Luana Persano

Optimization of electrospinning techniques for the realization of nanofiber plastic lasers

Electrospinning technologies for the realization of active polymeric nanomaterials can be easily up-scaled, opening perspectives to industrial exploitation, and due to their versatility they can be employed to finely tailor the size, morphology and macroscopic assembly of fibers as well as their functional properties. Light-emitting or other active polymer nanofibers, made of conjugated polymers or of blends embedding chromophores or other functional dopants, are suitable for various applications in advanced photonics and sensing technologies. In particular, their almost onedimensional geometry and finely tunable composition make them interesting materials for developing novel lasing devices. However, electrospinning techniques rely on a large variety of parameters and possible experimental geometries, and they need to be carefully optimized in order to obtain suitable topographical and photonic properties in the resulting nanostructures. Targeted features include smooth and uniform fiber surface, dimensional control, as well as filament alignment, enhanced light emission, and stimulated emission. We here present various optimization strategies for electrospinning methods which have been implemented and developed by us for the realization of lasing architectures based on polymer nanofibers. The geometry of the resulting nanowires leads to peculiar light-scattering from spun filaments, and to controllable lasing characteristics.

Electrospun Conjugated Polymer/Fullerene Hybrid Fibers: Photoactive Blends, Conductivity through Tunneling-AFM, Light Scattering, and Perspective for Their Use in Bulk-Heterojunction Organic Solar Cells

Hybrid conjugated polymer/fullerene filaments based on MEH-PPV/PVP/PCBM were prepared by electrospinning, and their properties were assessed by scanning electron, atomic and lateral-force, tunneling, and confocal microscopies, as well as by attenuated-total-reflection Fourier transform infrared spectroscopy, photoluminescence quantum yield, and spatially resolved fluorescence. Highlighted features include the ribbon shape of the realized fibers and the persistence of a network serving as a template for heterogeneous active layers in solar cell devices. A set of favorable characteristics is evidenced in this way in terms of homogeneous charge-transport behavior and formation of effective interfaces for diffusion and dissociation of photogenerated excitons. The interaction of the organic filaments with light, exhibiting specific light-scattering properties of the nanofibrous mat, might also contribute to spreading incident radiation across the active layers, thus potentially enhancing photovoltaic performance. This method might be applied to other electron donor–electron acceptor material systems for the fabrication of solar cell devices enhanced by nanofibrillar morphologies embedding conjugated polymers and fullerene compounds.

Electrostatic Mechanophores in Tuneable Light-Emitting Piezopolymer Nanowires

Electromechanical coupling through piezoelectric polymer chains allows the emission of organic molecules in active nanowires to be tuned. This effect is evidenced by highly bendable arrays of counter-ion dye-doped nanowires made of a poly(vinylidenefluoride) copolymer. A reversible redshift of the dye emission is found upon the application of dynamic stress during highly accurate bending experiments. By density functional theory calculations it is found that these photophysical properties are associated with mechanical stresses applied to electrostatically interacting molecular systems, namely to counterion-mediated states that involve light-emitting molecules as well as charged regions of piezoelectric polymer chains. These systems are an electrostatic class of supramolecular functional stress-sensitive units, which might impart new functionalities in hybrid molecular nanosystems and anisotropic nanostructures for sensing devices and soft robotics.

Tuning polymorphism in 2,3-thienoimide capped oligothiophene based field-effect transistors by implementing vacuum and solution deposition methods

We report on the investigation of the influence of the molecular packing and film morphology on the field-effect charge mobility in 2,3-thienoimide-based oligothiophenes semiconductors (Cn-NT4N). Organic field-effect transistors are realized by implementing both vacuum and solution methods in order to control the solid-state phase of the active layer. Thermal sublimation in a high vacuum chamber and supersonic molecular beam deposition were used as vacuum-based fabrication approaches for preparing thin films, while lithographically controlled wetting was used, as a solution-deposition technique, for the fabrication of the microstructured films. Thermal sublimation leads to thin films with a phase packing showing ambipolar behaviour, while supersonic molecular beam deposition enables, by varying the deposition rate, the formation of two different crystal phases, showing ambipolar and unipolar field-effect behaviours. On the other hand, lithographically controlled wetting enables the formation of Cn-NT4N microstructured active layers and their implementation in field-effect transistors.

3D printing of optical materials: an investigation of the microscopic properties

3D printing technologies are currently enabling the fabrication of objects with complex architectures and tailored properties. In such framework, the production of 3D optical structures, which are typically based on optical transparent matrices, optionally doped with active molecular compounds and nanoparticles, is still limited by the poor uniformity of the printed structures. Both bulk inhomogeneities and surface roughness of the printed structures can negatively affect the propagation of light in 3D printed optical components. Here we investigate photopolymerization-based printing processes by laser confocal microscopy. The experimental method we developed allows the printing process to be investigated in-situ, with microscale spatial resolution, and in real-time. The modelling of the photo-polymerization kinetics allows the different polymerization regimes to be investigated and the influence of process variables to be rationalized. In addition, the origin of the factors limiting light propagation in printed materials are rationalized, with the aim of envisaging effective experimental strategies to improve optical properties of printed materials.

Additive Manufacturing: Applications and Directions in Photonics and Optoelectronics

Abstract The combination of materials with targeted optical properties and of complex, 3D architectures, which can be nowadays obtained by additive manufacturing, opens unprecedented opportunities for developing new integrated systems in photonics and optoelectronics. The recent progress in additive technologies for processing optical materials is here presented, with emphasis on accessible geometries, achievable spatial resolution, and requirements for printable optical materials. Relevant examples of photonic and optoelectronic devices fabricated by 3D printing are shown, which include light‐emitting diodes, lasers, waveguides, optical sensors, photonic crystals and metamaterials, and micro‐optical components. The potential of additive manufacturing applied to photonics and optoelectronics is enormous, and the field is still in its infancy. Future directions for research include the development of fully printable optical and architected materials, of effective and versatile platforms for multimaterial processing, and of high‐throughput 3D printing technologies that can concomitantly reach high resolution and large working volumes.

Random optical media based on hybrid organic-inorganic nanowires: multiple scattering, field localization, and light diffusion

Random optical media (ROM) are a novel class of photonic materials characterized by a disordered assembly of the elementary constituents (such as particles, wires and fibers), that determines unique scattering, absorption and emission properties. The propagation of light in ROM is affected by the size and optical properties (refractive index, absorption and emission wavelengths) of their components, as well as by the overall 3-dimensional architecture. So far, most of the investigated ROM have been realized using liquid dispersions or bulk samples embedding colloidal nanoparticles or porous systems. While nanowire-based ROM are poorly investigated, such materials can feature new optical effects related to the elongated shape of their building blocks and to their light-transport properties. Here we report on the fabrication and on the morphological and spectroscopic characterization of hybrid organic-inorganic nanowires, realized by doping polymers with dielectric nanoparticles. We investigate light diffusion and multi-scattering properties of 3- dimensional ROM formed by organic and hybrid nanowires, as well as field localization in 2-dimensional networks. The influence of nanowire geometry and composition on the scattering properties is also discussed.

Control of photon transport properties in nanocomposite nanowires

Active nanowires and nanofibers can be realized by the electric-field induced stretching of polymer solutions with sufficient molecular entanglements. The resulting nanomaterials are attracting an increasing attention in view of their application in a wide variety of fields, including optoelectronics, photonics, energy harvesting, nanoelectronics, and microelectromechanical systems. Realizing nanocomposite nanofibers is especially interesting in this respect. In particular, methods suitable for embedding inorganic nanocrystals in electrified jets and then in active fiber systems allow for controlling light-scattering and refractive index properties in the realized fibrous materials. We here report on the design, realization, and morphological and spectroscopic characterization of new species of active, composite nanowires and nanofibers for nanophotonics. We focus on the properties of light-confinement and photon transport along the nanowire longitudinal axis, and on how these depend on nanoparticle incorporation. Optical losses mechanisms and their influence on device design and performances are also presented and discussed.