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International audience; Using a hydrothermal synthesis method, zinc oxide (ZnO) nanowires (NWs) have been grown on ZnO/Au/Ti/Si substrates. This method shows a low temperature (70 °C) approach for growing ZnO NWs on a ZnO seed layer. ZnO films of about 50 nm thickness were deposited by radio frequency (RF) magnetron sputtering at various sputtering powers (150, 100 and 65W). The formation of ZnO seed layers and ZnO NWs was clarified using X-ray diffraction (XRD), and the surface microstructure of the prepared ZnO NWs and ZnO films was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The results show that the growth behavior of ZnO NWs is strongly impacted by the sputtering power of ZnO seed layers. The decrease of RF sputtering power leads to a diminution in the grain size of ZnO seed layers and a smaller lattice mismatch between seed layers and NWs, thus inducing a morphological improvement and a better alignment of the obtained ZnO NWs. These works aim at optimizing the integration of ZnO NWs for mechanical energy harvesting applications, and to enhance the performance of such piezoelectric devices.

International audience; Well aligned crystalline zinc oxide (ZnO) nanowires (NWs) on ZnO/Au/Ti/Si substrates were grown by the so-called “hydrothermal synthesis”. ZnO seed layers with different thicknesses ranging from 5 to 100 nm, by controlling the deposition time, were prepared by radio-frequency sputtering followed by a post-annealing treatment in air at 400°C. The effects of deposition time and annealing treatment of ZnO seed layers on the subsequent growth of ZnO NWs were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The experimental results reveal that the quality and growth behavior of ZnO NWs are strongly dependent on both the thickness and the heat treatment of the ZnO seed layers. This work is an optimization step of the facile, cost-effective and industrially scalable process flow recently developed for the fabrication of a high performance nanocomposite-based stretchable nanogenerator (SNG) on polydimethylsiloxane (PDMS) substrate. The morphological improvement of hydrothermally grown ZnO NWs may therefore lead to higher performance SNGs for the targeted application of mechanical energy harvesting in order to supply flexible and wearable electronics.

International audience; From the multitude of nanostructures under active research, Zinc Oxide (ZnO) nanowires (NWs) have attracted enormous attention due to the materials’ unique electrical, optical, mechanical and piezoelectric properties. Since 10 years, piezoelectric nanocomposites based nanogenerators (NGs) have gained extensive attention for their applications in mechanical energy harvesters and self-powered tactile sensors. Hydrothermal approach is used for the synthesis of ZnO NWs and is a low cost manufacturing process, compatible with large area substrates. We present here a flexible and stretchable nanogenerator (SNG) which is manufactured thanks to a facile, cost-effective and industrially scalable process, on a polydimethylsiloxane (PDMS) substrate. The SNG exhibits excellent performance with a 35 μW peak output power achieved from a 8 cm2 device under a pressure of 100 kPa. The key issues of efficient NGs will be presented, in order to maximize the performance of these devices dedicated to low frequency mechanical energy harvesting.

International audience; We demonstrate that vertical well-aligned crystalline ZnO nanowires (NWs) arrays were grown on ZnO/Au/Ti/Si substrates by a hydrothermal method. Different thicknesses of ZnO seed layers were prepared by radio-frequency sputtering followed by a post-annealing treatment in air at 400°C. The effects of deposition time and annealing treatment of ZnO seed layers on the subsequent growth of ZnO NWs were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). In this work it was found that the morphology of ZnO NWs strongly depends on both the thickness and the heat treatment of the ZnO seed layers. The improved control of the morphology of ZnO nanowire arrays may lead to an enhanced performance of the resulting piezoelectric energy harvesting devices based on ZnO.

International audience; Bulk piezoelectric cantilever beams are mostly assembled under serial bimorph topologies where the piezoelectric material is laminated around an inner shim material. The two PZT layers exhibit an individual thickness ranging from few tens to hundreds of micrometers according to the targeted application and its miniaturization requests. Designing such devices becomes a challenge and one needs FEM (finite element model) to properly define material thicknesses and overall piezoelectric energy harvester geometries. Unfortunately, bulk piezoelectric material are characterized using thicker samples which provide results slightly different from the actual characteristics of thinned-bulk piezoelectric materials. Previous works have addressed an approach to study the thin piezoelectric material layer relying on the electrical admittance analysis. Then, the effective coefficients of thinned-bulk piezoelectric material are identified and entered in a 3D FEM of the PZT layer. The electrical admittance is calculated, showing a good agreement with the experimental measurements.

International audience; Les préoccupations liées à la récupération et à la conservation de l’énergie pour aider au développement de réseaux microélectroniques autonomes sont aujourd’hui au coeur des problématiques de recherche. Le développement des techniques de récupération d’énergie basées sur la thermoélectricité, le solaire ou l’éolien, cherchant à répondre aux enjeux énergétiques actuels, font cependant face à de nombreuses limitations, notamment la disponibilité intermittente de la source d’énergie. Par ailleurs, depuis plusieurs années, le développement de récupérateurs d’énergie mécanique attire l’attention des chercheurs du fait de la disponibilité de cette énergie dans l’environnement humain ou industriel. Ainsi, différents systèmes ont vu le jour en s’appuyant notamment sur les principes de triboélectricité [1] et de piézoélectricité [2]. Dans ce contexte, les nanogénérateurs (NGs) piézoélectriques [3], développés à partir d’une matrice composite de nanofils (NFs) de ZnO et de parylène, présentent aujourd’hui un grand potentiel pour la récupération d’énergie. La structure 1D des NFs de ZnO, matériau piézoélectrique, permet un accroissement de la sensibilité aux faibles sollicitations mécaniques [4] et laisse présager des performances électriques intéressantes lorsque intégrés dans les dispositifs de récupération d’énergie. Les caractéristiques prometteuses de ce matériau, alliées à sa non toxicité pour l’environnement ainsi qu’à la facilité des étapes de fabrication du NG, font de ce système un candidat idéal à l’industrialisation de ce type de générateur piézoélectrique.Nous présentons dans ce poster le développement et les performances associées à l’intégration des NFs de ZnO synthétisés par croissance hydrothermale [5] et encapsulés dans du parylène pour la fabrication de NGs piézoélectriques. Cette étude vise à la fois à l’amélioration des performances des NGs et à l’optimisation de leur design pour différentes applications.Références[1] Y. Yang, G. Zhu, H. Zhang, J. Chen, X. Zhong, ZH. Lin, Y. Su, P. Bai, X. Wen, et Z.L. Wang (2013) « Triboelectric Nanogenerator for Harvesting Wind Energy and as Self Powered Wind Vector Sensor System », ASC Nano, 7 (10), pp. 9461-9468.[2] Z.L. Wang, X. Wang, J. Song, J. Liu, et Y. Gao, (2008), « Piezoelectric nanogenerators for sel-powered nanodevices », IEE, 7(1), pp. 49-55.[3] Z. L. Wang, J. Song (2016) « Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays », Science, 312 (5771), pp. 242-246.[4] Z.L. Wang (2008) « Towards self-powered nanosystems: From nanogenerators to nanopiezotronics », Advanced Functional Materials, 18, pp.3553–3567.[5] S. Boubenia, A.S. Dahiya, G. Poulin-Vittrant, F. Morini, K. Nadaud, D. Alquier (2017) « A facile hydrothermal approach for the density tunable growth of ZnO nanowires and their electrical characterizations », Scientific Reports, 7, pp. 1-10.

International audience; La récupération d’énergie mécanique grâce aux NanoGénérateurs PiézoElectriques (PENGs) suscite depuis 2006 [1] un intérêt croissant pour le développement de microsystèmes autonomes. Les PENGs sont des composites intégrant des nanofils piézoélectriques, le plus souvent en oxyde de zinc ZnO ou en nitrure de gallium GaN (qui sont tous deux également semi-conducteurs), enrobés d’une matrice généralement constituée d’un matériau diélectrique souple. L'optimisation du matériau piézoélectrique nanostructuré, notamment sa morphologie (diamètre et longueur, celle-ci étant généralement de l’ordre du µm), sa densité (nombre de nanofils par unité de surface) et sa qualité (défauts structurels pouvant entraîner un dopage semiconducteur intrinsèque), est cruciale pour l'amélioration des performances des dispositifs PENGs. En effet, il a été démontré que le potentiel électrique et l'efficacité des dispositifs PENGs dépendent fortement de la morphologie du nanofil, qui affecte à la fois ses propriétés électriques et mécaniques [1], et dépendent également de la densité des nanofils [2]. Dans cette étude, nous nous sommes concentrés sur l’élaboration de nanofils de ZnO de dimensions contrôlées en utilisant la technique hydrothermale. La morphologie des nanofils a été contrôlée en agissant sur la sous couche de ZnO. Nous avons montré, en effectuant la synthèse de nanofils sur des sous couches de ZnO de différentes épaisseurs, que l’épaisseur de la sous couche influence nettement les propriétés des nanofils obtenus, de même qu’un traitement thermique effectué sur la sous couche avant la croissance des nanofils (Figure 1). Les propriétés structurales et morphologiques des nanofils obtenus avec ces différentes conditions de synthèse ont été étudiées par diffraction des rayons X et microscopie électronique à balayage (MEB). Finalement, pour chaque type de sous couche expérimentée, des dispositifs PENGs, constitués de ZnO (sous couche)/ZnO (nanofils)/Parylène, ont été fabriqués et caractérisés. Cela nous a permis de mieux comprendre les mécanismes électriques impliqués dans ce type de dispositif, ouvrant de nombreuses possibilités pour améliorer leurs performances.Références[1] Z. L. Wang, J. Song, « Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays », Science 2006, 312, 242-246. [2] A. Rinaldi, R. Araneo, S. Celozzi, M. Pea et A. Notargiacomo, « The clash of mechanical and electrical size-effects in ZnO nanowires and a double power law approach to elastic strain engineering of piezoelectric and piezotronic devices », Adv. Mater., 2014, 5976–5985.[3] R. Tao, M. Parmar, G. Ardila, P. Oliveira et D. Marques, « Performance of ZnO based piezo-generators under controlled compression », Semicond. Sci. Technol.,2017, 32, 64003.

International audience; Nanoscale mechanical energy harvesting using PiezoElectric NanoGenerators (PENGs) has been highlightedas a potential candidate for the realization of self-powered biomedical devices such as pacemakers andwearable communication devices. A PENG can be described as a stack of five layers (Figure 1) : a rigid orflexible substrate, a bottom electrode, an electroactive layer (piezoelectric semiconducting nanowires (NWs)surrounded by an insulating polymer), a top layer of insulating material and a top electrode [1]. For thecontinual performance improvement of PENG devices, optimization of the piezoelectric semiconductingnanomaterial, including morphology, density and quality, is of great interest and crucial. For example, it hasbeen theoretically shown that the output potential and efficiency of PENGs strongly depend on the electricalquality of the nanomaterial [2]. Among several nanomaterials, zinc oxide (ZnO) NWs can be synthesized byhydrothermal method, which is low temperature, industrially scalable and compatible with flexible substrates[3]. Different approaches have been used to model ZnO NW based PENGs including analytical models andnumerical models using finite element method (FEM) [1].In the present work, COMSOL software is used to model ZnO NW based PENGs. First, the piezoelectricpotential distribution in ZnO NWs is studied for different intrinsic doping concentrations. In terms ofpiezopotential magnitude, cylindrical shaped NWs with low doping completely outperformed their homologueNWs being highly doped leading to significant piezoelectric potential screening effect. Figure 2 shows that, atthe applied static pressure of 6.25 MPa, a cylindrical NW with a 1010 (/cm3) charge density presents amaximum piezoelectric potential of 2.9 V compared to 0.25 V reached by a 1015 (/cm3) doped NW. Moreover,in order to predict the complete PENG electrical characteristics and to reduce computation time, a newapproach using FEM simulation and analytical modelling has been developed [1]. This method combines FEMsimulation of a PENG unit cell (including one NW) in order to estimate the open-circuit voltage (without anyexternal electrical circuit), and an analytical model of the full PENG in order to predict the maximum powerand the corresponding optimal load. Further works will consist in studying the effect of the geometricalcharacteristics and electromechanical properties of the nanowire-polymer composite on the complete PENGperformance.References[1] Doumit N and Poulin-Vittrant G 2018 A new simulation approach for performance prediction ofvertically integrated nanogenerators Adv. Theory Simul. 1800033 1–8[2] Gao Y and Wang Z L 2009 Equilibrium potential of free charge carriers in a bent piezoelectricsemiconductive nanowire Nano Lett. 9 1103–10[3] Dahiya A S, Morini F, Boubenia S, Nadaud K, Alquier D, Poulin-Vittrant G 2017 Organic / inorganichybrid stretchable piezoelectric nanogenerators for self-powered wearable electronics AdvancedMaterials Technologies 1700249 11 pp.

International audience; Since the early 2000’s, many new applications related to nomad electronics need “portable” electrical energy. Huge research efforts on Lithium (Li) based batteries have led to performances that made this technology the most popular for rechargeable batteries with the best energy densities. Little by little, Li batteries are replacing the conventional batteries; they are thinner, safer and more reliable. These batteries are usually plugged and recharged through a standard AC/DC converter. Recharging them thanks to renewable energy systems, such as a photovoltaic cells or thermoelectric devices, has become a major challenge for electronic industry1. In some specific applications, like health monitoring, medical implants, smart clothes, autonomous sensors, etc… the later sources of energy are not directly available. In such cases, an alternative source of energy to charge the Li battery is necessary. The mechanical ambient energy trough low frequency “vibrations”, as for example human motion, liquid flow or engine vibrations, is a source of interest.Energy generation utilizing piezoelectric materials has been well-studied over the past two decades2-4. When subjected to a mechanical force, these materials become electrically polarized. Thus, they are able to convert unused mechanical energy of our surroundings into electrical energy. Ambient mechanical sources are encountered on vehicles, industrial machines, traffic ways (roads, bridges, tunnels), passage ways, wind turbines, pipelines... or on human beings (during walk or hand movements). In order to estimate the interest to harvest such “free” energy, some important questions must be asked, in particular on the frequency range and the amplitude of the mechanical excitation, its truly free nature, its continuous or intermittent availability, the proximity of the electrical network, the interest of wireless electronic devices...In this lecture, we will examine the key aspects of mechanical energy harvesting using piezoelectric materials, from the material choice to the design and manufacturing of the full device, including the dedicated electronics, that is usually interfacing the piezoelectric generator and the targeted electrical load. Some possible figures of merit will be presented, and the perspectives of standardization will also be discussed, illustrated with case study examples.

The present work describes the seedless approach for the growth of ZnO nanowires (NWs) on flexible polyethylene terephthalate (PET) substrates and fabrication of fully flexible ZnO NW based piezoelectric nanogenerators (NGs). The x-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) confirms the high quality of grown nanomaterial. The performance of the fabricated ZnO NW based NG device is measured under a force of 5N at a frequency of 3Hz. A peak output voltage of ~ 950mV is obtained that corresponds to a power density of ~ 1µW/cm 2. Our results clearly demonstrate the interest of ZnO NWs for the development of autonomous micro-nano systems (MNSs).

International audience

International audience; Nanowire source‐gated field‐effect transistors (NW SGT) are demonstrated using hydrothermally grown ZnO NWs. Device quality ZnO NWs with moderate n‐type doping are achieved by thermal annealing in ambient air at ∼550 °C. A single ZnO NW device with Au source‐drain contacts (s/d) is found to operate under source‐gating mode, with characteristics markedly different from a reference device with ohmic contacts. The NW SGT shows exceptionally early drain current–voltage saturation (IDSAT–VDSAT) below 1 V. The change in saturation with the gate voltage (VG) is over 80 times lower than a reference device with ohmic contacts. This device behavior is attributed to the source‐gate overlap, enabling gate field penetration inside the depleted source. Current modulation is obtained by a combination of gate‐induced image force barrier lowering and the high internal electric fields at source pinch‐off. Effective Schottky barrier heights are extracted from activation energy measurements, revealing systematic barrier lowering with increasing VG. These features of the device lead us to conclude that the single NW field‐effect transistor (FET) with Schottky contacts operated under SGT mode.

Since the market of portable electronic devices has been expanding for the last ten years, and their energy consumption has been decreasing in parallel, the idea of harvesting the energy of human movements is arousing a renewed interest.In this context, we conceived and made a prototype piezoelectric generator, in order to supply portable devices consuming between 10μW and 1 mW. The piezoelectric materials that we used are PZT ceramics, working in compression under a low frequency constraint (quasi-static mode). The generator consists in three conversion stages : the mechanical application device (MAD), the piezoelectric device (PD) and the harvesting system (HS).On the one hand, we modelled the PD in quasi-static mode, in order to highlight its working characteristics and estimate the delivered power, knowing the shape, amplitude and frequency of the mechanical constraint. On the other hand, we conceived, made and tested two different experimental devices, in order to demonstrate the feasibility of the concept. The delivered power reaches 13.2 μW for a 100 kOhms load, with a 55 Hz constraint of 2 N per piezoceramic. At last, we designed a harvesting electronic circuit, allowing to increase the converted energy. This circuit is dedicated to the PD, whose characteristics of impedance and current are special. Moreover, the use of low consumption components is necessary, with a view to carry out an autonomous generator., Avec l'expansion du marché des appareils électroniques portables et la diminution conjointe de leur consommation, l'idée de convertir l'énergie des mouvements humains en énergie électrique suscite un regain d'intérêt.Dans ce contexte, nous avons conçu et réalisé un générateur piézoélectrique prototype, destiné à alimenter des systèmes portables dont la puissance est comprise entre 10μW et 1 mW. Les matériaux utilisés sont des céramiques en PZT, fonctionnant en compression sous l'effet d'une contrainte de faible fréquence (régime quasi-statique). Le générateur est constitué de trois parties : le dispositif mécaniqued'application (DMA) de la contrainte, le système piézoélectrique (SP) et le système de récupération (SR).D'une part, nous avons modélisé le SP en régime quasi-statique afin de mettre en évidence ses caractéristiques de fonctionnement, et d'estimer la puissance générée sous l'effet d'une contrainte mécanique de forme, d'amplitude et de fréquence données. D'autre part, nous avons conçu, réalisé et testé deux maquettes expérimentales, afin de démontrer la faisabilité d'un tel générateur. La puissance délivrée atteint 13,2 μW sur une charge de 100 kOhms, pour une contrainte à 55 Hz et de 2 N par barreau piézoélectrique. Enfin, nous avons mis au point un circuit électronique de récupération, permettant d'accroître l'énergie électrique générée. Ce circuit a été spécialement conçu pour le SP, dont les caractéristiques d'impédance et de courants sont particulières. En outre, l'utilisation de composants à faible consommation est indispensable dans l'optique de réaliser un générateur autonome.

International audience; A vertically integrated nanogenerator (VING) refers to the new generation of mechanical energy harvesting devices: arrays of piezoelectric and semiconducting nanowires are grown perpendicularly to a substrate and appropriate electrodes enable charge circulation through an external electrical load. In this Letter, an equivalent circuit of VING is proposed. This model takes into account the effect of the polymer matrix that surrounds the nanowire lattice on the electromechanical behaviour of the generator. The active part of the material is then seen as a 1–3 piezocomposite. Simulations were made to study the VING performance in a context of body movement energy harvesting. In particular, it is shown that the strain amplitude and the strain rate of the piezocomposite have an impact on nanogenerator output voltage and output current, respectively

International audience; Novel mechanical energy micro-harvesters using ZnO nanowire (NW) arrays as active elements have recently appeared. The electrical power generation process is based on the coupling between piezoelectric and semiconducting properties of ZnO NWs. We have developed two specific models to simulate the electromechanical conversion that occurs in mechanically activated NWs and to predict the performance of NW-based microgenerators. These models, based on two distinct approaches, take into account both the piezoelectric effect and semiconducting properties of NWs. An original analytical numerical model is dedicated to the simulation of the intrinsic electromechanical conversion that occurs in a bent NW in the presence of free charge carriers. In addition, a modelling strategy was specifically developed in order to establish a dynamic model of NW-based microgenerators without falling into the different pitfalls relative to the simulation of ZnO nanowires. These two approaches are complementary and are helpful for the physical understanding of the device operation. Moreover, it indicates tendencies to optimise the generator performance. The energy conversion efficiency is maximised for high aspect ratio NWs and an optimal concentration of impurities, both in the case of a single deformed NW and for a NW array microgenerator.