Waveguiding, Microcavity Effects and Optically Pumped Lasing in Single Melt Processed Polyfluorene Nanowires
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0965-S03-05
Waveguiding, Microcavity Effects and Optically Pumped Lasing in Single Melt Processed Polyfluorene Nanowires Deirdre O'Carroll 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 melt-assisted wetting of nanoporous alumina membrane templates is reported. Polymer nanowires exhibiting blue photoluminescence emission with diameters of approx. 300 nm are obtained. Absorption and photoluminescence spectra of nanowire mats indicate that the nanowires consist of the semi-crystalline form of the polymer. Single wires exhibit active waveguide behavior whereby excited luminescence propagates towards and out-couples at the tips of the nanowires. Spectra of out-coupled emission exhibit multiple modes with mode spacing corresponding to longitudinal Fabry-Pérot cavities of equivalent lengths to those of the nanowires. Above a threshold incident pump energy spectral collapse to a single blue shifted peak with instrument limited line width occurs characteristic of single mode lasing. 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 quantum efficiencies and gain spectra that span the visible range [2]. In particular, poly(9,9-dioctylfluorene), PFO, is a thermally processable polymer showing thermotropic liquid-crystalline behavior with melting into a birefringent liquid occurring at ~ 156 o C [3]. In this paper, we report on the synthesis of emissive PFO polymer nanowires with homogeneous cylindrical geometries and well-defined end facets by melt-assisted template wetting [4]. Single PFO nanowires fabricated in this way exhibit distinct photonic functions such as active waveguiding and optically pumped lasing, demonstrating their potential for integration into future nanoscale photonic devices and systems. EXPERIMENTAL For synthesis of PFO nanowires by melt-assisted template wetting, a 40 mg / mL solution of PFO (American Dye Source, Inc.) in anhydrous THF was prepared. Drops (~ 30 µL) of this PFO solution were deposited onto glass substrates and allowed to dry to form layers of PFO. These layers were delaminated from the glass substrates by soaking in methanol or deionized water for a few seconds. The PFO layers were then placed on top of degassed 13 mm diameter porous anodic alumina membranes (alumina templates) with nominal pore diameters of 200 nm (Whatman Ltd.) and annealed under vacuum at 250 oC for 2 h to facilitate pore filling by the polymer. Subsequently, the PFO filled template was cooled at 0.2 oC/min to 50 oC, resulti
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