Dual color four-wave-mixing is used to visualize individual gold nanowires and single carbon nanotubes. The
strong nonlinear signals, which are detected at the anti-Stokes frequency, originate from the electronic response
of the nanostructures. In gold nanowires, the collective electron motions produce detectable coherent anti-Stokes signals that can be used to study the orientation and relative strength of the structure's plasmon resonances. In single walled carbon nanotubes, coherent anti-Stokes contrast can be used to map the orientation of the electronic resonances in single tubes. Coherent anti-Stokes imaging of the material's electronic response allows the first
close-ups of the coherent nonlinear properties of individual structures and molecules.
We report a fabrication technique that is potentially capable of producing arrays of individually addressable nanowire sensors with controlled dimensions, positions, alignments, and chemical compositions. The concept has been demonstrated with electrodeposition of palladium wires with 75 nm to 350 nm widths. We have also fabricated single and double conducting polymer nanowires (polyaniline and polypyrrole) with 100nm and 200nm widths using electrochemical direct growth. Using single Pd nanowires, we have also demonstrated hydrogen sensing. It is envisioned that these are the first steps towards nanowire sensor arrays capable of simultaneously detecting multiple chemical species.
Parallel arrays of long (> 500 m), dimensionally uniform nanowires composed of molybdenum, copper, nickel, gold, and palladium were electrodeposited. Nanowires with diameters in the range from 15 nm to 750 nm were obtained by electrodepositing these metals, or metal oxides, selectively at the step edges present on the surface of a highly oriented pyrolytic graphite electrode. Depending on the metal, either of two methods were used to carry out electrochemical step edge decoration (ESED): Nanowires of Ni, Cu or Mo were prepared by electrodepositing nanowires of a conductive metal oxide such as NiO, Cu<sub>2</sub>O, or MoO<sub>2</sub>. Nanowires of the parent metal were then obtained by reducing the metal oxide nanowires in hydrogen at elevated temperature. Nanowires composed of noble metals and some coinage metal can be obtained by direct electrodeposition of the metal at step edges. Direct electrodeposition involved the application of three voltage pulses in succession: An oxidizing "activation" pulse, a large amplitude, reducing "nucleation" pulse, and a small amplitude reducing "growth" pulse. These nanowires arrays were "portable": After embedding the nanowires in a polymer film, arrays of nanowires could be lifted off the graphite surface thereby facilitating the incorporation of these arrays in devices such as sensors.
Bilayers of cadmium stearate (Cd-stearate) and monolayers of dipalmitoyl phosphatidylcholine (DPPC) have been assembled on highly oriented pyrolytic graphite (HOPG) substrates and investigated in air using the scanning tunneling microscope (STM). STM images of Cd- stearate bilayers reveal a regular two-dimensional lattice of high tunneling current vertices which are separated by 4 - 6 angstroms. Few defects are observed in these lattices. 'High (tau) ' bilayers, which were deposited with a transfer ratio, (tau) approximately equals 2.0, have monoclinic lattices whereas the lattices of 'low (tau) ' bilayers, with (tau) equals 1.4 - 1.6, are orthorhombic. Molecular resolution STM data is also presented for DPPC monolayers prepared by the conventional Langmuir-Blodgett vertical deposition method. In monolayers of these structurally more complex amphiphiles, greater disorder is evident. Prolonged STM scanning of either Cd-stearate bilayers or DPPC monolayers induces modification of the 'native' crystalline structure presumably due to tip-film interactions.