Katarzyna Bejtka

Nanoconfinement: an effective way to enhance PVDF piezoelectric properties.

The dimensional confinement and oriented crystallization are both key factors in determining the piezoelectric properties of a polymeric nanostructured material. Here we prepare arrays of one-dimensional polymeric nanowires showing piezoelectric features by template-wetting two distinct polymers into anodic porous alumina (APA) membranes. In particular, poly(vinylidene fluoride), PVDF, and its copolymer poly(vinylidene fluoride-trifluoroethylene), PVTF, are obtained in commercially available APA, showing a final diameter of about 200 nm and several micrometers in length, reflecting the templating matrix features. We show that the crystallization of both polymers into a ferroelectric phase is directed by the nanotemplate confinement. Interestingly, the PVDF nanowires mainly crystallize into the β-phase in the nanoporous matrix, whereas the reference thin film of PVDF crystallizes in the α nonpolar phase. In the case of the PVTF nanowires, needle-like crystals oriented perpendicularly to the APA channel walls are observed, giving insight on the molecular orientation of the polymer within the nanowire structure. A remarkable piezoelectric behavior of both 1-D polymeric nanowires is observed, upon recording ferroelectric polarization, hysteresis, and displacement loops. In particular, an outstanding piezoelectric effect is observed for the PVDF nanowires with respect to the polymeric thin film, considering that no poling was carried out. Current versus voltage (I-V) characteristics showed a consistent switching behavior of the ferroelectric polar domains, thus revealing the importance of the confined and oriented crystallization of the polymer in monodimensional nanoarchitectures.

Ultrasensitive Ag-coated TiO2 nanotube arrays for flexible SERS-based optofluidic devices

In this study, a novel SERS sensor has been developed for repeatable detection of organic molecules and biological assays. Vertically oriented titania nanotube (TiO2 NT) arrays were grown by ultra-fast anodic oxidation of flexible titanium foils and then decorated with Ag nanoparticles (NPs) through d.c. sputtering deposition at room temperature. A parametric study was carried out taking into account the effect of sputtering parameters on the Ag NP arrangements on the NT surface. The structure morphology was investigated by means of scanning and transmission electron microscopy, evidencing the formation of hexagonal close-packed TiO2 NTs coated with Ag nanoparticles showing tunable diameter and distribution. The substrates were employed in a SERS optofluidic device, consisting of a polydimethylsiloxane cover irreversibly sealed to the silver-coated TiO2 NTs, able to detect Rhodamine molecules in ethanol over a wide range of concentrations down to 10−14 M, taking advantage of both electromagnetic and chemical enhancements. In order to evaluate the performances of the SERS substrates in terms of biosensing, an optimized protocol for the immobilization of oligonucleotide probes on the metal-dielectric surfaces was developed for verifying the hybridization events.

One-Dimensional ZnO/Gold Junction for Simultaneous and Versatile Multisensing Measurements

The sensing capabilities of zinc oxide nano/micro-structures have been widely investigated and these structures are frequently used in the fabrication of cutting-edge sensors. However, to date, little attention has been paid to the multi-sensing abilities of this material. In this work, we present an efficient multisensor based on a single zinc oxide microwire/gold junction. The device is able to detect in real time three different stimuli, UV-VIS light, temperature and pH variations. This is thanks to three properties of zinc oxide its photoconductive response, pyroelectricity and surface functionalization with amino-propyl groups, respectively. The three stimuli can be detected either simultaneously or in a sequence/random order. A specific mathematical tool was also developed, together with a design of experiments (DoE), to predict the performances of the sensor. Our micro-device allows reliable and versatile real-time measurements of UV-VIS light, temperature and pH variations. Therefore, it shows great potential for use in the field of sensing for living cell cultures.

Inkjet-printed PEDOT:PSS electrodes on plasma-modified PDMS nanocomposites: Quantifying plasma treatment hardness

Nanostructured polymeric composites are promising materials for the fabrication of piezoresistive devices because they show a huge variation in electrical resistance when subjected to mechanical deformation. Quantum tunneling composites feature a conduction mechanism occurring between the metallic filler and copper particles embedded in a polydimethylsiloxane (PDMS) insulating matrix, and the mechanism is enhanced by the spiky morphology of the particles. PEDOT:PSS electrodes are patterned on either side of the composite by inkjet printing, a technology that allows one-step fabrication processes. The adhesion and spreading of conductive printed ink drops are controlled and enhanced by pre-treating the samples surface in an atmospheric pressure plasma customized system. Because of an extremely high metal to polymer ratio, which results in the different surface and dielectric properties of the composite, conventional plasma conditions are not suitable to allow the control of spreading. The optimal plasma conditions for ink/surface compatibility were found using quantitative comparison based on image analysis and numerical interpretation of the adhesion/roughness properties such as bulging and spread.

Nanobranched ZnO Structure: p‐Type Doping Induces Piezoelectric Voltage Generation and Ferroelectric–Photovoltaic Effect

Dr. M. Laurenti, Dr. G. Canavese, Dr. A. Sacco, Dr. M. Fontana, Dr. K. Bejtka, Dr. M. Castellino, Prof. C. F. Pirri, Dr. V. Cauda Center for Space Human Robotics@PoliTo Istituto Italiano di Tecnologia C.so Trento 21 , 10129 Turin , Italy E-mail: valentina.cauda@iit.it Dr. G. Canavese, Dr. M. Fontana, Prof. C. F. Pirri Department of Applied Science and Technology Politecnico di Torino C.so Duca degli Abruzzi 24 , 10129 Turin , Italy

Comprehensive study of the templating effect on the ZnO nanostructure formation within porous hard membranes

ZnO nanowires were synthesized by combining the template-assisted method with three different ZnO growth approaches, i.e. sol–gel, aqueous chemical growth, and electrodeposition. We obtained nanostructures of 200, 50, and even 5 nm diameter in porous alumina and ordered mesoporous silica membranes, showing effective filling of the template channels and the formation of oriented ZnO nanostructures.

Ultraviolet mem-sensors: flexible anisotropic composites featuring giant photocurrent enhancement

By using two separate components, mem-sensing devices can be fabricated combining the sensitivity of a transducer with non-volatile memory. Here, we discuss how a mem-sensor can be fabricated using a single material with built-in sensing andmemory capabilities, based on ZnO microwires (MWs) embedded in a photocurable resin and processed from liquid by vertically aligning the MWs across the polymeric matrix using dielectrophoresis. This results in an ultraviolet (UV) photodetector, a device that is widely applied in fields such as telecommunication, health, and defense, and has so far implemented using bulk inorganic semiconductors. However, inorganic detectors suffer from very high production costs, brittleness, huge equipment requirements, and low responsivity. Here, we propose for the first time aneasy processable, reproducible, and low-cost hybrid UV mem-sensor. Composites with aligned ZnO MWs produce giant photocurrentscompared to the same composites with randomly distributed MWs. In particular, we efficiently exploit a mem-response where the photocurrent carries memory of the last electronic state experienced by the device when under testing. Furthermore, we demonstrate the non-equivalence of different wave profiles used during thedielectrophoresis: a pulsed wave is able to induce order in both the axis and the orientation of the MWs, whereas a sine wave only affects the orientation.

Synthesis of ferroelectric BaTiO3 tube-like arrays by hydrothermal conversion of a vertically aligned TiO2 nanotube carpet

In this study, the conversion of TiO2 nanotubes (NTs) into barium titanate (BaTiO3) exploiting a two-step low-temperature synthesis method is reported. Vertically oriented TiO2 NT arrays were grown by ultra-fast anodic oxidation of titanium foils in ammonium fluoride-based electrolytic solution and then converted into BaTiO3 exploiting a shape-preserving hydrothermal treatment. A parametric study was carried out taking into account the effect of alkalinity, temperature and reaction time on the converted materials. The crystalline phase and morphology of the nanostructured material were investigated by means of X-ray diffraction, scanning and transmission electron microscopy, evidencing the formation of ordered polycrystalline BaTiO3 arrays. The tetragonal phase of the obtained material was revealed by Raman spectroscopy and its ferroelectric behavior was confirmed by domain switching observation during electrical characterization. The tube-like nanostructured arrays of BaTiO3 could open the way for new applications of energy harvesting materials where easy and low temperature processing, controlled morphology and functional properties are required.

Ionic liquid-enhanced soft resistive switching devices

Resistive switching phenomena are of paramount importance in the area of memory devices. In the present study, we have fabricated a simple resistive switching device using a solution processable nanocomposite based on silver nitrate and poly(vinylidene fluoride-hexafluoropropylene). The change in resistance is ascribed to an initial ionic conduction, followed by a non-continuous field induced filament formation. The switching device fabricated with the above-mentioned active matrix displayed a volatile switching behavior. The addition of room temperature ionic liquid plays a fundamental role in triggering permanent memory and reducing the set voltage range up to ten-fold. The change in switching behavior with respect to the applied voltage bias and compliance level set during electrical characterization was studied thoroughly. The present work also gives a glimpse into the importance of device architecture on resistive switching phenomena.

Low-temperature atomic layer deposition of TiO2 thin layers for the processing of memristive devices

Atomic layer deposition (ALD) represents one of the most fundamental techniques capable of satisfying the strict technological requirements imposed by the rapidly evolving electronic components industry. The actual scaling trend is rapidly leading to the fabrication of nanoscaled devices able to overcome limits of the present microelectronic technology, of which the memristor is one of the principal candidates. Since their development in 2008, TiO2 thin film memristors have been identified as the future technology for resistive random access memories because of their numerous advantages in producing dense, low power-consuming, three-dimensional memory stacks. The typical features of ALD, such as self-limiting and conformal deposition without line-of-sight requirements, are strong assets for fabricating these nanosized devices. This work focuses on the realization of memristors based on low-temperature ALD TiO2 thin films. In this process, the oxide layer was directly grown on a polymeric photoresist, thus...