60 µm; Anodisc 13, Whatman Ltd.). Immediately afterwards, the wetted template was covered by a glass coverslip and a 2.5 kg weight was applied for } 12 h to ...
Polarized Luminescence from Single Polymer Nanowires and Aligned Nanowire Arrays Deirdre O'Carroll, Alan O'Riordan, and Gareth Redmond Nanotechnology Group, Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland
ABSTRACT The synthesis of poly(9,9-dioctylfluorene) conjugated polymer nanowires using the method of solution assisted wetting of nanoporous alumina membrane templates is reported. Polymer nanowires (approx. 109 per template) with a diameter of approx. 200 nm are obtained. Photoluminescence from isolated nanowires fluidically-aligned at glass substrates is found to be dominated by emission from the planar β-phase of the polymer. The wires also exhibit polarized light emission suggestive of axial alignment of β-phase segments within the nanowires. Dense arrays of aligned nanowires exhibiting anisotropic emission are also demonstrated. INTRODUCTION Semiconductor nanowires are an emerging class of nanostructures that represent attractive building blocks for nanoscale electronic and photonic devices [1]. However, while inorganic materials have been explored in depth, the challenge of fabricating organic nanowires has yet to be as comprehensively addressed. In this regard, polyfluorene-type π conjugated polymers have recently attracted interest due to their excellent charge transport properties, high photoluminescence (PL) quantum efficiencies and gain spectra that span the visible range [2]. In particular, poly(9,9-dioctylfluorene), PFO, is a thermally processable polymer that shows thermotropic liquid-crystalline behavior with melting into a birefringent liquid occurring at ~ 156 o C [3]. In this paper, we report on the novel synthesis of PFO polymer nanowires with anisotropic optical properties by solution-assisted wetting of porous alumina templates [4]. Since uptake of nanowires into mainstream applications will depend on the availability of manipulation methods that direct the positions of wires in a fast, reliable and scalable manner, we also report on shear-alignment of sparse and dense PFO nanowire arrays at glass substrates using fluidic flow. With this approach, polarized light emission from single nanowires and from nanowire arrays is demonstrated. EXPERIMENTAL PFO (poly(9,9-dioctylfluorenyl-2,7-diyl; American Dye Source, Inc.) nanowires were synthesized by solution-assisted template wetting. To this end, a drop (~ 50 µL) of a solution (typically 60 mg/mL) of PFO in tetrahydrofuran (THF) was deposited onto a porous anodic alumina membrane (alumina template, nominal pore diameters of 200 nm, membrane thickness ~ 60 µm; Anodisc 13, Whatman Ltd.). Immediately afterwards, the wetted template was covered by a glass coverslip and a 2.5 kg weight was applied for ~ 12 h to facilitate pore filling and slow solvent evaporation.
To extract the nanowires, a PFO filled template was soaked in 3 M NaOH for at least 20 min. Typically, the resulting wire residue was washed with deionized water followed by acetone and finally suspended in decane by sonication. Glass coverslips were acid cleaned (MeOH / HCl 1:1 v:v for 1 h; rinsed with deionized H2O; H2SO4 for 1.5 h; rinsed with deionized H2O). Some acid cleaned coverslips were subsequently treated with hexamethyldisilazane (HMDS) to increase their hydrophobicity. Contact angle measurements were acquired by depositing 2 µL of distilled water onto glass coverslips (OCA 20, DataPhysics Instruments GmbH). For preparation of isolated single nanowires, wires were deposited and flow aligned onto acid cleaned glass coverslips from dilute suspension (~108 nanowires/mL). The decane was allowed to evaporate giving an areal density of < 1000 nanowires/mm2. For preparation of more densely packed aligned nanowire arrays, a glass coverslip was angled at 45º and 5 µL of a more concentrated PFO nanowire suspension in decane (~1010 nanowires/mL) was deposited onto it. Immediately afterwards the coverslip was washed with ~ 0.5 mL of decane to facilitate fluidic alignment of the wires. Atomic force microscopy (AFM; Explorer, Veeco Instruments Ltd.) images were acquired in non-contact mode with commercial silicon probes. Epi-fluorescence and polarized optical microscope (POM) images were acquired using an Axioskop II Plus microscope (Carl Zeiss, Inc.) equipped with a 100 W halogen lamp, a TE cooled color CCD camera, a 100x objective and a rotatable stage, with appropriate filter sets and polarizing accessories. For far field polarization resolved photoluminescence images and spectra, a nanowire sample was placed on a motorized X-Y stage. A CW 405 nm laser diode was used for nanowire excitation in the far field. The laser power incident on the sample was less than 5 mW/cm2. The polarization of the laser light was adjusted by a sheet polarizer in the excitation path. PL emission was collected through the substrate using a 100x objective, directed through a long pass filter and into a 150 mm focal length imaging spectrometer equipped with a liquid nitrogen cooled CCD. A Glan Thompson polarizer was inserted between the long pass filter and the entrance to the spectrometer for polarized PL measurements. DISCUSSION The method of solution assisted template wetting was employed for polymer nanowire synthesis [4]. Briefly, a drop of a concentrated PFO solution was deposited onto the top surface of a porous alumina membrane. The wetted template was covered with a weight to facilitate penetration of the polymer solution into the template pores. Due to the high surface energy of the alumina membrane, a thin surface film rapidly covers the pore walls during the initial stages of wetting, forming polymer nanotubes. Nanowire formation occurs on longer time scales as the cohesive forces for pore filling are much weaker than the adhesive forces for wall wetting. [4]. To investigate the optical properties of the synthesized PFO nanowires, a PFO filled alumina template was dissolved in aqueous NaOH. Freed PFO wires were purified by decanting the supernatant, washing the insoluble nanowire material with deionized water several times and dispersing this material in decane by sonication. Isolated nanowires were prepared by drop deposition from dilute suspension and alignment on glass substrates as described earlier. A topographic AFM image of a single flow aligned PFO nanowire is shown in the inset of Figure 1a (length ~ 47 µm). Since height is unaltered by tip induced lateral broadening, measured height was taken to represent the actual nanowire diameter. From multiple radial line profiles, the mean wire diameter was determined to be ~ 200 nm.
To probe the optical properties of the nanowire shown in Figure 1a, far field polarization resolved photoluminescence imaging and spectroscopy measurements were undertaken. Polarization resolved PL images of this wire, acquired under axially polarized excitation at 405 nm, are shown in Figure 1b and Figure 1c. This data indicated that the PL emission from the wire body was most intense when the collection polarizer was oriented parallel to the nanowire long axis. Polarization resolved PL spectra of this wire are shown in Figure 1d. The spectra exhibited three peaks located at 442, 468 and 502 nm, with a shoulder at ~ 535 nm, corresponding to the 00 singlet exciton emission of the more planar, extended β-phase molecular conformation of PFO with 0-1, 0-2 and 0-3 vibronic replicas [3]. Formation of β-phase material in these nanowires may be due to the action of mechanical stresses that may have arisen within the polymer during solution assisted filling of the template pores and, afterwards, during solvent evaporation [3]. The spectral data confirmed that when the collection polarizer was parallel to the wire long axis, the PL emission was most intense. An emission dichroic ratio, DRE, of 5.6 (DRE = I|| / I⊥, where I|| and I⊥ are the parallel and perpendicular PL intensities, respectively) was determined at the 0-1 peak wavelength. The inset in Figure 1d shows the variation of DRE with wavelength. Overall, the anisotropic behavior of the wire emission suggested a degree of axial alignment of the emissive β-phase segments within the wires.
Figure 1 (a) Atomic force microscopy (AFM) image of an isolated PFO nanowire on a glass substrate. (b) Far field polarization resolved PL image of the PFO nanowire under axially polarized excitation measured with the collection polarizer oriented parallel to the wire long axis. (c) as for (b) but with collection polarizer oriented perpendicular to the wire long axis. Both PL images have the same intensity scale. Arrows indicate the excitation and collection polarizer orientations. (d) Corresponding polarized PL spectra acquired from the same nanowire. Inset: plot of emission dichroic ratio as a function of wavelength.
Figure 2a shows an epi-fluorescence image of a dense PFO nanowire array flow aligned on an acid cleaned glass substrate from decane suspension. The wires were not well dispersed with evidence of nanowire bundling and aggregation. In addition, there was not any obvious alignment of the nanowires along the macroscopic flow direction. A photograph of a water droplet on the surface of an acid cleaned glass substrate is shown in the inset to Figure 2a. The water contact angle was measured to be 13.4o indicating that the untreated substrate was highly hydrophilic. By contrast, Figure 2b shows an epi-fluorescence image of PFO nanowires flow deposited on a HMDS treated glass substrate (decane suspension). Wires were well isolated with little evidence of nanowire bundling or aggregation. There also appeared to be net alignment of the wires along to the macroscopic flow direction. The water contact angle for a HMDS treated substrate was measured to be 68.3o (Figure 2b, inset) indicating enhanced surface hydrophobicity (i.e., lower surface energy). The extent of nanowire alignment on each substrate was quantified by determining an orientational order parameter, S, defined as S = (1/2)(3〈Cos2φ 〉 − 1), where φ is the angle between a wire and the macroscopic flow direction, and brackets, 〈〉, denote an ensemble average. For wires flow deposited on an untreated substrate, S = 0.43, indicating a low degree of alignment (Figure 2c). For wires flow deposited on a HDMS treated substrate S increased substantially to 0.85 indicating a high degree of alignment (Figure 2d). The degree of nanowire alignment was markedly improved on the treated surface as the increased hydrophobicity provided stronger adhesion of polymer wires during flow deposition [5].
Figure 2 Epi-fluorescence image of an array of PFO nanowires flow deposited on (a) an untreated and (b) a HMDS treated glass substrate. Dashed white arrows indicate the macroscopic flow direction. Insets: Photograph of a water droplet on an untreated and a HMDS treated glass substrate, respectively. (c) and (d) Plots of angular distribution (about the macroscopic flow direction) of wires flow deposited on untreated and on HMDS treated glass substrates, respectively. The solid lines are Gaussian fits to the data. Polarized optical microscope (POM) images were also acquired for PFO nanowires flow deposited on a HMDS treated glass substrate; see Figure 3. When the sample was rotated between crossed polarizers, nanowire brightness exhibited maximum intensity when the macroscopic flow direction was oriented at either 45o or 135o to the analyzing polarizer (Figure 3a) and minimum intensity at either 0o or 90o relative to the analyzing polarizer (Figure 3b). This
periodic variation in optical brightness between crossed polarizers indicated that the nanowires were aligned on the substrate.
Figure 3 POM images of PFO nanowires flow aligned at a HMDS treated glass substrate acquired, as the sample was rotated in a plane between crossed polarizers, at (a) maximum brightness, (b) extinction. The solid white arrows indicate the orientation of the polarizer and analyzer with respect to the macroscopic flow direction indicated by the dashed white arrows.
Figure 4 Far field polarization resolved PL image of a PFO nanowire array flow deposited on a HMDS treated glass substrate measured with excitation and collection polarized (a) parallel and (b) perpendicular to the macroscopic flow direction. Solid white arrows indicate the polarizer orientations. (c) Corresponding polarized PL spectra acquired from the same wire array. Inset: AFM image of an array. Dashed white arrows indicate the macroscopic flow direction.
Polarization resolved PL images of nanowire arrays aligned on HMDS treated glass substrates were acquired as a function of excitation and collection polarizations; see Figure 4a and Figure 4b. This image data indicated that PL emission from the wire array was most intense when the excitation and collection polarizers were oriented parallel to the net nanowire orientation imposed by the flow alignment process. Polarization resolved PL spectra of the array are shown in Figure 4c while the inset shows a topographic AFM image of the array. As above, the emission spectra corresponded to singlet exciton emission by the PFO β-phase. The spectral data confirmed that when the excitation and collection polarizers were oriented parallel to the array orientation direction, the PL emission was most intense. The extent of emission anisotropy was quantified by determining a value for the emission dichroic ratio, DRE, of 2.2 (DRE = I|| / I⊥, where I|| and I⊥ are the parallel and perpendicular PL intensities, respectively) at the 0-1 peak wavelength. The low value of this dichroic ratio likely reflected the broad angular dispersion of nanowire orientations achieved relative to the intended macroscopic alignment direction. It is likely that the extent of this angular dispersion, apparent in both the PL and AFM image data of Figure 4, could be reduced if the uniformity of the aligning fluid flow was improved, by use of laminar flows and/or by confinement within micro-channels, for example. CONCLUSIONS The method of template wetting has been successfully employed for synthesis of poly(9,9-dioctylfluorene) nanowires in porous anodic oxide membranes. Synthesized nanowires were freed by chemical dissolution of the templates. Isolated single nanowires were deposited and flow aligned onto acid cleaned glass substrates from dilute suspension. Photoluminescence from these wires was dominated by polarized light emission from axially aligned β-phase PFO segments. Anisotropic optical properties of dense arrays of nanowires flow aligned at hydrophobized glass substrates from more concentrated suspensions was also demonstrated.
ACKNOWLEDGMENTS This work was supported by the Irish HEA PRTLI Programme and IRCSET Embark Initiative as well as the European Union Sixth Framework Programme (Nano3D project).
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