect nature of the nanowires and increased interfacial area for photogenerated charge transfer compared with thin films.

New Insight into Nanoscale Identification of the Polar Axis Direction in Organic Ferroelectric Films

Ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)] thin films have been deposited by spin-coating onto the Bi0.5Na0.5TiO3(BNT)/LNO/SiO2/Si heterostructure. The copolymer microstructure investigated by using grazing-incidence wide-angle X-ray diffraction (GIWAXD) and deduced from the (200)/(110) reflections demonstrates that the b-axis in the P(VDF-co-TrFE) orthorhombic unit cell is either in the plane or out of the plane, depending on the face-on or on the two types of edge-on (called I and II) lamellar structures locally identified by atomic force microscopy (AFM). For edge-on I lamellae regions, the electroactivity (dzzeff ∼ -50.3 pm/V) is found to be twice as high as that measured for both edge-on II or face-on crystalline domains, as probed by piezoresponse force microscopy (PFM). This result is directly correlated to the direction of the ferroelectric polarization vector in the P(VDF-co-TrFE) orthorhombic cell: larger nanoscale piezoactivity is related to the b-axis which lies along the normal to the substrate plane in the case of the edge-on I domains. Here, the ability to thoroughly gain access to the as-grown polar axis direction within the edge-on crystal lamellae of the ferroelectric organic layers is evidenced by combining the nanometric resolution of the PFM technique with a statistical approach based on its spectroscopic tool. By the gathering of information at the nanoscale, two orientations for the polar b-axis are identified in edge-on lamellar structures. These findings contribute to a better understanding of the structure-property relationships in P(VDF-co-TrFE) films, which is a key issue for the design of future advanced organic electronic devices.

The Preparation, Structural Design, and Application of Electroactive Poly(vinylidene fluoride)-Based Materials for Wearable Sensors and Human Energy Harvesters

Owing to their biocompatibility, chemical stability, film-forming ability, cost-effectiveness, and excellent electroactive properties, poly(vinylidene fluoride) (PVDF) and PVDF-based polymers are widely used in sensors, actuators, energy harvesters, etc. In this review, the recent research progress on the PVDF phase structures and identification of different phases is outlined. Several approaches for obtaining the electroactive phase of PVDF and preparing PVDF-based nanocomposites are described. Furthermore, the potential applications of these materials in wearable sensors and human energy harvesters are discussed. Finally, some challenges and perspectives for improving the properties and boosting the applications of these materials are presented.

Expedient secondary functions of flexible piezoelectrics for biomedical energy harvesting

Flexible piezoelectrics realise the conversion between mechanical movements and electrical power by conformally attaching onto curvilinear surfaces, which are promising for energy harvesting of biomedical devices due to their sustainable body movements and/or deformations. Developing secondary functions of flexible piezoelectric energy harvesters is becoming increasingly significant in recent years via aiming at issues that cannot be addressed or mitigated by merely increasing piezoelectric efficiencies. These issues include loose interfacial contact and pucker generation by stretching, power shortage or instability induced by inadequate mechanical energy, and premature function degeneration or failure caused by fatigue fracture after cyclic deformations. Herein, the expedient secondary functions of flexible piezoelectrics to mitigate above issues are reviewed, including stretchability, hybrid energy harvesting, and self-healing. Efforts have been devoted to understanding the state-of-the-art strategies and their mechanisms of achieving secondary functions based on piezoelectric fundamentals. The link between structural characteristic and function performance is unravelled by providing insights into carefully selected progresses. The remaining challenges of developing secondary functions are proposed in the end with corresponding outlooks. The current work hopes to help and inspire future research in this promising field focusing on developing the secondary functions of flexible piezoelectric energy harvesters.

The intrinsic piezoelectric properties of materials - a review with a focus on biological materials

Piezoelectricity, a linear electromechanical coupling, is of great interest due to its extensive applications including energy harvesters, biomedical, sensors, and automobiles. A growing amount of research has been done to investigate the energy harvesting potential of this phenomenon. Traditional piezoelectric inorganics show high piezoelectric outputs but are often brittle, inflexible and may contain toxic compounds such as lead. On the other hand, biological piezoelectric materials are biodegradable, biocompatible, abundant, low in toxicity and are easy to fabricate. Thus, they are useful for many applications such as tissue engineering, biomedical and energy harvesting. This paper attempts to explain the basis of piezoelectricity in biological and non-biological materials and research involved in those materials as well as applications and limitations of each type of piezoelectric material.

Piezoelectricity, a linear electromechanical coupling, is of great interest due to its extensive applications including energy harvesters, biomedical, sensors, and automobiles.

Improved Design via Simulation of Micro-Modified PVDF and its Copolymer Energy Harvester with High Electrical Outputs

In this work, micro-modified polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene-trifluoroethylene) (P(VDF-TrFE)) with salient enhancement in current output are demonstrated. The influence of surface-modified structure characteristics on electrical properties of energy harvester is systematically analyzed based on the finite element method. For vertical load mode, eight structures consisting of banded and disjunctive groups are compared to evaluate the voltage performance. The cylinder is proved to be the best structure of 3.25 V, compared to the pristine structure of 0.99 V (P(VDF-TrFE)). The relevant experiment has been done to verify the simulation. The relationship between radius, height, force and distance to the voltage output of the cylinder allocation is discussed. For periodical changing load mode, the cylinder modified structure shows a conspicuous enhancement in current output. The suitable resistance, current-voltage and frequency, the relationship between loading speed and current, and the ductility of current loading are studied. For 30 kHz, the peak current is 20 times larger than the flat plate structure. Tip shape mode and fusiform shape mode are found, which show the different shapes of the peak current-frequency curves. Four electrical loading circuit properties are also discussed: the suitable resistance of the system, synchronism of current and voltage, time delay nature of energy harvester and current-loading relationship. The simulation results can provide some theoretical basis for designing the energy harvester and piezoelectric nanogenerator (PENG).

Recent Structure Development of Poly(vinylidene fluoride)-Based Piezoelectric Nanogenerator for Self-Powered Sensor

As the internet of things (IoT) era approaches, various sensors, and wireless electronic devices such as smartphones, smart watches, and earphones are emerging. As the types and functions of electronics are diversified, the energy consumption of electronics increases, which causes battery charging and maintenance issues. The piezoelectric nanogenerator (PENG) received great attention as an alternative to solving the energy issues of future small electronics. In particular, polyvinylidene fluoride (PVDF) piezoelectric polymer-based PENGs are strong potential candidate with robust mechanical properties and a high piezoelectric coefficient. In this review, we summarize the recent significant advances of the development of PVDF-based PENGs for self-powered energy-harvesting systems. We discuss the piezoelectric properties of the various structures of PVDF-based PENGs such as thin film, microstructure, nanostructure, and nanocomposite.

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-Induced Crystallization at Lowered Temperature

Lead-free potassium and sodium niobate (KNN) nanofiber webs with random and aligned configurations were prepared by the electrospinning process from polymer-modified chemical solution. The crystallization process, structure, composition, dielectric, ferroelectric, and piezoelectric properties of the nanofibers and nanofiber webs were investigated. Theoretical analysis and experimental results showed that the surface-induced heterogeneous nucleation resulted in the remarkable lower crystallization temperature for the KNN nanofibers with the {100} orientation of the perovskite phase in contrast to the bulk KNN gel and thus well-controlled chemical stoichiometry. Low dielectric loss, large electric polarization, and high piezoelectric performance were obtained in the nanofiber webs. In particular, the aligned nanofiber web exhibited further improved piezoelectric strain and voltage coefficients and higher FOM than their thin film counterparts and is promising for high-performance electromechanical sensor and transducer applications.

Large Piezoelectric Strain in Sub-10 Nanometer Two-Dimensional Polyvinylidene Fluoride Nanoflakes

Functional polymers such as polyvinylidene fluoride (PVDF) and its copolymers, which exhibit room-temperature piezoelectricity and ferroelectricity in two-dimensional (2D) limit, are promising candidates to substitute hazardous lead-based piezoceramics for flexible nanoelectronic and electromechanical energy-harvesting applications. However, realization of many polymers including PVDF in ultrathin 2D nanostructures with desired crystal phases and tunable properties remains challenging due to ineffective conventional synthesis methods. Consequently, it has remained elusive to obtain optimized piezoelectric performance of PVDF particularly in sub-10 nm regimes. Taking advantage of its high flexibility and easy processing, we fabricate ultrathin PVDF nanoflakes with thicknesses down to 7 nm by using a hot-pressing method. This thermo-mechanical strategy simultaneously induces robust thermodynamic α to electroactive β-phase transformation, with β fraction as high as 92.8% in sub-10 nm flakes. Subsequently, piezoelectric studies performed by using piezoresponse force microscopy reveal an excellent piezoelectric strain of 0.7% in 7 nm film and the highest piezoelectric coefficient ( d₃₃) achieved is -68 pm/V for 50 nm-thick nanoflakes, which is 13% higher than the piezoresponse from 50 nm-thick PZT nanofilms. Our results further suggest thickness modulation as an effective strategy to tune the piezoelectric performance of PVDF and affirm its supremacy over conventional piezoceramics especially at nanoscale. This work aims not only to help understand fundamental piezoelectricity of pure PVDF in sub-10 nm regimes but also provides an opportunity to realize other polymer-based 2D nanocrystals.

Piezoelectric Polymer and Paper Substrates: A Review

Polymers and papers, which exhibit piezoelectricity, find a wide range of applications in the industry. Ever since the discovery of PVDF, piezo polymers and papers have been widely used for sensor and actuator design. The direct piezoelectric effect has been used for sensor design, whereas the inverse piezoelectric effect has been applied for actuator design. Piezo polymers and papers have the advantages of mechanical flexibility, lower fabrication cost and faster processing over commonly used piezoelectric materials, such as PZT, BaTiO₃. In addition, many polymer and paper materials are considered biocompatible and can be used in bio applications. In the last 20 years, heterostructural materials, such as polymer composites and hybrid paper, have received a lot of attention since they combine the flexibility of polymer or paper, and excellent pyroelectric and piezoelectric properties of ceramics. This paper gives an overview of piezoelectric polymers and papers based on their operating principle. Main categories of piezoelectric polymers and papers are discussed with a focus on their materials and fabrication techniques. Applications of piezoelectric polymers and papers in different areas are also presented.

Mechanical Energy Harvesting Performance of Ferroelectric Polymer Nanowires Grown via Template-Wetting

Nanowires of the ferroelectric co-polymer poly(vinylidenefluoride-co-triufloroethylene) [P(VDF-TrFE)] are fabricated from solution within nanoporous templates of both "hard" anodic aluminium oxide (AAO) and "soft" polyimide (PI) through a facile and scalable template-wetting process. The confined geometry afforded by the pores of the templates leads directly to highly crystalline P(VDF-TrFE) nanowires in a macroscopic "poled" state that precludes the need for external electrical poling procedure typically required for piezoelectric performance. The energy-harvesting performance of nanogenerators based on these template-grown nanowires are extensively studied and analyzed in combination with finite element modelling. Both experimental results and computational models probing the role of the templates in determining overall nanogenerator performance, including both materials and device efficiencies, are presented. It is found that although P(VDF-TrFE) nanowires grown in PI templates exhibit a lower material efficiency due to lower crystallinity as compared to nanowires grown in AAO templates, the overall device efficiency was higher for the PI-template-based nanogenerator because of the lower stiffness of the PI template as compared to the AAO template. This work provides a clear framework to assess the energy conversion efficiency of template-grown piezoelectric nanowires and paves the way towards optimization of template-based nanogenerator devices.

1D Piezoelectric Material Based Nanogenerators: Methods, Materials and Property Optimization

Due to the enhanced piezoelectric properties, excellent mechanical properties and tunable electric properties, one-dimensional (1D) piezoelectric materials have shown their promising applications in nanogenerators (NG), sensors, actuators, electronic devices etc. To present a clear view about 1D piezoelectric materials, this review mainly focuses on the characterization and optimization of the piezoelectric properties of 1D nanomaterials, including semiconducting nanowires (NWs) with wurtzite and/or zinc blend phases, perovskite NWs and 1D polymers. Specifically, the piezoelectric coefficients, performance of single NW-based NG and structure-dependent electromechanical properties of 1D nanostructured materials can be respectively investigated through piezoresponse force microscopy, atomic force microscopy and the in-situ scanning/transmission electron microcopy. Along with the introduction of the mechanism and piezoelectric properties of 1D semiconductor, perovskite materials and polymers, their performance improvement strategies are summarized from the view of microstructures, including size-effect, crystal structure, orientation and defects. Finally, the extension of 1D piezoelectric materials in field effect transistors and optoelectronic devices are simply introduced.

Piezoelectric Nanotube Array for Broadband High-Frequency Ultrasonic Transducer

Piezoelectric materials are vital in determining ultrasonic transducer and imaging performance as they offer the function for conversion between mechanical and electrical energy. Ultrasonic transducers with high-frequency operation suffer from performance degradation and fabrication difficulty of the demanded piezoelectric materials. Hence, we propose 1-D polymeric piezoelectric nanostructure with controlled nanoscale features to overcome the technical limitations of high-frequency ultrasonic transducers. For the first time, we demonstrate the integration of a well-aligned piezoelectric nanotube array to produce a high-frequency ultrasonic transducer with outstanding performance. We find that nanoconfinement-induced polarization orientation and unique nanotube structure lead to significantly improved piezoelectric and ultrasonic transducing performance over the conventional piezoelectric thin film. A large bandwidth, 126% (-6 dB), is achieved at high center frequency, 108 MHz. Transmission sensitivity of nanotube array is found to be 46% higher than that of the monolithic thin film transducer attributed to the improved electromechanical coupling effectiveness and impedance match. We further demonstrate high-resolution scanning, ultrasonic imaging, and photoacoustic imaging using the obtained nanotube array transducers, which is valuable for biomedical imaging applications in the future.

Enhanced piezoresponse of highly aligned electrospun poly(vinylidene fluoride) nanofibers

Well-ordered nanostructure arrays with controlled densities can potentially improve material properties; however, their fabrication typically involves the use of complicated processing techniques. In this work, we demonstrate a uniaxial alignment procedure for fabricating poly(vinylidene fluoride) (PVDF) electrospun nanofibers (NFs) by introducing collectors with additional steps. The mechanism of the observed NF alignment, which occurs due to the concentration of lateral electric field lines around collector steps, has been elucidated via finite-difference time-domain simulations. The membranes composed of well-aligned PVDF NFs are characterized by a higher content of the PVDF β-phase, as compared to those manufactured from randomly orientated fibers. The piezoelectric energy harvester, which was fabricated by transferring well-aligned PVDF NFs onto flexible substrates with Ag electrodes attached to both sides, exhibited a 2-fold increase in the output voltage and a 3-fold increase in the output current as compared to the corresponding values obtained for the device manufactured from randomly oriented NFs. The enhanced piezoresponse observed for the aligned PVDF NFs is due to their higher β-phase content, denser structure, smaller effective radius of curvature during bending, greater applied strain, and higher fraction of contributing NFs.

Flexible Semitransparent Energy Harvester with High Pressure Sensitivity and Power Density Based on Laterally Aligned PZT Single-Crystal Nanowires

A flexible semitransparent energy harvester is assembled based on laterally aligned Pb(Zr_0.52Ti_0.48)O₃ (PZT) single-crystal nanowires (NWs). Such a harvester presents the highest open-circuit voltage and a stable area power density of up to 10 V and 0.27 μW/cm², respectively. A high pressure sensitivity of 0.14 V/kPa is obtained in the dynamic pressure sensing, much larger than the values reported in other energy harvesters based on piezoelectric single-crystal NWs. Furthermore, theoretical and finite element analyses also confirm that the piezoelectric voltage constant g₃₃ of PZT NWs is competitive to the lead-based bulk single crystals and ceramics, and the enhanced pressure sensitivity and power density are substantially linked to the flexible structure with laterally aligned PZT NWs. The energy harvester in this work holds great potential in flexible and transparent sensing and self-powered systems.

Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE Templates and Its Application in Semiconducting/Ferroelectric Blend Resistive Memory

Ferroelectric polymer based devices exhibit great potentials in low-cost and flexible electronics. To meet the requirements of both low voltage operation and low energy consumption, thickness of ferroelectric polymer films is usually required to be less than, for example, 100 nm. However, decrease of film thickness is also accompanied by the degradation of both crystallinity and ferroelectricity and also the increase of current leakage, which surely degrades device performance. Here we report one epitaxy method based on removable poly(tetrafluoroethylene) (PTFE) templates for high-quality fabrication of ordered ferroelectric polymer thin films. Experimental results indicate that such epitaxially grown ferroelectric polymer films exhibit well improved crystallinity, reduced current leakage and good resistance to electrical breakdown, implying their applications in high-performance and low voltage operated ferroelectric devices. On the basis of this removable PTFE template method, we fabricated organic semiconducting/ferroelectric blend resistive films which presented record electrical performance with operation voltage as low as 5 V and ON/OFF ratio up to 10⁵.

High voltage generation from lead-free magnetoelectric coaxial nanotube arrays and their applications in nano energy harvesters

Harvesting energy from surrounding vibrations and developing self-powered portable devices for wireless and mobile electronics have recently become popular. Here the authors demonstrate the synthesis of piezoelectric energy harvesters based on nanotube arrays by a wet chemical route, which requires no sophisticated instruments. The energy harvester gives an output voltage of 400 mV. Harvesting energy from a sinusoidal magnetic field is another interesting phenomenon for which the authors fabricated a magnetoelectric energy harvester based on piezoelectric-magnetostrictive coaxial nanotube arrays. Piezoelectric K_0.5Na_0.5NbO₃ (KNN) is fabricated as the shell and magnetostrictive CoFe₂O₄ (CFO) as the core of the composite coaxial nanotubes. The delivered voltages are as high as 300 mV at 500 Hz and at a weak ac magnetic field of 100 Oe. Further tailoring of the thickness of the piezoelectric and magnetic layers can enhance the output voltage by several orders. Easy, single-step wet chemical synthesis enhances the industrial upscaling potential of these nanotubes as energy harvesters. In view of the excellent properties reported here, the lead-free piezoelectric component (KNN) in this nanocomposite should be explored for eco-friendly piezoelectric as well as magnetoelectric power generators in nanoelectromechanical systems (NEMS).

A flexible triboelectric-piezoelectric hybrid nanogenerator based on P(VDF-TrFE) nanofibers and PDMS/MWCNT for wearable devices

This paper studied and realized a flexible nanogenerator based on P(VDF-TrFE) nanofibers and PDMS/MWCNT thin composite membrane, which worked under triboelectric and piezoelectric hybrid mechanisms. The P(VDF-TrFE) nanofibers as a piezoelectric functional layer and a triboelectric friction layer are formed by electrospinning process. In order to improve the performance of triboelectric nanogenerator, the multiwall carbon nanotubes (MWCNT) is doped into PDMS patterned films as the other flexible friction layer to increase the initial capacitance. The flexible nanogenerator is fabricated by low cost MEMS processes. Its output performance is characterized in detail and structural optimization is performed. The device’s output peak-peak voltage, power and power density under triboelectric mechanism are 25 V, 98.56 μW and 1.98 mW/cm³ under the pressure force of 5 N, respectively. The output peak-peak voltage, power and power density under piezoelectric working principle are 2.5 V, 9.74 μW, and 0.689 mW/cm³ under the same condition, respectively. We believe that the proposed flexible, biocompatible, lightweight, low cost nanogenerator will supply effective power energy sustainably for wearable devices in practical applications.

Synthesis of semicrystalline nanocapsular structures obtained by Thermally Induced Phase Separation in nanoconfinement

Phase separation of a polymer solution exhibits a peculiar behavior when induced in a nanoconfinement. The energetic constraints introduce additional interactions between the polymer segments that reduce the number of available configurations. In our work, this effect is exploited in a one-step strategy called nanoconfined-Thermally Induced Phase Separation (nc-TIPS) to promote the crystallization of polymer chains into nanocapsular structures of controlled size and shell thickness. This is accomplished by performing a quench step of a low-concentrated PLLA-dioxane-water solution included in emulsions of mean droplet size <500 nm acting as nanodomains. The control of nanoconfinement conditions enables not only the production of nanocapsules with a minimum mean particle diameter of 70 nm but also the tunability of shell thickness and its crystallinity degree. The specific properties of the developed nanocapsular architectures have important implications on release mechanism and loading capability of hydrophilic and lipophilic payload compounds.

The potential of imogolite nanotubes as (co-)photocatalysts: a linear-scaling density functional theory study

We report a linear-scaling density functional theory (DFT) study of the structure, wall-polarization absolute band-alignment and optical absorption of several, recently synthesized, open-ended imogolite (Imo) nanotubes (NTs), namely single-walled (SW) aluminosilicate (AlSi), SW aluminogermanate (AlGe), SW methylated aluminosilicate (AlSi-Me), and double-walled (DW) AlGe NTs. Simulations with three different semi-local and dispersion-corrected DFT-functionals reveal that the NT wall-polarization can be increased by nearly a factor of four going from SW-AlSi-Me to DW-AlGe. Absolute vacuum alignment of the NT electronic bands and comparison with those of rutile and anatase TiO2 suggest that the NTs may exhibit marked propensity to both photo-reduction and hole-scavenging. Characterization of the NTs' band-separation and optical properties reveal the occurrence of (near-)UV inside-outside charge-transfer excitations, which may be effective for electron-hole separation and enhanced photocatalytic activity. Finally, the effects of the NTs' wall-polarization on the absolute alignment of electron and hole acceptor states of interacting water (H2O) molecules are quantified and discussed.