Francesca Di Benedetto

Patterning of light-emitting conjugated polymer nanofibres

Organic materials have revolutionized optoelectronics by their processability, flexibility and low cost, with application to light-emitting devices for full-colour screens, solar cells and lasers. Some low-dimensional organic semiconductor structures exhibit properties resembling those of inorganics, such as polarized emission and enhanced electroluminescence. One-dimensional metallic, III-V and II-VI nanostructures have also been the subject of intense investigation as building blocks for nanoelectronics and photonics. Given that one-dimensional polymer nanostructures, such as polymer nanofibres, are compatible with sub-micrometre patterning capability and electromagnetic confinement within subwavelength volumes, they can offer the benefits of organic light sources to nanoscale optics. Here we report on the optical properties of fully conjugated, electrospun polymer nanofibres. We assess their waveguiding performance and emission tuneability in the whole visible range. We demonstrate the enhancement of the fibre forward emission through imprinting periodic nanostructures using room-temperature nanoimprint lithography, and investigate the angular dispersion of differently polarized emitted light.

Laser emission from electrospun polymer nanofibers

althoughelectrospinning (ES), based on the stretching of a polymersolution under electrostatic forces, represents a practicallyunique technology that combines low cost and high through-put. Moreover, the addition of active components (i.e.,nanoparticles or molecular species) to the ES polymersolution allows one to obtain composite nanofibers withspecific functionalities.

Full color control and white emission from conjugated polymer nanofibers

The authors demonstrate full color tunability in the visible range, including white emission, by polymer nanofibers based on binary blends of conjugated materials. The nanofibers are realized by electrospinning and their emission is based on the dipole-dipole energy transfer from a blue-emitting donor and a red-emitting acceptor conjugated polymer. The fibers are characterized by scanning electron microscopy and time-resolved and cw photoluminescence. Light emission is tuned from blue to red, including bright white with color coordinates (0.38, 0.34) according to the standard of the Commission Internationale de l’Eclairage. Polymer nanofibers based on blends of conjugated compounds turn out to be a promising class of organic semiconductor building blocks for nanophotonics.

Polymeric distributed feedback lasers by room-temperature nanoimprint lithography

Room temperature nanoimprinting lithography is used to realize a distributed feedback laser by direct dry pressing of the conjugated polymer (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]). The laser device exhibits emission at 630nm with a pump threshold of 25μJ∕cm2 and a polarization contrast of the emitted light as large as 0.91. Therefore, room temperature nanoimprint lithography turns out to be very effective for producing stable patterns on light-emitting polymers for the one-step fabrication of nanopatterned optoelectronic devices.

Light-emitting nanocomposite CdS–polymer electrospun fibres via in situ nanoparticle generation

We report on the simple, in situ generation of CdS nanocrystals inside electrospun polymer fibres by thermal decomposition of a cadmium thiolate precursor, leading to nanocomposite light-emitting fibres. The modifications induced in the precursor by the thermal decomposition are investigated by a morphological, structural and spectroscopic analysis of the resulting nanocomposite fibres. This approach allows us to overcome nanofabrication difficulties related to disfavoured micro- or nanofluidic molecular flow as given by the direct incorporation of particles in the electrospinning solution. This method therefore enables the synthesis of luminescent, CdS-based composite fibres with emission peaked in the visible range, suitable as building blocks for nanophotonic devices based on light-emitting nanomaterials.

CdS-polymer nanocomposites and light-emitting fibers by in situ electron-beam synthesis and lithography.

A straightforward, electron-beam induced synthesis and patterning approach to the in situ generation of CdS nanocrystals in nanocomposite films and light-emitting electrospun nanofibers is used. Smartly combining room-temperature nanoimprinting, electrospinning, and electron-beam decomposition of nanocrystal precursors and subsequent nucleation of nanoparticles in a polymer matrix allows exploitation of the most favorable flow conditions of organics to produce various nanocomposite nanostructures.

Polymer nanofibers by soft lithography

The fabrication of polymeric fibers by soft lithography is demonstrated. Polyurethane, patterned by capillarity-induced molding with high-resolution elastomeric templates, forms mm-long fibers with a diameter below 0.3μm. The Young’s modulus of the fabricated structures, evaluated by force-distance scanning probe spectroscopy, has a value of 0.8MPa. This is an excellent example of nanostructures feasible by the combination of soft nanopatterning and high-resolution fabrication approaches for master templates, and particularly electron-beam lithography.

Sub-50-nm conjugated polymer dots by nanoprinting.

Methods for controlling, with high accuracy, the deposition of active molecular materials on surfaces play a fundamental role in the fabrication of functional devices. Different patterning techniques have been proposed for organic electronics, including optical methods, dip-pen, inkjet, and soft lithography, molecular self-assembly, nanoimprinting, and molding. However, among these approaches, only inkjet and dip-pen lithography have straightforward applications in organic pixel technology. These approaches allow one to realize matrices or arrays of separated features, although with limitations in terms of cost and serial throughput. In organic nanoand optoelectronics, a current challenging issue is the research of cheap, parallel-patterning approaches for fabricating i) addressable matrix arrangements of spatially discrete elements, and ii) high-quality emissive features with sub-100-nm size, to be employed as nanoscale light sources. High-resolution soft lithography is in principle able to fulfill these requirements. In particular, the recently developed particle replication in nonwetting templates (PRINT) method was applied to obtain particles using several materials, such as poly(ethylene glycol diacrylate), triacrylate resin, poly(lactic acid), and poly(pyrrole), possibly incorporating bioactive agents. PRINT utilizes a stamp which is not wetted by the target organic materials. If a liquid polymer is placed between a textured and a flat fluorinated surface, the organics tend to be confined within the recessed features of the mold. For this reason, in contrast to other soft lithographic methods, the presence of an

Polymer to polymer to polymer pattern transfer : Multiple molding for 100 nm scale lithography

We demonstrate a multiple molding procedure based on the combination of replica molding, in situ patterning of an ultraviolet curable epoxy resist, micromachining by elastomeric elements, and nanoimprinting lithography. The pattern, with features down to the 100nm scale, is sequentially transferred to several different polymers, allowing one to realize high-resolution organic molds for imprinting compounds of lower glass-transition temperature. The intimate integration of soft and nanoimprinting lithographies enables a combined, multistep mechanical patterning, which can be very useful for a great range of applications for molecular lithography and devices.

Rolling particle lithography by soft polymer microparticles

Elastomeric polymeric microspheres are employed as a direct-writing tool for the continuous delivery of molecular materials. The mechanical properties enabling patterning are investigated and modelled. The proposed approach provides a low cost and versatile lithographic method for transferring features with real-time dynamic control.