Superconducting MgB₂ Nanohelices Grown on Various Substrates

Masses of superconducting MgB2 nanohelices and nanowires were grown on various substrates by reaction of Mg metal with diborane (B2H6). The growth occurs via a self-catalyzed vapor−liquid−solid (VLS) mechanism. It is postulated that strain in the wires due to the presence of screw dislocations is responsible for the helical morphology. The several hundred micrometer long helices have a range of diameters and pitch and are made up of wires from 50 to 200 nm in diameter. Magnetic measurements indicate that the helices superconduct below ∼32 K. These nanohelices could be used to manufacture highly flexible superconducting cables or “nanosolenoids.

Combinatorial Investigation of the Effects of the Incorporation of Ti, Si, and Al on the Performance of α-Fe₂O₃ Photoanodes

The effect of adding small amounts of Ti, Si, and Al on the photoelectrochemical activity of α-Fe2O3 is investigated using a high-throughput combinatorial method. Quantitative ink jet printing is used to pattern iron oxide and dopant precursors onto conductive glass substrates. Subsequent pyrolysis yields a library of doped iron oxide electrodes that are screened for photoelectrolysis activity by immersing in an electrolyte and scanning a laser over the electrodes to map the photocurrent response. When Si and Al are individually added to iron oxide at the levels we studied, the photoelectrolysis activity decreased whereas low levels of Ti addition enhanced the photocurrents. Synergistic effects were observed resulting in enhanced photocurrents when multiple impurities were added to α-Fe2O3

Photosensitization of ZnO Crystals with Iodide-Capped PbSe Quantum Dots

Lead selenide (PbSe) quantum dots (QDs) are an attractive material for application in photovoltaic devices due to the ability to tune their band gap, efficient multiple exciton generation, and high extinction coefficients. However, PbSe QDs are quite unstable to oxidation in air. Recently there have been multiple studies detailing postsynthetic halide treatments to stabilize lead chalcogenide QDs. We exploit iodide-stabilized PbSe QDs in a model QD-sensitized solar cell configuration where zinc oxide (ZnO) single crystals are sensitized using cysteine as a bifunctional linker molecule. Sensitized photocurrents stable for >1 h can be measured in aqueous KI electrolyte that is usually corrosive to QDs under illumination. The spectral response of the sensitization extended out to 1700 nm, the farthest into the infrared yet observed. Hints of the existence of multiple exciton generation and collection as photocurrent, as would be expected in this system, are speculated and discussed

CdSe/ZnS Core/Shell Quantum Dot Sensitization of Low Index TiO₂ Single Crystal Surfaces

CdSe/ZnS Core/Shell Quantum Dot Sensitization of Low Index TiO2 Single Crystal Surface

Influence of the Aggregation of a Carbazole Thiophene Cyanoacrylate Sensitizer on Sensitized Photocurrents on ZnO Single Crystals

Dye sensitization of zinc oxide single crystals by a carbazole thiophene cyanoacrylate (MK-2) sensitizer deposited from THF and mixtures of THF and water was investigated. AFM images show the formation of larger aggregates, with the maximum size of 20–30 nm from mixtures of THF and water, compared with 8–12 nm from pure THF. Sensitized photocurrent spectra were correlated with the morphological results from AFM imaging and indicate that aggregation in water results in less efficient sensitization of the ZnO substrate. The presence of the aggregation in solution due to water content was confirmed by absorbance and fluorescence spectroscopies

Probing the Relative Photoinjection Yields of Monomer and Aggregated Dyes into ZnO Crystals

Cyanine dyes, often used in dye-sensitized solar cells (DSSCs), form a range of molecular species from monomers to large H and J aggregates in both solution and when adsorbed at a photoelectrode surface. To determine the relative capability of the different dye species to inject photoexcited electrons into a wideband gap oxide semiconductor, sensitization at a single-crystal zinc oxide surface was studied by simultaneous attenuated reflection (ATR) ultraviolet–visible (UV–vis) absorption and photocurrent spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface with simultaneous photocurrent spectroscopy to identify the contribution of the various dye forms to photocurrent signal. We study the dye 2,2′-carboxymethylthiodicarbocyanine bromide that is particularly prone to aggregation both in solution and at the surface of sensitized oxide semiconductors

Combinatorial Search for Improved Metal Oxide Oxygen Evolution Electrocatalysts in Acidic Electrolytes

A library of electrocatalysts for water electrolysis under acidic conditions was created by ink jet printing metal oxide precursors followed by pyrolysis in air to produce mixed metal oxides. The compositions were then screened in acidic electrolytes using a pH sensitive fluorescence indicator that became fluorescent due to the pH change at the electrode surface because of the release of protons from water oxidation. The most promising materials were further characterized by measuring polarization curves and Tafel slopes as anodes for water oxidation. Mixed metal oxides that perform better than the iridium oxide standard were identified

Preparation, Applications, and Digital Simulation of Carbon Interdigitated Array Electrodes

Carbon interdigitated array (IDA) electrodes with features sizes down to 1.2 μm were fabricated by controlled pyrolysis of patterned photoresist. Cyclic voltammetry of reversible redox species produced the expected steady-state currents. The collection efficiency depends on the IDA electrode spacing, which ranged from around 2.7 to 16.5 μm, with the smaller dimensions achieving higher collection efficiencies of up to 98%. The signal amplification because of redox cycling makes it possible to detect species at relatively low concentrations (10^–5 molar) and the small spacing allows detection of transient electrogenerated species with much shorter lifetimes (submillisecond). Digital simulation software that accounts for both the width and height of electrode elements as well as the electrode spacing was developed to model the IDA electrode response. The simulations are in quantitative agreement with experimental data for both a simple fast one electron redox reaction and an electron transfer with a following chemical reaction at the IDAs with larger gaps whereas currents measured for the smallest IDA electrodes, that were larger than the simulated currents, are attributed to convection from induced charge electrokinetic flow

Electrogenerated Chemiluminescence of BODIPY, Ru(bpy)₃²⁺, and 9,10-Diphenylanthracene Using Interdigitated Array Electrodes

Interdigitated array electrodes (IDAs) were used to produce steady-state electrogenerated chemiluminescence (ECL) by annihilation of oxidized and reduced forms of a substituted boron dipyrromethene (BODIPY) dye, 9,10-diphenylanthracene (DPA), and ruthenium­(II) tris­(bypiridine) (Ru­(bpy)₃²⁺). Digital simulations were in good agreement with the experimentally obtained currents and light outputs. Coreactant experiments, using tri-n-propylamine and benzoyl peroxide as a sacrificial homogeneous reductant or oxidant, show currents corresponding to electrode reactions of the dyes and not the oxidation or reduction of the coreactants. The results show that interdigitated arrays can produce stable ECL where the light intensity is magnified due to the larger currents as a consequence of feedback between generator and collector electrodes in the IDA. The light output for ECL is around 100 times higher than that obtained with regular planar electrodes with similar area

Templated Homoepitaxial Growth with Atomic Layer Deposition of Single-Crystal Anatase (101) and Rutile (110) TiO₂

Homoepitaxial growth of highly ordered and pure layers of rutile on rutile crystal substrates and anatase on anatase crystal substrates using atomic layer deposition (ALD) is reported. The epilayers grow in a layer-by-layer fashion at low deposition temperatures but are still not well ordered on rutile. Subsequent annealing at higher temperatures produces highly ordered, terraced rutile surfaces that in many cases have fewer electrically active defects than the substrate crystal. The anatase epitaxial layers, grown at 250 °C, have much fewer electrically active defects than the rather impure bulk crystals. Annealing the epilayers at higher temperatures increased band gap photocurrents in both anatase and rutile