Showing papers by "Nathan S. Lewis published in 1991"
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TL;DR: In this article, a unified treatment of the kinetics of the various charge transfer experiments and estimates of the timescales that can be expected for these reactions are presented, as well as their design and kinetic interpretation.
Abstract: Although a variety of heterogeneous rate constant data is now available
at metal electrodes (1, 2), relatively little kinetic information has been
obtained at semiconductor electrodes (3). Several recent experiments have
indicated that the rates of charge transfer at semiconductor/liquid interfaces
can vary from picoseconds to milliseconds (4-7); thus, a wide range
of timescales are apparently important in the dynamics of semiconductor
photoelectrochemistry. The experiments that yield long decay times have
often been interpreted in the same general framework as those that yield
short carrier lifetimes; yet, the various types of kinetic experiments actually
differ substantially in their design and kinetic interpretation. This review
presents a unified treatment of the kinetics of the various charge transfer
experiments and presents estimates of the timescales that can be expected
for these reactions.
166 citations
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TL;DR: In this article, the steady-state photoluminescence of (100)-oriented GaAs has been studied using x-ray photoelectron spectroscopy and steadystate photodynamic properties of GaAs surfaces exposed to inorganic and organic donors.
Abstract: Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2–(aq) concentration, variation of the pH of aqueous HS–solutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH=8, 0.1-M HS–(aq) solutions displaying comparable luminescence intensity relative to those treated with pH=14, 1.0-M Na2S·9H2O(aq). Organic thiols (R-SH, where R=–CH2CH2SH or –C6H4Cl) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of (100)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S3 overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of As2S3 is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electron-donating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2S3 and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The time-resolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HS– and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate.
163 citations
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TL;DR: In this article, a series of thiols were used to remove excess As0 and form detectable levels of As2S3-like phases, implying that neither of these factors is required for effective surface passivation chemistry.
Abstract: Exposure of GaAs crystals to solutions of organic thiols resulted in substantial reductions in nonradiative GaAs surface recombination rates. This process yielded improvements in steady state photoluminescence signals that were comparable to those obtained after a Na2S·9H2O (aqueous) treatment. Use of a series of thiols indicated that the chemically important surface electrical trap levels behaved as a polarizable, electron deficient center. X-ray photoelectron spectroscopy indicated that the thiols did not remove excess As0 nor form detectable levels of As2S3-like phases, implying that neither of these factors is required for effective surface passivation chemistry.
68 citations
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TL;DR: In this article, the authors showed that metal-insulator-semiconductor junctions with anomalously high barrier heights are difficult to obtain by such metalization and etching methods.
Abstract: Mechanistic studies of n-Si photoelectrodes have been conducted using aqueous and nonaqueous electrolytes. For (Me{sub 2}Fc{sup +}) between 0.3 mM and 0.050 M, the temperature dependence of V{sub oc} indicated that bulk-diffusion/recombination was the rate-limiting recombination process. In all of these experiments, conventional Shockley diode theory provided an excellent description of the solid/liquid junction properties. In a related set of experiments, HF-etched n-Si photoelectrodes ((100)- and (111)-oriented samples) and n-Si samples that had been metalized by several different methods all showed passivation in contact with Fe(CN){sub 6}{sup 3-/4-}{sub aq} or Br{sub 2}/Br{sup {minus}}{sub aq} or electrolytes. For several metalized Si samples, etches that had been reported to produce metal islands on the Si surface instead yielded Si surfaces free of metal. All Si samples that had been metalized by filament evaporation yielded I-V behavior in contact with the Ch{sub 3}OH-Me{sub 2}Fc{sup +/0} redox system that was characteristic of a pinned surface Fermi level, even for coverages of metal as low as 9 {Angstrom}. This indicates that proposed metal-insulator-semiconductor junctions with anomalously high barrier heights are difficult to obtain by such metalization and etching methods.
52 citations
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TL;DR: In this article, a 3-8 V pulses of 10-100 ps duration were used to generate 3-A-diam features on a pyrolytic graphite (HOPG) surface.
Abstract: Recently, atomic and molecular scale features have been produced on surfaces using scanning tunneling microscopy (STM) methods.‘-’ At room temperature, the smallest permanent lithographic process reported to date involves the formation of -4O-A-diam, =:3-A-deep pits on a highly ordered pyrolytic graphite (HOPG) surface.’ These pits were formed by the application of 3-8 V pulses of 10-100 ps duration, while within tunneling distances. Although a substantial fraction of the STM tips successfully generated hundreds of uniform pits when subjected to constant amplitude and constant duration bias pulses,’ the pulse threshold at which the smallest features were observed varied substantially from tip to tip. Additionally, the lithography process was observed to require a humid atmosphere and to exhibit daily fluctuations in pulse bias versus pit size. In order to elucidate the chemistry of this process, we have performed STM lithography studies of HOPG surfaces in contact with water and other organic liquids. For HOPG in H,O(Z), we have observed a welldefined pulse threshold of (4.0 *0.2)V, which reproducibly yielded 7-A-diam features that apparently protruded from the surface by 2 A. Larger voltage pulses yielded pits (with slightly larger dimensions) that were similar to those produced in air or in humid Nz(g) atmospheres. This work demonstrates that subnanometer scale lithography can be accomplished on graphite at room temperature, and that STM studies can provide mechanistic information regarding the chemistry of the liquid-coated and gas phase lithographic processes. A STM specifically designed for imaging surfaces immersed in liquids was employed for this work.’ Bias pulses with a duration of 20 ps and an amplitude
38 citations
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TL;DR: In this paper, the effect of the side group on the optical, electrical and materials properties of the polymer is discussed, as well as its properties in terms of conjugation and binding properties.
20 citations
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TL;DR: In this article, a quantitative study has been performed on the stability of GaAs surfaces to photoanodic corrosion in contact with the 1.0 M KOH(aq)-0.10 M K[sub 2]Se(aq) electrolyte.
Abstract: A quantitative study has been performed on the stability of GaAs surfaces to photoanodic corrosion in contact with the 1.0 M KOH(aq)-0.10 M K[sub 2]Se(aq)-0.01 M K[sub 2]Se[sub 2](aq) electrolyte. In this electrolyte, chemically etched n-type GaAs electrodes displayed significant photocorrosion in competition with faradaic charge transfer to Se[sup 2[minus]](aq). Chemisorption of group VIIIB metal ions, including complexes of Co(III), Os(III), and Ru(III), onto the etched GaAs surface yielded improved current-voltage (I-V) behavior of the GaAs photoanode and also resulted in a significant reduction in photocorrosion. This behavior implies that the chemisorbed metal ions act to increase the rate of hole transfer to the Se[sup 2[minus]] donor species, as opposed to the passivation of nonradiative surface recombination. Related experiments on n-GaAs photoanodes in contact with the 1.0 M KOH(aq)-0.3 M Te[sup 2[minus]](aq)-0.01 M Te[sub 2][sup 2[minus]](aq) electrolyte have also been performed. The I-V properties of n-GaAs, p-GaAs, and Sn-doped In[sub 2]O[sub 3] electrodes have been investigated in this electrolyte, and these measurements have been used to elucidate common mechanistic features of charge-transfer reactions for the Se[sup 2[minus]](aq) and Te[sup 2[minus]](aq) redox systems.
10 citations
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TL;DR: In this paper, the energy conversion efficiency of the n-type photoelectrodes was analyzed in the presence of CH{sub 3}CN-ferrocene{sup +/0} and KOH-Se{sup {minus}/2{minus}}(aq) electrolytes.
Abstract: Trends in open-circuit voltage (V{sub oc}), short-circuit current density (J{sub sc}), and energy conversion efficiency have been determined for the n-type Al{sub x}Ga{sub 1{minus}x}As series of photoelectrodes (x = 0.0, 0.09, 0.16, 0.24, 0.31) in contact with CH{sub 3}CN-ferrocene{sup +/0} and KOH-Se{sup {minus}/2{minus}}(aq) electrolytes. V{sub oc} increased linearly with increases in bandgap energy (E{sub g}) of the n-Al{sub x}Ga{sub 1{minus}x}As alloy electrodes, with {Delta}V{sub oc}/{Delta}E{sub g} = 0.45 {plus minus} 0.04 V eV{sup {minus}1} in CH{sub 3}CN and 0.41 {plus minus} 0.09 V eV{sup {minus}1} in KOH-Se{sup {minus}/2{minus}}(aq) at a light intensity sufficient to provide J{sub sc} = 1.0 mA cm{sup {minus}2}. J{sub sc} values under solar-simulated illumination decreased monotonically with increasing bandgap energy. The relatively low value of {Delta}V{sub oc}/{Delta}E{sub g} implies decreases in optimal energy conversion efficiency as the mole fraction of Al in the Al{sub x}Ga{sub 1{minus}x}As alloy is increased. This is in contrast to the behavior of the n-GaAs{sub x}P{sub 1{minus}x} alloy series in the same electrolytes.
9 citations
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01 Mar 1991TL;DR: In this paper, a ring-opening metathesis polymerization (ROMP) was used to synthesize poly-cyclooctatetraene (poly-COT), a material which is isostructural to polyacetylene.
Abstract: Recent developments in ring-opening metathesis polymerization (ROMP) have enabled the synthesis of poly-cyclooctatetraene (poly-COT), a material which is isostructural to polyacetylene. This liquid-phase polymerization method allows facile construction of interfaces, films, and devices with polyacetylene-like materials. The ROMP method also allows the preparation of soluble, yet highly conjugated polyacetylene analogs from substituted cyclooctatetraenes (R-COT). The redox characteristics of R-COT polymers were investigated at electrodes modified with thin polymer films. Voltammetric methods were used to characterize the redox response, band gap, electrochemical doping, and cis-trans isomerization properties of these polyenes. We have applied poly-COT technology to the fabrication of Schottky diodes and photoelectrochemical cells, by forming poly-COT films on semiconductor surfaces. The resultant semiconductor/organic-metal interfaces behave more ideally than semiconductor contacts with conventional metals, in that changes in the work function of the conducting polymer exert a large and predictable effect on the electrical properties of the resulting Schottky diodes. Transparent films of the solution-processible polymer poly- trimethylsilyl-cyclooctatetraene (poly-TMS-COT) have been cast onto n-silicon substrates and doped with iodine to form surface barrier solar cells. These devices produce photovoltages that are much larger than can be obtained from n-silicon contacts with conventional metals.
3 citations
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TL;DR: In this paper, nanometer-scale electrodes have been used for 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.
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.
3 citations
04 Nov 1991
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31 May 1991
TL;DR: In this article, the effects of tip-sample bias pulses while within tunneling distances have been investigated for graphite surfaces in contact with dry fluids, fluids containing water, and liquid water.
Abstract: The effects of tip‐sample bias pulses while within tunneling distances have been investigated for graphite surfaces in contact with dry fluids, fluids containing water, and liquid water. In dry ambients, no surface modifications were observed at pulse voltages in excess of ±10 V. In fluids containing water, bias pulses exceeding a threshold voltage produced pits in the graphite surface, but the threshold voltage exhibited daily fluctuations of as much as 5 V. At the threshold for surface modification, a typical pit was found to have a diameter of approximately 30 A and was 3 A in depth. The diameter and depth of the pits increased with pulse amplitude above the observed threshold. In pure water, a reproducible bias pulse threshold of 4.0±0.2 V was observed, and such pulses yielded dome‐like features on the surface. The domed features were found to have a diameter of ∼7 A and were ∼1.5 A high. Bias pulses greater than 4.0 V were found to produce pits of approximately the same diameter as those produced in humid gases. Although little is known at present about the chemical structure of the domed features, their conversion into pits with subsequent bias pulses of only 0.2 V suggests that they are structural intermediates in the pit formation process. This hypothesis implies that the domed features may be vulnerable to chemical attack; consequently, the domes may present a viable route to the localized, designed, chemical functionalization of a graphite surface.
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TL;DR: In this article, the first metalloporphyrin dihydrogen complex with a sterically bulky ligand bound between two metals was proposed as an intermediate in bimolecular elimination.
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.