Highly Polarized Luminescence from β-Phase-Rich Poly(9,9

Article pubs.acs.org/JPCA

Highly Polarized Luminescence from β‑Phase-Rich Poly(9,9dioctylfluorene) Nanofibers Daniela Iacopino,*,† Pierre Lovera,† Alan O’Riordan,† and Gareth Redmond‡ †

Tyndall National Institute, University College Cork, Dyke Parade, Cork, Ireland School of Physics, University College Dublin, Belfield, Dublin, Ireland

‡

S Supporting Information *

ABSTRACT: Novel poly(9,9-dioctlylfluorene) (PFO) nanofibers were fabricated by solution template wetting of anodic aluminum oxide (AAO) templates with a pore diameter of 25 nm. Individual nanofibers displayed a pronounced axially polarized luminescence with a typical emission dichroic ratio of 15 and low spread of the emissive species angular distribution. The strong optical characteristics were ascribed to intrachain reorientation of amorphous PFO to a more planar and elongated β-phase conformation induced by mechanical strain during polymer template pore infiltration. Absorption optical spectroscopy on nanofiber mats confirmed formation of 24% β-phase emissive segments, which dominated the nanofiber luminescence characteristics. X-ray diffraction measurements were used to confirm and quantify the extent of nanofiber internal molecular alignment.

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INTRODUCTION One-dimensional (1D) semiconductor conjugated polymer nanostructures are of current interest for their potential use as building blocks for nanophotonic and nanoelectronic applications. Due to their ease of processability and the possibility of incorporating functional molecules, 1D polymer nanostructures are particularly suitable for a number of applications such as nonlinear optics,1,2 light emission,3 or charge collection for solar cells.4 The performances of semiconductor polymer devices are generally correlated with the degree of intermolecular order.5 However, intramolecular crystallinity and long-range order become important at the nanoscale, with these ordered phases being likely to govern the physics of nanostructured materials. For example, molecular orientation is a critical parameter for the control and tuning of 1D polymer nanostructure physical properties because optical and electrical dipole orientations are polarized along the molecular chain axis.6,7 Accordingly, 1D polymer structures with a high degree of internal molecular orientation have been shown to possess desirable enhanced anisotropic optoelectronic properties such as polarized absorption and luminescence,8,9 enhanced hole mobilities,10,11 waveguiding,12 and improved organic laser performance.13,14 The establishment of reliable relationships between chemical structure and physical properties and the development of methodologies for the control of the degree of chain order formation would further enhance demonstrated performances and contribute to the development of nanoscale organic optoelectronic devices. Control of molecular orientation in organic nanostructures is commonly achieved by tuning material processing parameters such as moleculr weight, solvent type, or annealing temaperture.15,16 Recently, solution and melt wetting of porous © 2014 American Chemical Society

templates have been shown as convenient routes for the synthesis of high-yield 1D polymer nanostructures with markedly anisotropic optical properties.17,18 This phenomenon has been studied in detail by a number of groups, who have demonstrated that local stress imposed on the material as result of the geometric confinement occurring during polymer infiltration into porous templates resulted in formation of highly textured nanostructures.19−21 Recent advances in porous material fabrication have allowed fine control of material pore sizes ranging from tens to hundreds of nanometers. Polymer wetting of different diameter porous materials has been shown to produce organic nanostructures with different internal molecular organization.21 Therefore, template wetting would constitute an ideal technique for the systematic investigation and correlation between internal order and optical properties in 1D polymer structures. However, the influence of geometric confinement and nature of pore walls on the morphology of the resulting molded material, particularly in the case of systems exhibiting intrinsic anisotropy, has not yet been fully elucidated. Recently, our group has investigated the optical properties of β-phase-rich poly(dioctylfluorene) (PFO) nanowires and nanotubes obtained by melt and solution wetting of commercially available (pore diameter size, 230 nm) anodic aluminum oxide (AAO) templates. PFO nanostructures exhibited clear molecular orientation-dependent optical properties that reflected formation of a narrow distribution of emitting chain segments with increased conjugation length. The optical characteristics were associated with an intrachain reorientation Received: June 8, 2014 Revised: July 7, 2014 Published: July 8, 2014 5437

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3100, Veeco Instruments Inc.) operating in tapping mode. XRD measurements were performed on a Philips X’pert Pro diffractometer (Panalytical U.K. Ltd.), configured in a symmetric reflection coupled θ/2θ mode using a Cu Kα radiation source (λKα = 1.549 Å), with the surface of the template oriented perpendicular to the plane of the incident and scattered X-rays. Measurements were carried out with a step size of 0.02° and a scan integration time of 50 s/step. To assess the degree of alignment of PFO molecules with respect to the pores, Ψ scans were taken with 2θ fixed at 21.4, a step size of 0.02°, and an integration time of 117 min/step. Nanofiber Optical Characterization. All single nanofiber PL measurements were carried out using a time-resolved scanning confocal fluorescence microscope (MicroTime 200, PicoQuant). The horizontally polarized output of a 402 nm pulsed picosecond laser diode (70 ps; 40 MHz repetition rate) was spectrally filtered using a 405 nm band-pass filter, and its polarization state was controlled using a half wave plate. The collimated laser beam was then directed into the entrance port of an inverted microscope (IX 71, Olympus Corp.) using a dichroic mirror. A 100× oil immersion objective was used both for focusing the excitation light onto the sample and for collecting the resulting PL. Typically, the sample was composed of low-density nanofibers deposited onto a glass coverslip and sealed in a home-built vacuum cell. The collected luminescence was spatially filtered by focusing onto a 150 μm diameter pinhole. Single nanofiber emission intensity images were recorded by raster scanning the sample through the laser focus spot and collecting the luminescence using two avalanche photodiodes and a polarizing beam splitter for dual polarization PL image acquisition. All emission intensity images were recorded with a pixel integration time of 2 ms and an incident excitation power < 0.1 nW/cm2 to prevent sample photobleaching. Single nanofiber emission spectra were recorded by directing the PL from a single nanofiber onto the entrance slit of a monochromator (SP-2356, Acton Research) equipped with a thermoelectrically cooled, back-illuminated CCD. Spectra were typically recorded using an integration time of 15−40 s. UV−visible absorbance spectra were acquired using a scanning double beam spectrophotometer (UV-2401PC, Shimadzu) equipped with an integrated sphere. PL spectra (365 nm excitation) were recorded on a PerkinElmer LS 50 luminescence spectrometer equipped with a pulsed Xe discharge lamp and Monk−Gillieson monochromators. Emission microscopy images of nanofiber arrays were acquired using an epifluorescence microscope (Zeiss Axioskop II Plus, Carl Zeiss U.K.)

of the amorphous PFO molecular conformation into a more planar (low-energy) extended 21 helical β-phase conformation induced by local stress imposed on the material during the synthetic process.22−25 In this paper, we present the anisotropic optical properties of PFO nanofibers obtained by solution wetting of custom-made AAO templates (pore size, 25 nm). Imaging and photoluminescence (PL) microscopy of individual nanofibers revealed markedly anisotropic emission with a preferred axial polarization of nanofiber absorption and emission segments and a typical fiber emission dichroic ratio value of 15. Compared to previously synthesized larger 1D structures, nanofibers displayed higher optical anisotropy and narrowed emission peaks, confirming the formation of β-phase segments of increased and homogeneous conjugation length. The observed optical anisotropic characteristics were attributed to the strong geometric confinement imposed by the small size pore diameter template. Absorption and PL spectroscopic data acquired for nanofiber mats confirmed that 24% of PFO molecules adopted a more planar, extended β-phase molecular conformation. In particular, the luminescence behavior was dominated by this phase as the nanofibers behaved as selfdoped structures. The extent of molecular organization was quantified by X-ray diffraction (XRD) measurements, which confirmed the presence of a strong axial molecular alignment and allowed extraction of an orientational parameter of polymer chains ⟨P2⟩ = 0.98.

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EXPERIMENTAL SECTION Materials. Poly(9,9-dioctyl-2,7-dioctylflyuorene) (PFO), with a polydispersity index of 3.0 and mean molecular weight MW = 100 000 g/mol, was purchased from American Dye Source, Inc., and stored under nitrogen. High-purity (99,999%) aluminum foils 0.50 mm in thickness were purchased from Goodfellow, Cambridge Ltd. Oxalic acid, chromium oxide, copper chloride, and o-xylene were purchased from Aldrich. Anhydrous tetrahydrofuran (THF) and hexamethyldisilazane (HMDS) were also purchased from Aldrich and stored under nitrogen. Deionized water (Millipore Q; >18 MΩcm) was used for all aqueous solutions. Synthesis of Nanofibers. PFO nanofibers were fabricated using the technique of template synthesis. A drop of a 20 mg/ mL solution of PFO in THF was deposited onto custom-made AAO membranes with a calculated pore diameter of 25 nm. Immediately after, a glass microscope slide was pressed on the deposited solution, and a pressure of ∼50 kPa was applied over a period of 1 day in order to facilitate pore filling. Excess polymer was then removed from the membrane by polishing with a scalpel. In order to liberate nanofibers from the template, the polished PFO-filled templates were soaked in 3 M aqueous NaOH for 5 h; excess organic residue was then removed by three cycles of agitation, removal of the supernatant, and addition of H2O. Fibers were then dispersed in o-xylene. Nanofiber Structural Characterization. Scanning electron microscopy (SEM) images of nanofiber mats were acquired using a field emission SEM (JSM-6700F, JEOL U.K. Ltd.) operating at beam voltages between 3 and 5 kV. For imaging of nanofiber mats, nanofiber-filled AAO templates were mounted onto SEM stubs using carbon pads. A drop of NaOH solution (3 M, 30 min) was deposited onto the membrane to selectively dissolve the template and leave mats of nanofibers. The topography of individual nanofibers was characterized using a calibrated atomic force microscope (AFM) (Dimension

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RESULTS AND DISCUSSION PFO nanofibers were fabricated by solution wetting of custommade AAO templates with an average pore diameter of 25 nm. Figure 1a shows a SEM image of nanofibers deposited on SiO2 surfaces after dissolution of the template. Statistical analysis of diluted dispersions (o-xylene solvent; inset Figure 1a) gave an average length of 10.0 (±2.6) μm and diameter of 23.0 (±4.5) nm with an average aspect ratio of 400. A tapping-mode AFM image of a typical PFO nanofiber is shown in Figure 1b. In general, measured nanofibers exhibited smooth outer surface morphology with some degree of axial bending due to the fiber nature of the structure but not to structural defects. The bottom left inset of Figure 1b shows a topographic line profile measured across the nanofiber. A length of 6 μm and height of 23 nm were found for this 5438

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particular fiber, in good correlation with the 25 nm average diameter pore size calculated for the used AAO template. The top right inset of Figure 1b shows an epifluorescence image of an individual nanofiber displaying strong and uniform blue emission along the entire fiber length. In order to further investigate the optical characteristics, polarization PL imaging and spectroscopy measurements of individual nanofibers were undertaken under vacuum conditions. Polarization PL images of a nanofiber acquired under vertical (i.e., parallel to the nanofiber axis) and horizontal (i.e., perpendicular to the nanofiber axis) polarized excitation with either the vertical (VV, HV) or horizontal (VH, HH) polarized collection are shown in Figure 2a. Relatively uniform PL intensity was observed along the nanofiber, with low background emission. The nanofiber PL was anisotropic and was most intense when the collection polarizer was oriented parallel to the nanofiber long axis. Corresponding polarization-resolved PL spectra of the same nanofiber are shown in Figure 2b,c. Again, the fiber PL intensity was the greatest when the excitation and collection polarization was oriented parallel to the nanofiber long axis, indicating alignment of the transition dipoles of emission segments. Consistently, a fiber emission dichroic ratio, DRE = IVV/IVH, of 15 was determined, where IVV and IVH are the PL intensities of the 468 nm peak measured at the S1 → S0 0−1 peak (468 nm) wavelength for each polarization condition. The spectral features were characteristic of the S1 → S0 transition β-phase PFO with associated vibronic replicas.26 The narrow emission peaks were indicative of a narrow distribution of emitting PFO chain segments with increased and homogeneous conjugation length. The inset of

Figure 1. (a) SEM image of PFO nanofibers following template removal. (Inset) Higher SEM magnification of low-density PFO nanofibers. (b) Tapping-mode AFM image of a PFO nanofiber on a SiO2 substrate. (Inset) (bottom left) Topography line scan obtained along the black line indicated in the main panel. (Inset) (top right) Epifluorescence image of an individual PFO nanofiber. Scale bar 2 μm.

Figure 2. (a) Polarization PL image of a PFO nanofiber under vertically and horizontally polarized excitation measured with the collection polarizer vertically and horizontally. Scale bar = 1 μm. (b) Corresponding polarized PL spectra acquired for the same nanofiber under vertical excitation polarization with vertical (VV) and horizontal (VH) detection polarization. (Inset) Plot of the emission dichroic ratio as a function of the wavelength. (c) PL spectra acquired under horizontal excitation polarization with (HV) and horizontal detection polarization (HH). (d) Plot of the emission intensity versus the detection angle (θ) with a least-squares sin2 θ fit. 5439

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Figure 2b shows a plot of the polarization ratio, ρ, where ρ = (IVV − IVH)/(IVV + IVH). The ρ mean value was determined to be 0.75 across the spectrum of the fiber, confirming strong axial emission anisotropy. The value of ρ remained stable at all wavelengths, suggesting that the same population of chromophores was responsible for emission at all wavelengths (occurrence of complete excitation energy transfer from the disordered glassy phase to the β-phase).27 Polarization-resolved PL spectra measured for the fiber under HV and HH conditions are shown in Figure 2c. A fiber emission dichroic ratio IHV/IHH of 14 was determined, indicating that the emission from the nanofiber remained predominantly vertically polarized despite the horizontal polarization of the excitation light. Moreover, a ratio of 3.9 was calculated from the intensities of spectra measured under perpendicular and parallel polarized excitation and perpendicular polarized collection (IVV/IHV). This suggested that the anisotropic behavior of the fiber was associated with a strong degree of axial alignment of both emissive and absorbing segments within the fiber. Figure 2d shows the variation of the 0−1 peak emission intensity with the collection polarizer angle. A cos2 curve fit to these data indicated reproducible polarization-dependent behavior of the nanofiber PL. Axial emission polarization suggested that the emissive species within the nanofiber were well aligned. The almost zero intensity measured at 90° to the fiber long axis (VH) indicated again that most of the emissive molecules had a high degree of alignment. The spread in molecular orientation relative to the nanofiber long axis was estimated from the emission dichroic ratio according to28 DR E − 1 1 ≤ (3⟨cos2 θ ⟩ − 1)(3 cos2 β − 1) DR E + 2 4

infiltration.22 The optical anisotropy of larger nanowires was attributed to the formation of 21% elongated β-phase segments within the wires. Therefore, the increased optical anisotropy observed in PFO nanofibers could be indicative of formation of higher percentages of β-phase segments. In order to prove this point, we investigated the optical properties of nanofiber mats drop-deposited onto substrates from diluted o-xylene suspensions. Typical intensity normalized absorption and PL spectra are shown in Figure 3. The

Figure 3. Absorption and PL spectra of a PFO nanowire mat deposited onto a fused silica substrate. (Inset) Epifluorescence image of the PFO nanofiber mat.

absorption spectrum showed a shoulder at 386 nm and two peaks at 406 nm (full width at half-max (fwhm), 58) and 436 nm (fwhm, 19) characteristic of S0 → S1 0−2, 0−1, and 0−0 transitions of β-phase PFO, respectively. The fraction of βphase was estimated by fitting three Gaussian curves to the absorption spectrum (see Figure S2, Supporting Information). The amount of β-phase was taken to be the sum of the area under the fitted Gaussians at 406 and 436 nm divided by the total area under all Gaussians.32 In this way, a value of 24% was extracted, which explains the higher optical anisotropy characteristics observed in smaller diameter nanofibers. The PL spectrum of the PFO nanofiber mat exhibited peaks at 441, 468, 501, and ∼535 nm (fwhm of 7, 17, and 16 nm, respectively), characteristic of the S1 → S0 0−0 transition of βphase PFO with associated vibronic replicas. Overall, the absorption and PL spectra revealed that a large fraction of PFO molecules within the fibers adopted the more planar, extended β-phase PFO conformation. The nanofiber emission spectra were completely dominated by this phase due to a combination of direct excitation of the β-phase and either Förster-type energy transfer or singlet exciton migration from g-phase PFO to the lower energy β-phase.33−35 In order to obtain further information on the nature and extent of molecular organization within the fibers, XRD measurements were acquired for nanofibers embedded in AAO templates. The XRD profile exhibited a well-resolved peak at 21.3° 2θ, corresponding to the (008) reflection of semicrystalline PFO23 with a d spacing of 0.416 nm correlated with the interphenylene spacing along the polymer backbone and indicated alignment of polymer chains along the nanofiber main axes.36,37 To further quantify the extent of molecular alignment within the nanofibers, rocking angle Ψ scans were acquired. To this end, the Θ (source) and 2Θ (detector) angles were fixed at values corresponding to the maximum intensity of the (008) peak, and the resulting signal intensity was recorded as the sample was tilted in x increments through Ψ, the angle between the template surface normal and the plane defined by the incident and scattered X-ray beams (Figure 4, left inset). Intensity profiles were characterized by a maximum at Ψ = 0°

(1)

Where π

2

⟨cos θ ⟩ =

∫0 f (θ) cos2 θ sin θ dθ π

∫0 f (θ) sin θ dθ

θ is the off-axis angle of each emitter, and β is the emission transition dipole angle for PFO (19°);29 f(θ), the proportion of emitters at an angle, was represented by a normalized Gaussian distribution with a maximum at θ = 0 and a standard deviation σ. Fitting for σ gave an upper value of 4.8° for the spread in the emissive segments’ molecular orientation about the wire axis, a value remarkable lower than the 24° previously reported for larger nanowires.28 In addition eq 1 was used to calculate an orientational parameter ⟨P2⟩ = (DRE − 1)/(DRE + 2), useful to quantify the degree of internal molecular alignment within the nanofiber. For perfect alignment, ⟨P2⟩ = 1, and for isotropic alignment, ⟨P2⟩ = 0. A ⟨P2⟩ value of 0.98 was estimated for our nanofibers. This remarkable degree of axial texturing was ascribed to the strong geometrical confinement imposed on the polymer during filling of small pore size pore templates. In comparison, polymer nanofibers synthesized by electrospinning techniques have been reported to exhibit emission dichroic ratios up to 5 and polarization ratios of ∼0.25.30,31 Previously, synthesized PFO nanowires obtained from larger pore diameter templates (230 nm) showed lower alignment of emission segments (DRE = IVV/IVH = 5.6) and a poor degree of alignment of absorbing species (IVV/IHV of 1.7), as a result of the more limited mechanical stress imposed on the polymer material during pore 5440

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subwavelength photonic devices based on emissive polymers. This constitutes an important progression for the exploration and development of future nanoscale optoelectronic devices and systems.

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ASSOCIATED CONTENT

S Supporting Information *

Fabrication of AAO templates, calculation of the β-phase fraction, and polarized optical microscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

Figure 4. XRD diffractogram acquired for PFO nanofibers embedded in an alumina template. (Right inset) Schematic representation of the measurement configuration. (Left inset) Plot of the (008) peak intensity as a function of the template tilt angle, ψ, (solid squares), with a double Gaussian fit to the data (solid line).

*E-mail: [email protected]. Phone: 00353(0) 212346182. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Irish HEA PRTLI Nanoscience initiative.

and a fwhm of 6.6° (Figure 4, right inset). Double Gaussian fitting yielded an expression for the orientational distribution function, f(θ), of the polymer chains, where θ = Ψ is defined as the angle between a polymer chain axis and the nanofiber axis as follows 2

AUTHOR INFORMATION

REFERENCES

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2

f (θ ) = A1e(−0.5(θ / σ1) ) + A 2 e(−0.5(θ / σ2) )

The Gaussian amplitudes and standard deviations extracted from the least-squares fitting were A1 = 87 ± 32, A2 = 179 ± 128, σ1 = 4.1 ± 0.3, and σ2 = 5.8 ± 4. Furthermore, f(θ) was used to estimate a value for the second rank orientational parameter, ⟨P2⟩, defined as 1 ⟨P2⟩ = (3⟨cos2 θ ⟩ − 1) 2 A ⟨P2⟩ value of 0.98 was estimated from the XRD data, confirming the strong axial molecular alignment within the template-embedded nanofibers and in strong agreement with the ⟨P2⟩ value obtained from PL measurements.

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CONCLUSION In summary, the method of template wetting has been successfully employed for synthesis of polyfluorene nanofibers with markedly anisotropic PL emission. Polarization imaging and PL spectroscopy performed on single nanofibers showed emission dichroic ratios of 15, associated with a high degree of alignment of absorbing and emitting segments with the nanofiber long axes. The extent of internal molecular organization was attributed to an intrachain reorganization of the amorphous PFO phase to a more planar and elongated βphase. Evidence of β-phase formation was demonstrated by absorption and PL spectroscopy performed on nanofiber mats. The extent of internal molecular organization was confirmed by XRD measurements. The degree of β-phase chain alignment within the nanofibers was quantified by orientational parameter ⟨P2⟩, which was calculated from PL measurements on a single nanofiber and XRD on a nanofiber ensemble, giving the same high value of 0.98. The superior optical properties exhibited by PFO nanofibers in comparison to previously synthesized larger PFO nanostructures suggests that photophysical properties of PFO nanostructures are correlated with their morphological structure, and they can be consequentially controlled by synthetic manipulation. The understanding of structure− property correlation is highly desirable for the realization of 5441

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dx.doi.org/10.1021/jp505689y | J. Phys. Chem. A 2014, 118, 5437−5442