Nontemplated Routes

There are a number of synthetic routes that are capable of producing nanoparticles in the absence of a precursor substrate or a molecular template.

For example, He and co-workers [144] have described a technique wherein polyaniline is deposited between two nanoelectrodes to form a nanojunction. As the junction is decreased in size to just a few nanometers, abrupt switching characteristics are observed as opposed to the more gradual transition observed during electrochemical switching of polyaniline structures with larger dimensions. A further report from the same laboratory [145] added a new dimension by initiating growth of polyaniline between a scanning tunneling microscope (STM) tip and a gold substrate. The STM tip was modified to allow only a few nm to take part in the growth of the nanowire. During growth the tip and substrate were monitored at 20-100 nm resolution. After growth the nanowire could be stretched by moving the STM tip to produce wires up to 200 nm in length and with diameters as low as 6 nm. These structures had conductivities of about 5 S cm-1.

Electrodeposition of polyaniline containing [C60] fullerene as dopant [146, 147] has been shown to result in 2D or 3D fibrillar structures [147, 148]. Diameters from 10 to 100 nm with fibril lengths of up to 3000 nm were observed. The fib-rillar network had conductivities in the range 10-100 S cm-1. Two-dimensional networks have been prepared using pulsed potential techniques while 3D networks were observed to form over longer synthesis times. Via this approach there is apparent control over the density of contact points for the individual nanonetworks.

Others [148] have used electron bean lithography to create polythiophene-based nanopatterns withs linewidths down to 50 nm and gaps of 10 mm. They have shown that after exposure to a 50 kV electron beam poly(3-octylthiophene) becomes less soluble in chlorobenzene, enabling development of nanotracks that have conductivity of the order of 1 S cm-1.

Electrospinning [149, 150] is another recent nontemplated method. This simple approach is based on the electrostatic fiber spinning of composite fibers of polyaniline with polyethylene, oxide, polystyrene, or polyacrylonitrile. These fibers are formed when a high electric field (5-14 kV) is placed between the tip of a metallic anodic spinning needle loaded with the dissolved polymer solution (0.5-4 wt% PAn and 2-4 wt% host polymer) and an opposing cathode plate separated by 20 cm. The presence of the high electric field results in the surface tension of the polymer-loaded solution at the needle tip to be exceeded, expelling a polymer fiber from the surface toward the opposing cathodic plate. The transit time from the anode tip to the cathode plate is accompanied by a desolvation and drying process in part assisted by the electrostatic charges placed upon the solvent molecules causing electrostatic repulsion. The resulting nanofiber composite is reported to have lengths in the meter range and is collected as an interwoven mesh with large surface-to-volume ratios (~103 m2/g). These fibers are ohmic in nature. Fiber dimensions of less than 100 nm have been routinely produced by this technique. More recently [149] fibers of PAn have been directly spun from a 20 wt% solution of PAn (VersiconTM) in 98% sulfuric acid at 5 kV. The electrospinning method has also been extended to produce continuous polyaniline/polyethylene oxide monofilament nanofibers down to 60 nm diameter at a maximum spin rate of 1130 m/min and a conductivity of 33 S cm-1 [151].

Aligned polymer nanowire structures have been developed using an electrochemical deposition technique to form aligned polyaniline arrays on smooth and textured electrode substrates without the need for a porous templating structure [152, 153]. Oriented polymer nanowires were formed using a preprogrammed constant-current deposition method where the current density was stepped down throughout the nanowire growth. In the initial stage of nanowire growth, 50 nm polymer nuclei were deposited from an aqueous electrolyte containing 0.5 M aniline and 1.0 M perchloric acid at a current density of 0.08 mA cm-2 over 30 min. The current density was subsequently stepped down over a 3 hr period from 0.04 to 0.02 mA cm-2. Extended polyani-line growth in the initial nucleation phase resulted in the formation of a thick polymer deposit with randomly oriented, thick-branched polymer structures over the surface. Typical nanowire structures had diameters for 50 to 70 nm and lengths of approximately 0.8 / m. Complex surface geometries have been demonstrated using this technique where nanowires have been deposited onto ordered silica spheres deposited onto the electrode substrate as well as onto textured surfaces to produce hierarchical structures. The potential for these surfaces in chemical sensing has been demonstrated for H2O2 detection utilizing iron(III) hexacyanoferrate nanoparticle catalysts supported by the nanowire structure.

Polypyrrole nanowire have also been electrochemically synthesized on graphite/paraffin composite electrodes from a 0.2 M phosphate buffer (pH 6.86) electrolyte containing 0.2 to 2.0 M lithium perchlorate and 0.1 to 0.5 M pyrrole [154]. Nanowires were grown by cyclic voltamme-try and potential step methods. Nanorod growth is directed by inducing microstructure at the electrode surface. This was achieved by predigesting the graphite rod electrode in boiling 10% hydrochloric acid and then boiling 10% nitric acid for 30 min, respectively, resulting in a porous graphite rod surface which was subsequently in-filled with paraffin at 150 °C and finally polished. The resultant electrode surface was covered with nitro-groups to which the pyrrole monomer selectively adsorbs providing a nucleation zone for nanotube formation. The resultant tubes were unaligned and irregularly dispersed across the surface as an entangled mesh with diameters of ca. 50-200 nm.

Thin 50 to 100 nm freestanding polypyrrole nanofilms have been synthesized utilizing oxidative interfacial polymerization at a water-chloroform interface [155]. Thicker 3 to 4 /m polypyrrole films have been produced via this method [156]. Control over the final film thickness was achieved by removal of the immiscible solvents from above and below the nucleating film after a few minutes of polymerization. Stirring of the interfacial solutions resulted in the inhibition of polymer nucleation at the interface. Thin 5 nm polypyr-role films on mica and graphite surfaces have been formed via a admicellar polymerization route [157]. The thin films were formed due to the surfactant (CTAB and SDS) in solution adsorbing to the substrate surface as a thin film micelle containing the pyrrole monomer, which was subsequently polymerized by ferric chloride solution. On the graphite surface the polypyrrole morphology consisted of thin discs and interlinked islands. On the mica surface the morphology consisted of randomly scattered islands with no continuous film formation evident.

Dip-pen nanolithography has been performed on a number of surfaces via a coated atomic force microscope (AFM) tip at the 130 nm scale up to several micrometers using SPAN as the writing ink [158]. Transfer of the conducting polymer ink from the AFM tip to the substrate is achieved via capillary action upon tip contact to the substrate surface with 0.5 nN contact force. The deposited SPAN tracks were found to electroactive.

Near-field optical lithography has also been performed to draw nanostructures of poly(p-phenylene vinylene) (PVP) using light delivered from a scanning near-field fiber probe, with a 40 or 80 nm optical aperture, on a scanning near-field optical microscope AFM device [159]. Polymer features are drawn by exposing the PVP soluble precursor to ultraviolet light (325 nm HeCd laser, 0.2 mW at the sample surface) rendering the exposed precursor insoluble and permitting the dissolution of the remaining polymer. Following chemical conversion for the exposed PVP precursor a fully conjugated polymer can be achieved. Two-dimensional photonic crystal arrays have been demonstrated by this method with pillars 32 nm tall, 200 nm wide (at half height) and a lattice constant of 333 nm (ca. 30 x 30 dot lattice).

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