Search

Thermoelectric energy conversion based on flexible materials has great potential for applications in the fields of low-power heat harvesting and solid-state cooling. Here, we show that three-dimensional networks of interconnected ferromagnetic metal nanowires embedded in a polymer film are effective flexible materials as active Peltier coolers. Thermocouples based on Co-Fe nanowires exhibit much higher power factors and thermal conductivities near room temperature than other existing flexible thermoelectric systems, with a power factor for Co-Fe nanowire-based thermocouples of about 4.7 mW/K2m at room temperature. The effective thermal conductance of our device can be strongly and rapidly increased by active Peltier-induced heat flow, especially for small temperature differences. Our investigation represents a significant advance in the fabrication of lightweight flexible thermoelectric devices, and it offers great potential for the dynamic thermal management of hot spots on complex surfaces.

In this work, we report the preparation of nanostructured electrodes based on dense arrays of vertically-aligned copper (Cu) nanowires (NWs) to be subsequently covered by cadmium telluride (CdTe) thin films, with great potential to be used within “substrate”-type photovoltaic cells based on AII-BVI heterojunctions. In particular, the multi-step preparation protocol presented here involves an electrochemical synthesis procedure within a supported anodic aluminum oxide (AAO) nanoporous template for first generating a homogeneous array of vertically-aligned Cu NWs, which are then further embedded within a compact CdTe thin film. In a second stage, we tested three deposition methods (vacuum thermal evaporation, VTE; radio-frequency magnetron sputtering, RF-MS; and electrochemical deposition, ECD) for use in obtaining CdTe layers potentially able to consistently penetrate the previously prepared Cu NWs array. A comparative analysis was performed to critically evaluate the morphological, optical, and structural properties of the deposited CdTe films. The presented results demonstrate that under optimized processing conditions, the ECD approach could potentially allow the cost-effective fabrication of absorber layer/collecting electrode CdTe/Cu nanostructured interfaces that could improve charge collection mechanisms, which in turn could allow the fabrication of more efficient solar cells based on AII-BVI semiconducting compounds.

Due to their memristive properties nanowire networks are very promising for neuromorphic computing applications. Indeed, the resistance of such systems can evolve with the input voltage or current as it confers a synaptic behaviour to the device. Here, we propose a network of silver nanowires (Ag-NWs) which are grown in a nanopourous membrane with interconnected nanopores by electrodeposition. This bottom-up approach fabrication method gives a conducting network with a 3D architecture and a high density of Ag-NWs. The resulting 3D interconnected Ag-NW network exhibits a high initial resistance as well as a memristive behavior. It is expected to arise from the creation and the destruction of conducting silver filaments inside the Ag-NW network. Moreover, after several cycles of measurement, the resistance of the network switches from a high resistance regime, in the GOhm range, with a tunnel conduction to a low resistance regime, in the kOhm range., 8 pages, 5 figures

First-order reversal curve diagrams, or FORC diagrams, have been studied to determine if the widths of their distributions along the interaction and coercivity axes can be related to the mean-field magnetization dependent interaction field (MDIF). Arrays of nanowires with diameters ranging from 18 up to 100 nm and packing fractions varying from 0.4 to 12% have been analyzed. The mean-field MDIF has been measured using the remanence curves and used as a measuring scale on the FORC diagrams. Based on these measurements, the full width of the interaction field distribution and the full width at half maximum (FWHM) of the FORC distribution profile along the interaction field direction are shown to be proportional to the MDIF, and the relation between them is found. Moreover, by interpreting the full width of the coercive field distribution in terms of the dipolar induced shearing, a simple relation is found between the width of this distribution and the MDIF. Furthermore, we show that the width of the FORC distribution along the coercive field axis is equal to the width of the switching field distribution obtained by the derivation of the DC remanence curve. This was further verified with the switching field distribution determined using in-field magnetic force microscopy (MFM) for very low density nanowires. The results are further supported by the good agreement found between the experiments and the values calculated using the mean-field model, which provides analytical expressions for both FORC distributions.

In front of electromagnetic (EM) pollution of the environment, EM protection requires more and more efficient solutions. Multilayered films consisting of alternating stacked nickel nanowires (NWs)-based conductive layers separated by bulk polycarbonate films were fabricated as thin electromagnetic shield. The well-defined periodic arrangement of the multilayer structure with gradient NW concentration in the conductive layers demonstrates a remarkably high effective microwave absorption (reflection and transmission below $-$ 10 dB, up to 34.3 dB of absorption loss) by tuning the conductivity and magnetic permeability of each layer. The ferromagnetic nature of NWs enhances the absorption performances compared to nonmagnetic carbon nanotubes having similar conductivity. Our method provides a simple, compact, and efficient approach for the fabrication of electromagnetic interference shielding materials offering improved broadband microwave absorption.

Li-ion microbatteries are the frontline candidates to fulfill the requirements of powering miniature autonomous devices. However, it still remains challenging to attain the required energy densities of > 0.3mWh cm−2 μm−1 in a planar configuration. To overcome this limitation, 3D architectures of LIMBs have been proposed. However, most deposition techniques are poorly compatible with 3D architectures because they limit the choice of current collectors and selective deposition of the active materials. Electrodeposition was suggested as an alternative for rapidly and reproducibly depositing active materials under mild conditions, and with controlled properties. However, despite the huge potential, electrodeposition remains underexplored for LIMB cathode materials, partly due to challenges associated with the electrodeposition of Li-ion phases. Herein, we review advances in the electrodeposition of Li-ion cathode materials with the main focus set on the direct, one-step deposition of electrochemically active phases. We highlight the merits of electrodeposition over other methods and discuss the various classes of reported materials, including layered transition metal oxides, vanadates, spinel, and olivines. We offer a perspective on the future advances for the adoption of electrodeposition processes for the fabrication of microbatteries to pave the way for future research on the electrodeposition of cathode materials.

The versatility of the template-assisted electrodeposition technique to fabricate complex three-dimensional networks made of interconnected nanowires allows one to easily stack ferromagnetic and non-magnetic metallic layers along the nanowire axis. This leads to the fabrication of unique multilayered nanowire network films showing giant magnetoresistance effect in the current-perpendicular-to-plane configuration that can be reliably measured along the macroscopic in-plane direction of the films. Moreover, the system also enables reliable measurements of the analogous magneto-thermoelectric properties of the multilayered nanowire networks. Here, three-dimensional interconnected NixFe1−x/Cu multilayered nanowire networks (with 0.60≤x≤0.97) are fabricated and characterized, leading to large magnetoresistance and magneto-thermopower ratios up to 17% and −25% in Ni80Fe20/Cu, respectively. A strong contrast is observed between the amplitudes of magnetoresistance and magneto-thermoelectric effects depending on the Ni content of the NiFe alloys. In particular, for the highest Ni concentrations, a strong increase in the magneto-thermoelectric effect is observed, more than a factor of 7 larger than the magnetoresistive effect for Ni97Fe3/Cu multilayers. This sharp increase is mainly due to an increase in the spin-dependent Seebeck coefficient from −7 V/K for the Ni60Fe40/Cu and Ni70Fe30/Cu nanowire arrays to −21 V/K for the Ni97Fe3/Cu nanowire array. The enhancement of the magneto-thermoelectric effect for multilayered nanowire networks based on dilute Ni alloys is promising for obtaining a flexible magnetic switch for thermoelectric generation for potential applications in heat management or logic devices using thermal energy.

The versatility of the template-assisted electrodeposition technique to fabricate complex three-dimensional networks made of interconnected nanowires allows one to easily stack ferromagnetic and non-magnetic metallic layers along the nanowire axis. This leads to the fabrication of unique multilayered nanowire network films showing giant magnetoresistance effect in the current-perpendicular-to-plane configuration that can be reliably measured along the macroscopic in-plane direction of the films. Moreover, the system also enables reliable measurements of the analogous magneto-thermoelectric properties of the multilayered nanowire networks. Here, three-dimensional interconnected NixFe1−x/Cu multilayered nanowire networks (with 0.60≤x≤0.97) are fabricated and characterized, leading to large magnetoresistance and magneto-thermopower ratios up to 17% and −25% in Ni80Fe20/Cu, respectively. A strong contrast is observed between the amplitudes of magnetoresistance and magneto-thermoelectric effects depending on the Ni content of the NiFe alloys. In particular, for the highest Ni concentrations, a strong increase in the magneto-thermoelectric effect is observed, more than a factor of 7 larger than the magnetoresistive effect for Ni97Fe3/Cu multilayers. This sharp increase is mainly due to an increase in the spin-dependent Seebeck coefficient from −7 µV/K for the Ni60Fe40/Cu and Ni70Fe30/Cu nanowire arrays to −21 µV/K for the Ni97Fe3/Cu nanowire array. The enhancement of the magneto-thermoelectric effect for multilayered nanowire networks based on dilute Ni alloys is promising for obtaining a flexible magnetic switch for thermoelectric generation for potential applications in heat management or logic devices using thermal energy.

Electrochemical deposition of interconnected nanowires and nanotubes made of ferromagnetic metals into track-etched polycarbonate templates with crossed nanochannels has been revealed suitable for the fabrication of mechanically stable three-dimensional magnetic nanostructures with large surface area. These 3D networks embedded into flexible polymer membranes are also planar and lightweight. This fabrication technique allows for the control of the geometric characteristics and material composition of interconnected magnetic nanowire or nanotube networks, which can be used to fine-tune their magnetic and magneto-transport properties. The magnetostatic contribution to the magnetic anisotropy of crossed nanowire networks can be easily controlled using the diameter, packing density, or angle distribution characteristics. Furthermore, the fabrication of Co and Co-rich NiCo alloy crossed nanowires with textured hcp phases leads to an additional significant magnetocrystalline contribution to the magnetic anisotropy that can either compete or add to the magnetostatic contribution. The fabrication of an interconnected nanotube network has also been demonstrated, where the hollow core and the control over the tube wall thickness add another degree of freedom to control the magnetic properties and magnetization reversal mechanisms. Finally, three-dimensional networks made of interconnected multilayered nanowire with a succession of ferromagnetic and non-magnetic layers have been successfully fabricated, leading to giant magnetoresistance responses measured in the current-perpendicular-to-plane configuration. These interconnected nanowire networks have high potential as integrated, reliable, and stable magnetic field sensors, magnetic devices for memory and logic operations, or neuromorphic computing.

Tin‐based nanowire electrodes present desirable properties as lithium‐ion battery anodes, because they undergo volume changes without pulverization. However, they suffer from limited mass loading and propensity for surface parasitic reactions. Herein, we evaluate the electrochemical performances of interconnected nickel‐tin nanowire network electrodes (NiSn 3DNWN) with nanowire diameters of 40 nm, 105 nm, and 230 nm, respectively, that attain mass loadings of up to 3 mg cm‐2. To mitigate the surface parasitic reactions, the effects of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) additives are investigated as a function of nanowire diameter and additive concentration. The results show that FEC and VC of all compositions improve the capacity retentions and coulombic efficiencies of the NiSn 3DNWN electrodes. In 10 vol% FEC, the electrodes demonstrate a similar capacity of ∽ 550 mAh g‐1, but the capacity retentions after 100 cycles are 73.68%, 53.79%, 51.70% for the 40 nm, 105 nm, and 230 nm NiSn 3DNWN, respectively. However, the 105 nm‐diameter nanowire electrode has the highest average coulombic efficiency of 96.55%. Electrochemical impedance spectroscopy and post‐cycling investigations reveal that FEC has the most profound effect on the interphasial resistances, which is reflected in the rate performances of the tested electrodes. This article is protected by copyright. All rights reserved.

Recently, interconnected nanowire networks have been found suitable as flexible macroscopic spin caloritronic devices. The 3D nanowire networks are fabricated by direct electrodeposition in track-etched polymer templates with crossed nano-channels. This technique allows the fabrication of crossed nanowires consisting of both homogeneous ferromagnetic metals and multilayer stack with successive layers of ferromagnetic and non-magnetic metals, with controlled morphology and material composition. The networks exhibit extremely high, magnetically modulated thermoelectric power factors. Moreover, large spin-dependent Seebeck coefficients were directly extracted from experimental measurements on multilayer nanowire networks. This work provides a simple and cost-effective way to fabricate large-scale flexible and shapeable thermoelectric devices exploiting the spin degree of freedom.

NiFe alloy and NiFe/Cu multilayered nanowire (NW) networks were grown using a template-assisted electrochemical synthesis method. The NiFe alloy NW networks exhibit large thermopower, which is largely preserved in the current perpendicular-to-plane geometry of the multilayered NW structure. Giant magneto-thermopower (MTP) effects have been demonstrated in multilayered NiFe/Cu NWs with a value of 25% at 300 K and reaching 60% around 100 K. A large spin-dependent Seebeck coefficient of –12.3 μV/K was obtained at room temperature. The large MTP effects demonstrate a magnetic approach to control thermoelectric properties of flexible devices based on NW networks.

The difficulty to generate and control large strain gradients in materials hinders the investigation and application of flexoelectricity and flexomagnetism. This work demonstrates that thermal expansion can be used to induce very large non-uniform strains at the nanoscale, resulting in giant strain gradients at moderate temperatures. This is demonstrated in a nanopatterned multiferroic hybrid layer consisting of a regular array of ferromagnetic metallic nanocylinders embedded in a ferroelectric polymer matrix. The thermally-induced strain gradients can fully depolarize the ferroelectric component, and modify the magnetization of the ferromagnetic component via flexoelectric and flexomagnetic effects, respectively. Finiteelement analysis provides a quantitative view on thermal expansion-induced strains and strain gradients supporting the experimental findings. This work shows that nanoscale thermal strain engineering provides an additional degree of freedom to control electrical polarization and magnetization, which paves the way for the design and operation of novel functional devices and nanostructures.

The unique architectures and high degree of nanowire (NW) interconnectivity of three-dimensional (3D) NW networks make them attractive nanodevice components for a wide range of applications in energy harvesting/storage systems [1–3], electronic sensing devices and actuators [4–6], catalysts [7], electrochromic elements [8], solar cells [9], biosensors [10], and bioanalytical devices [11, 12]. However, the fabrication of 3D nanoarchitectures is extremely complicated with conventional lithography techniques and so far lacks the commercial viability. On the contrary, template-assisted synthesis is a versatile bottom-up approach for low-cost, reliable, and large-scale fabrication of NW networks with controlled size, geometry, composition, and surface morphology. Typically, these 3D networks are obtained by simple electrochemical deposition within the hierarchical nanopores of a suitable template. So far, various 3D nanoporous templates have been used for this purpose, including silica templates [13], diblock copolymers [8], 3D alumina nanoporous hosts [1, 14, 15], as well as track-etched polymeric membranes [7, 16]. The latter approach is the most promising as dense networks of crossed cylindrical nanopores can be obtained through sequential polycarbonate (PC) film irradiation with energetic heavy ions at different incidence angles, followed by selective chemical etching of the ion tracks within the polymer film [17]. The suitability of such 3D nanoporous polymer templates to form dense interconnected NW networks with tunable geometrical parameters in terms of NW size, density, and orientation opens up the possibility for a controlled synthesis of a large variety of complex 3D networks of high aspect-ratio nanostructures with different geometries and materials. Besides, the interconnected crossed nanowires (CNWs) architecture facilitates the ability to perform magneto-transport measurements and to investigate the interplay between electronic transport and magnetic properties [18, 19], which is of paramount importance for the development of magnetic sensors and for the storage and logic operation of information carried and processed by domain walls flowing along them [20]. Also, magnetic nanotubes (NTs) are interesting cylindrical nanostructures to be arranged in a 3D morphological architecture because its magnetic and magneto-transport behavior is different to that of solid NWs. The hollow core of the NTs provides an additional geometrical degree of freedom that is Magnetic Nano- and Microwires.

Magnetic nanowires are attractive materials because of their morphology-dependent remarkable properties suitable for various advanced technologies in sensing, data storage, spintronics, biomedicine and microwave devices, etc. The recent advances in synthetic strategies and approaches for the fabrication of complex structures, such as parallel arrays and 3D networks of one-dimensional nanostructures, including nanowires, nanotubes, and multilayers, are presented. The simple template-assisted electrodeposition method enables the fabrication of different nanowire-based architectures with excellent control over geometrical features, morphology and chemical composition, leading to tunable magnetic, magneto-transport and thermoelectric properties. This review article summarizing the work carried out at UCLouvain focuses on the magnetic and spin-dependent transport properties linked to the material and geometrical characteristics.

Recently, there has been increasing interest in the fabrication of flexible thermoelectric devices capable of cooling or recovering waste heat from hot surfaces with complex geometries. This paper reviews recent developments on three-dimensional networks of interconnected ferromagnetic nanowires, which offer new perspectives for the fabrication of flexible thermoelectric modules. The nanowire arrays are fabricated by direct electrodeposition into the crossed nanopores of polymeric templates. This low-cost, easy and reliable method allows control over the geometry, composition and morphology of the nanowire array. Here we report measured thermoelectric characteristics as a function of temperature and magnetic field of nanowire networks formed from pure metals (Co, Fe, Ni), alloys (NiCo, NiFe and NiCr) and FM/Cu multilayers (with FM = Co, Co50Ni50 and Ni80Fe20). Homogeneous nanowire arrays have high thermoelectric power factors, almost as high as their bulk constituents, and allow for positive and negative Seebeck coefficient values. These high thermoelectric power factors are essentially maintained in multilayer nanowires which also exhibit high magnetic modulability of electrical resistivity and Seebeck coefficient. This has been exploited in newly designed flexible thermoelectric switches that allow switching from an ‘off’ state with zero thermoelectric output voltage to an ‘on’ state that can be easily measured by applying or removing a magnetic field. Overall, these results are a first step towards the development of flexible thermoelectric modules that use waste heat to power thermally activated sensors and logic devices.

Large-scale, electrically interconnected three-dimensional (3-D) Ni crossed nanotube networks have been fabricated using an electrochemical dealloying method within the crossed nanopores of polymer host membranes. This method paves the way for the easy and cost-effective fabrication of 3-D magnetic NT networks with precise spatial arrangement and diameter and wall thickness of 10–100 nm controlled individually. The excellent control over geometrical parameters and morphological features of the Ni crossed nanotube networks leads to tunable magnetic and magneto-transport properties. Particularly, the low field magneto-transport behavior is consistent with the expected vortex-like states formed in different segments of the nanotube scaffold, whereas nucleation of domain walls at the intersection of the nanowire segments play a dominant role in the solid crossed nanowire networks counterpart. The present 3-D networks of nanomagnets are of special interest due to their potential for memory devices, computing architectures, sensing and biomedical applications.

International audience; Interfaces are a major issue when designing ferroelectric nanostructured materials with tailored properties. In a context of integration and multifunctionality in the field of electronics, several strategies have been developed to control the microstructure and defect chemistry of interfaces that strongly impact the macroscopic properties. The suitability of the core-shell approaches that allow a subtle tuning of interface phenomena at different scales has been widely demonstrated. We focus here on the flexibility of the core-shell approach devoted to the processing of nanostructured ferroelectric composites. Our strategy relies on the use of advanced synthesis processes to design ferroelectric grains coated with shells of different nature, morphology and crystallinity. Typical examples will be reviewed with a specific attention on their impact on both microstructure and dielectric properties. Our approach, based also on the use of fast sintering technique, provides a guidance to design 3D bulk nanostructured ferroelectrics while controlling and/or exploiting size, interface and defects chemistry. The contribution of specific spectroscopies to probe interfacial chemistry and defects is underlined. The high density of interfaces in core-shell materials is obviously an advantage to target additional functionality such as magneto-electric coupling. This is illustrated in 3D composites and one dimensional nanostructures that coaxially combine electric and magnetic materials. The core-shell approach described here could be transferred to a much broader range of materials covering many functionalities provided a deeper understanding of the interfaces at the atomic scale is achieved and a further development of low temperature processing is reached.

A method is presented that allows quantifying the average value of the interaction field in arrays of magnetic nanowires from the field difference between the isothermal remanence (IRM) and the DC demagnetizing (DCD) remanence curves when the normalized magnetization is equal to one third. Arrays of magnetic nanowires of different diameters and packing fractions are used to experimentally test the method. The results have been compared with those obtained using the method based on the difference between the remanence coercivity fields and with a mean-field expression for the interaction field, providing a very good agreement and thus validating the method. Additionally, it is shown that both the position (m0) and the shift along the magnetization axis of the intersection between the remanence curves with respect to the value of one third (δm = m0 − 1/3) provide qualitative information about the interaction field. The former indicates the type of interaction depending if the intersection is above (m0 > 1/3) or below (m0

The use of the spin degree of freedom in thermoelectric conversion offers new functionalities for electronic devices powered by waste heat and a promising solution for sustainable technologies. Here, thermocouples formed from two dissimilar arrays of interconnected magnetic nanowires embedded in a polymer film are used to realize flexible thermoelectric switches providing optimal magnetic-field-induced control of the sign and magnitude of the thermopower. By finetuning the composition of the homogeneous and multilayered nanowires forming the thermocouple legs, an ideal on/off ratio in the switching of the thermoelectric output voltage, or a simple sign reversal can be achieved in the presence of a magnetic field. This work paves the way for flexible spin-caloritronics devices exploiting residual thermal energy for the development of thermally activated sensors and logic devices.

In this work, a ferromagnetic resonance study on the influence of the electrolytic bath acidity on the magnetic anisotropy and hardness parameter of arrays of 50 nm diameter Ni nanowires in porous alumina membranes, is reported. Fine tuning the effective magnetic anisotropy of the arrays has been achieved by progressively changing the nanowires microstructure from polycrystalline textured to single-crystalline as the electrolyte acidity is reduced. The micro-structural analysis carried out by X-ray diffraction and high resolution transmission electron microscopy has revealed that a preferred crystal texture is not a sufficient condition for the enhancement of the magnetic anisotropy. Instead, a correlation between the increase of crystallite size and quality, and the increase of both the magnetic anisotropy and hardness parameter has been shown. Ferromagnetic resonance and alternating gradient magnetometry experiments have shown that no additional magnetic contributions take place in Ni NWs grown from highly acidic electrolytes (pH 1.6), whereas a significant additional anisotropy contribution progressively appears as the electrolyte acidity is reduced (pH 5.0). Besides the careful adjustment of electrodeposition conditions, achieving a very long length for the nanowires is also a key requirement to have a precise control on the effective magnetic anisotropy energy, which can be fine tuned in the wide range from 64 - 167 kJ · m−3 as a result of an energy enhancement of magnetoelastic origin. Ferromagnetic resonance measurements have demonstrated that nanocomposites based on arrays of nanowires made of low cost and abundant elements like Ni exhibit hardness parameters in the range 0.47–0.75 within the semihard region, where those with κ > 0.5 are suitable for permanent magnet applications. These features make arrays of low diameter Ni nanowires very appealing for their use in the development of agile microwave and spintronic devices.

Using arrays of bistable magnetic nanowires, we show that their microwave absorption properties can be reversibly configured using low magnetic field cycling by virtue of the double ferromagnetic resonance absorption spectra. These characteristics lead to a single bifunction microwave filter that can be configured to work as a bandstop or a bandpass filter, making these materials very attractive for compact multifunction planar microwave devices. The bifunction microwave properties of these systems are successfully predicted using a mean field model for their magnetic configuration-dependent effective permeability.

© 2019 American Physical Society. Reducing the size of a superconductor below its characteristic length scales can either enhance or suppress its critical temperature (Tc). Depending on the bulk value of the electron-phonon coupling strength, electronic and phonon confinement effects will play different roles in the modification of Tc. Experimentally disentangling each contribution has remained a challenge. We have measured both the phonon density of states and Tc of Sn nanowires with diameters of 18, 35, and 100 nm in order to quantify the effects of phonon confinement on superconductivity. We observe a shift of the phonon frequency towards the low-energy region and an increase in the electron-phonon coupling constant that can account for the measured increase in Tc. ispartof: PHYSICAL REVIEW B vol:99 issue:6 status: published

Magnetoresistance and thermopower of crossed NiCr and CoCr nanowire networks have been measured as a function of temperature and chromium content in dilute alloys. At low temperatures, it is found that the impurity effect leads to negative anisotropic magnetoresistance, an observation that even persists until room temperature in diluted CoCr alloy nanowires. The addition of a small amount of Cr in nickel nanowires also abruptly reverses the sign of the thermopower from −20 μV/K for pure Ni up to +18 μV/K for the dilute alloys, implying the switching from n- to p-type conduction. These results are consistent with pronounced changes in the density of states for the majority spin electrons. The high room-temperature power factors of these magnetic nanowire networks (in the range of 1–10 mW/K2 m) provide interesting perspectives for designing n- and p-type legs for flexible spin thermoelectric devices.

Spin-related effects in thermoelectricity can be used to design more efficient refrigerators and offer novel promising applications for the harvesting of thermal energy. The key challenge is to design structural and compositional magnetic material systems with sufficiently high efficiency and power output for transforming thermal energy into electric energy and vice versa. Here, the fabrication of large-area 3D interconnected Co/Cu nanowire networks is demonstrated, thereby enabling the controlled Peltier cooling of macroscopic electronic components with an external magnetic field. The flexible, macroscopic devices overcome inherent limitations of nanoscale magnetic structures due to insufficient power generation capability that limits the heat management applications. From properly designed experiments, large spin-dependent Seebeck and Peltier coefficients of $-9.4$ $\mu$V/K and $-2.8$ mV at room temperature, respectively. The resulting power factor of Co/Cu nanowire networks at room temperature ($\sim7.5$ mW/K$^2$m) is larger than those of state of the art thermoelectric materials, such as BiTe alloys and the magneto-power factor ratio reaches about 100\% over a wide temperature range. Validation of magnetic control of heat flow achieved by taking advantage of the spin-dependent thermoelectric properties of flexible macroscopic nanowire networks lay the groundwork to design shapeable thermoelectric coolers exploiting the spin degree of freedom., Comment: 11 pages, 7 figures

There is a proliferation of autonomous microscale devices, which are increasingly in need of power sources of commensurate form factors. The trade-off between the areal energy and power capabilities of miniature energy storage devices restrict their abilities to meet the requirements of these microscale devices. However, three-dimensional (3D) micro- and nanoarchitectures have the potential to meet the energy requirements of autonomous microscale devices, while retaining the power capabilities of thin-film systems. In this paper, we present a novel full-cell architecture and fabrication strategy for 3D microscale energy storage devices based on template-assisted electrodeposition of three-dimensionally interconnected metallic nanowire network, and its subsequent laser-patterning to form interdigitated nanostructured electrodes. The patterned nanowire networks are functionalized with the polyaniline active electrode material via an electroless deposition procedure. We show that a footprint areal capacitance/energy enhancement of more than 5 times is achieved by just 10 μm-high nanowire networks, in addition to enhanced power capabilities.

We developed a versatile template-based fabrication method for growing large arrays of three-dimensionally interconnected polypyrrole nanotubes (or nanowires) with easily tunable geometrical dimensions and spatial arrangement. Such a macroscopic network made up of conducting polymer elongated nanostructures provides an extremely large active surface with increased electrical connectivity, as well as an enhanced structural integrity of the flexible network. The three-dimensional array of polypyrrole nanofibers exhibited an excellent sensitivity towards gaseous ammonia, providing reliable and accurate detection at gas concentrations as low as 1 ppm. The novel preparation approach offers a cost-effective alternative for large-scale production of easily integrable chemiresistive sensors for different applications.

We report the facile and low-cost preparation as well as detailed characterization of dense arrays of passivated ferromagnetic nickel (Ni) nanotubes (NTs) vertically-supported onto solid Au-coated Si substrates. The proposed fabrication method relies on electrochemical synthesis within the nanopores of a supported anodic aluminum oxide (AAO) template and allows for fine tuning of the NTs ferromagnetic walls just by changing the cathodic reduction potential during the nanostructures&rsquo, electrochemical growth. Subsequently, the experimental platform allowed further passivation of the Ni NTs with the formation of ultra-thin antiferromagnetic layers of nickel oxide (NiO). Using adequately adapted magnetic measurements, we afterwards demonstrated that the thickness of the NT walls and of the thin antiferromagneticNiO layer, strongly influences the magnetic behavior of the dense array of exchange-coupled Ni/NiO NTs. The specific magnetic properties of these hybrid ferromagnetic/antiferromagnetic nanosystems were then correlated with the morpho-structural and geometrical parameters of the NTs, as well as ultimately strengthened by additionally-implemented micromagnetic simulations. The effect of the unidirectional anisotropy strongly amplified by the cylindrical geometry of the ferromagnetic/antiferromagnetic interfaces has been investigated with the magnetic field applied both parallel and perpendicular to the NTs axis.

In this work, three dimensional (3D) interconnected Ni 1 - x Co x ( 0 ⩽ x ⩽ 100 %) crossed nanowire (CNW) networks with different scale ratios have been fabricated in order to investigate the influence of size reduction of the whole 3D networks structure on their crystal orientation and magnetic properties. Interconnected porous polycarbonate membranes, each one with different nanopore diameters but with the same porosity, have been designed to obtain porous architectures with different scale ratios. Using these membranes, CNW networks with the same packing fraction of about 22%, but different nanowires (NWs) diameters have been grown by electrodeposition. As a result, the magnetostatic (MS) anisotropy in the saturated state is the same for all the 3D networks, which allows identifying size dependent magnetic anisotropies. Reducing the diameter of the nanowires leads to changes on their magnetic properties, so that Co-rich CNW networks show a magnetocrystalline (MC) anisotropy contribution originated by a preferential hexagonal close packed orientation that competes with the MS anisotropy field. In turn, low diameter Ni-rich CNW networks show an anisotropy contribution of different nature than the MS and MC fields. The strategy presented in this work to accurately separate magnetic contributions leads to a better understanding of the magnetic behavior of complex 3D nanoarchitectures and to their potential use in the development of magnetic, sensing and spintronic applications.

The effects of the deposition potential on the structural and magnetic properties of Ni nanowires with a diameter of 40 nm and lengths varying from 16 up to 56 μm have been studied. The results show that very long NWs exhibit a large enhancement of their uniaxial magnetic anisotropy. This anisotropy reaches values as high as 1.6 × 10 6 erg/cm3 that is comparable with the value due to the magnetostatic anisotropy contribution ( ≈ 5 × 10 6 erg/cm3) and as shown from the results this large anisotropy is practically insensitive to the deposition potential. Moreover, as shown on shorter NWs, this enhancement of the anisotropy is not observed and interestingly the potential does induce structural changes that relate to their magnetic properties. The enhanced anisotropy observed in long NWs is a robust effect as it is independent of the potential, making this property attractive for applications where high uniaxial anisotropies are required.

This paper presents an innovative method to synthesize nanowire-based microwave devices integrated inside a nanoporous alumina membrane. A laser treatment is used to destroy locally the template surface porosity, preventing the nanowire growth in the modified regions. We have realized a substrate integrated waveguide (SIW) in which the vertical walls consist of nanowire arrays. The waveguide can then be modified to achieve different types of microwave devices by properly placing nanowire arrays inside the SIW. The so-obtained devices combine the advantages of nanowire arrays (compactness, tunable permittivity and permeability,…) with those of substrate integrated waveguides (low losses). Our fabrication approach enables wide range of devices and we present promising results for integrated waveguide isolators.

We reported an efficient way of preparing large and dense ferromagnetic arrays of vertically-aligned Ni nanotubes (NTs). The synthesis method allowed then for a direct, effective and controllable thermal conversion of the as-prepared Ni NTs into heterostructured Ni/NiO NTs, without compromising the nanostructures' verticality to the substrate and their initial tubular integrity. Next, we demonstrated the ability to tune the dipolar interaction field (DIF) in this array of one-dimensional (1D) Ni/NiO heterostructures, just by slightly changing the thickness of the antiferromagnetic NiO layer. Furthermore, the proposed fabrication method assures full compatibility with the standard Si processing, providing thus a direct pathway towards large-scale integrability and manufacturing of a new generation of magnetic devices relying on dense arrays of vertical 1D magnetic nanostructures.

Magnetoelectric layers with a strong coupling between ferroelectricity and ferromagnetism offer attractive opportunities for the design of new device architectures such as dual-channel memory and multiresponsive sensors and actuators. However, materials in which a magnetic field can switch an electric polarization are extremely rare, work most often only at very low temperatures, and/or comprise complex materials difficult to integrate. Here, we show that magnetostriction and flexoelectricity can be harnessed to strongly couple electric polarization and magnetism in a regularly nanopatterned magnetic metal/ferroelectric polymer layer, to the point that full reversal of the electric polarization can occur at room temperature by the sole application of a magnetic field. Experiments supported by finite element simulations demonstrate that magnetostriction produces large strain gradients at the base of the ferroelectric nanopillars in the magnetoelectric hybrid layer, translating by flexoelectricity into equivalent electric fields larger than the coercive field of the ferroelectric polymer. Our study shows that flexoelectricity can be advantageously used to create a very strong magnetoelectric coupling in a nanopatterned hybrid layer.

In this work, we evaluated the piezoelectric properties of the ZnO nanorods (NRs), subsequently grown by a template-free electrochemical deposition (ECD) approach. The one-step preparation method yielded in a relatively short-time synthesis large arrays of high aspect-ratio ZnO NRs, vertically aligned to the substrate and exhibiting a well-defined hexagonal symmetry coupled with an excellent crystallinity. Piezoresponse force microscopy (PFM) was employed to extract the effective piezoelectric coefficient ( d 33 ) of the ZnO [0 0 0 1]-oriented NRs. A mean effective d 33 coefficient of 11.8 pm/V was measured over several individual NRs. The results have shown an increased piezoelectric response of the ZnO NRs, with ∼18% in respect to the ZnO bulk material, and with a factor ranging from 28% to 167% compared to similar ZnO nanostructures prepared by other counterpart techniques. Both morphology and crystalline quality of the obtained ZnO NRs, as well as the piezoelectric investigations, ranked the ECD as a promising low-cost pathway for possibly producing high quality nanostructured ZnO-based piezoelectric devices with enhanced performance.

International audience; A reliable and flexible synthesis route was used for processing high density Ni–BaTiO3 nanocable arrays based on wet chemical impregnation and subsequent electrodeposition within a highly ordered unidirectional porous alumina membrane. The core–shell structure was carefully investigated by bright field scanning transmission electronic microscopy coupled with energy dispersive X-ray spectroscopy. The strength of the dipolar interaction arising from the packing density of the magnetic nanowires was correlated with the BaTiO3 wall thickness through magnetometry and ferromagnetic resonance measurements. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties.

A new strategy to elaborate (1-3) type multiferroic nanocomposites with controlled dimensions and vertical alignment is presented. The process involves a supported nanoporous alumina layer as a template for growth of free-standing and vertically aligned CoFe

The fundamental properties of 2D arrays of magnetic nanowires and nanotubes (NWs and NTs) have been well described theoretically, but real systems are subject to certain flaws and imperfections which strongly affect the mechanism and dynamics of magnetization reversal. Therefore, before exploiting the vast potential of magnetic NWs and NTs, there is a need to investigate the materials parameters as well as the field dependent magnetic properties in real systems. Amongst the various issues at stake for a comprehensive understanding of these arrays is the influence of long-range dipolar interactions because these interactions strongly influence the magnetization reversal and thus the switching field distributions (SFD) which plays a significant role for information storage. Particularly, the width of the SFD is important since smaller values of this parameter leads to less recording errors and it is also a measure of the quality of the recording media. Consequently, the understanding and evaluation of the distinct parameters influencing the interactions and the intrinsic SFD in the magnetization reversal process of such arrays is critical for the development of magnetic recording media. It is shown that the intrinsic SFD mostly originates from nonuniformities of the geometrical parameters such as the aspect ratio and the shape of the magnetic nanostructures. This chapter provides an overview of the MFMcharacterization studies undertaken to understand the reversal behavior of 2D arrays of magnetic NWs and NTs at the nanoscale and the motivations to investigate isolated single domain NWs and NTs.

International audience; A new strategy to elaborate (1-3) type multiferroic nanocomposites with controlled dimensions and vertical alignment is presented. The process involves a supported nanoporous alumina layer as a template for growth of free-standing and vertically aligned CoFe2 nanopillars using a room temperature pulsed electrodeposition process. Ba0.70Sr0.30TiO3–CoFe2O4 multiferroic nanocomposites were grown through direct deposition of Ba0.7Sr0.3TiO3 films by radio-frequency sputtering on the top surface of the pillar structure, with in situ simultaneous oxidation of CoFe2 nanopillars. The vertically aligned multiferroic nanocomposites were characterized using various techniques for their structural and physical properties. The large interfacial area between the ferrimagnetic and ferroelectric phases leads to a magnetoelectric voltage coefficient as large as ~320 mV cm−1 Oe−1 at room temperature, reaching the highest values reported so far for vertically architectured nanocomposite systems. This simple method has great potential for large-scale synthesis of many other hybrid vertically aligned multiferroic heterostructures.

3-D alloyed and multilayered interconnected nanofiber networks have been fabricated by electrodeposition techniques, allowing a controlled composition and 3-D structural topology. These features have been found crucial to tailor their magnetic and magneto-transport properties. Their interplay along with the use of a simple analytical model based on the particular interconnected topology of the networks has allowed to accurately determine the anisotropic magnetoresistance (AMR) ratio. The as-obtained AMR ratio for interconnected nanofiber networks is consistent with an average that results from all the nanowires orientations in the membrane. The careful choice of magnetic and non-magnetic layer thicknesses has been decisive for the fabrication of Co/Cu multilayered interconnected nanofiber networks with giant magnetoresistive response as high as 19%. Interconnected nanofiber networks with controlled material composition and specific structural features are very attractive for the development of mechanically stable superstructures suitable for potential technological device applications.

A bottom-up approach for the fabrication of an assembly of electrodeposited nanowires has been combined to single nanowire electrical connection techniques to investigate the spin-transfer-torque and microwave emission of specially designed nanowires containing Co/Cu/Co pseudo spin-valves (SVs). Porous alumina templates are used for the growth by electrodeposition of metallic in-series connected SVs. Under specific magnetic field and injected current conditions, emission of microwave current is detected with frequency in the GHz range and linewidth as low as 1.8[Formula: see text]MHz. Microwave signals have been obtained even at zero magnetic field and high frequency versus magnetic field tunability was demonstrated. Our findings are in good agreement with micromagnetic simulations. In addition, it appears that in our particular geometry, the microwave emission is generated by the vortex gyrotropic motion which occurs in, at least, one of the two magnetic layers of our SV structures.

tIn this work, large and dense arrays of zinc oxide (ZnO) nanorods (NRs) were fabricated by an improvedtemplate-free electrodeposition (ECD) route on top of home-made Al-doped ZnO (AZO) thin filmsdeposited on glass. The AZO layer, acting as transparent conducting oxide, was found to display enhancedelectrical and morphological properties compared to other commercially available substrates, facilitat-ing the subsequent electrochemical growth of the ZnO nanostructures. In addition, its intrinsic columnarnature strengthened the vertical orientation of the obtained ZnO NRs. The one-step low-temperatureelectrochemical method did not require any additional seeding layer, being suitable for applications inwhich the process simplicity and substrate’s thermal toughness are essential requirements. The ECDprotocol was optimized as a function of the process temperature and duration, demonstrating the pos-sibility to yield reasonable aspect-ratio ZnO NRs in a relatively short time synthesis. Further structuralstudies confirmed the excellent orientation along c-axis of the ZnO Wurtzite phase and have shown apolycrystalline nature of the NRs, with a well-defined hexagonal symmetry of the crystallites.

This paper compares two laser-assisted processes developed by the authors for the fabrication of microwave devices based on nanowire arrays loaded inside porous alumina templates. Pros and cons of each process are discussed in terms of accuracy, reproducibility and ease of fabrication. A comparison with lithography technique is also provided. The efficiency of the laser-assisted process is demonstrated through the realization of substrate integrated waveguide (SIW) based devices. A Nanowired SIW line is firstly presented. It operates between 8.5 and 17 GHz, corresponding to the first and second cut-off frequency of the waveguide, respectively. Next, a Nanowired SIW isolator is demonstrated. It shows a nonreciprocal isolation of 12 dB (corresponding to 4.4 dB/cm), observed in absence of a DC magnetic field, and achieved through an adequate positioning of ferromagnetic nanowires inside the waveguide cavity.

Unbiased ferromagnetic nanowires arrays were used to conceive and fabricate planar microwave circulators working at dedicated frequencies in the X-band (8-12 GHz) for airborne applications. We investigated the dependence of the microwave behavior to the nanowires length with the fabrication of four circulators based on NiFe nanowires arrays having different nanowires lengths and found that it is possible to control precisely the circulation frequency. We also proceeded to a numerical optimization of the devices parameters (nanowires nature, quality of alumina membrane and geometrical dimensions). Numerical resulting performances meet the commercial standard requirements and confirm the high potential of the nanowired substrates for on-board systems in aerospace applications.

A nanopatterned hybrid layer is designed, wherein the electric polarization can be flipped at room temperature by a magnetic field aided by an electrical field. This is achieved by embedding ferromagnetic nanopillars in a continuous organic ferroelectric layer, and amplifying the magnetostriction-generated stress gradients by scaling down the supracrystalline cell of the material.

A large enhancement of the magnetic anisotropy of Ni nanowires (NWs) embedded in anodic aluminium oxide porous membranes is obtained as a result of an induced magnetoelastic (ME) anisotropy contribution. This unusual large anisotropy enhancement depends on the diameter of the NWs and exceeds the magnetostatic (MS) contribution. As a consequence, it leads to effective magnetic anisotropy energies as large as 1.4 × 10(6) erg cm(-3), which are of the same order of magnitude and comparable to the MS energies of harder magnetic materials like Co NWs. Specifically, from ferromagnetic resonance experiments, the magnetic anisotropy of the NWs has been observed to increase as its diameter is decreased, leading to values that are about four times larger than the corresponding value when only the MS anisotropy is present. Our results are consistent with the recently proposed growth mechanism of Ni NWs that proceeds via a poly-crystalline stage at the bottom followed by a single-crystalline stage with texture [110] parallel to the axis of the NWs. A strong correlation between reducing the diameter of the NWs with the decrease of the length of the poly-crystalline segment and the enhancement of the effective magnetic anisotropy has been shown. Magnetization curves obtained from alternating gradient magnetometry experiments show that the average ME anisotropy results from the competition between the magnetic anisotropies of both crystalline segments of the NWs. Understanding the influence of size and confinement effects on the magnetic properties of nanocomposites is of prime interest for the development of novel and agile devices.

The electronic conductivity anisotropy of $\mathrm{T}{\mathrm{i}}_{3}\mathrm{Si}{\mathrm{C}}_{2}$ is directly evidenced from data collected on (i) a thin film epitaxially grown on a $(11\overline{2}0)$-oriented SiC single crystal and (ii) a single crystal. Density functional theory calculations, including the linear-response approach and coupled to a Bloch-Gr\"uneisen model, show that the electron-phonon interactions are mainly responsible for the observed anisotropy. Detailed analysis of the electron-phonon coupling constants allows for the rationalization of these scattering processes in terms of the $\mathrm{T}{\mathrm{i}}_{3}\mathrm{Si}{\mathrm{C}}_{2}$ nanostructure, giving insights into the possibility of modifying the electron-phonon interaction in this system by substitution effects.

International audience; A major challenge in the development of efficient magnetoelectric nanocomposites is the adequate control of the interfaces, in order to avoid the formation of undesirable interphases and to ensure an optimal strain mediated coupling. In this work we used a combination of low-cost impregnation and electrodeposition processes within the ordered nanochannels of a porous anodic alumina template, to elaborate CoFe2O4–PbZr0.52Ti0.48O3 core/shell nanocable arrays with high interfacial areas between the ferroelectric and the magnetic phases. We have shown in this way that the thermal annealing steps required for phase's crystallization and oxidation of metallic CoFe2 into spinel CoFe2O4 are critical with respect to interdiffusion phenomena through the interfaces. The impact of the processing temperature on the morphological and structural features of the nanocables was then discussed. We demonstrated that an optimization of process variables in the synthesis of CoFe2O4–PbZr0.52Ti0.48O3 nanocables allows a significant improvement in their microstructure and ensures the chemical integrity of the two-phase materials, an important concern when seeking for enhanced multiferroic properties.