Conducting organic-metallic composite submicrometer rods based on ionic liquids.

Molecular self-assembly is an emerging subdiscipline of material science with important potential technological applications. Self-assembly is known in both molecular chemistry (for example, aggregation of molecules, molecular crystal or phase-separated polymers) and biology (for example, bilayer formation, protein folding and unfolding). Molecular self-assembly typically relies on amphiphilic molecules that can be assembled into various geometric structures (for example, spherical, cylindrical, or planar) by triggering their insolubility in a medium through the adjustment of environmental conditions, such as temperature, pH value, or solvent composition. Self-assembly plays a key role in the electronics industry in the fabrication of organic memory cells. Extensive efforts have been directed at preparing nanostructures for use in memory cells, with self-assembled structures of specified size and shape containing incorporated doping materials. The main advantages of self-assembly for electronic applications are the simple, scalable, and affordable processing methods that avoid expensive lithography techACHTUNGTRENNUNGniques. Various nanomaterials are being investigated to address these applications, including pyridinium salt crystals with ferroelectric properties prepared by slow evaporation. Self-assembled organic-based nanostructures can show excellent electrical properties. The synthesis of nanostructures at room temperature should improve process scalability without adversely impacting on the control over size and shape. Room-temperature ionic liquids (RTILs) are molten salts that are becoming increasingly important solvents due to their nonvolatility, thermal stability, and designable miscibility with cosolvents. RTILs have been used for various chemical reactions, nanomaterials synthesis, separations, and electrochemical applications, as well as biopolymer and molecular self-assembly. The synthesis of metal nanoparticles, hollow TiO2, porous SiO2, and CuCl platelets in RTIL solvents/templates is already known. Here we demonstrate the use of an RTIL for the synthesis of pyridiniumbased organic–metal composite nanostructures by electrostatically induced self-assembly. Using a simple approach, we assemble N-butyl-4-methylpyridinium tetrafluoroborate ([bmp]ACHTUNGTRENNUNG[BF4]) and chloroauric acid into hexagonal rods of Au–RTIL. This assembly results from the electrostatic complexation of the RTIL with chloroauric acid to form nanorods. Gold nanoparticles were also found embedded in these self-assembled rods, probably resulting from the photoreduction of chloroauric acid during the reaction or from reduction by ethanol during the extraction process. The aspect ratio of the Au–RTIL rods can be varied by changing the relative molar ratio of the RTIL and chloroauric acid. To our knowledge, this represents the first report of salt-assisted self-assembly of metal–organic nanostructures. The resulting rods containing gold nanoparticles are conductive and exhibit typical organic-memory-cell behavior. Anisotropic Au–RTIL particles were obtained directly by dissolving chloroauric acid in the RTIL, as observed by scanning electron microscopy (SEM). Figure 1A–C shows SEM images of drop-coated films of Au–RTIL after washing with ethanol. These SEM images show well-defined hexagonal rods with very smooth surface morphology. The rods are quite uniform, and the rod size, diameter, and aspectratio distributions are shown in Figure 1D–F. Gaussian fits to the histograms yield an average rod length of ACHTUNGTRENNUNG(3 1.5) mm, a diameter of (700 150) nm, and an aspect ratio of 5 1, respectively. There are also some fines present with the rods; these correspond to immature rods formed during the reaction. The chemical composition of these rods was determined by X-ray photoemission electron spectroscopy (XPS). The general scan spectrum of the film at room temperature showed the presence of C 1s, N 1s, B 1s, F 1s, and Au 4f core levels with no evidence of impurities. The film was sufficiently thick and, therefore, no signal was measured from the substrate (Si 2p core level). The Au 4f spectrum was recorded from the film deposited at room temperature (Figure 2A). The Au 4f spectrum could be resolved into a two [*] Dr. A. Kumar, Dr. V. Pushparaj, C. Soldano, Dr. O. Nalamasu, Prof. P. M. Ajayan Department of Material Science and Engineering Rensselaer Polytechnic Institute 110, 8th Street, Troy, NY 12180 (USA) Fax: (+1)518-2768554 E-mail: [email protected]