Showing papers by "Nathan S. Lewis published in 1990"

Fabrication and Use of Nanometer-Sized Electrodes in Electrochemistry

Abstract: Electrodes with electrochemical dimensions as small as 10 angstroms have been fabricated and used for electrochemical studies. These nanometer-scale electrodes have enabled the measurement of electron-transfer rate constants, k_(het), that are two orders of magnitude faster than k_(het) values accessible with any other electrochemical method.

Thin Films of n-Si/Poly-(CH3)3Si-Cyclooctatetraene: Conducting-Polymer Solar Cells and Layered Structures

Abstract: The optical and electronic properties of thin films of the solution-processible polymer poly-(CH(3))(3)Si-cyclooctatetraene are presented. This conjugated polymer is based on a polyacetylene backbone with (CH(3))(3)Si side groups. Thin transparent films have been cast onto n-doped silicon (n-Si) substrates and doped with iodine to form surfacebarrier solar cells. The devices produce photovoltages that are at the theoretical limit and that are much greater than can be obtained from n-Si contacts with conventional metals. Two methods for forming layered polymeric materials, one involving the spincoating of preformed polymers and the other comprising the sequential polymerization of different monomers, are also described. An organic polymer analog of a metal/insulator/metal capacitor has been constructed with the latter method.

Effects of redox potential, steric configuration, solvent, and alkali metal cations on the binding of carbon dioxide to cobalt(I) and nickel(I) macrocycles

Abstract: The binding of CO_2 to metal macrocycles has been determined electrochemically by using cyclic voltammetry or differential pulse polarography. The CO_2 binding constants, K for a series of Co(I) tetraammacrocycle complexes in (CH3)_2SO showed a strong correlation with the Co(II/I) redox potential. Although meso and d,l stereoisomers of [Co^I(Me_6[14]4,11- diene)]^+ had identical redox potentials, K_(CO_)2 differed by a factor of 10^2 for these stereoisomers, suggesting a large steric effect on CO_2 binding. Binding of CO_2 to Ni(I) tetraazamacrocycles in (CH_3)_2SO yielded a redox potential dependence similar to that of Co(I) macrocycles. A weak correlation between K_(CO_2) and the solvent dielectric constant was found for d,l-[Co(Me_6[ 14]-4,11 -diene)]^+. In tetrahydrofuran (THF), d,l-[CO(Me_6[14]4,11-diene)]^+ was found to bind CO_2 more strongly in the presence of 0.1 M Li^+ than in the presence of 0.1 M tetrabutylammonium (TBA^+). Electrochemically determined K_(CO_2)'s for Co(salen)^- and Co(Me_2salen)^- in THF indicated a strong dependence on redox potential and electrolyte cation.

Electronic properties of junctions between silicon and organic conducting polymers

Abstract: Diodes formed from semiconductor/metal interfaces often display non-ideal electronic properties. For instance, silicon/metal (Schottky) diodes made from n-type silicon and a variety of contacting metals exhibit only small differences in their rectification properties, despite theoretical and practical expectations that changes in the metal should effect changes in device properties. Similarly, Schottky diodes formed on p-type silicon generally exhibit ohmic behaviour with poor rectification characteristics. This lack of electrical response to changes in the properties of the contacting metal phase is generally attributed to interfacial reactions that take place during the high-temperature thermal or electron-beam deposition of metals onto silicon. Here we describe the fabrication of diodes using a low-temperature chemical procedure, in which contact to the semiconductor is made by a layer of the conducting organic polymer, polyacetylene. Unlike conventional metals, the electrical properties of polyacetylene can be manipulated through choice of the polymer dopant. The resultant organic/inorganic interfaces behave more ideally than contacts with conventional metals, in that changes in the electrical properties of the conducting polymer exert a large and predictable effect on the electrical properties of the resulting semiconductor/polymer diodes.

Mechanistic studies of light-induced charge separation at semiconductor/liquid interfaces

Abstract: The energy crisis of the early 1970s stimulated numerous investigations of semiconductor/liquid junctions for the conversion and storage of solar energy Although similar in concept to solid-state photovoltaic devices, semiconductor /liquid junctions offered the potential for inexpensive, chemically based energy-conversion devices, with the accompanying potential to effect the direct conversion of light into chemical fuels

Unprecedented, bridged dihydrogen complex of a cofacial metallodiporphyrin and its relevance to the bimolecular reductive elimination of hydrogen

Abstract: Recently, the authors characterized the first metalloporphyrin dihydrogen complex, Os(OEP)(H{sub 2}), and proposed that a similar ruthenium porphyrin dihydrogen complex, Ru(OEP)(H{sub 2}), is involved as an intermediate in the catalytic H/D isotopic exchange between water and hydrogen. They now report that treatment of a metal-metal-bonded cofacial ruthenium porphyrin dimer with a sterically bulky ligand in the presence of hydrogen gas has yielded the first known complex containing a dihydrogen ligand bound between two metals. Such a bridged dihydrogen complex is proposed as an intermediate in the bimolecular elimination of dihydrogen from two metalloporphyrin hydrides.

Dihydrogen complexes of metalloporphyrins: characterization and hydrogen-transfer reactivity

Abstract: La protonation d'hydrures de porphyrines metalliques aboutit a des complexes inedits dihydrogenes

XPS and EXAFS studies of the reactions of Co(III) ammine complexes with GaAs surfaces

Abstract: The chemisorption of Co(II) and Co(III) complexes onto GaAs surfaces has been investigated by x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy, and radiotracer techniques XPS of (100)-oriented n-GaAs single crystals exposed to aqueous (pH > 10) (Co{sup III}(NH{sub 3}){sub 5}X){sup n+} (X = NH{sub 3}, Br{sup {minus}}, HO{sup {minus}}) solutions indicated the deposition of a Co(II)-oxo overlayer of approximate stoichiometry Co(OH){sub 2}, with a coverage of (02-24) {times} 10{sup {minus}8} mol of Co/cm{sup 2} projected area of GaAs The reaction stoichiometry between Co(III) and GaAs was confirmed by quantitative chemical and x-ray fluorescence analysis, as well as by XPS analysis X-ray absorption spectroscopy indicated that the oxidation state of the adsorbed Co (from either Co(III) or Co(II) complexes) from aqueous pH = 12 electrolytes was Co(II)

Real-time measurements of interfacial charge transfer rates at silicon/liquid junctions

Abstract: La vitesse de transfert de charge a l'interface electrode constituee de silicium de conductivite de type n et electrolyte (methanol) est mesuree grâce a des techniques micro-ondes et de conductivite radio-frequence

Film thickness dependence of critical current density and microstructure for epitaxial YBa2Cu3O7-x films

Abstract: Films of nominal composition YBa2Cu3O7-x(YBCO) were made on (100) SrTiO3 substrates by coevaporation and furnace annealing. Film thickness was in the range 0.2-2.4 mu m. Chemical analysis by inductively coupled plasma emission spectroscopy reveals that the thinner films had an excess of Cu and Ba relative to Y, which led to Cu- and Ba-rich particles on the film surface. Transmission electron microscopy was used to define the epitaxial nature of the films: a continuous layer of about 0.4 mu m thickness adjacent to the substrate and with the c axis normal to the substrate plane, followed by material with the c-axis in the film plane. A simple model based on microstructural observations and the strongly anisotropic transport properties of YBCO is compared with the measurements of room-temperature resistivity and critical current density at 77 K as a function of film thickness. The critical current density is found to decrease as a function of increasing film thickness by a much larger factory than the simple model predicts.

Fabrication of minority‐carrier‐limited n‐Si/insulator/metal diodes

Abstract: A photoelectrochemical anodization technique has been used to fabricate n-Si/insulator/metal (MIS) diodes with improved electrical properties. MIS structures fabricated with Au have provided the first experimental observation of a solid-state n-Si surface barrier device whose open circuit voltage Voc is controlled by minority-carrier bulk diffusion/recombination processes. For these diodes, variation of the minority-carrier diffusion length and majority-carrier dopant density produced changes in Voc that were in accord with bulk diffusion/recombination theory. Additionally, the variation in Voc in response to changes in the work function of the metal overlayer indicated that these MIS devices were not subject to the Fermi level pinning restrictions observed for n-Si Schottky structures. X-ray photoelectron spectroscopic characterization of the anodically grown insulator indicated 8.2±0.9 A of a strained SiO2 layer as the interfacial insulator resulting from the photoanodization process.

Short-wavelength spectral response properties of semiconductor/liquid junctions

Abstract: We report the first measurements of photocurrent quantum yields for semiconductor/liquid junctions in the short-wavelength region of the spectrum (200-600 nm). The key feature of this wavelength region is the short penetration depth for the absorbed photon (less than 100 A), which allows measurement of the majority carrier collection velocity. Spectra have been obtained for semiconductor/liquid, semiconductor/metal, and semiconductor/insulator/metal junctions. For all semiconductors studied (n-Si, p-Si, n-GaAs, n-InP, a-Si:H), the spectral responses of the liquid junctions showed higher quantum yields than the metal junctions, indicating greater majority carrier losses at the metal junctions. This general trend was independent of redox species, solvent, supporting electrolyte, and metal overlayer. The spectral response data can also be used to distinguish Schottky barrier behavior from electrocatalytic behavior of metal overlayers, and this approach has been used to study several semiconductor/metal film junctions in contact with electrolytes.

Studies of silicon photoelectrochemical cells under high injection conditions

Abstract: The behavior of Si/CH3OH-dimethylferrocene+/0 junctions has been investigated under high injection conditions. Open circuit voltages of (626±5) mV were obtained at short circuit photocurrent densities of 20 mA/cm^2 for samples with an n + -diffused back region, point contacts on the back surface, and with a base of thickness 390 µm and a 1 ms hole lifetime. The diode quality factor and recombination current density were 1.8±0.1 and (2.6±1.5)×10–8 A/cm^2, respectively. These data are consistent with recombination dominated by the base and back contact regions, and not at the Si/CH3OH interface.

Reaction Zone Growth in Ti-Base/SiC Composites

Abstract: Reaction zone phases and kinetics over a temperature range of 650-1200°C have been characterized for Ti-14Al-21Nb/SiC and Ti-6Al-2Sn-4Zr-2Mo/SiC fibrous composites. The matrix of the Ti-1421/SiC materials was α2 with varying amounts of beta phase and transformed beta phase, and the Ti-6242 matrix consisted of alpha plus beta. The reaction zone in the as-HIP Ti-1421/SiC contained fine-grained TiC, a region of coarser-grained TiC, a carbide layer containing Ti, Al and Nb, and a region of(Ti, Nb) 5 (Si, Al) 3 . The matrix adjacent to the reaction zone was α2-Ti 3 Al with no beta phase. In the as-HIP Ti-6242/SiC composites, the reaction zone contained TiC near the fiber, and a layer of (Ti,Zr) 5 Si 3 adjacent to the matrix. In heat-treated samples, the reaction zone thicknesses varied in a linear fashion with the square-root of aging time, indicating diffusion-controlled growth. Incubation periods for reaction zone growth were observed in Ti-1421/SiC samples aged at 760-1000°C. The overall kinetics for the two matrix alloys were approximately the same, and the activation energy was measured as 63-73 kcal/mole for Ti-1421/SiC and 73kcal/mole for Ti-6242/SiC.

Correlations between the interfacial chemistry and current‐voltage behavior of n‐GaAs/liquid junctions

Abstract: Correlations between the surface chemistry of etched, (100) oriented n-GaAs electrodes and their subsequent photoelectrochemical behavior have been probed by high-resolution x-ray photoelectron spectroscopy. GaAs photoanodes were chemically treated to prepare either an oxide-free near stoichiometric surface, a surface enriched in zero-valent arsenic (As0), or a substrate-oxide terminated surface. The current-voltage (I-V) behavior of each surface type was subsequently monitored in contact with several electrolytes.

Synthesis, Characterization, and Applications of Substituted Polyacetylenes Derived from Ring-Opening Metathesis Polymerization of Cyclooctatetraenes

Abstract: Polyacetylene and partially substituted derivatives of polyacetylene are synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) and its derivatives. The physical properties and characterization of these polymers are reported. In particular, certain poly-COT derivatives afford soluble, highly conjugated polyacetylenes. Nonlinear optical properties are highlighted by the fact that, in several cases, scattering losses are greatly minimized. These polymers have also been used in device applications such as Schottky barrier diodes and solar cells.

The Application of Ring-Opening Metathesis Polymerization to the Synthesis of Substituted Polyacetylenes

Abstract: Ring-opening metathesis polymerization of substituted cyclooctatetraenes has been used to synthesize substituted polyacetylenes. The spacing between the side groups in the polymer allows the polymer to attain a conjugated conformation, while the twisting of the main chain induced by the substituents renders the polymer soluble. Poly(trimethylsilylcyclooctatetraene) has been used to make Schottky-type solar cells.

Short-Wavelength Spectral Response Properties of Semiconductor/Liquid Junctions.

Abstract: We report the first measurements of photocurrent quantum yields for semiconductor/liquid junctions in the short-wavelength region of the spectrum (200-600 nm). The key feature of this wavelength region is the short penetration depth for the absorbed photon (less than 100 A), which allows measurement of the majority carrier collection velocity. Spectra have been obtained for semiconductor/liquid, semiconductor/metal, and semiconductor/insulator/metal junctions. For all semiconductors studied (n-Si, p-Si, n-GaAs, n-InP, a-Si:H), the spectral responses of the liquid junctions showed higher quantum yields than the metal junctions, indicating greater majority carrier losses at the metal junctions. This general trend was independent of redox species, solvent, supporting electrolyte, and metal overlayer. The spectral response data can also be used to distinguish Schottky barrier behavior from electrocatalytic behavior of metal overlayers, and this approach has been used to study several semiconductor/metal film junctions in contact with electrolytes.

Effects of Redox Potential, Steric Configuration, Solvent, and Alkali Metal Cations on the Binding of Carbon Dioxide to Cobalt(I) and Nickel(I) Macrocycles.

Abstract: The binding of CO_2 to metal macrocycles has been determined electrochemically by using cyclic voltammetry or differential pulse polarography. The CO_2 binding constants, K for a series of Co(I) tetraammacrocycle complexes in (CH3)_2SO showed a strong correlation with the Co(II/I) redox potential. Although meso and d,l stereoisomers of [Co^I(Me_6[14]4,11- diene)]^+ had identical redox potentials, K_(CO_)2 differed by a factor of 10^2 for these stereoisomers, suggesting a large steric effect on CO_2 binding. Binding of CO_2 to Ni(I) tetraazamacrocycles in (CH_3)_2SO yielded a redox potential dependence similar to that of Co(I) macrocycles. A weak correlation between K_(CO_2) and the solvent dielectric constant was found for d,l-[Co(Me_6[ 14]-4,11 -diene)]^+. In tetrahydrofuran (THF), d,l-[CO(Me_6[14]4,11-diene)]^+ was found to bind CO_2 more strongly in the presence of 0.1 M Li^+ than in the presence of 0.1 M tetrabutylammonium (TBA^+). Electrochemically determined K_(CO_2)'s for Co(salen)^- and Co(Me_2salen)^- in THF indicated a strong dependence on redox potential and electrolyte cation.