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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.

Triboelectric generators are excellent candidates for smart textiles applications due to their ability to convert mechanical energy into electrical energy. Such devices can be manufactured into yarns by coating a conductive core with a triboelectric material, but current triboelectric yarns lack the durability and washing resistance required for textile-based applications. In this work, we develop a unique triboelectric yarn comprising a conducting carbon nanotube (CNT) yarn electrode coated with poly(vinylidene fluoride) (PVDF) fibers deposited by a customized electrospinning process. We show that the electrospun PVDF fibers adhere extremely well to the CNT core, producing a uniform and stable triboelectric coating. The PVDF–CNT coaxial yarn exhibits remarkable triboelectric energy harvesting during fatigue testing with a 33% power output improvement and a peak power density of 20.7 μW cm–2 after 200 000 fatigue cycles. This is potentially due to an increase in the active surface area of the PVDF fiber coating upon repeated contact. Furthermore, our triboelectric yarn meets standard textile industry benchmarks for both abrasion and washing by retaining functionality over 1200 rubbing cycles and 10 washing cycles. We demonstrate the energy harvesting and motion sensing capabilities of our triboelectric yarn in prototype textile-based applications, thereby highlighting its applicability to smart textiles.

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.

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.

Balancing forces within weight-bearing joints such as the hip during joint replacement surgeries is essential for implant longevity. Minimising implant failure is vital to improve patient wellbeing and alleviate pressure on healthcare systems. With improvements in surgery, hip replacement patients are now often younger and more active than in previous generations, and their implants correspondingly need to survive higher stresses. However, force balancing currently depends entirely on surgical skill: no sensors can provide quantitative force feedback within the hip joint’s small, complex geometry. Here, we solve this unmet clinical need by presenting a thin and conformable microfluidic force sensor, which is compatible with the standard surgical process. We optimised the design using finite element modelling, then incorporated and calibrated our sensor in a model hip implant. Using a bespoke testing rig, we demonstrated high sensitivity at typical forces experienced during hip replacements. We anticipate that these sensors will aid implant positioning, increasing the lifetime of hip replacements, and represent a powerful new surgical tool for a range of orthopaedic procedures where force balancing is crucial.

The re-wiring of photosynthetic bio-machineries to electrodes is a forward-looking semi-artificial route for sustainable bio-electricity and fuel generation. Currently, it is unclear how the bio-material interface can be designed to meet the complex requirements for high bio-photoelectrochemical performance. Here, we developed an aerosol jet printing method for generating hierarchical electrode structures using indium tin oxide nanoparticles. We printed libraries of micropillar array electrodes varying in height and sub-micron surface features and studied the energy/electron transfer processes across the bio-electrode interfaces. When wired to the cyanobacterium Synechocysis sp. PCC 6803, micropillar array electrodes with micro-branches exhibited favourable biocatalyst loading, light utilisation and electron flux output, ultimately almost doubling the photocurrent of state-of-the-art porous structures of the same height. When the micropillars’ heights were increased from 50 to 600 µm, milestone mediated photocurrent densities of 245 µA cm–2 (the closest thus far to theoretical predictions) and external quantum efficiencies of up to 29% could be reached. This study demonstrates how bioenergy from photosynthesis could be more efficiently harnessed in the future and provide new tools for 3D electrode design.

Access to clean water is a global challenge, and fog collectors are a promising solution. Polycarbonate (PC) fibers have been used in fog collectors but with limited efficiency. In this study, we show that controlling voltage polarity and humidity during the electrospinning of PC fibers improves their surface properties for water collection capability. We experimentally measured the effect of both the surface morphology and the chemistry of PC fiber on their surface potential and mechanical properties in relation to the water collection efficiency from fog. PC fibers produced at high humidity and with negative voltage polarity show a superior water collection rate combined with the highest tensile strength. We proved that electric potential on surface and morphology are crucial, as often designed by nature, for enhancing the water collection capabilities via the single-step production of fibers without any postprocessing needs.

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.

Summary Force sensors that are thin, low-cost, flexible, and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, and robotics. By leveraging a combination of microfluidics and capacitive sensing, we develop a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes coated with a thin, insulating polymer layer. When a force is applied to the microfluidic channel reservoir, the fluid is displaced along the channel over the electrodes, thus inducing a capacitance change proportional to the applied force. The microfluidic molds themselves are made of low-cost sacrificial materials deposited via aerosol-jet printing, which is also used to print the electrode layer. We envisage a large range of industrial and biomedical applications for this force sensor., Graphical abstract, Highlights Highly flexible, capacitive force sensors based on microfluidics Linear capacitance response to applied force Rapid prototyping of low-cost and robust force sensors via aerosol-jet printing Tunable design for various sensitivities and force measurement ranges, Jing et al. report a thin, conformable force sensor based on a combination of microfluidics and capacitive sensing. The low-cost sensor is fabricated via aerosol-jet printing and shows a highly linear response with adjustable design parameters for tunable sensitivities and force ranges.

We studied the magnetic properties of ferromagnetic-semiconductor composites based on nickel and porous-GaN, motivated by the effort to couple magnetic and semiconductor functionality. Nickel-infiltrated and nickel-coated (laminated thin-film) porous GaN structures were fabricated by electrodeposition, and their magnetic properties were subsequently examined collectively, by vibrating sample magnetometry and on the nanoscale, by magnetic force microscopy. We successfully demonstrated the ability to realize nickel infiltrated porous GaN, where the magnetic properties were dominated by the infiltrated material without a measurable surface contribution. We found that the structure and magnetization of electrodeposited porous-GaN/Ni composites depended on GaN degree of porosity and the amount of deposited nickel. The magnetization evolves from a nearly isotropic response in the infiltrated structures, to a shape-anisotropy controlled magnetic thin-film behaviour. Furthermore, both infiltrated and thin-film nickel electrodeposited on porous GaN were found to have low (< 0.1%) strain and corresponding low coercivity: < 6.4 and < 2.4 kA/m for infiltrated and thin-film, correspondingly. The most likely cause for the lowered strain is increased compliance of the porous GaN compared to bulk. These results encourage deeper investigation of magnetic nanostructure property tuning and of magnetic property coupling to GaN and similar materials., Supplamentary information included

Plasmonic metafilms have been widely utilized to generate vivid colors, but making them both active and flexible simultaneously remains a great challenge. Here flexible active plasmonic metafilms constructed by printing electrochromic nanoparticles onto ultrathin metal films (, Commercially available printing techniques offer fullyprinted flexible plasmonic metafilms, which show electrically‐tunable omni‐ or bi‐directional color dynamics engineered with structural geometries. Such activelycontrolled directional color dynamics in flexible plasmonics significantly unlocks the full potential of plasmonics for flexible/wearable devices.

Biosensors are powerful analytical tools for biology and biomedicine, with applications ranging from drug discovery to medical diagnostics, food safety, and agricultural and environmental monitoring. Typically, biological recognition receptors, such as enzymes, antibodies, and nucleic acids, are immobilized on a surface, and used to interact with one or more specific analytes to produce a physical or chemical change, which can be captured and converted to an optical or electrical signal by a transducer. However, many existing biosensing methods rely on chemical, electrochemical and optical methods of identification and detection of specific targets, and are often: complex, expensive, time consuming, suffer from a lack of portability, or may require centralised testing by qualified personnel. Given the general dependence of most optical and electrochemical techniques on labelling molecules, this review will instead focus on mechanical and electrical detection techniques that can provide information on a broad range of species without the requirement of labelling. These techniques are often able to provide data in real time, with good temporal sensitivity. This review will cover the advances in the development of mechanical and electrical biosensors, highlighting the challenges and opportunities therein.

α-Phase nylon-11 nanowires exhibit unprecedented dipole alignment, leading to high-performance triboelectric generators., Dipole alignment in ferroelectric polymers is routinely exploited for applications in charge-based applications. Here, we present the first experimental realization of ideally ordered dipole alignment in α-phase nylon-11 nanowires. This is an unprecedented discovery as dipole alignment is typically only ever achieved in ferroelectric polymers using an applied electric field, whereas here, we achieve dipole alignment in as-fabricated nanowires of ‘non-ferroelectric’ α-phase nylon-11, an overlooked polymorph of nylon proposed 30 years ago but never practically realized. We show that the strong hydrogen bonding in α-phase nylon-11 serves to enhance the molecular ordering, resulting in exceptional intensity and thermal stability of surface potential. This discovery has profound implications for the field of triboelectric energy harvesting, as the presence of an enhanced surface potential leads to higher mechanical energy harvesting performance. Our approach therefore paves the way towards achieving robust, high-performance mechanical energy harvesters based on this unusual ordered phase of nylon-11.

It has become increasingly evident that the mechanical and electrical environment of a cell is crucial in determining its function and the subsequent behavior of multicellular systems. Platforms through which cells can directly interface with mechanical and electrical stimuli are therefore of great interest. Piezoelectric materials are attractive in this context because of their ability to interconvert mechanical and electrical energy, and piezoelectric nanomaterials, in particular, are ideal candidates for tools within mechanobiology, given their ability to both detect and apply small forces on a length scale that is compatible with cellular dimensions. The choice of piezoelectric material is crucial to ensure compatibility with cells under investigation, both in terms of stiffness and biocompatibility. Here, we show that poly-l-lactic acid nanotubes, grown using a melt-press template wetting technique, can provide a "soft" piezoelectric interface onto which human dermal fibroblasts readily attach. Interestingly, by controlling the crystallinity of the nanotubes, the level of attachment can be regulated. In this work, we provide detailed nanoscale characterization of these nanotubes to show how differences in stiffness, surface potential, and piezoelectric activity of these nanotubes result in differences in cellular behavior.

Magnetoelectric (ME) composite materials, in which a coupling between magnetostrictive and piezoelectric effects is achieved, are potential candidates for multifunctional devices where the interpla...

Dynamic microscale surface topographies are desired in smart optics, controlling surface wettability and preventing marine biofouling. Voltage-controlled reversible responses have demonstrated potential for reliable reproducibility and stability, fast response, and for actuating thin films fixed over large solid surfaces. To obtain reversible deformation with regular geometric patterns, however, electrical methods have had to be coupled with mechanical stretching/bending and other ways to induce anisotropy. There is a great need and potential for on-demand electrical generation of programmable complex responsive surface patterns. Here we demonstrate a responsive polymer coating over an underlying pattern of counter electrodes which can be activated selectively. We present a patternable electrode printing method to achieve localized and structured wrinkling deformation without mechanical pre-force deformation. We discover that below a minimal separation distance, electrodes below the polymer act as a single electrode. We establish parameters that govern the alignment of wrinkles and quantify the regularity and direction of the new patterns between electrodes separated by larger than this distance. We analyze and quantify the regularity of the formed wrinkling patterns by four disorder metrics: box-counting fractal dimension, tortuosity, angle distribution, and branch number. We demonstrate the application of such electrode/wrinkles-on-demand patterning with a working multi-state light reflection–diffusion–grating device.

Force sensors that are thin, low cost, flexible and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, robotics and in various “smart control” technologies. By leveraging a novel combination of microfluidics and capacitive sensing we have developed a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes that are coated with a thin polymer layer for insulation. When a force is applied to a fluid reservoir at the base of the channel, the fluid is displaced along the channel, passing over the electrodes and thus inducing a capacitance change, which is proportional to the amount of force applied. The microfluidic molds themselves are made of low-cost sacrificial material inks deposited via aerosol-jet printing, which is also used to print the electrode layer. The sensors exhibit a nearly linear change in capacitance in response to external load, with a sensitivity of up to 3.8 pF/N and a force range of up to 9 N achieved with the present design. We quantify the effects of changing the electrode morphology, liquid permittivity, sensor thickness and geometry, as well as the thickness of the insulation layer over the electrodes, on sensor range and sensitivity, using a combination of experiments and finite element analysis simulations. We further demonstrate how the sensor can be used to remotely control a robotic clamp. The conformable force sensors developed here are thin, flexible and facilitate easy signal processing. The fabrication processes are low-cost, amenable to fast prototyping and mass manufacturing. We envisage a large range of industrial and biomedical applications for this novel force sensor.

Controlling nanomaterial shape beyond its basic dimensionality is a concurrent challenge tackled by several growth and processing avenues. One of these is strain engineering of nanowires, implemented through the growth of asymmetrical heterostructures. Here, we report metal&ndash, organic molecular beam epitaxy of bent InP/InAs core/shell nanowires brought by precursor flow directionality in the growth chamber. We observe the increase of bending with decreased core diameter. We further analyze the composition of a single nanowire and show through supporting finite element simulations that strain accommodation following the lattice mismatch between InP and InAs dominates nanowire bending. The simulations show the interplay between material composition, shell thickness, and tapering in determining the bending. The simulation results are in good agreement with the experimental bending curvature, reproducing the radius of 4.3 &micro, m (&plusmn, 10%), for the 2.3 &micro, m long nanowire. The InP core of the bent heterostructure was found to be compressed at about 2%. This report provides evidence of shape control and strain engineering in nanostructures, specifically through the exchange of group-V materials in III&ndash, V nanowire growth.

Resistance-based thermometry is a simple and widely used technique, though we still lack a single sensor that spans a broad temperature range without losing sensitivity. This article presents an alloy that is useful for thermometry from room temperature down to about 100 mK with little variation in its dimensionless sensitivity. Its resistivity varies logarithmically from approximately 200 K down to approximately 200 mK, allowing for simple calibration, and it can be patterned on a chip alongside experimental devices. This material might be quite useful in producing a chip-integrated thermometer operating at stable sensitivity over three decades in temperature.

The requirements for modern electronic devices, particularly those intended for wearable or human health monitoring applications, have rapidly evolved to being both flexible and stretchable. Hence devices, as well as interconnects, need to be capable of retaining functionality even when being mechanically deformed. Most approaches towards achieving this rely on printing or transferring structures onto elastomeric substrates that can withstand stretching. However, the processing involved can often be cumbersome, and the structures themselves tend to suffer from poor fatigue and/or are limited by the mechanical properties of the underlying substrate. Here, we introduce an aerosol-jet printing technique by which fully freestanding functional structures can be built up layer by layer, which are stable and robust upon repeated stretching. The process involves printing a combination of layers of different materials with the desired functionality, onto a substrate coated with a sacrifical film that is subsequently dissolved to release the printed structure. Using this method, we demonstrate freestanding conductive wires can be used as stretchable interconnects/electrodes, and that also function as strain-sensors. We also show that a freestanding capacitive structure functions as a robust, stretchable humidity sensor, paving the way for the development of other multi-layer, multifunctional stretchable devices and sensors.

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. info:eu-repo/grantAgreement/EC/FP7/267226

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 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.

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, FoM33 X, 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- (Fij X), or strain-driven (Fij x).

Semiconducting piezoelectric materials have attracted considerable interest due to their central role in the emerging field of piezotronics, where the development of a piezo-potential in response to stress or strain can be used to tune the band structure of the semiconductor, and hence its electronic properties. This coupling between piezoelectricity and semiconducting properties can be readily exploited for force or pressure sensing using nanowires, where the geometry and unclamped nature of nanowires render them particularly sensitive to small forces. At the same time, piezoelectricity is known to manifest more strongly in nanowires of certain semiconductors. Here, we report the design and fabrication of highly sensitive piezotronic pressure sensors based on GaAs nanowire ensemble sandwiched between two electrodes in a back-to-back diode configuration. We analyse the current–voltage characteristics of these nanowire-based devices in response to mechanical loading in light of the corresponding changes to the device band structure. We observe a high piezotronic sensitivity to pressure, of ~7800 meV MPa−1. We attribute this high sensitivity to the nanowires being fully depleted due to the lack of doping, as well as due to geometrical pressure focusing and current funnelling through polar interfaces.

Cellulose, a major constituent of our natural environment and a structured biodegradable biopolymer, has been shown to exhibit shear piezoelectricity with potential applications in energy harvesters, biomedical sensors, electro-active displays and actuators. In this regard, a high-aspect ratio nanofiber geometry is particularly attractive as flexing or bending will likely produce a larger piezoelectric response as compared to axial deformation in this material. Here we report self-assembled cellulose nanofibers (SA-CNFs) fabricated using a template-wetting process, whereby parent cellulose nanocrystals (CNCs) introduced into a nanoporous template assemble to form rod-like cellulose clusters, which then assemble into SA-CNFs. Annealed SA-CNFs were found to exhibit an anisotropic shear piezoelectric response as directly measured using non-destructive piezo-response force microscopy (ND-PFM). We interpret these results in light of the distinct hierarchical structure in our template-grown SA-CNFs as revealed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (TEM).

The increasing demand for portable and low-power electronics for applications in self-powered devices and sensors has spurred interest in the development of efficient piezoelectric materials, via which mechanical energy from ambient vibrations can be transformed into electrical energy for autonomous devices, or which can be used in strain-sensitive applications. Semiconducting piezoelectric materials are ideal candidates in the emerging field of piezotronics and piezophototronics, where the development of a piezopotential in response to stress/strain can be used to tune the band structure of the semiconductor and hence its electronic and/or optical properties. Furthermore, research into nanowires of these materials has intensified due to the enhancement of piezoelectric properties at the nanoscale. In this regard, nanowires of ZnO and the III-nitrides have been extensively studied, but the piezoelectric properties of non-nitride III–V semiconductor nanowires remain less-explored. Indeed, direct measurements of the piezoelectric properties of single III–V nanowires are tellingly rare due to the difficulties associated with measurements of piezoelectric properties of nanoscale objects using conventional scanning probe microscopy techniques. This review addresses the challenges related to the study of piezoelectricity in III–V nanowires and the opportunities that lie therein in terms of device applications.

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.

© 2018 Elsevier Inc. Piezoelectric semiconducting nanowires have generated much interest due to the interplay of their mechanical, electrical, and optical properties, which paves the way for potential applications in mechanical energy harvesting as well as sensing. The nature of piezoelectricity in these nanowires is governed by the crystalline phases present, which in turn can be controlled during the nanowire growth process. This chapter provides insight into the manifestation of piezoelectricity in semiconducting nanowires, the effect of growth on their piezoelectric properties, and importantly, how piezoelectricity is characterized at the nanoscale in these materials. Energy-related applications of semiconducting piezoelectric nanowires are described in detail, including their incorporation into nanogenerators for energy harvesting, as well as in piezotronic and photo-piezotronics devices based on the electromechanical and opto-electromechanical interactions taking place in piezoelectric semiconductor-nanowire junction-based devices. Advances in nanofabrication, nanoscale characterization, and device engineering, coupled with a greater understanding and control of piezoelectricity in semiconducting nanowires, will ultimately help unlock the full potential of these fascinating nanomaterials.

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.

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.

Stretchable electronics represent an emerging class of devices that can be compressed, twisted and conform to very complicated shapes. The mechanical and electrical compliances of the technology promise to open up applications for healthcare, energy and entertainment purposes. However, advancement in the field has been hindered by material related constraints. Moreover, the current microfabrication facilities are optimized for rigid substrates such as silicon, which have significant different properties compared to elastomers. In this paper, four categories of enabling technologies for stretchable electronics commercialization are critically reviewed, namely: the novel design of stretchable structures, use of non-conventional materials, state-of-art printing techniques and also the patterning of electrodes or metal interconnects via conventional manufacturing techniques.

Ferroelectrics are important technological materials with wide-ranging applications in electronics, communication, health, and energy. While lead-based ferroelectrics have remained the predominant mainstay of industry for decades, environmentally friendly lead-free alternatives are limited due to relatively low Curie temperatures (T ) and/or high cost in many cases. Efforts have been made to enhance T through strain engineering, often involving energy-intensive and expensive fabrication of thin epitaxial films on lattice-mismatched substrates. Here, a relatively simple and scalable sol–gel synthesis route to fabricate polycrystalline (BaCa)(ZrTi)O nanowires within porous templates is presented, with an observed enhancement of T up to ≈300 °C as compared to ≈90 °C in the bulk. By combining experiments and theoretical calculations, this effect is attributed to the volume reduction in the template-grown nanowires that modifies the balance between different structural instabilities. The results offer a cost-effective solution-based approach for strain-tuning in a promising lead-free ferroelectric system, thus widening their current applicability., This work was financially supported by the European Research Council through an ERC Starting Grant (Grant No. ERC–2014–STG–639526, NANOGEN). S.K.‐N., A.D. and Y.C are grateful for financial support from this same grant. S‐L.S acknowledges funding through the EPSRC grant EP/M010589/1. C.O acknowledges studentship funding from the China Scholarship Council and the Cambridge Commonwealth, European and International Trust. M.F. acknowledges collaboration with the AFLOW Consortium (http://www.aflow.org) under the sponsorship of DOD‐ONR (N000141310635 and N000141512266). P.E.S.‐J. was supported by a Marie Curie–Junta de Andalucia Postdoc Talentia grant. Supporting data for this paper is available at the DSpace@Cambridge data repository (https://dx.doi.org/10.17863/CAM.9457).

International audience; Semiconducting III–V materials exhibiting piezoelectric properties are much sought after due to their potential applications in piezotronic and piezo-phototronic devices. Nanowires (NWs) 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, as opposed to the zinc blende (ZB) structure that is typically observed in the bulk. However, performing direct measurements of the piezoelectric properties of III–V NWs 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 NWs, 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 NW, scan direction, as well as NW orientation on the resulting PFM signal. We also report PFM data from single GaAs and InP NWs 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, d33 = 0.7–1 pm V−1. The methods and analysis described here are particularly relevant for the investigation of piezoelectric properties in a wide range of semiconducting III–V NW systems.

There has been tremendous interest in piezoelectricity at the nanoscale, for example in nanowires and nanofibers where piezoelectric properties may be enhanced or controllably tuned, thus necessitating robust characterization techniques of piezoelectric response in nanomaterials. Piezo-response force microscopy (PFM) is a well-established scanning probe technique routinely used to image piezoelectric/ferroelectric domains in thin films, however, its applicability to nanoscale objects is limited due to the requirement for physical contact with an atomic force microscope (AFM) tip that may cause dislocation or damage, particularly to soft materials, during scanning. Here we report a non-destructive PFM (ND-PFM) technique wherein the tip is oscillated into “discontinuous” contact during scanning, while applying an AC bias between tip and sample and extracting the piezoelectric response for each contact point by monitoring the resulting localized deformation at the AC frequency. ND-PFM is successfully applied to soft polymeric (poly-L-lactic acid) nanowires, as well as hard ceramic (barium zirconate titanate–barium calcium titanate) nanowires, both previously inaccessible by conventional PFM. Our ND-PFM technique is versatile and compatible with commercial AFMs, and can be used to correlate piezoelectric properties of nanomaterials with their microstructural features thus overcoming key characterisation challenges in the field.