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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

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.

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.

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Gaseous nitric acid removal by surfaces in experimental systems and in the atmospheric boundary layer is rapid. However, neither the form of HNO3 on surfaces nor its impact on the properties of the thin surface film are known. We report here studies of surfaces that have been exposed at room temperature (295 ± 2 K) to gaseous mixtures of water vapor with HNO3 at concentrations from 46 ppb to 4 × 103 ppm. The surfaces were probed using a combination of Fourier transform infrared spectrometry (FTIR), non-contact atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Exposure of borosilicate glass, quartz, and thin Teflon films to mixtures of gaseous HNO3 and water vapor leads to the subsequent uptake of much larger amounts of water than occurs on the corresponding unexposed surfaces. Infrared spectra show evidence for the formation of nitric acid–water complexes on the surface that leads to this enhanced water uptake. On borosilicate glass, exposure to the nitric acid–water vapor mixture results in surface segregation of the trace metal oxides and their nitrates formed from reaction with HNO3. The majority of these oxides can be removed by rinsing with water; however, smaller, segregated regions of ZnO remain on the surface. The implications for heterogeneous reactions in thin films on surfaces in laboratory systems and in the atmosphere are discussed.

We have recently described the preparation and properties of ionically conductive Nafion-impregnated Gore-tex (NIGT) composite membranes. In this paper we describe electronically conductive composite membranes prepared by electropolymerizing either pyrrole or thiophene within the ionically conductive NIGT membrane. These composites have the mechanical properties of the host membrane yet the electronic conductivity of the NIGT/polypyrrole composite membrane is essentially identical to that of polypyrrole. For comparison, we have also attempted to electropolymerize pyrrole in nafion membranes and films and in bare Gore-tex and poly(vinyl chloride)-impregnated Gore-tex. We have found that the rate of polymerization of pyrrole at a NIGT-coated Pt electrode is higher than the rate at bare Pt. The origins of this rate enhancement are probed.

We describe a new ultramicroelectrode fabrication technique wh ich has allowed the preparation of conical and hemispherical platinum-iridium ultramicroelectrodes with radii of 0.5 micro m-10 micro m. These electrodes have been characterized by scanning electron microscopy, cyclic voltammetry and chronoamperometry. The smallest radii electrodes (r < 2 micro m) exhibited diffusional behavior in accord with a hemispherical geometry, while larger radii electrodes exhibited conical diffusion behavior. A first order mathematical approach to diffusion at conical surfaces has been developed to explain the results for these electrodes. Conical, Hemispherical ultramicroelectrodes, Construction, and Characterization.

SUMMARY The total current between the tip and the sample in a scanning tunnelling microscopy study of a solid/liquid interface can be dominated by Faradaic charge transfer currents. In such a situation, feedback control of the tunnelling gap, and imaging, is precluded. In this contribution we describe the preparation of glass and polymer coated STM tips that possess < 100 A2 of exposed metal. These tips effectively discriminate against Faradaic current and enable STM imaging in the presence of reversible electroactive solution species at appreciable tip/sample biases.

A series of experiments has been performed to determine whether nuclear fusion processes occur in palladium rods that have been electrochemically charged with deuterium. With a variety of metallurgical pretreatment procedures and different electrolytes, no evidence has been obtained for any excess enthalpy, neutron, gamma ray, tritium or helium production during electrolysis of D_20 with palladium cathodes.

The theoretical and experimental aspects of a new current-pulse method for electrochemical investigations of charge-transport rates in thin redox active films on electrode surfaces are described. This method yields an apparent diffusion coefficient, D app , which is a quantitative measure of the rate of the charge-transport process in the film. This method is ideally suited for solving the myriad problems associated with electrochemical analyses of electronically conductive polymers (e.g. polypyrrole, polyaniline, etc.). When applied to such polymers, the D app obtained is a quantitative measure of the rate of the insulator-to-conductor switching reaction. We have used this method to obtain what we believe are the first reliable electrochemically-generated values of D app for the polymer polypyrrole.

An ac impedance method was used to determine apparent diffusion coefficients associated with the oxidation of polypyrrole. This method was applied to the reduced (nonconductive) form of the polymer, and the polymer remained in the nonconductive state throughout the duration of the experiment. Apparent diffusion coefficients obtained were similar in magnitude to coefficients obtained by a related current-pulse method. Diffusion coefficients obtained from finite diffusion data were ca. a factor of 3.4 times larger than coefficients obtained from semiinfinite diffusion data.

Procedures are described for insulating metal scanning tunneling microscope (STM) tips with either glass or polymer coatings. In solutions containing 0.10 M of a reversible redox couple, Fe(CN) - 3/-46 , the faradaic limiting current to polymer coated tips was 200–500 pA and that for glass coated tips was

Electronically conductive polymers are a fascinating new class of materials with unique electronic, electrochemical, and optical properties (1). One of the most interesting aspects of these polymers is that they can be reversibly “switched” between electronically insulating and electronically conductive states. Numerous researchers in a variety of conductor transition (1). In addition to being an interesting scientific phenomenon, this switching process plays an integral role in nearly all of the proposed chemical applications of electronically conductive polymers (2–13)

A hybrid electrochemical/chemical (E/C) synthesis of zinc oxide (ZnO) nanoparticles and films is described. The E/C procedure involves two steps: Zinc metal was first electrochemically deposited at basal-plane oriented graphite electrode surfaces from dilute aqueous solutions; then this deposit was permitted to spontaneously oxidize and dehydrate at open circuit in the pH = 1.0 plating solution. Deposition was size-selective, and wurtzite phase ZnO nanocrystallites having mean diameters in the range from 15 to 100 A were obtained using this approach. Relative standard deviations of the particle diameter for ZnO particle dispersions varied from 25 to 50%. Polycrystalline ZnO films of 100−400 A in thickness were also obtained by depositing larger quantities of zinc metal in the first step of the synthesis. For ZnO particles (dia. < 80 A), electron diffraction analysis revealed a preferred orientation for ZnO crystallites where the c-axis of the wurtzite unit cell was oriented perpendicular to the plane of ...

A number of heterogeneous reactions of atmospheric importance occur in thin water films on surfaces in the earth's boundary layer. It is therefore important to understand the interaction of water with various materials, both those used to study heterogeneous chemistry in laboratory systems, as well as those found in the atmosphere. We report here studies at 22 degreesC to characterize the interaction of water with such materials as a function of relative humidity from 0 - 100%. The surfaces studied include borosilicate glass, both untreated and after cleaning by three different methods (water, hydrogen peroxide and an argon plasma discharge), quartz, FEP Teflon film, a self assembled monolayer of n-octyltrichlorosilane (C8 SAM) on glass, halocarbon wax coatings prepared by two different methods, and several different types of Teflon coatings on solid substrates. Four types of measurements covering the range from the macroscopic level to the molecular scale were made: ( 1) contact angle measurements of water droplets on these surfaces to obtain macroscopic scale data on the water-surface interaction, (2) atomic force microscopy measurements to provide micron to sub-micron level data on the surface topography, ( 3) transmission FTIR of the surfaces in the presence of increasing water vapor concentrations to probe the interaction with the surface at a molecular level, and ( 4) X-ray photoelectron spectroscopy measurements of the elemental surface composition of the glass and quartz samples. Both borosilicate glass and the halocarbon wax coatings adsorbed significantly more water than the FEP Teflon film, which can be explained by a combination of the chemical nature of the surfaces and their physical topography. The C8 SAM, which is both hydrophobic and has a low surface roughness, takes up little water. The implications for the formation of thin water films on various surfaces in contact with the atmosphere, including building materials, soil, and vegetation, are discussed.

Metal (copper and cadmium) nanocrystallites -electrochemically deposited on the graphite basal plane- have been converted on a particle-by-particle basis to nanocrystals of a semiconducting metal salt (Cul and CdS). Deposition of the metal nanocrystals from a dilute ([M n+ ] 1.0 mM) plating solution is accomplished using a potentiostatic pulse with an overpotential of -500 mV and a duration in the range 50-200 ms. Non-contact atomic force microscopy images of the surface show that defect sites, such as step edges, and apparently atomically smooth terraces on the graphite basal plane both are covered with metal particles at an areal density of 10 9 -10 10 cm -2 . Each of these metal nanocrystallites is then converted into a semiconducting salt particle in two subsequent steps: first, metal nanocrystals are chemically or electrochemically oxidized at the appropriate pH to yield either metal oxide or hydroxide nanocrystals; secondly, the oxygen or hydroxide is displaced with an anion to yield the semiconducting salt of interest. This final displacement step is always chemical in nature. The CuI and CdS nanocrystals prepared using this E/C approach possess an excellent lattice match to graphite and epitaxially oriented nanocrystals are obtained with both materials. The particle size distribution for E/C-synthesized particles is determined by that of the parent metal particle distribution. Both CdS and CuI nanocrystals prepared by the E/C method are strongly luminescent at the band edge.

Methode pour forcer les polymeres conducteurs a adopter une morphologie fibrillaire, macroporeuse

Ion transporting membranes are required in a variety of electrochemical processes and devices including chlorine-caustic production, batteries, fuel cell, electrolyzers, etc. Nafion, a perfluorosulfonate ionomer marketed by Du Pont has proved useful for a number of these applications. Nafion has a number of attractive features including good chemical and mechanical stability and high perselectivity. In addition, however, it has two potentially significant disadvantages - it is a cation transporting membrane (i.e., there is no anion transporting equivalent) and it is rather expensive. These difficulties have prompted efforts to develop anion and cation transporting materials which have Nafion's desirable chemical and mechanical features but which can be produced more cheaply. It occurred to us that a cheaper and more versatile membrane material might be prepared by doping an inexpensive, nonconductive material with a small amount of an ion transporting polymer. In an effort to demonstrate the feasibility of this approach, we have impregnated commericially available porous polytetrafluoroethylene membranes (Gore-Tex) with small quantities of Nafion, to make cation transporting composites. Electrode surfaces were covered with these composite membranes electrochemical techniques were used to study the membranes transport properties. We have shown that the Nafion-doped Gore-Tex does, indeed, transport cations and that the rate of transport can be much higher than the transport rate in a pure Nafion membrane. Keywords include: Ionic conduction, Membranes, Transport.

The construction and operation of ionomer-film modified electrodes are described, including the fundamentals of the ion-exchange processes. Ion-exchange selectivity coefficients for a series of alkylsubstituted pyridinium cations in Nafion have been measured and the large values of these coefficients are attributed to the hydrophobic effect; a free-energy relationship relating the coefficient to the size of the ion has been established. The use of Nafion-coated electrodes for the in vivo measurement of neurotransmitters and for the preconcentration of analytes present in low concentration is discussed. Experiments on the dynamics of the response of ionomer modified electrodes are reported and very high ionic transport rates can be achieved for novel ionically conductive composite polymer membranes. The covalent attachment of redox groups such as ferrocene allows one to control the release of counterions by switching the redox state of the ferrocene.