Polarized Luminescence from Single Polymer Nanowires and Aligned Nanowire Arrays
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0948-B09-04
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 c
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