Nanowires composed of the thermoelectric material Bi2Te3 were synthesized on highly oriented pyrolytic graphite (HOPG) electrodes using the electrochemical step edge decoration (ESED) method. Nanowire synthesis was initiated by applying a voltage pulse of -0.75 V versus SCE for 5 ms to an HOPG electrode in an aqueous solution containing both Bi3+ and TeO22-, thereby producing nuclei at the step edges. Bi2Te3 was electrodeposited onto these nuclei using a cyclic electrodeposition-stripping scheme that involved the electrodeposition of bismuth-rich Bi2Te3 on a negative-going voltammetric scan (to -0.05 V) and the subsequent anodic stripping of excess bismuth from these nanowires during a positive-going scan (to +0.35 V). When this cycle was repeated 10-50 times, Bi2Te3 nanowires in the 100-300-nm-diameter range were obtained. These nanowires were narrowly dispersed in diameter (RSDdia = 10-20%), were more than 100 microm in length, and were organized into parallel arrays containing hundreds of wires. Smaller nanowires, with diameters down to 30 nm, were obtained by electrooxidizing 150-nm-diameter Bi2Te3 nanowires at +0.37 V under conditions of kinetic control. This oxidation process unexpectedly improved the uniformity of Bi2Te3 nanowires, and X-ray photoelectron spectroscopy (XPS) shows that these nanowires retain a Bi2Te3 core but also have a thin surface layer composed of Bi and Te oxides. The ability of Bi2Te3 nanowires to generate electrical power was assessed by transferring ensembles of these nanowires onto cyanoacrylate-coated glass surfaces and evaporating 4-point nickel contacts. A dimensionless figure of merit, ZT, ranging from 0 to 0.85 was measured for fresh samples that were less than 1 day old. XPS reveals that Bi2Te3 nanowires are oxidized within a week to Bi2O3 and TeO2. These oxides may interfere with the application by evaporation of electrical contacts to these nanowires.

Cadmium selenide (CdSe) nanowires, 30-300 nm in diameter, were synthesized using electrochemical step edge decoration on highly oriented pyrolytic graphite (HOPG) surfaces. These CdSe nanowires were more than 100 mu m in length and were organized into parallel arrays of hundreds following the length and organization of the HOPG step edges used to nucleate these nanowires. The synthesis of CdSe nanowires involved a method derived from the cyclic electrodeposition/stripping scheme described by Sailor and co-workers (Chem. Mater. 1991, 3, 1015). Stoichiometric CdSe was obtained by electrodepositing CdSe together with excess elemental cadmium and selenium, followed by the selective oxidative stripping of both excess cadmium and selenium from these nanowires. This synthesis method also afforded precise control of the nanowire diameter over the above-mentioned range. CdSe nanowires were characterized by scanning electron microscopy, transmission electron microscopy (TEM), selected area electron diffraction, photoluminescence, X-ray photoelectron spectroscopy, and Raman spectroscopy. The TEM results showed that the CdSe nanowires were composed of nanocrystalline, cubic CdSe with a crystallite size that decreased with increasing pH. CdSe nanowires showed band edge photoluminescence at 1.74 eV that increased in intensity by a factor of 15 when these wires were covered by a shell of CdS by exposure to gaseous H2S at 300 degrees C.

Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoO(x) wires (a mixture of MoO(2) and MoO(3)) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoO(x) precursor wires were converted to MoS(2) by exposure to H(2)S either at 500-700 degrees C, producing "low-temperature" or LT MoS(2) nanowires that were predominantly 2H phase, or above 800 degrees C producing "high-temperature" or HT MoS(2) ribbons that were predominantly 3R phase. The majority of these MoS(2) wires and ribbons were more than 50 microm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS(2) nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV-visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS(2) nanowires were structurally distinct: LT MoS(2) wires were hemicylindrical in shape and nearly identical in diameter to the MoO(x) precursor wires from which they were derived. LT MoS(2) wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS(2); HT MoS(2) ribbons were 50-800 nm in width and 3-100 nm thick, composed of planar crystallites of 3R-MoS(2). These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS(2) unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS(2) wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS(2) nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS(2). These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS(2) layers). The energy shifts were proportional to 1/ L( parallel)(2), and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS(2) nanostructures were probed as a function of temperature from -33 degrees C to 47 degrees C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.

Ensembles of silver nanowires (AgNEs) with diameters ranging from 200 nm to 1.0 microm have been prepared by electrochemical step edge decoration. These AgNEs showed a rapid (< 5 s), reversible increase in resistance upon exposure to the vapor of ammonia, trimethylamine, and pyridine. The amplitude of the resistance change was up to +3000% (DeltaR/Ro)-more than 2 orders of magnitude larger than can be explained based on boundary layer scattering effects. We experimentally probe the mechanism for this resistance modulation in the case of ammonia, and we propose a model to describe it. Conductive tip atomic force microscopy was used to probe individual sections of nanowires in AgNEs; these data revealed that the resistance change caused by NH(3) exposure was concentrated within a minority (approximately 10%) of the 5-microm wire segments that were probed--not uniformly distributed along each nanowire. All AgNEs showed a temperature dependence of their resistance, alpha, that was smaller than expected for silver metal. Highly sensitive AgNEs sometimes showed a negative alpha, characteristic of semiconductors, but negative alpha values were never observed for AgNEs with a low sensitivity to NH3. AgNEs did not respond to hydrocarbons, O2, H2O, N2, CO, or Ar, but a large (DeltaR/Ro > |-50%|) irreversible decrease in resistance was seen upon exposures to acids including HCl, HNO3, and H2SO4. Based on these and other data, we propose a model in which oxidized constrictions in silver nanowires limit the conductivity of the wire and provide a means for "gating" conduction based on the protonation state of the oxide surface.

Wires composed of AgxO (1 < x < 2) with diameters ranging frorn 0.7 to 1.1 mu m were prepared by electrochemical step edge decoration on highly oriented pyrolytic graphite (HOPG) electrode Surfaces. AgxO rnicrowires were obtained by the oxidative electrodeposition of AgxO from a pH = 6 acetate-based plating Solution. Step edge selectivity, Coupled with the high nucleation density necessary for wire formation, required that the electrodeposition be carried Out within a potential window of just 10 mV. The resulting rnicrowires were characterized by scanning electron microscopy and electron diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. AgxO rnicrowires were transferred from the HOPG surface to a glass surface, and electrical contacts were applied to ensembles of between 5 and 100 transferred rnicrowires. The resistance of these microwire ensembles increased reversibly by up to 5000% upon exposure to NH3 vapor, whereas an irreversible decrease in the wire resistance was seen upon exposure to the vapors of strong acids. We propose that the mechanism responsible for this resistance modulation is identical to that proposed recently [Murray et A, Anal. Chem. 2005, 77, 52051 to account for resistance modulation by ammonia of electrodeposited silver metal nanowire ensembles.

Polycrystalline bismuth telluride (Bi2Te3) nanowires have been prepared by the step edge selective electrodeposition of Bi2Te3 on highly oriented pyrolytic graphite (HOPG) surfaces. Bi2Te3 nanowires were obtained from an aqueous plating solution containing Bi3+ and HTeO2+ using a three-step procedure: (1) potentiostatic oxidation of the graphite surface at +0.8 V (vs; saturated calomel electrode, SCE), (2) potentiostatic nucleation Of Bl(2)Te(3) at -0.6 V for 5 ms, (3) growth of Bi2Te3 from these nuclei by cyclic electrodeposition (of both Bi2Te3 and bismuth) and stripping (of bismuth only) between +0.30 V and -0.05 V at 20 mV s(-1). Control of the number of electrodeposition/stripping scans allowed the diameter of Bi2Te3 nanowires to be specified in the range from 100 to 300 nm. Bi2Te3 nanowires were narrowly dispersed in diameter (RSDdla = 10-20%), were up to 1.0 mm in length, and were organized into parallel arrays containing hundreds of wires.

Molybdenum disulfide nano- and microribbons were synthesized by a two-step, electrochemical/chemical synthetic method in which MoO2 “precursor” nanowires were first electrodeposited size-selectively on a highly oriented pyrolytic graphite (HOPG) surface. These precursor wires were then converted to MoS2 by exposure to H2S at 800−900 °C. The MoS2 ribbons prepared using this method had the 2H crystal structure of bulk MoS2, were organized into parallel arrays of hundreds of ribbons, and were up to a millimeter in length. The electronic properties of these nanoribbon arrays were probed after transferring them onto an insulator surface.