Ordered poly(9,9-dioctylfluorene) nanopillars with β-phase morphology were obtained by replicating from nanoporous alumina via template wetting at room temperature. As explained in the experimental section, the template were fabricated by two-step anodization process in phosphoric acid, yielding templates with an average pore depth of 500 nm and pore diameter of 225 nm (Figure ). A scanning electron microscopy (ESEM) image of a PFO nanopillar array acquired after the template dissolution is shown in Figure .
Figure 1 ESEM images of a top view of the self-ordered alumina template (Inset: cross section of the template), b PFO nanopillars after removing the template. c The chemical structure of a segment of a PFO chain in the β-phase conformation. The rotational (more ...)
Figure shows the UV–Vis absorbance spectra of a PFO solution in chloroform and a PFO film. The nanopillars spectrum was not possible measured due to the high concentration of the solution that we used in order to fabricate the structures. The solution absorption spectra exhibited a band at 373–395 nm assigned to the S0
0-0 transition of PFO. The film spectrum was red-shifted and broader with a band at 388 nm and a low-energy shoulder at 435 nm characteristic of β-phase PFO [4
a UV–Vis absorption and PL spectra of PFO solution, PFO film and PFO nanopillars. b X-ray diffraction patterns of PFO film and PFO nanopillars (curves are offset for clarity).
The photoluminescence (PL) spectra of a PFO solution in chloroform, PFO film and an entire sample of ordered PFO nanopillars are also shown in Figure . The solution PL spectrum exhibited a characteristic vibronic progression with peaks located at 418, 437 and 463 nm due to S0
0-0 singlet exciton transition of solution PFO with 0-1, 0-2 and 0-3 vibronic replicas. The PFO film and PFO nanopillar PL spectrum were red-shifted compared to those to the solution, suggesting a narrowed distribution of PFO chain segments with increased conjugation lengths [19
]. The nanopillars spectrum exhibited the emission peaks at 439, 464 and 496 nm, which agree with the PL spectrum of β-phase PFO film described in the literature [20
]. If we compare the film spectra and the nanostructure spectra, we can observe different intensity in the emission peak at 439 nm. This result seems to indicate that the optical properties of the polymer nanostructure could be affected by the polymer morphology into the nanopores. It could be attributed to ordering or axial alignment of polymer chains into the nanopores of the template during the infiltration process [21
Grazing incidence X-ray diffraction (GIXRD) profiles acquired for a PFO film and PFO nanopillars are shown in Figure . The peaks appear at 2θ of 6.98° and 21.16° for PFO film and at 2θ of 7.00° and 21.10° for ordered PFO nanopillars. The peak at 7.0 degrees corresponds to (200) plane, which is in agreement with the XRD peak of the β-phase PFO [20
These results obtained from PL and X-ray measurements reveal that the ordered PFO nanopillars fabricated via template wetting using nanoporous alumina as template are obtained with β-phase morphology.
Raman spectroscopy characterization is also performed in order to study the different polymer conformation into the nanoporous template. Raman studies can provide structural information on conjugated polymers, necessary for understanding of optical and electronic properties and the development of devices in which they are used as active layers. We have studied the effect on the Raman spectrum of the polymer chain orientation. The nanostructured samples were excited with the laser beam polarized parallel to the orientation direction of the pillars and the Raman signal measured polarized either parallel or perpendicular to the polymer backbones. Figure shows the Raman spectra in the 1,000–1,700 cm-1
range for PFO film and PFO nanopillars. The most intense band is located at 1,604 cm-1
and is assigned to the phenyl intra-ring C–C stretch mode [4
]. We observed that the intensity of the signal polarized parallel to the excitation is higher than that polarized perpendicular when the spectra is acquired for PFO nanopillars, but there are no changes in the intensity of the peaks when the sample is a PFO film. These Raman spectra indicate that the chains of the polymer are mainly parallel to the pillar in the nanostructure, but in the film, the chains are not aligned with respect to the laser beam. These results are in agreement with the PL measurements showed previously, where the emission spectrum is affected by the nanostructure.
Raman spectra of PFO film and nanopillars. The excitation laser is polarized parallel to the polymer chains and detection is polarized parallel (dashed line) and perpendicular (solid line).