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Funder: Royce Cluster Tool, Operando electrochemical transmission electron microscopy (ec‐TEM) is a promising tool for advanced characterization of energy systems close to their operating conditions. However, reliable sample preparation for the technique is particularly challenging due to spatial constraints. Here, a novel approach of manufacturing such samples is introduced and demonstrated for industrially relevant battery powders: NMC811 and LTO (LiNi0.8Mn0.1Co0.1O2 and Li4Ti5O12). Aerosol‐jet printing is used to deposit microscale patterns of battery active material particles on the electrodes of commercial ec‐TEM chips. This method provides high spatial resolution (line width < 30 µm) and accuracy, ease of ink preparation, flexibility of materials, and ability to manufacture complex structures (layered or mixed materials). Size selectivity of aerosol jet printing and its positive implications for ec‐TEM sample preparation are also discussed. This procedure is a promising solution to the difficult problem of micro‐battery fabrication for ec‐TEM and for other applications where high spatial resolution deposition from powders is required.

Triboelectric energy harvesting from ambient mechanical sources relies on motion-generated surface charge transfer between materials with different electron affinities. In order to achieve highly efficient energy harvesting performance, choosing materials with a high surface charge density is crucial, and odd-numbered polyamides (Nylons), such as Nylon-11, are particularly promising due to their strong electron-donating characteristics and the possibility to achieve dipolar alignment leading to high surface potential. The use of Nylon-11 as a material for triboelectric energy harvesting has been rather limited due to the extreme processing conditions required for film fabrication, and the high voltage poling process required for dipole alignment. However, several methods to achieve “self-poled” Nylon-11 nanowires via facile nanoconfinement techniques have been demonstrated recently, leading to highly efficient Nylon-11 nanowire-based triboelectric nanogenerators. Here we review the most recent advances in the fabrication of Nylon-11 nanowires, with a focus on how nanoconfinement-based fabrication methods can be used to control phase and crystallinity. These growth methods lead to self-poled nanowires without the requirement for subsequent electrical poling, facilitating their integration into triboelectric energy harvesting devices. Strategies to fabricate Nylon-11 nanowires for applications in triboelectric devices can be extended to other polymeric families as well.

Molecular ordering in polymers can have a drastic effect on their properties, and can be used to induce or enhance functionality. In the case of poly-L-lactic acid (PLLA), which is a widely used polymer in biomedicine, sensors and actuators, preferential orientation of chains can lead to significantly enhanced electromechanical properties. In this context, template-wetting is a straightforward method of producing polymer nanostructures, which can lead to some degree of molecular order in the polymer. Template-wetting of PLLA has not been fully explored, especially in terms of morphological and/or structural characterisation. In this work, PLLA nanotubes are grown via a modification of the template-wetting process, referred to here as melt-press template-wetting. The nanotubes are thoroughly characterised with wide-angle X-ray diffraction, isothermal differential scanning calorimetry and polarised light optical microscopy. This characterisation indicates that the polymer chains in these PLLA nanotubes are aligned parallel to the cylindrical axis of the nanotube, which may be beneficial in certain applications.

Magnetoelectric (ME) composite materials, in which the coupling between magnetostricitve and piezoelectric effects is achieved, are potential candidates for multifunctional devices where the interplay between electrical, magnetic and mechanical properties of these structures can be fully exploited. Nanostructured composites are particularly interesting due to the enhancement of ME coupling expected at the nanoscale. However, direct studies of ME coupling in nanocomposites by scanning probe techniques are rare due to the complex interplay of forces at play, including those arising from electrostatic, magnetic and electromechanical interactions. In this work, the ME coupling of coaxial nickel - polyvinylidene fluoride trifluoroethylene [Ni-P(VDF-TrFE)] composite nanowires, fabricated by a scalable template-wetting based technique, is studied using a systematic sequence of scanning probe techniques. Individual ME nanowires were subjected to an electric field sufficient for ferroelectric poling in piezo-response force microscopy (PFM) mode, while magnetic force microscopy (MFM) was used to measure localised changes in magnetization as a result of electrical poling. Kelvin probe force microscopy (KPFM) measurements of surface potential were conducted to eliminate for the effect of contact potential differences during these measurements. An inverse, static, magnetoelectric coupling coefficient of ~1 x 10-11 s m-1 was found in our coaxial nanocomposite nanowires, comparable to other types of planar composites studied in this work, despite having an inferior piezoelectric-to-magnetostrictive volume ratio. The efficient ME coupling in our coaxial nanowires is attributed to the larger surface-to-volume interfacial contact between Ni and P(VDF-TrFE), and is promising for future integration into ME composite devices such as magnetic field sensors or energy harvesters.

Semiconducting III-V materials exhibiting piezoelectric properties are much sought after due to their potential applications in piezotronic and piezo-phototronic devices. Nanowires of III-V semiconductors are particularly interesting in this respect due to the occurrence of the wurtzite (WZ) structure commonly associated with enhanced piezoelectric properties in these materials, as opposed to the zinc blende (ZB) structure that is typically observed in the bulk. However, direct measurements of the piezoelectric properties of III-V nanowires using piezo-response force microscopy (PFM) is challenging, and the analysis and interpretation of such measurements is far from trivial. Here we present detailed finite element simulations of single GaAs nanowires, with both WZ and ZB crystalline structure, scanned by an atomic force microscope tip in PFM mode, demonstrating the effect of the non-uniform electric field between the tip and nanowire, scan direction as well as nanowire orientation on the resulting PFM signal. We also report PFM data from single GaAs and InP nanowires with both ZB and WZ structure, grown by molecular beam epitaxy, based on a novel non-destructive intermittent contact PFM mode. We explain our experimental data in the framework of our simulations, and for the first time, extract an experimental value for the axial piezoelectric coefficient of WZ InP, d 33 = 0.7−1 pm/V. The methods and analysis described here are particularly relevant for the investigation of piezoelectric properties in a wide range of semiconducting III-V nanowire systems.

Piezoelectric polymers are capable of interconverting mechanical and electrical energy, and are therefore candidate materials for biomedical applications such as sensors, actuators, and energy harvesters. In particular, nanowires of these materials are attractive as they can be unclamped, flexible and sensitive to small vibrations. Poly-$\small \text{L}$-lactic acid (PLLA) nanowires have been investigated for their use in biological applications, but their piezoelectric properties have never been fully characterised, even though macroscopic films and fibres have been shown to exhibit shear piezoelectricity. This piezoelectric mode is particularly interesting for $\textit{in vivo}$ applications where shear forces are especially relevant, and is similar to what has been observed in natural materials such as bone and DNA. Here, using piezo-response force microscopy (PFM), we report the first direct observation of shear piezoelectricity in highly crystalline and oriented PLLA nanowires grown by a novel template-wetting method. Our results are validated using finite-element simulations and numerical analysis, which importantly and more generally allow for accurate interpretation of PFM signals in soft nanostructured materials. Our work opens up the possibility for the development of biocompatible and sustainable piezoelectric nanogenerators and sensors based on polymer nanowires.

Triboelectric generators have emerged as potential candidates for mechanical energy harvesting, relying on motion-generated surface charge transfer between materials with different electron affinities. In this regard, synthetic organic materials with strong electron-donating tendencies are far less common than their electron-accepting counterparts. Nylons are notable exceptions, with odd-numbered Nylons such as Nylon-11, exhibiting electric polarisation that could further enhance the surface charge density crucial to triboelectric generator performance. However, the fabrication of Nylon-11 in the required polarised δ′-phase typically requires extremely rapid crystallisation, such as melt-quenching, as well as “poling” via mechanical stretching and/or large electric fields for dipolar alignment. Here, we propose an alternative one-step, near room-temperature fabrication method, namely gas-flow assisted nano-template (GANT) infiltration, by which highly crystalline “self-poled” δ′-phase Nylon-11 nanowires are grown from solution within nanoporous anodised aluminium oxide (AAO) templates. Our gas-flow assisted method allows for controlled crystallisation of the δ′-phase of Nylon-11 through rapid solvent evaporation and an artificially generated extreme temperature gradient within the nanopores of the AAO template, as accurately predicted by finite-element simulations. Furthermore, preferential crystal orientation originating from template-induced nano-confinement effects leads to self-poled δ′-phase Nylon-11 nanowires with higher surface charge distribution than melt-quenched Nylon-11 films, as observed by Kelvin probe force microscopy (KPFM). Correspondingly, a triboelectric nanogenerator (TENG) device based on as-grown templated Nylon-11 nanowires fabricated via GANT infiltration showed a ten-fold increase in output power density as compared to an aluminium-based triboelectric generator, when subjected to identical mechanical excitations.

Triboelectric generators rely on contact-generated surface charge transfer between materials with different electron affinities to convert mechanical energy into useful electricity. The ability to modify the surface chemistry of polymeric materials can therefore lead to significant enhancement of the triboelectric performance. Poly(methyl methacrylate) (PMMA) is a biocompatible polymer commonly used in medical applications, but its central position on the triboelectric series, which empirically ranks materials according to their electron-donating or electron accepting tendencies, renders it unsuitable for application in triboelectric generators. Here, we show that the surface potential of PMMA fibers produced by electrospinning can be tailored through the polarity of the voltage used during the fabrication process, thereby improving its triboelectric performance, as compared to typically spin-coated PMMA films. The change in surface chemistry of the electrospun PMMA fibers is verified using X-ray photoelectron spectroscopy, and this is directly correlated to the changes in surface potential observed by Kelvin probe force microscopy. We demonstrate the enhancement of triboelectric energy harvesting capability of the electrospun PMMA fibers, suggesting that this surface potential modification approach can be more widely applied to other materials as well, for improved triboelectric performance.

A flexible and robust piezoelectric nanogenerator (NG) based on a polymer-ceramic nanocomposite structure has been successfully fabricated via a cost-effective and scalable template-assisted hydrothermal synthesis method. Vertically aligned arrays of dense and uniform zinc oxide (ZnO) nanowires (NWs) with high aspect ratio (diameter ∼250 nm, length ∼12 μm) were grown within nanoporous polycarbonate (PC) templates. The energy conversion efficiency was found to be ∼4.2%, which is comparable to previously reported values for ZnO NWs. The resulting NG is found to have excellent fatigue performance, being relatively immune to detrimental environmental factors and mechanical failure, as the constituent ZnO NWs remain embedded and protected inside the polymer matrix., The authors thank Yeonsik Choi for discussions and experimental support. S.K.-N., C.O., and A.D. are grateful for financial support from the European Research Council through an ERC Starting Grant (Grant no. ERC-2014-STG-639526, NANOGEN). F.L.B. and R.A.W. thank the EPSRC Cambridge NanoDTC, EP/G037221/1, for studentship funding. P.S.J. acknowledges the support of TEP-1900 and Talentia Postdoc Program, cofunded by the European Union’s Seventh Framework Program, Marie Skłodowska-Curie actions (COFUND Grant Agreement 267226) and the Ministry of Economy, Innovation, Science and Employment of the Junta de Andalucía. S-L.S acknowledges support through the EPSRC grant EP/M010589/1, This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acsami.6b04041.

Electrical polarization phenomena in GaN are important as they have significant impact on the operation of modern day energy efficient lighting and are fundamental to many GaN-based high power and high frequency electronics. Controlling polarization is beneficial for the optimization of these applications. GaN is also piezoelectric, and therefore mechanical stress and strain are possible handles to control its polarization. Nonetheless, polar semiconductors in general, and GaN in particular, are weak piezoelectric materials when compared to ceramics, and are therefore not considered for characteristic electromechanical applications such as sensing, actuation and mechanical energy harvesting. Here, we examine the effect of nanoscale porosity on the piezoelectricity of initially conductive GaN. We find that for 40% porosity, the previously conductive GaN layer becomes depleted, and exhibits enhanced piezoelectricity as measured using piezoresponse force microscopy, as well as by using a mechanical energy harvesting setup. The effective piezoelectric charge coefficient of the porous GaN, d33,eff, is found to be about 8 pm/V which is 2 3 times larger than bulk GaN. A macroscale device comprising a porous GaN layer delivered 100 nW/cm2 across a resistive load under a 150 kPa mechanical excitation. We performed finite element simulations to analyze the evolution of the piezoelectric properties with porosity. The simulations suggest that increased mechanical compliance due to porosity gives rise to the observed enhanced piezoelectricity in GaN. Furthermore, the simulations show that for stress-based excitations, the porous GaN electromechanical figure of merit is increased by an order of magnitude and becomes comparable to that of barium titanate piezoceramics. In addition, considering the central role played by GaN in modern electronics and optoelectronics, our study validates a very promising research direction when considering stress-based electromechanical applications which combine GaN’s semiconducting and piezoelectric properties.

Piezoelectric polymers, capable of converting mechanical vibrations into electrical energy, are attractive for use in vibrational energy harvesting due to their flexibility, robustness, ease, and low cost of fabrication. In particular, piezoelectric polymers nanostructures have been found to exhibit higher crystallinity, higher piezoelectric coefficients, and "self-poling," as compared to films or bulk. The research in this area has been mainly dominated by polyvinylidene fluoride and its copolymers, which while promising have a limited temperature range of operation due to their low Curie and/or melting temperatures. Here, the authors report the fabrication and properties of vertically aligned and "self-poled" piezoelectric Nylon-11 nanowires with a melting temperature of ≈200 °C, grown by a facile and scalable capillary wetting technique. It is shown that a simple nanogenerator comprising as-grown Nylon-11 nanowires, embedded in an anodized aluminium oxide (AAO) template, can produce an open-circuit voltage of 1 V and short-circuit current of 100 nA, when subjected to small-amplitude, low-frequency vibrations. Importantly, the resulting nanogenerator is shown to exhibit excellent fatigue performance and high temperature stability. The work thus offers the possibility of exploiting a previously unexplored low-cost piezoelectric polymer for nanowire-based energy harvesting, particularly at temperatures well above room temperature.

Triboelectric generators have emerged as potential candidates for mechanical energy harvesting, relying on motion-generated surface charge transfer between materials with different electron affinities. In this regard, synthetic organic materials with strong electron-donating tendencies are far less common than their electron-accepting counterparts. Nylons are notable exceptions, with odd-numbered Nylons such as Nylon-11, exhibiting electric polarisation that could further enhance the surface charge density crucial to triboelectric generator performance. However, the fabrication of Nylon-11 in the required polarised δ′-phase typically requires extremely rapid crystallisation, such as melt-quenching, as well as “poling” via mechanical stretching and/or large electric fields for dipolar alignment. Here, we propose an alternative one-step, near room-temperature fabrication method, namely gas-flow assisted nano-template (GANT) infiltration, by which highly crystalline “self-poled” δ′-phase Nylon-11 nanowires are grown from solution within nanoporous anodised aluminium oxide (AAO) templates. Our gas-flow assisted method allows for controlled crystallisation of the δ′-phase of Nylon-11 through rapid solvent evaporation and an artificially generated extreme temperature gradient within the nanopores of the AAO template, as accurately predicted by finite-element simulations. Furthermore, preferential crystal orientation originating from template-induced nano-confinement effects leads to self-poled δ′-phase Nylon-11 nanowires with higher surface charge distribution than melt-quenched Nylon-11 films, as observed by Kelvin probe force microscopy (KPFM). Correspondingly, a triboelectric nanogenerator (TENG) device based on as-grown templated Nylon-11 nanowires fabricated via GANT infiltration showed a ten-fold increase in output power density as compared to an aluminium-based triboelectric generator, when subjected to identical mechanical excitations.

Aerosol-jet printing allows functional materials to be printed from inks with a wide range of viscosities and constituent particle sizes onto various substrates, including the printing of organic thermoelectric materials on flexible substrates for low-grade thermal energy harvesting. However, these materials typically suffer from relatively poor thermoelectric performance, compared to traditional inorganic counterparts, due to their low Seebeck coefficient, S, and electrical conductivity, σ. Here, we demonstrate a modified aerosol-jet printing technique that can simultaneously incorporate well dispersed high S Sb2Te3 nanoflakes, and high-σ multi-walled carbon nanotubes (MWCNTs) providing good inter-particle connectivity, to significantly enhance the thermoelectric performance of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) structures on flexible polyimide substrates. A nominal loading fraction of 85 wt.% yielded a power factor of ~41 µW/mK2, which is among the highest for printed organic-based structures. Rigorous flexing and fatigue tests were performed to confirm the robustness and stability of these aerosol-jet printed MWCNT-based thermoelectric nanocomposites.

Thermoelectric generators (TEGs) operate in the presence of a temperature gradient, where the constituent thermoelectric (TE) material converts heat into electricity via the Seebeck effect. However, TE materials are characterised by a thermoelectric figure of merit (ZT) and/or power factor (PF), which often has a strong dependence on temperature. Thus, a single TE material spanning a given temperature range is unlikely to have an optimal ZT or PF across the entire range, leading to inefficient TEG performance. Here, we demonstrate compositionally graded organic-inorganic nanocomposites, where the composition of the TE nanocomposite is systematically tuned along the length of the TEG, in order to optimise the PF along the applied temperature gradient. The nanocomposite composition can be dynamically tuned by an aerosol-jet printing method with controlled in-situ mixing capability, thus enabling the realisation of such compositionally graded thermoelectric composites (CG-TECs). We show how CG-TECs can be realised by varying the loading weight percentage of Bi2Te3 nanoparticles or Sb2Te3 nanoflakes within an organic conducting matrix using bespoke solution-processable inks. The enhanced energy harvesting capability of these CG-TECs from low-grade waste heat (

Harvesting energy from ambient mechanical sources in our environment has attracted considerable interest due to its potential to power applications such as ubiquitous wireless sensors and Internet of Things devices. In this context, piezoelectric and/or triboelectric materials offer a relatively simple means of directly converting mechanical energy from ubiquitous ambient vibrating sources into electrical power for microscale/nanoscale device applications. In particular, nanoscale energy harvesters, or nanogenerators, are capable of converting low-level ambient vibrations into electrical energy, thus are vital to the realization of the next generation of self-powered devices. Polymer-based nanogenerators are attractive as they are inherently flexible and robust, making them less prone to mechanical failure which is a key requirement for vibrational energy harvesters. They are also lightweight, easy and cheap to fabricate, lead-free and biocompatible, but in many cases their energy harvesting performance is found lacking in comparison to more commonly studied inorganic materials. Recent advances have been made in developing scalable nanofabrication techniques for flexible and low-cost polymer-based nanogenerators with improved energy conversion efficiency, including the incorporation of high-quality polymer nanowires with enhanced crystallinity, piezoelectric and/or surface charge properties. In this review, we discuss aspects of nanomaterials growth and energy harvester device design, including those involving nanowires of polymers of polyvinylidene fluoride and its co-polymers, Nylon-11, and poly-lactic acid for scalable piezoelectric and triboelectric nanogenerator applications, as well as the design and performance of polymer-ceramic nanocomposite nanogenerators. In particular, we highlight the effects of growth parameters, nanoconfinement, self-poling, surface polarization, crystalline phases, and device assembly on the energy harvesting performance of a range of recently reported nanostructured polymer-based materials and devices.

© 2019, The Author(s). Piezoelectrics are an important class of materials for mechanical energy harvesting technologies. In this paper we evaluate the piezoelectric harvesting process and define the key material properties that should be considered for effective material design and selection. Porous piezoceramics have been shown previously to display improved harvesting properties compared to their dense counterparts due to the reduction in permittivity associated with the introduction of porosity. We further this concept by considering the effect of the increased mechanical compliance of porous piezoceramics on the energy conversion efficiency and output electrical power. Finite element modelling is used to investigate the effect of porosity on relevant energy harvesting figures of merit. The increase in compliance due to porosity is shown to increase both the amount of mechanical energy transmitted into the system under stress-driven conditions, and the stress-driven figure of merit, FoM33X, despite a reduction in the electromechanical coupling coefficient. We show the importance of understanding whether a piezoelectric energy harvester is stress- or strain-driven, and demonstrate how porosity can be used to tailor the electrical and mechanical properties of piezoceramic harvesters. Finally, we derive two new figures of merit based on the consideration of each stage in the piezoelectric harvesting process and whether the system is stress- (FijX), or strain-driven (Fijx).

In this work we study the influence of the thermal contact resistance on the temperature of a typical nanostripe used in current induced magnetic domain wall movement or depinning. The thermal contact resistance arises from an imperfect heat transport across the interface between the metallic ferromagnetic nanostripe and the substrate. We show that this parameter, which is likely non-zero in any experimental device, increases the temperature in the nanostripe considerably. When the current is injected in the nanostripe in nanosecond long pulses, the larger temperature also implies a reduction of the effective current density delivered by the pulse generator. Both the thermal contact resistance and the dynamic response of the pulse generator are usually neglected in theoretical estimations of the influence of spin transfer torque on domain wall displacement and depinning. Here we show that only if the thermal contact resistance and the electric resistivity of the ferromagnetic nanostripe are optimized to the best values reported in the bibliography, the Joule heating may not be so crucial for current densities of the order of 108 A cm−2. Also, the use of physical constrictions (notch) to pin the magnetic domain wall may complicate the interpretation of the results as they always come together with relevant thermal gradients., This work has been partially funded by the Spanish Ministerio de Economía y Competitividad through the projects MAT2014-52477-C5-1-P and MAT2014-52477-C5-3-P and by the Engineering and Physical Sciences Research Council through grant code EPSRC EP/E03389X.

Flexible electronics are of great interest and importance due to their applications in a range of fields, from wearable electronics to solar cells. While resolutions of printed flexible electronics have been improving in recent years, there remain problems with mechanical fatigue and substrate cost, curtailing the use of such devices due to performance-related issues, and resulting in increased cost and waste products. Here we present a novel method for improving the fatigue resistance of printed flexible electronics by a factor of ~ 40 by sintering the electronic circuits prior to transferring them into low-cost polymer substrates with low melting temperatures, such that the circuits remain embedded in the substrates. This method is demonstrated using circuits printed using silver nanoparticulate ink with an aerosol jet printer, and could be applicable to multiple different metallic inks and polymer substrates. Importantly, this method can be used to transfer-print circuits into polymer substrates with low melting temperatures, without subjecting the polymers to the otherwise detrimentally high sintering temperatures required for ink curing.

Piezoelectric materials can directly transduce electrical and mechanical energy, making them attractive for applications such as sensors, actuators and energy harvesting devices. The piezoelectric effect is often associated with ceramic materials, yet piezoelectric behaviour is also observed in many polymers. The flexibility, ease of processing and biocompatibility of piezoelectric polymers mean that they are often preferable for certain applications, despite their reduced piezoelectric coefficients. This review will focus on the underlying mechanisms governing piezoelectricity in polymers from both a theoretical and practical perspective, including ways by which the effect can be enhanced through processing and structural modifications, and an outlook on specific areas in biology where piezoelectric polymers can be applied. Given that many biological materials are themselves piezoelectric, there is much interest in studying how artificial piezoelectric materials influence and stimulate cellular function. Understanding and developing piezoelectric polymers is therefore relevant for applications in energy, sensing and biology, as well as for high level, fundamental research in the physical and life sciences.

Funder: Emmanuel College (University of Cambridge); doi: http://dx.doi.org/10.13039/501100000609, Triboelectric devices capable of harvesting ambient mechanical energy have attracted attention in recent years for powering biomedical devices. Typically, triboelectric energy harvesters rely on contact-generated charges between pairs of materials situated at opposite ends of the triboelectric series. However, very few biocompatible polymeric materials exist at the ‘tribopositive’ end of the triboelectric series. In order to further explore the use of triboelectric energy harvesting devices within the body, it is necessary to develop more biocompatible tribopositive materials and look into ways to improve their triboelectric performance in order to enhance the harvested power output of these devices. Poly-L-lactic acid (PLLA) is a tribopositive biocompatible polymer, frequently used in biomedical applications. Here, we present a way to improve the triboelectric output of nanostructured PLLA through fine control of its crystallinity via a customised template-assisted nanotube (NT) fabrication process. We find that PLLA NTs with higher values of crystallinity (∼41%) give rise to a threefold enhancement of the maximum triboelectric power output as compared to NTs of the same material and geometry but with lower crystallinity (∼13%). Our results thus pave the way for the production of a viable polymeric and biocompatible tribopositive material with improved power generation, for possible use in implantable triboelectric nanogenerators.