Wenbo Yan

High-throughput fabrication of photoconductors with high detectivity, photosensitivity, and bandwidth.

Nanocrystalline cadmium selenide (nc-CdSe) was electrodeposited within a sub-50 nm gold nanogap, prepared by feedback-controlled electromigration, to form a photoconductive metal-semiconductor-metal nanojunction. Both gap formation and electrodeposition were rapid and automated. The electrodeposited nc-CdSe was stoichiometric, single cubic phase with a mean grain diameter of ∼7 nm. Optical absorption, photoluminescence, and the spectral photoconductivity response of the nc-CdSe were all dominated by band-edge transitions. The photoconductivity of these nc-CdSe-filled gold nanogaps was characterized by a detectivity of 6.9 × 10(10) Jones and a photosensitivity of 500. These devices also demonstrated a maximum photoconductive gain of ∼45 and response and recovery times below 2 μs, corresponding to a 3 dB bandwidth of at least 175 kHz.

A chemically-responsive nanojunction within a silver nanowire.

The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1-5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH(3)) induced a rapid (<30 s) and reversible resistance change that was as large as ΔR/R(0) = (+)138% in 7% NH(3) and observable down to 500 ppm NH(3). Exposure to water vapor produced a weaker resistance increase of ΔR/R(0,H(2)O) = (+)10-15% (for 2.3% water) while nitrogen dioxide (NO(2)) exposure induced a stronger concentration-normalized resistance decrease of ΔR/R(0,NO(2)) = (-)10-15% (for 500 ppm NO(2)). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type Ag(x)O is formed within the CRJ and the binding of molecules to this Ag(x)O modulates its electrical resistance.

A lithographically patterned capacitor with horizontal nanowires of length 2.5 mm.

A symmetrical hybrid capacitor consisting of interdigitated, horizontal nanowires is described. Each of the 750 nanowires within the capacitor is 2.5 mm in length, consisting of a gold nanowire core (40 × ≈200 nm) encapsulated within a hemicylindrical shell of δ-phase MnO2 (thickness = 60-220 nm). These Au@δ-MnO2 nanowires are patterned onto a planar glass surface using lithographically patterned nanowire electrodeposition (LPNE). A power density of 165 kW/kg and energy density of 24 Wh/kg were obtained for a typical nanowire array in which the MnO2 shell thickness was 68 ± 8 nm. Capacitors incorporating these ultralong nanowires lost ≈10% of their capacity rapidly, during the first 20 discharge cycles, and then retained 90% of their maximum capacity for the ensuing 6000 cycles. The ability of capacitors consisting of ultralong Au@δ-MnO2 nanowires to simultaneously deliver high power and high capacity with acceptable cycle life is demonstrated.

Water Oxidation Using a Cobalt Monolayer Prepared by Underpotential Deposition

Development of electrocatalysts for the conversion of water to dioxygen is important in a variety of chemical applications. Despite much research in this field, there are still several fundamental issues about the electrocatalysts that need to be resolved. Two such problems are that the catalyst mass loading on the electrode is subject to large uncertainties and the wetted surface area of the catalyst is often unknown and difficult to determine. To address these topics, a cobalt monolayer was prepared on a gold electrode by underpotential deposition and used to probe its efficiency for the oxidation of water. This electrocatalyst was characterized by atomic force microscopy, grazing-incidence X-ray diffraction, and X-ray photoelectron spectroscopy at various potentials to determine if changes occur on the surface during catalysis. An enhancement of current was observed upon addition of PO4(3-) ions, suggesting an effect from surface-bound ligands on the efficiency of water oxidation. At 500 mV overpotential, current densities of 0.20, 0.74, and 2.4 mA/cm(2) for gold, cobalt, and cobalt in PO4(3-) were observed. This approach thus provided electrocatalysts whose surface areas and activity can be accurately determined.