Stacked electrospun polymer nanofiber heterostructures with tailored stimulated emission

We present stacked organic lasing heterostructures made by different species of light-emitting electrospun fibers, each able to provide optical gain in a specific spectral region. A hierarchical architecture is obtained by conformable layers of fibers with disordered two-dimensional organization and three-dimensional compositional heterogeneity. Lasing polymer fibers are superimposed in layers, showing asymmetric optical behavior from the two sides of the organic heterostructure, and tailored and bichromatic stimulated emission depending on the excitation direction. A marginal role of energy acceptor molecules in determining quenching of high-energy donor species is evidenced by luminescence decay time measurements. These findings show that non-woven stacks of light-emitting electrospun fibers doped with different dyes exhibit critically-suppressed Förster resonance energy transfer, limited at joints between different fiber species. This leads to the obtaining of hybrid materials with mostly physically-separated acceptors and donors, thus largely preventing donor quenching and making it much easier to achieve simultaneous lasing from multiple spectral bands. Coherent backscattering experiments are also performed on the system, suggesting the onset of random lasing features. These new organic lasing systems might find application in microfluidic devices where flexible and bidirectional excitation sources are needed, optical sensors, and nanophotonics.


Introduction
In the last decade, light-emitting polymer or organic/inorganic nanobers have received continuously increasing interest, in view of their possible use in various elds including photonics. 1,2 Examples of recently demonstrated applications include the embedment of nanobers in micro-lasers, 3-5 their use as turbid media for random lasing, 6,7 or as efficient materials for advanced lighting purposes. 8 Nanobers realized by electrospinning, [9][10][11][12] a method in which polymer solutions are ejected at very high strain rates by an external applied electric eld, are especially suited to this aim. Advantages of electrospun bers include the enhanced alignment of molecular backbones along the longitudinal axis of the organic laments, which might lead to polarized emission from conjugated polymers, 13,14 the possibility of depositing coatings on large areas and with low cost, which makes the method compatible with manufacturing technologies for organic optoelectronics, and the high chemical versatility, which allows bers to be realized with various polymer matrices, luminescent nanocrystals, 15 and light-emitting molecules. For instance, the variety of different organic dyes available for polymer doping makes the emission from electrospun bers able to cover from the near ultraviolet to the near infrared without needing any complex chemistry or fabrication methods. 8,16,17 However, while electrospun nanobers can be arranged in various hierarchical architectures, 18-20 the congurations of light-emitting devices and lasers based on them, which have been tested so far, are still limited. Multilayer deposition by electrospinning has been performed by sequentially producing different populations of nanobers onto the same collecting surface, which results in vertically-stacked heterostructures. 21 These multilayered brous meshes have been largely applied to obtain scaffold components for tissue engineering. In molecular electronics and photonics, stacking multiple organics in a networked structure might rise to enhanced charge transport, excitation migration, and multiple light scattering across the complex material. Due to the intimate and distributed interfaces between different polymer laments, multicomponent electrospun heterostructures might nd application within chemical reactors working at ultra-small scale (10 À21 mol), 22 and within light-emitting devices and lasers.
In this work, we demonstrate a hierarchical electrospun polymer architecture which exhibits light-emission and optical gain, composed by conformable layers with random two-dimensional organization of organic laments, and vertical, three-dimensional compositional heterogeneity. The individual layers are doped with two different laser dyes, i.e. rhodamine 6G (Rh6G) and cresyl violet (CrV). These molecules could form a Förster resonance energy transfer (FRET) pair as acceptor and donor component upon polymer co-doping through blends. Such FRET effect was previously observed in blend micro-bers. 23 Energy transfer mechanisms clearly cause stimulated emission (STE) to occur mainly from acceptors, and donor emission to be correspondingly quenched. These aspects lead to disadvantages in terms of spectral control, since multi-band STE in blend systems is hard to obtain and only associated to specic and narrow donor/acceptor relative concentrations. Similar conclusions can be drawn for other FRET pairs upon codoping of polymer matrices, including thin-lms and organic distributed feedback lasers. 24 Here, our approach relies on nonwoven stacks of bers with critically-suppressed FRET since acceptors and donors are mostly physically-separated, thus limiting donor-acceptor energy transfer events at joints between different ber species, and making much easier to achieve simultaneous STE from multiple spectral bands.
The here presented systems show a signicantly asymmetric optical behavior of its two sides, bichromatic STE which can be tuned in its spectral shape and threshold depending on the excitation direction, as well as incoherent random lasing features promoted by optical gain and interplaying lightscattering in the complex mat, as investigated by coherent backscattering (CBS) measurements. These organic lightemitting heterostructures might easily nd application as novel excitation sources within lab-on-chips, sensors, and nanophotonic devices and architectures.

Experimental section
Poly(methyl methacrylate) (PMMA, number-average molecular weight, M n ¼ 120 000 g mol À1 ), chloroform, and N,N-dimethylformamide (DMF) are purchased from Sigma-Aldrich. Rh6G and CrV are from exciton. The dyes are separately dissolved in CHCl 3 : DMF (4 : 1) solutions of PMMA (30% polymer/solvent w/v concentration). In both cases the concentration of dye is kept at 1% w/w with respect to PMMA, which is found to be adequate to provide good optical gain for reliable STE measurements. Electrospinning is carried out for a duration of 15 min and performed with a voltage bias of 18 kV applied over a distance of 25 cm from the 21 gauge needle of a syringe containing the solution to a copper target. Coverslips placed on the target are used for realizing samples suitable for the subsequent optical characterization. Applied ow rates during electrospinning are of 0.7 mL h À1 , achieved by a programmable syringe pump (Harvard Apparatus). Following the deposition of a rst layer composed of Rh6G-doped lament, the second one (with CrV) is deposited with identical electrospinning parameters. The samples have areas of about 14 cm 2 and exhibit bright light emission under cw UV illumination, with different colors observed from the two sides composing the heterostructure ( Fig. 1a-d).
Further solutions of the two dyes are prepared in tetrahydrofuran (THF) for spectrophotometric and spectrouorometric characterization of Rh6G and CrV (concentrations y 10 À4 M). Absorption spectra are obtained by a Jasco V-670 UV-VIS/NIR spectrophotometer. The absorption of Rh6G approximately ranges between 400 and 575 nm with maximum located at 537.5 nm, whereas the emission covers the yellow-orange part of the visible spectrum with maximum at 558 nm and Full Width at Half of Maximum (FWHM) equal to 40 nm. In CrV, the broadband absorption spectrum approximately ranges between 450 and 650 nm with maximum at 598.5 nm, and the maximum of emission is located at 631 nm with FWHM equal to 42 nm (Fig. 1e).
Electrospun materials are inspected by an inverted optical microscope featuring a confocal system (A1R MP, Nikon), by a Hitachi F4500 spectrouorometer, and by scanning electron microscopy (SEM, JSM 6610LVnx, JEOL Ltd). For confocal measurements, emission spectra are excited through a 20Â objective (numerical aperture 0.50) with a cw Ar + laser operating at the wavelength (l) of 514.5 nm. Micrographs are collected at different depths (z d ) into the sample (of thickness d y 40 mm), where z d y 0 mm represents the position of the top (CrV) layer, whereas z d y 40 mm refers to bottom (Rh6G).
CBS measurements are performed by the experimental setup described in ref. 25. STE from the realized heterostructures is excited by the second harmonic (l ¼ 532 nm) of a Nd:YAG laser (Surelite II, Continuum) operating at repetition rate of 10 Hz with pulse duration 5 ns, and analyzed by a spectral detection unit with a multi-anode photomultiplier. To control the intensity of pumping light, the excitation beam is driven through a half-wave plate and a Glan-Laser polarizer with vertical electric vector transmission. The beam of 3.3 mm diameter is incident perpendicular onto the plane of deposited bers, and the emission is acquired from the sample using a USB 2000 ber spectrometer (Ocean Optics, 4 nm resolution). Emission lifetime measurements are carried out by Time Correlated Single Photon Counting (TCSPC) technique, using a Becker & Hickl system that includes a TCSPC module (SPC-130-EM), a hybrid PMT detector (HPM-100-06) with detector control card (DCC 100) mounted on a Princeton Instruments Spectrograph (Acton SpectraPro-2300i), and excitation delivered by a picosecond, 516 nm laser diode (BDL-516-SMC). Fig. 2a shows confocal images of planes with different vertical coordinate (z) in the electrospun sample, as well as the correspondingly collected emission spectra. The yellow emission attributed to Rh6G dominates the uorescence spectra from the rstly electrospun (bottom) layer (corresponding to z d y 40 mm). When the focal plane is moved towards higher z values (i.e., towards the top of the sample and at lower z d depth), the emission at the wavelength of 620 nm (attributed to the CrV uorescence) emerges out of the overall spectra (Fig. 2b).

Results and discussion
The top and bottom layers, imaged at higher magnication, are shown in Fig. 3a and b, respectively, which better highlight the different colors emitted from individual bers composing the layers. The laments mostly have ribbon shape with typical crosssection (w Â h) of 2.3-5.0 mm Â 1.1-2.0 mm, with no preferential orientation within each layer (Fig. 3c).
A marked asymmetric behavior is found for the light emission from the heterostructure. This is highlighted by excitation spectra of bers which are measured at a given emission wavelength (660 nm, i.e. mainly occurring from CrV) while continuously varying the excitation wavelength (dotted lines in Fig. 4). When the system is excited from the CrV side, the band centered at the wavelength of 600 nm, referring to CrV absorption, is mostly accentuated in the excitation spectrum, while the Rh6G band is hidden and manifests as a shoulder on the short-wavelength tail of the spectrum. In the opposite situation, when excitation photons rstly impinge on Rh6Gbased bers, the excitation band consists instead of two maxima, namely a more intense attributed to Rh6G absorption (at 539 nm) and a second peak from CrV (again at about 600 nm).
Similarly one would expect an inuence of the direction of incidence of the excitation light on STE. Under sufficient pumping intensity from Nd:YAG second harmonic (532 nm), STE is found from the brous heterostructure at the wavelengths of 595 nm and 655 nm as displayed in Fig. 4 and in insets of Fig. 5a and b. The high-energy STE peak is associated with Rh6G whereas that at low-energy is due to CrV. More interestingly, when the excitation beam is rstly incident on the side of less absorbing bers (i.e., with CrV dye), the bers containing Rh6G molecules become efficiently pumped leading to a STE threshold uence of (515 AE 12) mJ cm À2 (Fig. 5a). For sake of comparison, we recall that this value is a relatively low one compared to other systems with Rh6G described elsewhere. [25][26][27][28] This can be related to various concomitant and Paper interplaying effects, such as moderate light-scattering of excitation photons through the brous structures 29 and photon waveguiding 30 within the complex mats, possibly gathering excitation priming STE on the laser dyes. The FWHM of emission is narrowed down to 10 nm above the pumping threshold as typically found for STE. The corresponding threshold uence for the CrV STE band is of (1061 AE 6) mJ cm À2 , and the FWHM for this band is reduced down to 13 nm.
When the sample is illuminated from the Rh6G side, the threshold for Rh6G STE does not change signicantly (508 AE 9) mJ cm À2 , whereas the threshold for the CrV STE more than doubles compared to the previous case, rising to (2584 AE 5) mJ cm À2 (Fig. 5b). The same value of Rh6G threshold found for different pumping sides clearly suggests that the extinction of directional excitation photons through CrV-based bers does not affect Rh6G STE in a signicant way. On the other hand, the signicant increase of the threshold of CrV STE with pumping photons rstly impinging on Rh6G-based bers indicates that Rh6G absorption is more effective in subtracting excitation photons before they can reach CrV-based bers. Related to this, the relative intensity of the two STE peaks is also dependent on   the pumping side. When bers containing CrV dye are excited at rst, the ratio of intensities between Rh6G and CrV bands is $2.2, in the other case this value is reaches $3.6. These ndings agree with data shown in Fig. 1e, evidencing that the extinction coefficient for Rh6G at the wavelength of pumping laser (532 nm) is about four times larger than that of CrV. In both the excitation congurations, however, pumping of low-energy STE mediated by FRET from Rh6G-based bers cannot be ruled out in principle, though limited at the interface between different species of bers which involved junctions regions with a quite low density per heterostructure unit volume.
To analyze the STE mechanisms and the eventual effect of FRET more in depth, we measure involved emission lifetimes by the TCSPC technique. Emission decay curves are presented in Fig. 6a together with mono-exponential ts following deconvolution of Instrument Response Function (IRF). The efficiency of energy transfer process is determined through the equation where t DA is the uorescence lifetime of donor (Rh6G) in presence of acceptor (CrV) and t D is the uorescence lifetime without acceptor (only Rh6G). The obtained values of t DA are 2.46 or 2.41 ns for excitation photons impinging on CrV-based or on Rh6G-based bers, respectively. A t D value equal to 2.67 ns is measured on a reference sample consisting of Rh6G-based bers only. In this way, a FRET efficiency, h ¼ 10%, is found when excitation is carried out from the side with CrV-based bers, and h ¼ 8%, for the excitation from the side with Rh6G-based bers, conrming a marginal role of FRET in affecting the overall STE from the heterostructure. For this reason, it can be concluded that the STE from CrV when pumping photons reach the sample from the side made of Rh6G-based bers ( Fig. 4 and 5a) is mainly due to residual light component at 532 nm or to reabsorption of photons emitted by rhodamine.
Finally, we point out that the complexity of the bi-layered electrospun system suggests that the STE observed in our experiments is in fact incoherent random lasing. 32 To study this aspect, the transport mean free path (L t ) for photons in the material can be calculated by estimating the FWHM (Du) of CBS angular dependencies as 33 ; where l is the wavelength of scattered light. CBS measurements carried out for both sides of pumping lead to the value of 19 mm for the photon transport mean free path (Fig. 6b). The obtained L t well agrees with the hypothesis that emitted light would undergo multiple scattering when travels in the plane of bers lm, which would support incoherent random lasing in association with large excitation areas (spot diameter y 3.3 mm). The excited region, being much larger than transport mean free path for photons, would naturally lead to a high number of overlapping optical modes. The applications of asymmetric, light-emitting heterostructures made of electrospun bers and showing multi-band STE and lasing could be numerous, including the development of low-cost sources laminated in transparent microuidic devices featuring overlapping channels, of exible, conformable or free-standing chemosensing surfaces with double-side optical response, and of tailored systems for parallel imaging. In addition, the compositional complexity of light-emitting electrospun heterostructures can be further increased, by depositing many layers featuring different bers, to nely tune achieved colors and lasing threshold.

Conclusion
We have realized a lasing heterostructure made of layered electrospun bers. A textile material with complex light-emission and STE behavior is obtained, merging the optical features of two laser dyes embedded in different ber species. The use of these bers results in reduced quenching of a donor dye, therefore exciting the two species with the same wavelength leads to a donor emission not seriously affected by the presence of the Fig. 5 Dependence of the emission intensity on the excitation energy fluence, for STE from Rh6G-based fibers (red lines) and from CrVbased fibers (black lines), respectively, and for excitation photons impinging on the heterostructure side made of CrV-based fibers (a) or on the side made by Rh6G-based fibers (b). Thresholds for STE are highlighted by arrows. Insets: emission spectra for varying pumping fluence.
acceptor, and to double-band STE from the system. The threshold of the emission as well as intensity ratio of STE bands are dependent on the pumping side in the layered geometry, showing that the spatial distribution of bers also affects the optical properties of the obtained laser system. All of these experiments clearly indicates the signicance of light-emitting bers for realizing novel, low-cost media for random lasers and multi-band STE sources with versatile optical behavior. It can be concluded that a wide variety of dye-doped bers and of electrospun heterostructures might be used for fabricating optical and laser materials, making possible to tailor light-scattering and optical gain properties in exible molecular systems.

Conflicts of interest
There are no conicts to declare.