Jeremiah K. N. Mbindyo

Electric-field assisted assembly and alignment of metallic nanowires

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 SiO2 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 4.5×10−6 Ω cm.

DNA-Directed Assembly of Gold Nanowires on Complementary Surfaces**

branes. The tubes are well ordered but somewhat tilted due to their relatively low mechanical strength compared with cylinders. The Raman spectra of the tubes have shown two broad peaks centered on 1345 and 1565 cm ‐1 , indicating that the tubes consist mainly of DLC. All the nanotubes have a uniform outer diameter of about 300 nm. The wall thickness of the tubes is very small, which can be controlled by optimizing the growth conditions. The tube length was about 7 lm, as in the case of cylinders. All the tubes have uniform height and are found to be hollow inside. Such nanotube arrays are probably useful for field emitters as well as for depositing metal catalysts or enzymes in the tubes, which in turn may be useful for new technologies. Other applications presumably include their use as porous electrodes in electrochemistry, as conductive diamond is known to exhibit outstanding electrochemical properties such as low background current, wide electrochemical potential window and high resistance to deactivation. [19] The present technique is a simple one for producing highly ordered diamond nanocylinders and nanotubes in high yield. The dimensions of these nanofibers are easily controllable by varying the pore dimensions of the alumina membrane. This technique enables others to adopt it easily and study the physical properties of these arrays for various applications in fieldemission displays, photonic bandgap materials, composite materials, and electrochemistry.

Layer-by-layer self-assembly strategy for template synthesis of nanoscale devices

Abstract A combined membrane replication/layer-by-layer synthetic approach to preparing nanoscale rod-shaped rectifiers is described. Alumina and polycarbonate (PC) membranes (pore diameters 200, 100 and 70 nm) were used as templates for the electrochemical preparation of free-standing Au nanowires several microns in length. Wet layer-by-layer self-assembly of nanoparticle (TiO2 or ZnO)/polymer multilayer films was performed inside the membrane pores in two ways. (1) Growing the film between metal electrodeposition steps to give in-wire junctions; (2) first coating the membrane walls with multilayer films, and then growing nanowires inside the resulting tubules to give concentric structures. TiO2/PSS, ZnO/PSS (PSS=polystyrenesulfonate) and ZnO/PAN (PAN=polyaniline) assembly was driven by electrostatic and covalent-coordination interactions, respectively. The current–voltage (I–V) characteristics of nanowires containing semiconductor nanoparticles show current rectifying behavior. Current rectification appears to arise at the oxide semiconductor–metal interface. Switching behavior and hysteresis, which was found in all devices, was particularly evident in junctions containing anionic PSS and cationic TiO2 particles, and less evident in ZnO-containing devices.

Room temperature negative differential resistance in molecular nanowires

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.

Template synthesis of polymer-insulated colloidal gold nanowires with reactive ends

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.

Pollutant Decomposition with Simultaneous Generation of Hydrogen and Electricity in a Photogalvanic Reactor

A small laboratory-scale reactor employing a nanocrystalline titanium dioxide anode and a platinum black cathode was evaluated for pollutant decomposition with simultaneous reduction of water to produce hydrogen. The reactor requires only light as an energy input, and operates as a photogalvanic cell, producing electricity. Oxidative photodegradation of 4-chlorophenol, 2,4,5-trichlorophenol, and 4,4{prime}-dichlorobiphenyl were achieved. Solutions of 0.1 mM chlorophenols were decomposed in 2 to 3 h, with an average turnover of 5.4 {times} 10{sup 15} molecules/cm{sup {minus}2} s{sup {minus}1}. Complete degradation of chlorophenols to carbon dioxide, water, and chloride ion was achieved in less than 6 h. Poorly water soluble 4,4{prime}-dichlorobiphenyl adsorbed onto soil and suspended in a pH 13 anode solution was also decomposed. Hydrogen gas was produced at the cathode at a rate 1.4 ml/h cm{sup 2} of electrode when using a pH 13 anode solution and a pH 1 cathode solution. The average reactor potential under these conditions was 1.53 V. Power output was 0.36 mW at a current density of 2 mA/cm{sup 2}.

CURRENT-VOLTAGE CHARACTERIZATION OF ALKANETHIOL SELF-ASSEMBLED MONOLAYERS IN METAL NANOWIRES

An electric-field assisted assembly has been used to place rod-shaped, metal-organic, molecule-metal nanowires onto lithographically defined metal pads allowing the electrical characterization of metal-molecule self-assembled monolayer-metal containing nanowires. Our results show that the parameters of metal-molecule metal junctions are close to previously published data, so we have constructed systems containing insulating monolayers with reasonable properties.

Electrochemical Synthesis of Multi-Material Nanowires as Building Blocks for Functional Nanostructures

Nanostructures are electrochemically deposited into alumina or polycarbonate templates resulting in monodisperse, anisotropic particles with a range of tunable sizes. These particles have been synthesized with diameters of 20-250 nm and with lengths of 1-10 µm. Currently, structures have been made with stripes of Au, Ag, CdSe, Co, Cu, Ni, Pd, and Pt. These materials offer a variety of different properties. In particular, many of the metals in this group are excellent conductors, meaning these particles can actually be used as nanowires. Co and Ni are ferromagnetic and may be used for separation or assembly. CdSe is a semiconductor, possibly allowing for the synthesis of electronic devices such as transistors. Furthermore, many of these materials have different surface chemistries, making the orthogonal functionalization and assembly of these nanowires more accessible. This research focuses on increasing the number of materials available, especially semiconductors, incorporating these potentially useful materials into multilayered nanowires and evaluating their electrical properties, either individually or in small bundles. In addition, the surface chemistry of the various materials in the nanowires is being compared to aid in orthogonal self-assembly of functional nanostructures such as memory devices. The work presented will demonstrate the effects of rod composition on electrical properties. In particular, the effects of changing the work function of the materials on either side of a semiconductor to form Schottky junctions or ohmic contacts will be shown.

Layer-by-layer Assembly of Nanotubes and Nanofilms from Nanoparticle and Polymer Blocks for Electronic Applications

The near-future trends in computing, electronics, optoelectronics have put forward the necessity in radically new concepts of fabricating ultrahigh density electronic circuits. A perspective approach to this problem is constructing the circuits from the ground up using nanoscale There have been numerous reports of molecule- and polymer-based devices that can function as rectifiers, transistors, and switches [1,2]. Some elaborate logic circuits based on molecular diodes [3], and nanoscale crossbar arrays that contain bistable memory elements [4] have recently been proposed. Obviously, current rectification is needed at each crossing point in order to isolate individual memory elements from each other. Real challenge to practical use of these devices is the problem of making the appropriate connections between nanoscale devices, and their integration into the larger scale circuits.