This letter describes an electric-field assisted assembly technique used to position individual nanowires suspended in a dielectric medium between two electrodes defined lithographically on a SiO 2 substrate. During the assembly process, the forces that induce alignment are a result of nanowire polarization in the applied alternating electric field. This alignment approach has facilitated rapid electrical characterization of 350-and 70-nm-diameter Au nanowires, which had room-temperature resistivities of approximately 2.9 and 4.5ϫ10 Ϫ6 ⍀ cm.

The coverage of Au surfaces with Au nanowires by linking them with DNA offers great prospects for the assembly of wire structures with particular connectivity. This work presents the first example of using DNA hybridization to control the assembly of micrometer‐size inorganic particles on surfaces. The Figure shows the optical microscopy image of Au nanowires modified with DNA.

Metal nanowires containing in-wire monolayer junctions of 16-mercaptohexanoic acid were made by replication of the pores of 70 nm diameter polycarbonate track etch membranes. Au was electrochemically deposited halfway through the 6 microm long pores and a self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid was adsorbed on top. A thin layer of Au was then electrolessly grown to form a metal cap separated from the bottom part of the wire by the SAM. Electron micrographs showed that the bottom and top metal segments were separated by an approximately 2 nm thick organic monolayer. Current-voltage measurements of individual nanowires confirmed that the organic monolayer could be contacted electrically on the top and bottom by the metal nanowire segments without introducing electrical short circuits that penetrate the monolayer. The values of the electrical properties for zero-bias resistance, current density, and breakdown field strength were within the ranges expected for a well-ordered alkanethiol SAM of this thickness.

Metal-CdSe-metal (metal ) Au, Ni) nanowires were grown by electrochemical replication of porous aluminum oxide and polycarbonate track etch membranes with pore diameters of 350 and 70 nm, respectively. The lengths of the individual segments of the nanowires were controlled by varying the amount of charge that was passed. The composition of the CdSe segments was characterized by energy-dispersive X-ray spectroscopy. A 1:1 ratio could be obtained, and Cd-and Se-rich stoichiometries were also made by adjusting the concentrations of Cd 2+ and SeO 2 in the aqueous plating solutions. X-ray powder diffraction showed the presence of both zinc blende and wurzite phases, and grain sizes on the order of 10 nm were observed by TEM. The nanowires were resistive in the dark but showed pronounced visible light photoconductivity.

Alumina membranes containing 200 nm diameter pores were replicated electrochemically with metals (Au and Ag) to make free-standing nanowires several microns in length. Wet layer-by-layer assembly of nanoparticle (TiO2 or ZnO)/polymer thin films was carried out in the membrane between electrodeposition steps to give nanowires that contained rectifying junctions. Concentric structures with similar properties were prepared by first coating the membrane walls with multilayer films, and then growing nanowires inside the resulting tubules, or by growing films on the exposed surface of the nanowires after dissolution of the membrane. The I−V characteristics of nanowires prepared by either technique show current rectifying behavior. The electronic properties of Au(MEA)/(ZnO/PSS)19ZnO/Ag (MEA = mercaptoethylamine) devices indicate that rectification is determined by charge injection at the metal/ZnO/PSS-film interface rather than by a tunneling mechanism. In the case of Ag(TiO2/PSS)9TiO2/Au devices, switching behavior and hysteresis that could not be described by Schottky or Fowler−Nordheim characteristics was found. The combined replication/layer-by-layer synthetic approach allows one to control both the geometry and the chemical composition of diode nanowire devices.

Electric-field assisted assembly has been used to place rod-shaped metal nanowires containing 4-{[2-nitro-4-(phenylethynyl)phenyl]ethynyl}benzenethiol molecules onto lithographically defined metal pads. In these systems the current-voltage characteristics exhibited negative differential resistance at room temperature with on-off peak-to-valley ratios of 1.80 to 2.21.

Alternate adsorption of anionic and cationic polyelectrolytes creates smooth organic films on the walls of template-grown Au nanowires, which can be made chemically reactive on their ends by removal of a sacrificial Ag layer. † Electronic supplementary information (ESI) available: colour versions of Scheme 1 and Figs. 1 and 2. See http://www.rsc.org/suppdata/cc/b0/ b007999p/ Scheme 1 Strategy for formation of a multilayer polyelectrolyte film on a gold nanowire array.

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