Microcavity effects and optically pumped lasing in single conjugated polymer nanowires DEIRDRE O’CARROLL 1 , INGO LIEBERWIRTH 2 AND GARETH REDMOND 1 * 1 Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland 2 Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany *e-mail: gareth.redmond@tyndall.ie Published online: 25 February 2007; doi:10.1038/nnano.2007.35 Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications 1,2 . Conjugated polymer lasers, in a variety of resonator geometries such as microcavity 3 , micro-ring 4 , distributed feedback 5 and photonic bandgap 6 structures, have been fabricated using a range of coating and imprinting techniques. Currently, one-dimensional nanowires are emerging as promising candidates for integrated, subwavelength active and passive photonic devices 7–10 . We report the first observation of optically pumped lasing in single conjugated polymer nanowires. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry–Pe ´rot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-mode excitonic laser action. Fluorene-based conjugated polymers are attractive photonic materials because they exhibit high photoluminescence quantum efficiencies, large stimulated emission cross- sections and chemically tuneable emission wavelengths 11 . Poly(9,9-dioctylfluorene) (PFO) is a prototypical main-chain liquid-crystalline homopolymer that emits in the blue and exhibits polymorphic behaviour, with striking implications for its photophysical properties 12 . Solid-state room-temperature photoluminescence quantum efficiencies of more than 60% have been measured for various thin-film phases of the polymer and very large stimulated emission cross-sections (10 215 cm 2 ) have been reported, making PFO a desirable laser medium 11,13 . One-dimensional nanostructures fabricated from conjugated polymers are the subject of much research with respect to their physical, chemical, electronic and photonic properties. A variety of methods have been developed for fabrication of these nanostructures, including electospinning 14 , assembly 15 and solution chemistry 16 techniques. Recently, a new method for formation of organic nanotubes and nanowires based on wetting of porous alumina templates has been reported 17 . We used a melt-assisted template wetting method to synthesize arrays of semicrystalline PFO nanowires (see Methods). To this end, polymer material is placed on a porous alumina membrane and heated above its glass transition temperature or melting point. Due to the high surface energy of the template, a thin surface film covers the pore walls during initial wetting, forming nanotubes with well-defined wall thickness. Nanowire formation occurs on longer time scales because the cohesive forces for pore filling are considerably weaker than the adhesive forces for wall wetting 17 . A scanning electron microscope (SEM) image of a wire array (Fig. 1a), acquired following dissolution of the alumina template (with aqueous NaOH), confirms that aligned forests of close-packed PFO nanowires are successfully generated by this method (10 9 nanowires per template). Individual nanowires, liberated from arrays by sonication in methanol, were imaged by tapping-mode atomic force microscopy (AFM) following deposition on glass substrates (Fig. 1b). Nanowire diameter, d, was found to range between 150 nm and 400 nm, with 85% of wires having diameters between 250 nm and 350 nm. This diameter dispersity reflected the inhomogeneity of internal pore diameters within the original templates. Nanowire length, L, ranged between 1 mm and 35 mm, depending on dispersion solvent and sonication time. A typical length of 6 mm was found for wires dispersed in methanol with 1 min sonication. All wires exhibited smooth outer surface morphology without pronounced bending or obvious structural defects. High-resolution SEM imaging of PFO nanowires deposited onto gold substrates (Fig. 1c) indicated that many wire tips also exhibited regular features, being cleanly fractured with terraced or planar (35%) end facets. Cryogenic transmission electron microscopy (TEM) imaging of single nanowires (Fig. 1d) indicated that the wires were semicrystalline (dark-field mode), and selected-area electron diffraction showed [00l ] zone patterns (Fig. 1e) and, in particular, (200) reflections that indicated preferential orientation of PFO chains perpendicular to the long axes of the nanowires (with interchain packing distance governed by alkyl side chain length) 18 . Intensity-normalized absorption and photoluminescence spectra of a PFO solution and of a random PFO nanowire mat are shown in Fig. 2a. Above 300 nm, the solution absorption spectrum exhibited a single band at 385 nm assigned to the inhomogeneously broadened vibronic S 0 ! S 1 transition of pristine PFO. The nanowire spectrum was considerably broadened, with a low-energy shoulder at 425 nm, which is characteristic of semicrystalline PFO (refs 12 and 18). LETTERS nature nanotechnology | VOL 2 | MARCH 2007 | www.nature.com/naturenanotechnology 180