Near-field electrospinning of conjugated polymer light-emitting nanofibers

The authors report on the realization of ordered arrays of light-emitting conjugated polymer nanofibers by near-field electrospinning. The fibers, made by poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], have diameters of few hundreds of nanometers and emission peaked at 560 nm. The observed blue-shift compared to the emission from reference films is attributed to different polymer packing in the nanostructures. Optical confinement in the fibers is also analyzed through self-waveguided emission. These results open interesting perspectives for realizing complex and ordered architectures by light-emitting nanofibers, such as photonic circuits, and for the precise positioning and integration of conjugated polymer fibers into light-emitting devices.


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The resulting relative concentration of MEH-PPV/PEO is ∼1:100 (w/w), and this solution is stirred and ultrasonically agitated, thus becoming homogeneous. SiO 2 /Si (oxide thickness = 800 nm), cleaned with acetone in an ultrasonic bath and dried with pure air, is used as substrate. The substrate is placed on a grounded metallic plate (collector), 500 µm below the needle tip to utilize the stable liquid jet region for a controllable deposition of the fibers.
The aligned fibers are deposited by a home-built NF-ES apparatus, schematized in Figure 1. The NF-ES set-up is composed by a plastic syringe equipped with a µm-diameter tip Tungsten spinneret in a 26 gauge needle, a syringe pump (Harvard Apparatus 22), a high voltage power-supply (Innotec A2K5-20HR), a grounded collector and x-y (C-865 PILine Ultrasonic Piezomotor Controller) and z stages (C-862 Mercury DC-motor Controller), allowing to control the collector movement by dedicated software. The solution is loaded into the syringe, whose metallic needle is connected to the positive electrode of the power-supply and fed through the syringe needle at constant rate (50 µL/h). The applied electrostatic voltage is about 1.3 kV. During the deposition, the x-y stage velocity is 50 cm/s. With these values of voltage and stage velocity, and at a tip-substrate distance of 500 μm, straight, continuous and uniform fibers are deposited.
Lower stage velocities determine the formation of spiraling and buckling nanofibers, whereas non-continuous nanofibers would be deposited by using higher velocities. As a general rule, smooth and continuous fibers are formed when the stage translational velocity is as much as possible comparable to the deposition rate. 16 The experiments are carried out at room temperature and in air atmosphere. The morphology of the electrospun nanofibers is examined by Scanning Electron Microscopy (SEM), by using a FEG-SEM (LEO 1530) at 5-10 kV beam energy. Atomic force microscopy is performed by using a Multimode system equipped with a Nanoscope IIIa electronic controller (Veeco Instruments). Si cantilevers with a resonance frequency of 250 kHz are used to image the nanofiber surface topography in Tapping mode.

Optical properties
Fluorescence micrographs of the nanofibers are acquired by a laser confocal microscope, composed by a scan head (A1R MP, Nikon) and an inverted microscope (Eclipse Ti, Nikon). Samples are excited by a CW diode laser (Melles Griot 56 ICS series, λ = 408 nm). The emission is collected by a 20× (Plan Fluor Numerical Aperture, NA=0.50, Nikon) or a 60× (oil immersion Plan Apo, NA=1. 40,Nikon) objective and the intensity is measured by either a photomultiplier or a spectral detection unit equipped with a multi-anode photomultiplier (Nikon).
The single-nanofiber emission is analyzed by using a micro-photoluminescence (μ-PL) set-up, based on an inverted microscope (IX71, Olympus) equipped with a 60× oil immersion objective (Plan ApoN, NA=1.42, Olympus). The PL is excited by a diode laser (λ = 405 nm), coupled to the microscope by a dichroic mirror and focused on the sample through the microscope objective (spot diameter about 3 μm). The fiber emission is collected by the same microscope objective and analyzed by a monochromator (Jobin Yvon, iHR320), equipped with a Charged Coupled Device (CCD) detector (Jobin Yvon, Symphony). This μ-PL system is also used for characterizing fiber waveguiding. Part of the light emitted by the fluorescent conjugated polymer, excited by the tightly focused laser beam, is coupled into the fiber and waveguided. The fiber optical losses can be evaluated by measuring the intensity of PL signal diffused by the fiber surface, as a function of the distance, d, from the exciting laser spot. 26 To this aim, the spatially-resolved emission intensity map of a single light-emitting fiber is measured by a CCD camera (Leica DFC 490).
For polarized excitation spectroscopy, the fibers are excited by a collimated, linearly polarized laser beam (diameter about 2 mm), normally incident on samples. The beam polarization can be adjusted with respect to the nanofiber longitudinal axis by a λ/2 waveplate.
The nanofiber emission is collected by an optical fiber and analyzed by a spectrometer.

Results and Discussion
The key factor of NF-ES is the exploitation of the stable region of the extruded jet close to the metallic needle (h=500 μm in our experiments, Fig. 1). Such approach allows the onset of the jet instabilities to be avoided, and arrays of polymer nanofiber with high spatial precision to be consequently deposited. Figure 2(a) shows optical bright-field images of ordered arrays of MEH-PPV/PEO nanofibers, composed by parallel, roughly equally-spaced fibers (deposited area about 25 mm 2 ). The realized parallel fibers feature a very high degree of mutual alignment. The distribution of the angles formed by the fiber axes and the stage translational axis is characterized by a standard deviation<1°, much lower than the typical values reported in uniaxially aligned fibers realized by standard electrospinning systems. 28 In Figure 2  fibers have a ribbon shape, with height below 100 nm.
In Figure 4 we show PL micrographs of the produced MEH-PPV/PEO nanofibers, imaged by confocal microscopy. The data show uniform and almost regular arrays of fluorescent fibers, confirming the spatial control achieved in depositing electrospun fibers into predefined geometries, and the high effectiveness of the near-field technique for fabricating ordered arrays of emitting nanofibers. In some samples, high-resolution confocal maps of single fibers [ Fig. 4(e)] evidence a complex structure internally to the electrospun   The packing and possible alignment of MEH-PPV macromolecules within the electrospun nanofibers is investigated through polarized excitation and PL spectroscopies. The polarized excitation measurements are accomplished by collecting the nanofiber fluorescence intensity, I(θ), excited by a laser beam, whose linear polarization direction forms an angle, θ, with respect to the fiber axis. By this method the polarization dependence of the absorption process through the emission intensity can be probed. 45 Figure 5 evidencing that the emission is mainly polarized along the nanofiber axis (the peak emission with polarization parallel to the fiber axis is about twice the emission peak intensity with polarization perpendicular to the fiber axis), further supporting a prevalent orientation of the MEH-PPV optical transition dipoles along the fiber. The degree of ultimately achievable orientation of the active macromolecules in fibers produced by NF-ES is likely related to the solvent evaporation rate. Indeed, the alignment of MEH-PPV molecules could be adversely affected by the presence of residual solvent in the deposited fibers. 16 In fact, the short needle-collector distance can disfavor the complete evaporation of the solvents from the spun jet. As a consequence, the presence of solvent residues in the deposited fibers can allow the polymer molecules to still relax to reach a more isotropic configuration following deposition and during solidification, 32 thus decreasing the fluorescence degree of polarization. For light-emitting macromolecules, this eventual relaxational dynamics should also be related to the solvent quality, with good or poor solvents for the used conjugated polymer favoring typically more elongated or aggregated configurations, respectively. 36,37 Fig. 6 Normalized PL intensity guided by a single fiber as function of distance, d, from the excitation spot. The continuous line is a fit to the data by an exponential decay I=I 0 exp(-αd), where α is the optical loss coefficient. Inset: fluorescence micrograph of a MEH-PPV fiber. The PL is excited by a focused laser (red circle) and a part of emitted light is coupled into the fiber. Scale bar = 2 μm.
Finally, we investigate the waveguiding properties of the light-emitting nanofibers, in view of their possible exploitation as active waveguides. Figure 6 displays the spatial decay of the self-waveguided emission. The decay is almost exponential, with a loss coefficient of the order of 10 3 cm -1 , comparable to similar active systems. 26,45 We attribute optical losses mainly to scattering from the surface fiber microstructure evidenced in Fig. 3. The measured surface roughness (root mean squared, RMS) is of about 10 nm (Fig. 3). Another contribution to the measured losses is attributable to self-absorption that typically affects waveguiding in conjugated polymer nanostructures and films. We anticipate that the optical losses can be reduced to cm -1 level, suitable for on-chip waveguiding application, by using specifically-designed host-guest donor-acceptor systems. 46 These experiments are currently in progress in our laboratories.