Highly Anisotropic Luminescence from Poly(9,9-dioctylfluorene) Nanowires Doped with Orientationally Ordered β-Phase Polymer Chains

Abstract

We quantitatively assess the nature and extent of emissive polymer chain organization within individual poly(9,9-dioctylfluorene) nanowires fabricated by solution-assisted template wetting. A minority fraction of planar, extended β-phase polymer chains occurs within the nanowires, which dominates their luminescence behavior. Nanowires exhibit pronounced optical birefringence and detailed polarized optical microscopy analysis indicates preferential axial alignment of polymer chains. Polarization-resolved near-field photoluminescence imaging and spectroscopy methods are employed to probe local spatial variations in light emission intensity along individual nanowires. Data indicate strong axially polarized nanowire emission, with locally measured dichroic ratios up to 8.9, consistent with preferential axial alignment of emissive β-phase polymer chains. To visualize the extent of β-phase chain alignment within a typical nanowire, we generated image maps of the orientational order parameter, ⟨P_2⟩, from measured near-field emission intensity image data. Numerous domains of long-range order up to 2 μm in size with ⟨P_2⟩ values ranging from 0.47 to 0.76 are identified, indicating extensive axial alignment of emissive β-phase chains within the nanowires. Poly(9,9-dioctylfluorene) nanowires exhibiting micrometer-scale order make them attractive elements for future subwavelength organic opto-electronic devices.