Ignazio Roppolo

3D Printing of Conductive Complex Structures with in Situ Generation of Silver Nanoparticles

Coupling the photoreduction of a metal precursor with 3D-printing technology is shown to allow the fabrication of conductive 3D hybrid structures consisting of metal nanoparticles and organic polymers shaped in complex multilayered architectures. 3D conductive structures are fabricated incorporating silver nitrate into a photocurable oligomer in the presence of suitable photoinitiators and exposing them to a digital light system.

Silver nanoparticle ink technology: state of the art

Printed electronics will bring to the consumer level great breakthroughs and unique products in the near future, shifting the usual paradigm of electronic devices and circuit boards from hard boxes and rigid sheets into flexible thin layers and bringing disposable electronics, smart tags, and so on. The most promising tool to achieve the target depends upon the availability of nanotechnology-based functional inks. A certain delay in the innovation-transfer process to the market is now being observed. Nevertheless, the most widely diffused product, settled technology, and the highest sales volumes are related to the silver nanoparticle-based ink market, representing the best example of commercial nanotechnology today. This is a compact review on synthesis routes, main properties, and practical applications.

Radical diffusion engineering: tailored nanocomposite materials for piezoresistive inkjet printed strain measurement

We present a comprehensive study related to UV-curable nanocomposite (NC) materials, based on acrylic matrix containing Ag nanoparticles (NPs) formed by in situ reduction and co-formulated with titania NPs. Addition of titania produces a diffusion limited aggregation of in situ formed Ag NPs during photocuring due to radical propagation, allowing to obtain electromechanical percolation at very low solid content. Keeping low the solid content is important, considering the cost of raw materials. Compared to NCs based on spherical fillers, where percolation is reached at very high solid contents (around 70%), by radical engineering we could approach it by adding 5 to 30% of Ag precursor (Ag content 2 to 10%). These NCs are characterized by a low viscosity at room temperature, allowing full processability by means of inkjet printing (IjP), as well as good electrical properties after curing, ranging from metallic to dissipative, in their annealed state. We present morphological, chemo-physical and electrical characterisation, as well as outstanding piezoresistive properties of these materials in the thin film state and after direct patterning by means of IjP. The goal was to realize low cost printed strain-gages featuring improved characteristics when compared to available commercial products. We obtain diffusion-engineered unstructured materials featuring gauge factors (GF) as high as 13.4, corresponding to a seven-fold increase with respect to commercial metallic alloys. Measurements performed on structured NC IjP strain gauges produce GF up to 220, corresponding to a hundred-fold increase in comparison with commercial devices.

3D Printed PEG-Based Hybrid Nanocomposites Obtained by Sol–Gel Technique

In this work, three-dimensional (3D) structured hybrid materials were fabricated combining 3D printing technology with in situ generation of inorganic nanoparticles by sol-gel technique. Those materials, consisting of silica nanodomains covalently interconnected with organic polymers, were 3D printed in complex multilayered architectures, incorporating liquid silica precursors into a photocurable oligomer in the presence of suitable photoinitiators and exposing them to a digital light system. A post sol-gel treatment in acidic vapors allowed the in situ generation of the inorganic phase in a dedicated step. This method allows to build hybrid structures operating with a full liquid formulation without meeting the drawbacks of incorporating inorganic powders into 3D printable formulations. The influence of the generated silica nanoparticle on the printed objects was deeply investigated at macro- and nanoscale; the resulting light hybrid structures show improved mechanical properties and, thus, have a huge potential for applications in a variety of advanced technologies.

Luminescence thermochromism of acrylic materials incorporating copper iodide clusters

By incorporating molecular copper iodide clusters of formula [Cu4I4L4] (L = PPh2(CH2)2CH3) into an UV-polymerizable acrylic resin, namely BEDA (Bisphenol-a-EthoxylateDiAcrylate), transparent and highly emitting photoluminescent composite materials have been synthesized. In these materials, the original luminescence properties of the copper iodide cluster and the transparency and processability of the acrylic matrix are combined. Study of the photopolymerization kinetics shows that the clusters incorporated in low concentration have limited influence on the polymerization reaction leading to a highly cross-linking polymeric matrix. These composite materials exhibit thermochromic luminescence properties with intense emissions varying with the temperature. A perfectly controlled luminescence thermochromism is observed due to a ‘protecting effect’ of the matrix preventing the non-radiative phenomenon of the cluster luminescence. The patterning of these UV-polymerizable photoluminescent films has been also realized by the UV-NIL technique to optimize the light-emitting properties of these materials. The surface patterning acts as a diffraction grating to extract the light which was previously guided inside the film. Moreover, the nanopatterning allows tuning of the emission color of the film as a function of the viewing angle. These composite materials present potential applications as photoactive systems with emission wavelength sensitive to the temperature and the surface nanostructuration.

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.

In Situ Thermal Generation of Silver Nanoparticles in 3D Printed Polymeric Structures

Polymer nanocomposites have always attracted the interest of researchers and industry because of their potential combination of properties from both the nanofillers and the hosting matrix. Gathering nanomaterials and 3D printing could offer clear advantages and numerous new opportunities in several application fields. Embedding nanofillers in a polymeric matrix could improve the final material properties but usually the printing process gets more difficult. Considering this drawback, in this paper we propose a method to obtain polymer nanocomposites by in situ generation of nanoparticles after the printing process. 3D structures were fabricated through a Digital Light Processing (DLP) system by disolving metal salts in the starting liquid formulation. The 3D fabrication is followed by a thermal treatment in order to induce in situ generation of metal nanoparticles (NPs) in the polymer matrix. Comprehensive studies were systematically performed on the thermo-mechanical characteristics, morphology and electrical properties of the 3D printed nanocomposites.

3D printable light-responsive polymers

New photo-curable polymers for 3D printing are provided, exhibiting mechanical light-responsivity upon laser irradiation. Azobenzene moieties are employed both as dyes in the 3D printing process and as active groups providing the desired light responsivity. The incorporation of azobenzene units into polymeric matrices allows a reversible and controllable change of the Youngs modulus of a crosslinked micrometric structure. Depending on the temperature of operation, laser irradiation induces either a decrease (photo-softening) or an increase (photo-hardening) of the Youngs modulus. Such a behaviour can be spatially controlled in order to locally modify the mechanical features of 3D printed objects such as microcantilevers.

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.