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The present study reports on the synthesis of a new alkoxysilane-bearing light-responsive cinnamyl group and its application as a surface functionalization agent for the development of SiO2 nanoparticles (NPs) with photoreversible tails. In detail, cinnamic acid (CINN) was activated with N-hydroxysuccinimide (NHS) to obtain the corresponding NHS-ester (CINN-NHS). Subsequently, the amine group of 3-aminopropyltriethoxysilane (APTES) was acylated with CINN-NHS leading to the generation of a novel organosilane, CINN-APTES, which was then exploited for decorating SiO2 NPs. The covalent bond to the silica surface was confirmed by solid state NMR, whereas thermogravimetric analysis unveiled a functionalization degree much higher compared to that achieved by a conventional double-step post-grafting procedure. In light of these intriguing results, the strategy was successfully extended to naturally occurring sepiolite fibers, widely employed as fillers in technological applications. Finally, a preliminary proof of concept of the photoreversibility of the obtained SiO2@CINN-APTES system has been carried out through UV diffuse reflectance. The overall outcomes prove the consistency and the versatility of the methodological protocol adopted, which appears promising for the design of hybrid NPs to be employed as building blocks for photoresponsive materials with the ability to change their molecular structure and subsequent properties when exposed to different light stimuli.

The force (F) and the power consumption (P) of a magnetic actuator are modeled, measured and optimized in the context of developing micro-actuators for large arrays, such as in portable tactile displays for the visually impaired. We present a novel analytical approach complemented with finite element simulation (FEM) and experiment validation, showing that the optimization process can be performed considering a single figure of merit F/√P. The magnetic actuator is a disc-shaped permanent magnet displaced by planar microcoil. Numerous design parameters are evaluated, including the width and separation of the coil traces, the trace thickness, number of turns and the maximum and minimum radius of the coil. We obtained experimental values of F/√P ranging from 2 to 12 mN/√W using up to 2-layer coils of both microfabricated and commercial printed circuit board (PCB) technologies. This performance can be further improved by a factor of two by adopting a 6-layer technology. The method can be applied to a wide range of electromagnetic actuators.

Wind and fluid flow represent some of the most attractive renewable energy sources for addressing climate change, pollution and energy insecurity issues. Wind harvesting technologies, in particular, are the fastest-growing electric technologies in the world because of their efficiency and lower environmental impact with respect to traditional energy sources, despite exhibiting major drawbacks such as big infrastructure investment and environment invasiveness, producing high levels of noise and requiring the need of large areas for their installation. A single wind turbine can produce megawatts of power and they have the potential to cover the entire world's energy demand in the next few years, but they have a technological limit in a cut-in wind speed of about 4 m s-1, below which the turbines do not operate, excluding them as an energy source for slow air flows. At the same time most of the wind available in the environment is below the turbines' threshold speed. In this paper we show that small flags, made by piezoelectric thin film on flexible polymers and whose shape resembles the dry leaves of trees, can efficiently act as harvesters of energy from wind at extremely low speed, such as from a gentle blow or breath. We demonstrate that piezoelectricity on flexible polymers is achievable by depositing a thin film of piezoelectric aluminium nitride (AlN), sandwiched between metal electrodes with columnar grains coherent through the polycrystalline layers, on Kapton substrates. The prototype flags have a natural curling due to the release of the residual stress of the layers. While the curling is essential for the activation of the maximum flag oscillation, this system is so elastic and light that oscillations start at a cut-in flow speed of 0.4 m s-1, the lowest reported so far, with an open circuit peak to peak voltage of 40 mV. The voltage increases to 1.2 V when the flag is flattened and parallel to the fluid flow lines, with a generated power of 0.257 mW cm-3. This journal is

In this work the micro-fabrication of flexible MicroElectroMechanical transducers based on the piezoelectric effect is reported. We developed the technological protocol to realize a piezoelectric transducer composed by a Molybdenum (Mo) top electrode, the Aluminum Nitride active layer and a Mo bottom electrode on a polymeric tape. The process starts from the DC sputtering deposition of the Molybdenum layer at room temperature on Kapton HN. The Molybdenum is chosen not only for its electrical properties but also because it enhances the crystal orientation of AlN. The next step is the deposition of AlN that occurs at high temperature, around 250^oC. Temperature and physical sputtering enhanced by applying a DC bias on the substrates are two important parameters to improve the crystal orientation of the film. These extreme growth conditions guarantee a very good crystal structure without damaging the Kapton substrate. Then a final Mo layer is sputtered at room temperature. SU8-25 thick photo resist is used to define the top electrode and the AlN layer, and in a second mask step the Mo bottom electrode. The developer, the PG remover and SU8 negative resist itself have shown a chemical compatibility with Kapton HN. We measured the piezoelectric response on a capacitor test structure: through the Dynamic Mechanical Analyzer we applied controlled forces, and at the same time, by an LCR meter we performed measurements of the capacitance.

Graphical abstractDisplay Omitted Highlights? In this work we fabricate flexible piezoelectric cantilevers. ? We provide the optimized process for AlN smart integration on Kapton. ? The resonance frequency is characterized by piezoresponse AFM. ? The predicted frequency of flexible cantilever is obtained by Comsol Multiphysics. In this work we present for the first time the fabrication and the characterization of flexible micro cantilevers based on Aluminum Nitride (AlN) as piezoelectric active layer and polyimide as elastic substrate. The AlN thin film, embedded into two layers of Molybdenum (Mo), is grown by sputtering deposition and presents highly c-axis oriented hexagonal crystal structure. The flexible structures are successfully realized by a two masks process, exploiting a silicon support to perform device key fabrication steps together with optimized processes for peeling off and patterning of the flexible layer. The realized flexible cantilevers present a bending downwards because of the residual compressive stress of the Mo/AlN/Mo multilayer on polyimide. The mechanical response of the realized flexible cantilevers has been investigated by piezoresponse measurements and the experimentally obtained first resonance frequency resulted to be around 15kHz. This value has been compared with simulations of the structures performed by finite element method.

This work presents modeling, fabrication and characterization of planar microcoils for wireless power transfer in medical implanted devices, proposing integrated technology as a way to reduce the dimensions and achieve higher efficiency. The wireless power transfer (WPT) architecture is composed by: a primary coil carrying the alternating current signal to generate a magnetic field and the receiving coil to convert magnetic field in current, that is located parallel to the primary one with a gap between the two inductors. In the proposed design the two microcoils are circular with short-circuited turns to minimize electrical resistance. The electrical measurements on the fabricated test structures with different pitch sizes show a dramatic reduction of coil resistance to few ohms with respect to classic coil design. The experimental values of the resistance and inductance (from 24 to 45 nH) are in good agreement with the analytical model. The efficiency of fabricated microcoils for wireless power transfer is predicted by Finite element method (FEM) at 10 kHz modeling in terms of coupling factor ranging the distance of the inductors from 0 to 1 mm. FEM results show that the transfer efficiency can be further enhanced by the introduction of a ferromagnetic material on the back side of each coil in order to confine the magnetic field. High coupling factor above 65% can be achieved with this shielding layer, even with lowest pitch value and high coil distances.

Laser ablation technique is employed in order to generate polydimethylsiloxane (PDMS)/Ag NPs in situ, starting from a silver target in a solution of PDMS prepolymer and toluene. The produced surfactant-free nanoparticles are characterized by high resolution transmission electron microscopy (HRTEM) and scanning TEM-high angle annular dark field (STEM-HAADF) imaging modes, showing the majority of them to be of the order of 4 nm in diameter with a small percentage of larger Ag-AgCl multidomain NPs, embedded into a PDMS matrix. Low concentrations of carbon onion-like nanoparticles or larger fibers are also formed in the toluene-PDMS prepolymer solution. In accordance with this, UV-vis spectra shows no peak from silver NPs; their small size and their coverage by the PDMS matrix suppresses the signal of surface plasmon absorption. Inductively coupled plasma measurements reveal that the concentration of silver in the polymer is characteristically low, ~0.001% by weight. The electrical properties of the PDMS nanocomposite films are modified, with current versus voltage (I-V) measurements showing a low current of up to a few tenths of a pA at 5 V. The surface resistivity of the films is found to be up to ~10(10) Ω/sq. Under pressure (e.g. stress) applied by a dynamic mechanical analyzer (DMA), the I-V measurements demonstrate the current decreasing during the elastic deformation, and increasing during the plastic deformation.

The integration of a polycrystalline material such as aluminum nitride (AlN) on a flexible substrate allows the realization of elastic tactile sensors showing both piezoelectricity and significant capacitive variation under normal stress. The application of a normal stress on AlN generates deformation of the flexible substrate on which AlN is grown, which results in strain gradient of the polycrystalline layer. The strain gradient is responsible for an additional polarization described in the literature as the flexoelectric effect, leading to an enhancement of the transduction properties of the material. The flexible AlN is synthesized by sputtering deposition on kapton HN (poly 4,4'-oxydiphenyl pyromellitimide) in a highly oriented crystal structure. High orientation is demonstrated by X-ray diffraction spectra (FWHM = 0.55° of AlN (0002)) and HRTEM. The piezoelectric coefficient d(33) and stress sensitive capacitance are 4.7 ± 0.5 pm V(-1) and 4 × 10(-3) pF kPa(-1), respectively. The parallel plate capacitors realized for tactile sensing present a typical dome shape, very elastic under applied stress and sensitive in the pressure range of interest for robotic applications (10 kPa to 1 MPa). The flexibility of the device finalized for tactile applications is assessed by measuring the sensor capacitance before and after shaping the sensing foil on curved surfaces for 1 hour. Bending does not affect sensor's operation, which exhibits an electrical Q factor as high as 210, regardless of the bending, and a maximum capacitance shift of 0.02%.

We report a new approach to fabricate suspended composite membranes consisting of carbon-coated iron nanopowder uniformly dispersed in a polydimethylsiloxane matrix. The approach is based on the use of selectively photopolymerized thick SU-8 layer wherein the unexposed to ultraviolet light area represents the sacrificial layer. Circular composite membranes of different sizes are designed, fabricated, and characterized. The membranes' deflections upon the application of an external magnetic field, measured by optical microscopy, show a quadratic dependence on the diameter, in agreement with the theory. Tensile tests proved that the fillers do not affect the elastic properties of the matrix, since the Young's modulus of the composite material is similar with the one of the pure polymer. The versatile approach presented in this work allows to easily and rapidly fabricate suspended composite membranes that provide diaphragm materials with excellent performances, which can find broad applicability in microfluidic applications (valves, micro-pumps) and in MEMS technology (tactile displays, mobile devices).

This work presents a promising ultrasound wearable technology based on a piezoelectric transducer, realized on flexible highly oriented Aluminum Nitride, with significant mechanical displacement in spite of being attached on a rigid support. Circular membranes with different radius sizes, based on 1-μm-thick AlN thin film as the active piezoelectric layer, are designed, fabricated, and characterized. The AlN is deposited on kapton HN substrate with a low sputtering deposition temperature that allows the integration and the compatibility with flexible electronics. Mechanical and thermal stability of kapton makes this polyimide based sheet a potential substrate for flexible piezoelectric thin film technology. The actuation at low voltage (1-10 V) of the AlN membranes is studied in air in the range of ultrasound frequencies, from 0 Hz up to 2 MHz; the voltage amplitude, the shape and displacement of the flexure mode (0, 1) is studied by a Laser Doppler Vibrometer to characterize the mechanical properties of the device. © 2014 Elsevier B.V. All rights reserved.

Tertiary amines are useful for the production of high purity low-oxygen content R2Zn dopant sources. Trace oxygen levels in Me2Zn are significantly reduced by formation of the Me2Zn.NEt3 adduct, which is a more controllable and reproducible source than Me2Zn/H2 gas mixtures. Me2Zn and Et2Zn form 2∶1 adducts with the macrocyclic tertiary amines 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (N4-aza crown) and 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane (N6-aza crown), and the structure of these compounds has been determined by single crystal x-ray crystallography. Mild thermal dissociation of R2Zn-aza crown adducts yields base-free R2Zn, giving them potential applications in the purification of Me2Zn and Et2Zn dopant sources for the MOVPE of III–V materials.

This paper presents a new AlN-based MEMS devices suitable for vibrational energy harvesting applications. Due to their particular shape and unlike traditional cantilever which efficiently harvest energy only if subjected to stimulus in the proper direction, the proposed devices have 3D generation capabilities solving the problem of device orientation and placement in real applications. Thanks to their particular shape, the realized devices present more than one fundamental resonance frequencies in a range comprised between 500 Hz and 1.5 kHz, with a voltage generation higher than 300?V and an output power up to 0.4 pW for single MEMS device.

In this work we investigate the influence of the substrate resistivity and of different buffer layers on the out-of-band insertion losses in AIN SAW devices fabricated on (100) silicon substrates. The measurements have shown that the out-of-band insertion loss is improved by increasing the substrate resistivity but also by the introduction of an insulating buffer layer material. An out-of-band insertion loss as low as -75 dB has been achieved by the device realized on a silicon nitride buffer layer. This can be explained in terms of inhibition of the diffusion of silicon atoms in the AIN film. Our results show that silicon nitride represents a good solution in terms of low out-of-band insertion loss and cost effectiveness for the realization of nitride-based piezoelectric MEMS devices on silicon substrates. (c) 2007 Elsevier B.V. All rights reserved.

In this paper, we propose a flexible piezoelectric MEMS transducer based on aluminum nitride thin film grown on polyimide soft substrate and developed for tactile sensing purposes. The proposed device consists of circular micro-cells, with a radius of 350 μm, made of polycrystalline c-axis textured AlN. The release of compressive stress by crystalline layers over polymer substrate allows an enhanced transduction response when the cell is patterned in circular dome-shaped geometries. The fabricated cells show an electromechanical response within the full scale range of 80 mN (200 kPa) both for dynamic and static load. The device is able to detect dynamic forces by exploiting both piezoelectric and flexoelectric capabilities of the aluminum nitride cells in a combined and synergistic sensing that occurs as voltage generation. No additional power supply is required to provide the electrical readout signals, making this technology suitable candidate when low power consumption is demanding. Moreover a capacitance variation under constant stress is observed, allowing the detection of static forces. The sensing ability of the AlN-based cells has been tested using an ad hoc setup, measuring both the applied load and the generated voltage and capacitance variation.

In this work we have fabricated and characterized GaN-based surface acoustic wave filters grown by metal organic chemical vapour deposition (MOCVD) on sapphire substrates. The acoustic wave propagation velocity and piezoelectric electromechanical coupling constant have been determined for different wavelengths. The acoustic velocity and high order propagation mode have been analyzed for different penetration levels of the acoustic wave in the sapphire substrate. The RF filters exhibit an acoustic velocity around 5000 m/s, which is independent from the GaN layer resistivity. In contrast, a strong improvement of the noise level and propagation loss is found in devices fabricated on high resistive GaN. A noise level as low as −80 dB was measured on prototype devices, which is of the order of the instrument’s intrinsic noise. The fundamental Rayleigh mode is accompanied by a high order mode, which is known as the Sezawa mode. The propagation velocity of the Sezawa acoustic wave, ascribed to the presence of sapphire substrate, was almost twice that of the Rayleigh wave, which makes it very interesting for very high frequency applications.

In this work we have fabricated and characterized GaN based surface acoustic wave (SAW) delay lines grown by metal organic chemical vapor deposition (MOCVD) on sapphire substrate. The acoustic wave velocity of 0th Rayleigh and Sezawa modes, and the piezoelectric electromechanical coupling constant have been measured for different wave numbers in a 2 mum-thick layer. The acoustic velocity resulted to be independent from the layer resistivity, which strongly affects the noise level. Through the introduction of a highly resistive GaN buffer layer, a noise level as low as -70 dB has been measured. This result has been attributed to a reduced coupling between the input and output terminals.

Force sensors are key components in a wide range of high-controlled environments and operations. Force sensors based on piezoelectric transduction mechanism are particular attractive since they are able to provide electrical signals even without power supply. Among the most investigated materials for force sensing, aluminum nitride (AlN) has the peculiar properties of high isolation and the highest thermal conductivity. Despite the piezoelectric coefficient d33 is moderate, AlN thin films integrated on polymeric substrates improve the piezoelectric response of the sensor as a whole. This work presents a promising technology based on piezoelectric highly c-axis oriented aluminum nitride integrated on Kapton substrate, which provides a self-induced three-dimensional structure dome shaped. The flexible piezoelectric transducers realized by this technology detect normal contact forces in the load range of interest for robotics application, with an improvement of the output voltages with respect to similar sensors based on silicon.