Andrea Camposeo

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.

Local Mechanical Properties of Electrospun Fibers Correlate to Their Internal Nanostructure

The properties of polymeric nanofibers can be tailored and enhanced by properly managing the structure of the polymer molecules at the nanoscale. Although electrospun polymer fibers are increasingly exploited in many technological applications, their internal nanostructure, determining their improved physical properties, is still poorly investigated and understood. Here, we unravel the internal structure of electrospun functional nanofibers made by prototype conjugated polymers. The unique features of near-field optical measurements are exploited to investigate the nanoscale spatial variation of the polymer density, evidencing the presence of a dense internal core embedded in a less dense polymeric shell. Interestingly, nanoscale mapping the fiber Young’s modulus demonstrates that the dense core is stiffer than the polymeric, less dense shell. These findings are rationalized by developing a theoretical model and simulations of the polymer molecular structural evolution during the electrospinning process. This model predicts that the stretching of the polymer network induces a contraction of the network toward the jet center with a local increase of the polymer density, as observed in the solid structure. The found complex internal structure opens an interesting perspective for improving and tailoring the molecular morphology and multifunctional electronic and optical properties of polymer fibers.

Electrospun dye-doped polymer nanofibers emitting in the near infrared

The authors report on the fabrication and characterization of near infrared fluorescent nanofibers. The nanofibers are composed by an organic dye dispersed in a poly(methylmethacrylate) inert matrix and realized by electrospinning. They exhibit diameters down to 70nm, with average values in the range of 170–480nm, depending on the process parameters, and photoluminescence emission peaked at 865nm. The temporal behavior of the emission under ultraviolet excitation in air can be described by an oxygen diffusion model with a characteristic time τ in the range of 400–1200s, depending on the fiber size, which correspond to a photostability longer than (0.4–1.2)×105 excitation laser pulses. These results open the way for large volume and cost-effective realization of infrared-emitting nanofibers, which are promising candidates as nanoscale infrared light sources.

Electrospun light-emitting nanofibers as excitation source in microfluidic devices

We introduce the integration of organic, polarised light-emitting electrospun nanofibers and lab-on-a-chip microchannel geometries. The alignment and spinning electric field leads to ordered mesoscopic active areas, up to many mm(2), which exhibit polarised light emission and are fully compatible with microlithographies and microfluidics. We utilise the nanofibers demonstrating the photo-excitation of flowing dye chromophores in microchannels. This leads to easy decoupling the excitation and sample emission by polarisation analysers, thus remarkably increasing the imaging signal to background noise ratio.

Rotational dynamics of optically trapped nanofibers.

We report on the experimental evidence of tilted polymer nanofiber rotation, using a highly focused linear polarized Gaussian beam. Torque is controlled by varying trapping power or fiber tilt angle. This suggests an alternative strategy to previously reported approaches for the rotation of nano-objects, to test fundamental theoretical aspects. We compare experimental rotation frequencies to calculations based on T-Matrix formalism, which accurately reproduces measured data, thus providing a comprehensive description of trapping and rotation dynamics of the linear nanostructures.

Near-infrared imprinted distributed feedback lasers

The authors report on the fabrication and characterization of an organic distributed feedback laser operating in the near infrared. The device, fabricated by room-temperature nanoimprint lithography, is based on an organic dye hosted by a poly(methylmethacrylate) matrix. The laser emission from an imprinted 620nm period grating is peaked at 918nm with a linewidth of 8A and a pumping threshold of 37μJ∕cm2, and it is strongly polarized with a polarization contrast as high as 0.99. The lasing wavelength is tunable in the range of 890–930nm by adjusting the grating period, and the operational lifetime is up to 6×103 excitation pulses in vacuum environment. These results demonstrate the possibility of realizing imprinted organic-based near-infrared lasers, thus approaching spectral regions relevant for optical communication applications.

Bright Light Emission and Waveguiding in Conjugated Polymer Nanofibers Electrospun from Organic Salt Added Solutions

Light-emitting electrospun nanofibers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(N,N′-diphenyl)-N,N′-di(p-butyl-oxy-phenyl)-1,4-diaminobenzene)] (PFO–PBAB) are produced by electrospinning under different experimental conditions. In particular, uniform fibers with average diameter of 180 nm are obtained by adding an organic salt to the electrospinning solution. The spectroscopic investigation assesses that the presence of the organic salt does not alter the optical properties of the active material, therefore providing an alternative approach for the fabrication of highly emissive conjugated polymer nanofibers. The produced nanofibers display self-waveguiding of light, and polarized photoluminescence, which is especially promising for embedding active electrospun fibers in sensing and nanophotonic devices.

Single light-emitting polymer nanofiber field-effect transistors

We report on single nanofiber field-effect transistors made by the light-emitting polymer, poly(2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene). We measure electrical performances comparable to or better than those of thin-film transistors by the same organic semiconductor, due to the molecular alignment induced by electrospinning, such as hole mobility of the order of 10(-3) cm(2) V(-1) s(-1) and on/off current ratios up to 780. In addition, we observe controllable photoluminescence intensity quenching by varying the gate voltage up to -40 V with device operation in the luministor mode. Single light-emitting polymer nanofiber transistors coupling electrical and optical functionalities open the way towards low cost and flexible one-dimensional switches and nanofiber-based light-emitting transistors.

Metal-Enhanced Near-Infrared Fluorescence by Micropatterned Gold Nanocages

In metal-enhanced fluorescence (MEF), the localized surface plasmon resonances of metallic nanostructures amplify the absorption of excitation light and assist in radiating the consequent fluorescence of nearby molecules to the far-field. This effect is at the base of various technologies that have strong impact on fields such as optics, medical diagnostics, and biotechnology. Among possible emission bands, those in the near-infrared (NIR) are particularly intriguing and widely used in proteomics and genomics due to its noninvasive character for biomolecules, living cells, and tissues, which greatly motivates the development of effective and, eventually, multifunctional NIR-MEF platforms. Here, we demonstrate NIR-MEF substrates based on Au nanocages micropatterned with a tight spatial control. The dependence of the fluorescence enhancement on the distance between the nanocage and the radiating dipoles is investigated experimentally and modeled by taking into account the local electric field enhancement and the modified radiation and absorption rates of the emitting molecules. At a distance around 80 nm, a maximum enhancement up to 2–7 times with respect to the emission from pristine dyes (in the region 660–740 nm) is estimated for films and electrospun nanofibers. Due to their chemical stability, finely tunable plasmon resonances, and large light absorption cross sections, Au nanocages are ideal NIR-MEF agents. When these properties are integrated with the hollow interior and controllable surface porosity, it is feasible to develop a nanoscale system for targeted drug delivery with the diagnostic information encoded in the fluorophore.