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Triboelectric nanogenerators (TENGs) are the most viable solution to harvest energy from low-frequency mechanical motions. Here, a triboelectric nanogenerator, an electromagnetic generator (EMG) and a piezoelectric nanogenerator (PENG) were hybridized and implemented inside a shoe sole to harvest energy from human walking. To optimize the TENG, we developed and studied three different structures (parallel, arcked and zigzag triboelectric plates) based on the contact-separation mode and suitable to be assembled in footwear. The parallel-plate structure generated the largest electrical outputs, so that the distance between triboelectric layers and the number of tribo-pair in this configuration were also optimized. This resulted in a significant increase on the output performance of the TENG and enabled the charging of different capacitors. To further enhance energy generation properties, and through an effective conjugation of triboelectrification, electromagnetic induction and piezoelectricity, we fabricated a hybridized nanogenerator that increased 20% the charging capacity of the TENG system alone. This optimized device opens new horizons for ways to produce and store wasted energy and, in a near future, to power wireless sensors or electronic gadgets.

Orthoferrites perovskites are an important family of materials in the field of nanotechnology, particularly for cathodes in solid oxide fuel cells. Doping these materials enables the enhancement and control of their properties. In this context, we studied the effect of Ti doping on different properties of the La0.7Ga0.3Fe1-xTixO3 (x = 0, 0.1, 0.2 and 0.3) lanthanum orthoferrites. X-Ray Diffraction (XRD) measurements proved the presence of two orthorhombic phases with Pbnm and Pnma space groups. SEM images corroborate the XRD results, clearly signaling the presence of two phases in these samples, with EDS analysis allowing the estimation of each phase fraction. FTIR and Raman spectroscopies allowed us to study the vibrational modes related with the La-O, Fe-O and Ti-O bonds. We note a shift towards higher wavenumber associated with the increase in Ti. Magnetic measurements showed a transition from antiferromagnetism to weak ferromagnetism with the introduction of Ti. The zero-field cooled (ZFC) and field cooled (FC) magnetization versus temperature curves do not superimpose, proving the heterogeneity of the magnetic properties of the samples. The saturation magnetization Ms, the remanent magnetization Mr and the coercive field Hc decrease with both increasing temperature and Ti content.

A triboelectric nanogenerator (TENG) is a new energy harvester that converts small scale mechanical motions into electrical energy by a combination of triboelectrification and electrostatic induction through the periodic contact-separation and/or sliding movement between two tribo-materials with different abilities of gaining or losing electrical charges. This new approach to harvest mechanical energy can produce high power outputs capable of supplying equipment and sensors deployed in remote offshore locations and of supporting offshore activities whilst being able to be used in conjunction with traditional energy harvesting technologies. This review describes the fundamentals of TENGs and the existing energy harvesting modes, with focus on those more suitable for marine applications. Moreover, the equipment and offshore activities whose energy needs can be satisfied by TENGs are described and implementation schemes presented. We conclude that TENGs have high potential for numerous maritime applications, ranging from the demand of electronics used for metocean monitoring, signalling and surveillance, to activities such as offshore aquaculture or oil and gas exploration. The advantages of such systems as an alternative to currently existing solutions are also discussed, along with insights concerning applications that can take advantage of their high efficiency harvesting low amplitude and low frequency wave energy.

The authors wish to add the following information to the acknowledgements section of theirpaper published in Nanomaterials [1] [...]

Highly ordered anodic hafnium oxide (AHO) nanoporous or nanotubes were synthesized by electrochemical anodization of Hf foils. The growth of self-ordered AHO was investigated by optimizing a key electrochemical anodization parameter, the solvent-based electrolyte using: Ethylene glycol, dimethyl sulfoxide, formamide and N-methylformamide organic solvents. The electrolyte solvent is here shown to highly affect the morphological properties of the AHO, namely the self-ordering, growth rate and length. As a result, AHO nanoporous and nanotubes arrays were obtained, as well as other different shapes and morphologies, such as nanoneedles, nanoflakes and nanowires-agglomerations. The intrinsic chemical-physical properties of the electrolyte solvents (solvent type, dielectric constant and viscosity) are at the base of the properties that mainly affect the AHO morphology shape, growth rate, final thickness and porosity, for the same anodization voltage and time. We found that the interplay between the dielectric and viscosity constants of the solvent electrolyte is able to tailor the anodic oxide growth from continuous-to-nanoporous-to-nanotubes.

Ordered arrays of gold micro-mushrooms were electrodeposited inside lithographically patterned micro-pores with around 2 μ m in diameter and 1 μ m in depth. By using a commercial electrolytic gold bath and adjusting the deposition time (3–30 min) and potential (−0.8 to −1.3 V), one was able to tune the mushrooms’ morphology. Flattened caps with large grain sizes were obtained at lower potentials and short depositions; rounded caps with smaller grains occurred at −1.0 V and longer depositions; and umbilicated caps with smoother surfaces appeared at −1.3 V. The formation of a central depression at the latter structures was ascribed to the abundant hydrogen evolution during the deposition process and to the non-uniform current density profile across the micropore. Aiming to fabricate perfectly hemispherical caps, the effect of the deposition temperature (25, 30 and 35 °C) on the micro-mushrooms’ formation at −1.0 V was also studied, allowing the estimation of different deposition rates for the cap’s height and radius. The final results provide a simple route for the fabrication of hemispherical caps by adjusting the growth conditions (time, temperature and applied potential), which is highly important for the expedite development and implementation of gold micro-mushrooms as high-performance micro-electrodes.

Resistive switching devices are being intensively studied aiming a large number of promising applications such as nonvolatile memories, artificial neural networks and sensors. Here, we show nanoscale bipolar resistive switching in Pt/Si/Ag/TiW structures, with a dielectric barrier thickness of 20 nm. The observed phenomenon is based on the formation/rupture of metallic Ag filaments in the otherwise insulating Si host material. No electroforming process was required to achieve resistive switching. We obtained average values of 0.23 V and −0.24 V for the Set and Reset voltages, respectively. The stability of the switching was observed for over 100 cycles, together with a clear separation of the ON (103 Ω) and OFF (102 Ω) states. Furthermore, the influence of the Set current compliance on the ON resistance, resistances ratio and Set/Reset voltages percentage variation was also studied.

Due to their high tunnel magnetoresistance (TMR) ratios at room temperature, magnetic tunnel junctions (MTJs) with a crystalline MgO insulating barrier and CoFeB ferromagnetic (FM) layers are the best candidates for novel magnetic memory applications. To overcome impedance matching problems in electronic circuits, the MgO barrier must have an ultra-low thickness (∼1 nm). Therefore, it is mandatory to optimize the MTJ fabrication process, in order to prevent relevant defects in the MgO barrier that could affect the magnetic and electrical MTJ properties. Here, a smoothing process aiming to decrease the roughness of the buffer surface before the deposition of the full MTJ stack is proposed. An ion beam milling process was used to etch the surface of an MTJ buffer structure with a Ru top layer. The morphologic results prove an effective decrease of the Ru surface roughness with the etching time. The electrical and magnetic results obtained for MTJs with smoothed buffer structures show a direct influence of the buffer roughness and coupling field on the improvement of the TMR ratio.

In present study, we report a facile synthesis of crystalline, small size Pd nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Pd/RGO for electrocatalytic oxidation of formic acid (FA). Here, first graphene oxide (GO) was reduced by the green method using l -ascorbic acid and citric acid and further Pd NPs were decorated on RGO by a facile method without using any reducing agents. The reduction of GO to RGO and synthesis of Pd NPs was confirmed by the X-ray diffraction (XRD) and X-ray photoelectrons (XPS) techniques. Surface morphology of Pd/RGO nanocomposite was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The electrocatalytic behavior of Pd/RGO nanocomposite was tested by using of cyclic voltammetric (CV) technique for electro-oxidation of FA in mixed solution of 0.5 M HCOOH + 0.5 H 2 SO 4 at RT. Results shows that the higher electrocatalytic activity of Pd/RGO nanocomposite compare to Pd NPs.

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωμm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (P out ) in the range of 1–40 nW. Here, P o u t values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωμm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (J STT ) was observed, without any further change in the magnetic configuration.

The control and understanding of magnetization reversal mechanisms and magnetostatic interactions in nanomagnet arrays is a critical point that has to be overcome in order to reach industrial applications. However, usual magnetic characterization techniques can only provide information on the overall array behaviour and are not able to discriminate relevant local properties. In this work, we show that the first-order reversal curve (FORC) method is a unique tool to identify weakly-interacting uniaxial single domain (SD) particles in systems with complex mixed magnetic states. We compare the FORC diagrams of two sets of Ni nanowire (NW) arrays electrodeposited in hexagonally ordered nanoporous alumina templates: A) with a uniform length distribution (interacting SD particles), and B) with a non-uniform length distribution. For non-uniform length distributions, regions of isolated NWs occur in the array, creating a complex mixture of strongly and weakly-interacting SD particles. These weakly-interacting NWs are here identified by a characteristic ridge along the coercive field axis of the FORC diagram. Micromagnetic simulations confirmed the presence of this weakly-interacting magnetic behaviour in the array with non-uniform length distributions. Combining FORC results with micromagnetic simulations, we show that the magnetization reversal of interacting wires occurs by the nucleation of transverse domain walls at the NWs' ends, while isolated wires nucleate vortex domain walls. This work thus highlights the importance of the FORC method for the accurate identification and understanding of local magnetic behaviours in nanomagnet arrays., MPP and CTS acknowledge FCT for grants SFRH/BPD/84948/2012 and SFRH/BPD/82010/2011, respectively, supported by funding POPH/FSE. JV acknowledges financial support through FSE/POPH and PTDC/CTM-NAN/3146/2014. The authors acknowledge funding from FCT through the Associated Laboratory IN and project UID/NAN/50024/2013, from FEDER and ON2 through project Norte-070124-FEDER-000070, from Spanish MINECO under project MAT2013-48054-C2-1-R and from Comunidad Madrid under project Nanofrontmag S2103/MIT-2850.

Bi2Te3 is the most commonly used thermoelectric material in modern solid-state refrigerators and power generators based on this basic principle. Due to predictions of significant improvements in their efficiency by using nanostructured materials, a thorough study on thin films and nanowires deposited by the electrodeposition method are here presented. The study of the deposition applied potential effect on the morphology, stoichiometry and crystallinity of both thin films and nanowires has been conducted. The morphology and stoichiometry was found to highly depend on the deposition potential, where by increasing it one was able to accurately control the Te% content of the deposits. X-ray diffraction measurements have shown the presence of a strong relation between the material's crystallinity and the deposition potential, where samples ranged from monocrystalline, at very low potentials, to almost completely amorphous, at high potentials. Finally, nanowire diameters were seen to diminish with the applied potential, in conjunction with the general array. Keywords: Electrodeposition, Thin films, Nanowires, Energy harvesting, Morphology

The recent realization of memristors, nanodevices exhibiting non-volatile resistive switching, has sparked tremendous interest for applications in fields such as nonvolatile memories. Here we report unipolar resistive switching in Pt/MgO/Ta/Ru structures, with an oxide barrier thickness of only 15 nm. No electroforming process was required to achieve resistive switching and an ohmic conduction mechanism is associated with the ON state. We observed an inverse dependence of the ON state resistance on the SET current compliance and average values of 1.61 V and 1.38 V for the SET and RESET voltages, respectively. We show the stability of the switching for over 40 cycles and a clear separation of the ON (10¹ Ω) and OFF (10² Ω) states during at least 10⁴ s.

Resistive switching in metal–insulator–metal nanosctructures is being intensively studied for nonvolatile memory applications. Here, we report unipolar resistive switching in Pt/MgO/Ta/Ru structures, with a 30 nm oxide barrier. A forming process was needed to initiate the resistive switching, which was then observed for all Set and Reset voltage polarity combinations. We studied the influence of the voltage polarity on the variability of the Set/Reset voltages and ON/OFF resistances and revealed the importance of a thin TaOx layer working as an oxygen revervoir for resistive switching. The mechanism behind this phenomenon can be understood in terms of conductive filaments formation/rupture with a contribution from Joule heating. Resistance change is thus caused by a voltage-driven oxygen vacancy motion in the MgO layer and a filament model was proposed for each polarity mode. A OFF/ON resistance ratio of at least 2 orders of magnitude was obtained with resistive states stable up to 104 s. Our results open the prospect to improve switching performance in other resistive switching systems, by proving a better understanding of the differences between operation modes.

A series of Zn and Co-substituted nickel ferrite nanoparticles of nominal composition Ni1−xZnxFe2−xCoxO4 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) have been synthesized by the PVA assisted sol–gel method. The reported structural and magnetic properties of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM). Also, the effect of these substitutions on vacancy distribution is investigated with positron annihilation lifetime spectroscopy (PALS). The cation distribution is inferred from the X-ray diffraction using the MAUD program to investigate ion occupancy, the exact coordinates of the atoms and unit cell dimensions. XRD analysis using the Rietveld refinement technique proves the crystalline structure and the growth of pure, single-phase and nanosize substituted nickel ferrite. These results are confirmed by the performed FESEM and FTIR studies. Magnetic measurements revealed that the values of the saturation magnetization (Ms) in the Ni1−xZnxFe2−xCoxO4 samples gradually increased for doping levels up to x = 0.4, then decreasing for further content. Also, the coercivity showed an overall decreasing trend with increasing doping. The positron lifetime spectra of the substituted nickel ferrite were decomposed into three-lifetime components, τ1, τ2, τ3. First of all, τ1, is depends on the positrons that do not get trapped by the vacancy defects. The second lifetime component, τ2, is attributed to positrons annihilation in vacancy-type defects in ferrite nanoparticles. The final lifetime, τ3, is due to the annihilation of positrons across nanoparticles in the free volume of ferrite structure. Also, magnetic behaviors could be explained by positron annihilation lifetime spectroscopy.

The exponential rise of power dissipation in electric devices has so far vastly surpassed the performance of power converter systems. New thermal management solutions are therefore in high demand to maintain devices at their optimal temperature and efficiency. Here, we present an innovative magnetically actuated thermal switch (MATS) based on nanofluids capable of controlling both the magnitude and direction of the heat flux. When in the OFF state (zero applied magnetic field), the nanofluid captures the heat being generated at the heat source. When an external magnetic field is applied (ON state), the nanofluid is transported from the heat source to the heat sink where it releases the carried heat. This work (i) provides a detailed description of the MATS operating principles, (ii) studies the effect of thickness and operation frequency in its cooling power, (iii) presents numerical simulations to forecast MATS behavior and (iv) demonstrates the MATS applicability in the thermal management of a 10 W Light Emitting Diode. The versatility, simple design, unique operating mode and high performance of the MATS suggests that the proposed approach is a promising technology to be used in a vast number of electronic devices, thermal storage systems and thermal management applications.

Triboelectric nanogenerators are a new promising energy harvesting technology that has a simple working mechanism, produces high output power and has the ability of harvesting energy from a wide range of sources and under various conditions. Here, we present a rotary triboelectric nanogenerator (TENG) that harvests energy from water flows using polytetrafluoroethylene (PTFE) and Nylon 6.6 as triboelectric materials. The operation mechanism of the fabricated rotary TENG is a hybridization of the contact and sliding modes by using the triboelectrification and electrostatic induction effects. The optimization of the developed setup for the effective generation of electricity showed that the best configuration consists in a triboelectric structure with four Nylon and one PTFE plate for the maximum used water flow of 44 L/min. With this configuration, the rotary TENG delivers a mean voltage value of ∼102.2 V, a short-circuit current density ∼120 mA/m2 and a maximum power density of ∼6.1 W/m2. Finally, with this device it was possible to light up more than 50 serial-connected light emitting diodes, fully charge a 1.0 μF capacitor with 15.2 V in just 65 s and feed a commercial temperature and humidity sensor. The developed rotary TENG can then be applied to autonomous sensors that monitor water supply systems using the energy produced by water movement in plumbing.

Novel catalyst, cobalt boride @ nickel/reduced graphene oxide abbreviated as Co-B@Ni/RGO nanocomposite was synthesized by precipitation of Co2+ ions on Ni/RGO. In the first step, Ni/RGO nanocomposite was synthesized by a facile method. In the next step, Co-B NPs were decorated on Ni/RGO support which led the superficial attachment and maximum exposure of them for catalytic reaction. The structure, composition and morphology of catalyst were fully characterized using various techniques. Co-B@Ni/RGO nanocomposite showed the high catalytic activity for hydrolysis of alkaline sodium borohydride (NaBH4) solution. Moreover, Co-B@Ni/RGO nanocomposite showed the high current response and stability of catalyst under cyclic voltammetry (CV) experiment, suggested as promising low cost nanocatalyst for future energy applications due to its high catalytic activity, charge storage capability and robustness.

A novel catalyst, palladium on nickel boride supported with reduced graphene oxide abbreviated as Pd@NixB/RGO nanocomposite was synthesized by a facile method. NixB and RGO were synthesized by a single step via the simultaneous reduction of graphene oxide (GO) and Ni2+ ions in presence of alkaline sodium borohydride (NaBH4, pH at 12.5). In the next step, Pd nanoparticles (NPs) were synthesized on the surface of NixB/RGO in presence of anhydrous methanol (CH3OH) without using any specific reducing agents, which led the formation of small Pd NPs with homogeneous size distribution, suggesting the facile and green synthesis of the nanocomposite. The structure and morphology of Pd@NixB/RGO nanocomposite were characterized by several techniques. Cyclic voltammetric (CV) measurements of Pd@NixB/RGO nanocomposite towards ethanol oxidation reaction (EOR) show their promising electrocatalytic behaviour. CV shows the high current density which was modified and increased upon cycling after 50 cycle, which indicates the high catalytic activity and CO tolerance capability when compared to the unsupported Pd@NixB NPs. Further, chronoamperometric (CA) measurements show the higher stability of the Pd@NixB/RGO nanocomposite compare to the Pd@NixB NPs.

A novel method for the synthesis of non-noble metal based catalyst, copper–nickel-reduced graphene oxide (Cu@Ni/RGO) is established using two step methodology. First, the Ni/RGO nanocomposite was synthesized by the simultaneous reduction of Ni 2+ ions and graphene oxide (GO) under reducing conditions, and after that Cu nanoparticles (Cu NPs) were encapsulated on them. We discuss the role of in-situ generated hydrogen radical (H ▪ ) through spillover of H 2 on Ni/RGO which led the formation of crystalline Cu NPs, easily. The structure, composition and morphology of Cu@Ni/RGO nanocomposite were characterized by several related techniques. Further, we explored the practical application of non-noble metal based catalyst Cu@Ni/RGO for catalysis reaction due to its high catalytic performance towards reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). It showed a remarkable high activity 90 s at room temperature (RT) with excellent stability up to 5 cycles.

In an industrial context, the re-utilization of resources is very important to monetize processes, reducing the production costs without overlooking a proper performance of the devices. For specific applications such as dye-sensitized solar cells (DSC), a high-aspect-ratio configuration of the produced TiO2 nanotubes (NTs) is essential to achieve higher conversion efficiencies. In this work, we report the effect of electrolyte re-utilization in the diameter, length and morphology of TiO2 nanotubes (NTs) produced by electrochemical anodization. A detailed study of the bias voltage effect on the anodic NTs for new and re-utilized electrolytes was performed. Also, the possibility of obtaining NT arrays with ribs morphology just by re-using the electrolyte was unveiled.

We have investigated numerically the dependence of the temperature span and optimum operating frequency of solid state magnetic refrigerators on the most important physical properties of magnetocaloric materials. We concluded that the temperature span depends inversely on the thermal conductivity of the magnetocaloric material and is proportional to the square root of its density, heat capacity and maximum adiabatic temperature variation upon the application/removal of a magnetic field ( Δ T a d m ). Also, and contrary to general belief, the increase of the thermal conductivity is not the only factor able to increase the optimum operating frequency. Reducing the density, heat capacity, Δ T a d m or broadening of the Δ T a d ( T ) curve can result in larger operating frequencies. Our results show that a compromise between the maximum achievable temperature span and optimum operating frequency must be reached, as the increase of one reduces the other.

In this work, we studied the DC electrochemical deposition of zinc oxide (ZnO) thin films on a conducting and flexible substrate, for their application in energy harvesting piezoelectric nanodevices. The deposition process was performed by varying the zinc nitrate concentration (c) in the electrolyte, its temperature (T), and the applied deposition potential (V), and subsequently tracing the influence of such parameters on the morphology (analyzed by scanning electron microscopy), crystallography (X-ray diffraction), and thickness (using the deposition current transient curves) of the ZnO thin films. The variation of the electrodeposition parameters led to the formation of different micro- and nano-structures, such as flat layers, microflowers, nanospheres, webs, and microramifications. Furthermore, the analysis of the deposited charge (by integrating the deposition current transients) illustrated an increase in the deposition rate with the increase of T and c, and a decrease of V. Finally, the maximum ZnO film thickness (~5 \(\upmu {\mathrm{m}}\)) was obtained for T = 80 °C, c = 0.1 M, and V = −1.5 V. This study provides us important tools to tune the electrochemical growth of ZnO thin films.

Substituted barium hexaferrite nanoparticles with nominal composition of BaMg x /2 Mn x /2 Co x Ti 2 x Fe 12−4 x O 19 ( x =0–0.5) were synthesized by a co-precipitation method. Then, the structural, magnetic and microwave characteristics of the representative samples were examined by employing X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and vector network analyzer (VNA). The XRD, along with FTIR evaluations confirmed the successful substitution of Mg, Mn, Co and Ti cations in the barium hexaferrite lattice. The microstructure evaluations also proved that the mean particle size decreases and typical morphologies are gradually varying from almost platelet ( x =0) to irregular shapes ( x =0.5) with increasing dopant concentration. Hysteresis loops revealed that the saturation magnetization increased up to substitution level of x =0.2, and then decreased abruptly. In addition, the coercivity exhibited a decreasing trend from 3669 Oe to 708 Oe with increasing amount of substitution. Finally, microwave measurement showed that the substituted barium hexaferrite sample with x =0.5 could be used as an efficient microwave absorption material with an appropriate absorption bandwidth of 6 GHz in the 10–16 GHz frequency range.

In this work silver-rich and sulfur-rich silver sulfide (Ag2S) thin films were fabricated using a diversified set of experimental methods, namely ion beam deposition and atmosphere- and solution-based sulfurizations. The composition of the Ag2S thin films was studied using X-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. We found that it strongly depends on the fabrication conditions, such as sulfurization time and temperature. These conditions, in turn, affect the electrical characteristics of the thin films, namely the resistivity and resistive switching. We were able to control the Ag2S stoichiometry and infer its dependence on the fabrication parameters for all the followed methods.

Electrical conductivity and Seebeck coefficient of freestanding paper of graphene oxide (GO) and GO reduced by heat treatment at 180 degrees C (rGO) are tuned by addition of carbon nanotubes (CNT). The electrical conductivity of rGO with 50% of CNT is raised to similar to 2940 S/m from similar to 613 S/m for pure rGO or similar to 34 S/m for GO with 50% of CNT without reduction. Moreover, Seebeck coefficient of rGO increases with addition of CNT from -7.04 mu V/K for pure rGO to -20.82 mu V/K for rGO with 50% of CNT, presenting n-type conductivity. As result, calculated power factor PF is enhanced with increasing CNT concentration and the highest PF similar to 1.28 mu W/(K(2)m) is obtained for rGO with 50% of CNT., O.O. and G.G. also thank Portuguese Foundation for Science and Technology (FCT) for the GrantsNo. SFRH/BPD/77704/2011 and SFRH/BPD/84419/2012, respectively, as well as UID/EMS/00481/2013-FCT and CENTRO-01-0145-FEDER-022083. C.D. is thankful to FCT for grant SFRH/BD/101661/2014. E.M.F.V. is grateful for financial support through the FCT grant SFRH/BPD/95905/2013. A.T. acknowledges funds by FEDER through Programa Operacional Factores de Competitividade - COMPETE and national funds through FCT within CICECO Project No. - FCOMP-01-0124-FEDER-037271 (FCT Ref. PEst-C/CTM/LA0011/2013) and independent researcher grant No. IF/00602/2013. Seebeck coefficient measurements were supported by FCT with the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 - Programa Operacional Competitividade e Internacionalizacao (POCI) with the reference project POCI-01-0145-FEDER-006941 and by the project PTDC/CTM - NAN/5414/2014 (POCI-01-0145-FEDER-016723). M Ferro is acknowledged for STEM analysis.

Mobile malware is growing and affecting more and more mobile users around the world. Malicious developers and organisations are disguising their malware payloads on apparently benign applications and pushing them to large app stores, such as Google Play Store, and from there to final users. App stores are currently losing the battle against malicious applications proliferation and existing malware. Detection methods based on signatures, such as those of an antivirus, are limited, new approaches based on machine learning start to be explored to surpass the limitations of traditional mobile malware detection methods, analysing not only static characteristics of the app but also its behaviour. This paper contains an overview of the existing machine learning mobile malware detection approaches based on static, dynamic and hybrid analysis, presenting the advantages and limitations of each, and a comparison between the reviewed methods. info:eu-repo/semantics/acceptedVersion

Hybrid conjugates of graphene with metallic/semiconducting nanostructures can improve the sensitivity of electrochemical sensors due to their combination of well-balanced electrical/electrocatalytic properties and superior surface-to-volume ratio. In this study, the synthesis and physical characterization of a hybrid conjugate of reduced graphene oxide and nickel nanoparticles (rGO-Ni NPs) is presented. The conjugate was further deposited onto a glassy carbon electrode as a nanocomposite film of chitosan and glucose oxidase. The electrochemical response and morphology of the films were investigated using SEM, CV, and EIS, and their applications as a glucose biosensor explored for the first time in proof-of-concept tests. The low operating potential along with the good linearity and sensitivity (up to 129 μA cm(-2) mM(-1)) found in the sub-millimolar range suggest potential applications in the self-management of hypoglycemia from blood samples or in the development of non-invasive assays for body fluids such as saliva, tears or breath.

In the present study, we report a facile and efficient method for the anchoring of cobalt nanoparticles (Co NPs) on reduced graphene oxide (RGO) sheets in presence of cationic surfactant cetyltrimethyl ammonium bromide (CTAB) and dispersing agent polyvinylpyrrolidone (PVP). Co NPs were decorated on RGO by the simultaneous reduction of Co 2+ ions and graphene oxide (GO) under reducing condition of sodium borohydride (NaBH 4 ) at room temperature (RT). The formation Co NPs and the reduction of GO sheets was confirmed by the X-ray diffraction (XRD) technique. The surface morphology was characterized by scanning electron microscopy (SEM) technique. The catalytic activity of Co/RGO nanocomposites was investigated for removal of methylene blue (MB) dye from water. Moreover, I/V characteristics of Co/RGO showed remarkable increment in current value compare to GO under applied bias range of -10 to + 10 V at RT (with typical on/off the resistive current), suggests their use for nanoelectronic device fabrication also.

It is well established that electrical methods, such as the application of a high-density (10(10)-10(11) A m(-2)) current, can be employed for the switching of magnetization in thin ferromagnetic films and hetero-structures after their patterning at the nanoscale. We find that magnetization is robustly switchable, i.e., at macroscopic scales (up to a few millimeters in lateral directions), in a single similar to 30 nm-thick layer of alpha-Fe upon discharging a capacitor through the sample. Interestingly, samples sputtered at enhanced base pressures (similar to 10(-6) mbar) exhibited much lower switching thresholds with no bias field, while their electrical conductivity is anomalously high ( higher than that in bulk of alpha-Fe). We argue that the observed enhancement of conduction relates to the mixed oxide/oxyhydroxide (FeO/FeOOH) layer formed naturally on the top of our alpha-Fe films. This provides an explanation for the magnetization switching upon discharging a capacitor in terms of the Oersted field generated by the discharge current. We analyze the ratio needed between the conductions of the FeO/FeOOH layer and the underlying alpha-Fe for the effective action of the Oersted field.

This work demonstrates the fabrication of Fe 3 O 4 nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Fe 3 O 4 -RGO nanocomposite by a green and facile method. In this work, first we synthesized the small sized uniformly dispersed Fe 3 O 4 NPs by high-temperature solution phase reaction of iron (III) acetylacetonate, Fe (acac) 3 . The formation of magnetite phase (Fe 3 O 4 ) was confirmed by the X-ray diffraction (XRD) technique. The surface morphology and size of Fe 3 O 4 NPs were evaluated by transmission electron microscopy (TEM). Then, Fe 3 O 4 NPs were dispersed with graphene oxide (GO) under reducing condition of citric acid and L-ascorbic acid. The dispersion of Fe 3 O 4 NPs with RGO was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques, both showing the well intercalation and decoration of Fe 3 O 4 NPs with graphene sheets.

This paper presents a mobile gamified application encouraging positive coping strategies for patients of serious and possibly fatal illnesses. The application concept is based on the analogy between one’s lifetime memories and future bucket lists and a journey in the sea of remembrance, traveling back and forth between past and present positive moments and future wish-lists, whilst being aware that life as water, goes on. This concept was co-designed by a team of UX researchers, engineers, and domain experts applying the principles of Eudaimonic design. The iterative development process brought to the final prototype: a multimedia diary with a gameful interface, which is thoroughly described, along with the relative heuristic and empirical evaluations, in this paper.

Novel applications for memory devices demand nanoscale flexible structures. In particular, resistive switching (RS) devices are promising candidates for wearable and implantable technologies. Here, the Pt/Si/Ag/TiW metal-insulator-metal structure was fabricated and characterized on top of flexible substrates using a straightforward microfabrication process. We also showed that these substrates are compatible with sputtering deposition. RS was successfully achieved using both commercial cellulose cleanroom paper and bacterial cellulose, and polymer (PET) substrates. The bipolar switching behavior was observed for both flat and bent (under a radius of 3.5 mm) configurations. The observed phenomenon was explained by the formation/rupture of metallic Ag filaments in the otherwise insulating Si host layer. FEDER (Fundo Europeu de Desenvolvimento Regional) funds through the COMPETE 2020 Operational Programme for Competitiveness and Internationalisation (POCI), Portugal 2020 FCT (Fundacao para a Ciencia e a Tecnologia)Portuguese Foundation for Science and Technology Ministerio da Ciencia, Tecnologia e Inovacao [POCI-010145 FEDER007274] Associated LaboratoryInstitute of Nanoscience and Nanotechnology FSE/POPHEuropean Social Fund (ESF) FCTPortuguese Foundation for Science and TechnologyEuropean Commission [SFRH/BD/101661/2014, CEECIND/00464/2017] FCT through the IN Associated Laboratory National Infrastructure Roadmap [NORTE-01-0145-FEDER-22090] [PTDC/CTM-NAN/122868/2010] [PTDC/CTM-NAN/3146/2014] [PTDC/NAN-MAT/31688/2017] [UID/CTM/50011/2019] [UID/EEA/50008/2019] [POCI-01-0145-FEDER-016623] info:eu-repo/semantics/publishedVersion

With the ever increasing power dissipation in electrical devices, new thermal management solutions are in high demand to maintain an optimal operating temperature and efficient performance. In particular, recently developed magnetically-activated thermal switches (MATSs) provide an alternative to existing devices, using the magnetic and thermal properties of superparamagnetic nanofluids to dissipate heat in a controlled manner. However, the presence of moving parts is a major drawback in these systems that must still be addressed. Herein, we present a compact and automatized MATS composed by an encapsulated superparamagnetic nanofluid and an electromagnet allowing to activate the MATS without any moving part. We investigate the effect of different temperature gradients ( 10 , 26 and 40 °C) and powers applied to the coil (6.5, 15, 25 and 39 W) on the performance of this novel MATS. The results show that the highest ( 44.4 % ) and fastest ( 0.6 °C/s) temperature variation occur for the highest studied temperature gradient. On the other hand, with increasing power, there is also an increase in the efficiency of the heat exchange process between the two surfaces. These results remove one of the main barriers preventing the actual application of magnetic thermal switches and opens new venues for the design of efficient thermal management devices.

The exchange bias effect at the interface between antiferromagnetic (AFM) and ferromagnetic (FM) layers is of paramount importance in state-of-the-art spintronic devices. However, a complete account of the physics behind exchange bias remains elusive and new effects are constantly unraveled. In particular, a bimodal distribution of blocking temperatures was recently discovered, associated with the bulk of the AFM layer and interfacial AFM/FM regions with spin-glass-like properties. Here we study exchange bias in MnIr (25, 60 A)/CoFe (50 A) bilayers annealed at high temperatures (623 K and 673 K). We observe, for all samples and annealing temperatures, the existence of a large exchange bias variation at low temperatures associated with interfacial disorder. Such variation is more significative in the thinnest samples, where it is found to be independent on annealing temperature. On the other hand, in the thickest samples the contribution of the low temperature distribution largely increases with annealing temperature, due to enhanced disorder arising from Mn diffusion. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Among the different phases of complex design processes, early design is the most dynamic and unpredictable stage since it involves a great deal of uncertainty, concurrency of activity streams, collaborative design iterations, and distributed and adaptive decision-making behaviour in response to both organizational commitments and to the occurrence of unforeseen events. This paper argues that current activity-based modelling approaches have limited ability to capture the dynamics of complex early design processes and explores novel modelling approaches. The development of an Agent Model for Planning and rEsearch of eaRly dEsign (AMPERE) aiming to capture various facets of uncertainty, iteration, collaboration and adaptation is described. The model was developed to tackle early design phases of complex systems, with the ability to deal with changes in requirements coming in and affecting the subsequent design evolution while design tasks are on-going. Initial results from agent-based simulations are presented, showing how the agent-based approach can support industrial organizations evaluating likely early design project performance and understanding complex cause–effect relationships that may affect project outcomes. Early design planning support from the agent model is demonstrated through an investigation to the likely project performance for varying levels of externally driven requirements change.

Este artigo descreve uma nova abordagem ao desenho do espaço público aberto, baseada numa análise multidimensional orientada para a tomada de decisão e para o desenvolvimento de soluções arquitetônicas assentes em evidências. Apresenta uma visão geral das variáveis mais relevantes de projeto e suas restrições, fornecendo assim informações úteis para o desenvolvimento de um desenho urbano mais sustentável, que considera os valores sociais e culturais locais. Este trabalho propõe fornecer uma ferramenta de orientação holística para o desenvolvimento de melhores propostas de projeto em ambientes urbanos contemporâneos; mais especificamente, visa caracterizar de forma síncrona os espaços urbanos através de análises multiescala e multidimensional, quantitativas e qualitativas, incorporando as contribuições da sintaxe espacial, dos estudos de vida pública, da ciência da construção e da análise ambiental/conforto em espaços públicos abertos. Várias ferramentas digitais avançadas são utilizadas para gerir os dados, gerar e testar iterativamente as diferentes propostas do projeto. A metodologia proposta se baseia em diversos testes e análises executados no processo de desenvolvimento de um novo projeto experimental desenvolvido para o Largo da Graça, uma praça urbana localizada no centro histórico de Lisboa que permitiu o teste de diferentes tipos de propostas. Este experimento gerou uma estrutura processual digital para o desenho urbano da praça, no qual se registaram todas as etapas relativas à resolução de vários problemas de projeto, incluindo os objetivos de eficiência (centralidade, conectividade, compacidade, conforto térmico, segurança, igualdade e interação). O processo abarca uma sequência de propostas testadas e comparadas, bem como as escolhas feitas em relação às informações latentes, subjacentes às condições de mudança no uso do espaço público e à adaptabilidade programática da praça portuguesa. A estratégia de projeto adotada e os exemplos extraídos da estrutura processual digital são usados para ilustrar a abordagem prática da metodologia proposta, pretendendo assim demonstrar como o vínculo entre investigação aplicada e desenho urbano sustentável pode ser o adequado para o planejamento e o desenvolvimento de melhores soluções de espaço público.

The recent realization of memristors opened the possibility to fabricate novel neuromorphic computational systems, including highly scalable and low power artificial neural networks. In fact, it has been shown that memristors can be used as an artificial synapse or to build the spiking core of an artificial neuron. The high resemblance between memristor and synaptic dynamics offers exciting possibilities in two major research fields: on one hand, memristors can be used to advance our understanding of the human brain, by supporting very-large-scale integration (VLSI) models where experiments can be performed and hypothesis tested in an in silico testbed. On the other hand, memristors have the potential to support novel advances in computing by providing the building blocks to bio-inspired computing paradigms, alternative to the von Neumann architecture, where storage and processing are supported by the same substrate. This chapter reviews the neuromorphic properties of memristors, comparing them with the key players of neuronal computations, synapses and neurons. The presentation is extended to more complex systems, where multiple computing units are combined in networks to achieve more elaborated dynamics. Emphasis is given to memristive-based associative memories, a bio-inspired content addressable memory system which relevant properties such as distributed storage and noise correction.

The extraction of multi-word relevant expressions has been an increasingly hot topic in the last few years. Relevant expressions are applicable in diverse areas such as Information Retrieval, document clustering, or classification and indexing of documents. However, relevant single-words, which represent much of the knowledge in texts, have been a relatively dormant field. In this paper we present a statistical language-independent approach to extract concepts formed by relevant single and multi-word units. By achieving promising precision/recall values, it can be an alternative both to language dependent approaches and to extractors that deal exclusively with multi-words.

The transient electrical behavior of an electrode/electrolyte interface based on an electrode micro-structured with gold protruding mushroom shapes is measured and modeled. The application of a voltage pulse train with changing frequency generates a corresponding train of current pulses with an offset current that approaches a saturation limit exponentially. The transient behavior of the electrical current was accurately reproduced using the Nernst-Planck-Poisson model and the geometric parameters of the mushroom shape. Spatiotemporal patterns of the electric field demonstrate that the current transient is caused by a slow kinetics of charges accumulating at the gold/electrolyte interface and under the micro-mushroom cap. The accumulation of charges establishes an internal electrical potential that drives a Faradic current through the mushroom/electrolyte interface. The aim of this work is to understand how this charge dynamics in the vicinity of the three-dimensional electrode surfaces is established and how it can be minimized to prevent damage to cells and tissues under electrical stimulation.The transient electrical behavior of an electrode/electrolyte interface based on an electrode micro-structured with gold protruding mushroom shapes is measured and modeled. The application of a voltage pulse train with changing frequency generates a corresponding train of current pulses with an offset current that approaches a saturation limit exponentially. The transient behavior of the electrical current was accurately reproduced using the Nernst-Planck-Poisson model and the geometric parameters of the mushroom shape. Spatiotemporal patterns of the electric field demonstrate that the current transient is caused by a slow kinetics of charges accumulating at the gold/electrolyte interface and under the micro-mushroom cap. The accumulation of charges establishes an internal electrical potential that drives a Faradic current through the mushroom/electrolyte interface. The aim of this work is to understand how this charge dynamics in the vicinity of the three-dimensional electrode surfaces is established and...

With the consistent shrinking of devices, micro-systems are, nowadays, widely used in areas such as biomedics, electronics, automobiles, and measurement devices. As devices shrunk, so too did their energy consumptions, opening the way for the use of nanogenerators (NGs) as power sources. In particular, to harvest energy from an object’s motion (mechanical vibrations, torsional forces, or pressure), present NGs are mainly composed of piezoelectric materials in which, upon an applied compressive or strain force, an electrical field is produced that can be used to power a device. The focus of this work is to simulate the piezoelectric effect in different ZnO nanostructures to optimize the output potential generated by a nanodevice. In these simulations, cylindrical nanowires, nanomushrooms, and nanotrees were created, and the influence of the nanostructures’ shape on the output potential was studied as a function of applied parallel and perpendicular forces. The obtained results demonstrated that the output potential is linearly proportional to the applied force and that perpendicular forces are more efficient in all structures. However, nanotrees were found to have an increased sensitivity to parallel applied forces, which resulted in a large enhancement of the output efficiency. These results could then open a new path to increase the efficiency of piezoelectric nanogenerators.

We report fabrication of reduced graphene oxide (rGO) films using chemical reduction by hydrazine hydrate and rGO paper-like samples using low temperature treatment reduction. Structural analysis confirms the formation of the rGO structure for both samples. Current-voltage (I-V) measurements of the rGO film reveal semiconductor behavior with the maximum current value of similar to 3 x 10(-4) A. The current for the rGO paper sample is found to be, at least, one order of magnitude higher. Moreover, bipolar resistance switching, corresponding to memristive behavior of type II, is observed in the I-V data of the rGO paper. Although precise values of the rGO film conductivity and the Seebeck coefficient could not be measured, rGO paper shows an electrical conductivity of 6.7 x 10(2)S/m and Seebeck coefficient of -6 mu V/degrees C. Thus, we demonstrate a simplified way for the fabrication of rGO paper that possesses better and easier measurable macroscopic electrical properties than that of rGO thin film. Published by AIP Publishing.

The idea to use not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10(12) Hz) emission spectroscopy and exploiting the spin-orbit interaction, we demonstrate the optical generation of electric photocurrents in metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.

We present a detailed study on domain imaging, Kerr effect magnetometry (MOKE) and magnetoresistance (MR), for a series of 20 nm Co73.8Fe16.2B10 thin films, both as-deposited (amorphous) and annealed (crystalline). By considering the two different (orthogonal) in-plane magnetization components, obtained by MOKE measurements, we were able to study the uniaxial anisotropy induced during CoFeB-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axis. MOKE magnetic imaging enabled us to observe the dominant magnetization processes, namely domain wall motion and moment rotation. These processes were correlated with the behavior of the magnetoresistance, which depends both on short-range spin disorder electron scattering and on the angle between the electrical current and the spontaneous magnetization ( M S ). A simple numerical treatment based on Stoner–Wolfarth model enables us to satisfactorily predict the magnetization behaviour observed in these films. A comparison between the results in Co73.8Fe16.2B10 films and the previous ones obtained in annealed Co80Fe20 films, show that the introduction of boron in CoFe reduces significantly the coercive and saturation fields along the easy axis (e.g. H c from ∼ 2 kA m−1 down to ∼ 0.5 kA m−1). Also, the magnetization along the hard axis saturates at lower fields. We conclude that amorphous and nanocrystalline CoFeB films show low coercive fields and abrupt switching, as well as absence of short-range spin disorder effects after switching when compared with Co80Fe20.

Electrical properties and magnetic properties of graphene oxide and reduced graphene oxide films with addition of Ni nanoparticles were investigated. It is found that reduction of GO by Hydrazine hydrate (HH) and subsequent mixing with Ni NPs yields a higher current (10(-5)A) compared to the samples where GO is first mixed with Ni NPs and further reduced (10(-6)A). At the same time, GO sample prepared by addition of Ni NPs to GO with subsequent reduction by HH presents the largest hysteresis with Mr as high as 11.8emu/g. Moreover, mixing of Ni NPs with HH before addition to GO or addition of Ni NPs to already reduced GO yields smaller values of the magnetization. Thus, the optimal procedure to obtain the magnetic response requested for particular application can be chosen. (C) 2015 Elsevier Ltd. All rights reserved.

High aspect ratio Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited inside ordered arrays of self-assembled pores (approximately 50 nm in diameter and approximately 50 μm in length) in anodic alumina templates by a potentiostatic method. The current transients monitored during each process allowed us to distinguish between NW and NT formation. The depositions were long enough for the deposited metal to reach the top of the template and form a continuous Ni film. The overfilling process was found to occur in two steps when depositing NWs and in a single step in the case of NTs. A comparative study of the morphological, structural, and magnetic properties of the Ni NWs and NTs was performed using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometry, respectively., M. P. Proença and C. T. Sousa are thankful to FCT for the doctoral and postdoctoral grants SFRH/BD/43440/2008 and SFRH/BPD/82010/2011, respectively. J. Ventura acknowledges the financial support through FSE/POPH. M Vázquez thanks the Spanish Ministry of Economia y Competitividad, MEC, under project MAT2010-20798-C05-01. J. P. Araújo also thanks the Fundação Gulbenkian for its financial support within the ‘Programa Gulbenkian de Estímulo à Investigação Científica’. The authors acknowledge the funding from FCT through the Associated Laboratory - IN and project PTDC/FIS/105416/2008.

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions (TJs) with ferromagnetic (FM) electrodes and attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier. The CIS effect is here studied in TJs with two thin (20 \AA) non-magnetic (NM) Ta electrodes inserted above and below the insulating barrier. We observe resistance (R) switching for positive applied electrical current (flowing from the bottom to the top lead), characterized by a continuous resistance decrease and associated with current-driven displacement of metallic ions from the bottom electrode into the barrier (thin barrier state). For negative currents, displaced ions return into their initial positions in the electrode and the electrical resistance gradually increases (thick barrier state). We measured the temperature (T) dependence of the electrical resistance of both thin- and thick-barrier states ($R_b$ and R$_B$ respectively). Experiments showed a weaker R(T) variation when the tunnel junction is in the $R_b$ state, associated with a smaller tunnel contribution. By applying large enough electrical currents we induced large irreversible R-decreases in the studied TJs, associated with barrier degradation. We then monitored the evolution of the R(T) dependence for different stages of barrier degradation. In particular, we observed a smooth transition from tunnel- to metallic-dominated transport. The initial degradation-stages are related to irreversible barrier thickness decreases (without the formation of pinholes). Only for later barrier degradation stages do we have the appearance of metallic paths between the two electrodes that, however, do not lead to metallic dominated transport for small enough pinhole radius., Comment: 10 pages, 3 figures