New perspectives for light-emitting polymer nanofibers

The current miniaturization trend in photonics requires components that are able to generate, guide, and detect light, but are smaller than the wavelength of the light with which they interact. Among several proposed approaches to achieve this goal, the exploitation of 1D nanomaterials is one of the most promising since such systems often show new or enhanced figures of merit compared to the alternatives.^1,2 For instance, light-emitting 1D nanomaterials made by conjugated polymers have unique features because they combine optoelectronic properties of semiconductors with structural properties of polymers. Organic nanowires, nanofibers, and nanotubes can be produced by different techniques, including template-assisted synthesis³ and vacuum sublimation.⁴ However, the low throughput of many of these approaches limits their application in photonic integrated systems.

We fabricate light-emitting polymer nanofibers by electrospinning,⁵ a high-throughput method, to realize 1D structures with typical diameters down to tens of nanometers. Electrospinning stretches a viscous polymer solution along one axis by applying an electrostatic field between a metallic needle and a collector. Applying the method to conjugated polymers is particularly challenging due to the limited solubility and relatively poor viscoelasticity of these compounds. However, some of the challenges can be partially overcome by making the final solution more suitable for processing.⁵ Light-emitting nanofibers can be fabricated with different conjugated polymers, co-polymers, and their blends, displaying emissions tunable in the whole visible range.⁶ Figure 1 shows typical photoluminescence confocal and scanning electron micrographs of conjugated polymer fibers, exhibiting average diameters in the range 0.1–10μm depending on the processing parameters (applied voltage, solution feeding rate, and needle-collector distance).⁵ Emission can be further tuned by Förster energy transfer, i.e. by means of the dipole-dipole non-radiative energy transfer of the excitation from a donor molecule to a suitably chosen acceptor molecule. In particular, white light emission can be achieved.⁶

Figure 1.Photoluminescence confocal micrograph of a mat of electrospun fibers made with a conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]. Inset: Scanning electron micrograph of fibers made with a blend of a blue-emitting host polymer, and a red-emitting guest, with guest:host ratio 1:1000.

The optical properties of the nanofibers can also be tailored by including a periodic, wavelength-scale microstructure in the fibers. This can be achieved through specifically developed lithographic techniques such as room-temperature nanoimprint lithography (RT-NIL).^5,7 This technology is based on the deformation, under applied pressure, of a polymeric material by a nanostructured silicon template in contact with the target polymer surface. Being performed at room temperature, the process preserves the emission properties of the conjugated materials. In fact, these compounds are usually very sensitive to degradation effects caused by elevated temperatures or by the interaction with high-energy light (UV) or charged particles, which are used in most lithographic techniques. By RT-NIL, we obtained 1D gratings with periods between 500nm and 650nm on light-emitting fibers (see Figure 2), thereafter showing an increased forward emission due to Bragg scattering of waveguided modes.⁵

Figure 2.(a) Atomic force micrograph and (b) corresponding height profile of a conjugated polymer fiber, patterned by room-temperature nanoimprint lithography. The master template used is a silicon grating with period 560nm fabricated by electron beam lithography and reactive ion etching. Inset: Photoluminescence confocal microscope image of a structured fiber evidencing two adjacent unstructured and patterned regions. X: Fiber cross-sectional coordinate.

Furthermore, we found that we could control the polarization of the emission by choosing the grating orientation with respect to the fiber axis.⁷ As a consequence of the high stretching of the polymer solution during the electrospinning process, the polymer backbone typically aligns with the fiber axis. The emission polarized along the same axis is enhanced as a result. We observed that, by imprinting a 1D grating parallel or perpendicular to the fiber axis, the polarization of the emission along the fiber axis can be enhanced or depressed, respectively.

In summary, the possibility of fabricating organic semiconductor, light-emitting nanofibers by a simple, low-cost, and high-throughput technique may open new perspectives in plastic photonics. We exploit such building blocks within lasers⁸ and field effect transistors,⁹ and as miniature, polarized light sources in prototype lab-on-a-chip devices.¹⁰ Future work will include photonic integrated circuits where fibers can act as light sources, waveguides, and detectors.

We acknowledge the support from the Regional Strategic Project PS_016 and from Italy's Basic Research Investment Fund through projects RBIP06SH3W and RBFR08DJZI within the program ‘Futuro in Ricerca’.