Cleavage In Air Research Articles

Palladium-catalyzed denitrogenative cross-coupling of aryl halides with arylhydrazines under mild reaction conditions

A greener approach for the synthesis of various functionalized biaryl frameworks in good to excellent yield through palladium-catalyzed denitrogenative cross-coupling of aryl halides with arylhydrazines under mild reaction conditions was developed. Catalytic system is free from the aid of expensive ligands and external oxidants. Biogenically prepared palladium nanoparticles (Pd NPs) immobilized cellulose based dip catalyst displayed excellent reactivity and selectivity toward the synthesis of a broad array of symmetrical and unsymmetrical biaryls through C–N bond cleavage in air as green oxidant. In addition, recyclability in denitrogenative cross-coupling reaction was also studied which showed excellent recycling performance and the dip catalyst remained stable even after several reuses. Thus, our newly developed strategy was successfully applied for constructing wide-ranging functional groups tolerated biaryls using arylhydrazines and aryl halides as coupling partners which is most useful for practical applications in synthetic chemistry.

On the Abnormal Conductivity of the Surface (001) of Alkali Halide Crystals

Surfaces (001) of alkali halide crystals (KCl, NaCl), which were obtained by cleavage in air and ultrahigh vacuum, have been studied by Auger electron spectroscopy. It is established that the surface obtained by cleavage in ultrahigh vacuum has an anomalously high conductivity, which is broken down during heating and depositing an atomic layer of metal (Au).

Improved characteristics of mesa-type intrinsic Josephson junctions by vacuum cleavage process for Bi2Sr2CaCu2O8+δ/Au contacts

Improvement in the quality of stacked intrinsic Josephson junctions (IJJs) was attempted using Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals. The introduction of a clean Bi-2212/Au interface reduced contact resistivity (ρc), decreased low frequency noise, and suppressed the drop in the maximum Josephson current (Ic), which are very important factors for applications such as a superconducting quantum interference device. The ρc of samples produced by crystal cleavage in air or vacuum were, respectively, 8.2 × 10−5 and 7.0 × 10−6 Ω cm2 at 300 K and 8.2 × 10−5 and 2.1 × 10−6 Ω cm2 at 77 K. Random-telegraph voltage noise observed with the air-cleaved samples did not occur for the samples cleaved in a vacuum. The degradation of Ic was suppressed for IJJs produced by the vacuum cleavage process.

Mononuclear Fe(ii)-N4Py complexes in oxidative DNA cleavage: structure, activity and mechanism

A series of monotopic N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, 1) derived ligands have been prepared and evaluated in the iron catalyzed oxidative cleavage of pUC18 DNA, in the presence and absence of external reducing agent DTT. The mononuclear iron(II) complexes induce efficient DNA cleavage in air with a low catalyst loading. It was demonstrated that covalent attachment of 9-aminoacridine, ammonium group or 1,8-naphthalimide leads to increased DNA cleavage activity in the presence of a reductant. Also some complexes displayed a small degree of double-strand DNA cleavage activity. In contrast, in the absence of reducing agent, no beneficial effect of the covalently attached DNA binding moieties was observed, which was attributed to the reduction from Fe(III) to Fe(II), which is required for oxygen activation, becoming rate limiting. Mechanistic investigations revealed an important role for superoxide radicals. A proposed mechanism involves the formation of an Fe(III)-OOH intermediate as the active species or precursor.

How flat is an air-cleaved mica surface?

Muscovite mica is an important mineral that has become a standard substrate, due to itseasy cleavage along the {001} planes, revealing a very flat surface that is compatible withmany biological materials. Here we study mica surfaces by dynamic atomic forcemicroscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum(UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFMdue to large surface charges; however, cleavage in air yields much less surfacecharge and allows for NC-AFM imaging. We present highly resolved NC-AFMimages of air-cleaved mica surfaces revealing a rough morphology originatingfrom a high density of nanometre-sized particles. Among these particles, we findregularly shaped structures indicating the growth of crystallites on the surface. Thecontamination layer cannot be removed by degassing in UHV; even prolonged heating at atemperature of 560 K under UHV conditions does not yield an atomically flat surface.

ＭｇＯ（００１）劈開面への水曝露による水酸化の観察

Hydroxylation on cleaved MgO(001) surface by water exposure has been studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The used samples are MgO surfaces cleaved in air, exposed to water vapor and immersed in water. The immersed times are 0.01, 0.03, 0.1, 0.3, 1 and 3 h. Water molecules could not be found in the XPS spectra of all samples. Hydroxyl exists on the surface immediately after cleaved in air. The hydroxyl component in the XPS spectra becomes large with the immersed time up to 0.03 h. At the 0.1 h of the immersed time the component of Mg(OH)2 appears in the XPS spectrum and the component becomes larger with the immersed time. These results indicate that the hydroxylation makes progress by dissociative adsorption of water right after cleavage in air and the Mg(OH)2 component begin to grow from about 0.1 h of the immersed time.

Where the bulk terminates: Experimental evidence for restructuring, chemibonded OH − and H + , adsorbed water and hydrocarbons on calcite surfaces

Better understanding of mineral surface reactivity can improve control of crystal growth and dissolution which is central to biomineralisation, design of advanced materials and preventing transport of toxic substances in the environment. Some experimental observations of fundamental processes on calcite using high-resolution techniques (AFM, XPS, TOF-SIMS, LEED) are summarised. Surfaces, cleaved in ultrahigh vacuum (UHV) in the absence of reactive species, relax. Cleavage in air results in hydration and hydrolysis, covering the surface with chemisorbed CaOH and CO 3 H. Water adsorbed from air promotes recrystallisation on apparently dry surfaces. Organic components can inhibit dissolution and precipitation; even in the cleanest laboratory, surfaces have at least a monolayer of hydrocarbons. On commercial calcite powders contaminated by organic additives during manufacture, we do not see layer-by-layer and atom-by-atom growth or dissolution and behaviour contradicts the theoretical models developed from macroscopic experiments with these materials. Instead, steps are rounded and many atomic layers high.

Degradation of the CaF 2(111) surface by air exposure

We investigate the electronic and geometric structure of CaF 2(111) surfaces produced by cleavage in air in comparison to surfaces cleaved in an ultra-high vacuum. An analysis with ultraviolet photoelectron spectroscopy reveals a broadening of the fluorine 2p valence band peak, the formation of a broad band of occupied electronic states extending into the band gap and an increase in electron scattering after the surface has been exposed to air. We attribute these effects to the incorporation of gas molecules into the crystal and chemical reactions at the surface. Surface modifications appear as stable, nanometer-sized patches of reactants that can be directly observed by scanning force microscopy. These patches are found to be randomly distributed over the surface and have a height never exceeding the minimum possible step height on the CaF 2(111) surface. Furthermore, it is demonstrated that oxygen in the crystal can be transformed into CaO when metallising the surface and near surface bulk by low-energy electron irradiation. Oxidation is strongly enhanced by electron-stimulated diffusion of oxygen. By monitoring oxidation in a shallow surface layer with X-ray photoelectron spectroscopy, we also monitor thermally activated diffusion of oxygen to the surface after electron irradiation.

Photoemission from pure and electron irradiated CaF 2

We describe changes of the electronic structure of CaF 2 (1 1 1) surfaces caused by electron irradiation, with an emphasis on radiation stimulated oxidation. CaF 2 crystals cleaved in air and in ultra high vacuum (UHV) were investigated with ultraviolet photoelectron spectroscopy using HeI light (21.2 eV). Surfaces produced by cleavage in air show a large density of states in the band gap region, in contrast to those cleaved in UHV. When irradiating the air cleaved crystals with low energy electrons, we observed the formation of a new peak in the band gap region that is attributed to oxygen. From the electron dosage dependence of this new peak, we conclude that it is mainly formed during irradiation by oxygen already present at the crystal surface. For crystals cleaved in UHV, no new features are observed following irradiation, although a metallic film on the crystal surface is visible with the bare eye. However, dosage with oxygen also yields the band gap peak. The oxidized surface is modeled by ab-initio calculations, based on the Hartree–Fock method. Results are compatible with experimental observations.

Direct Measurement of Colloidal Forces between Mica and Silica in Aqueous Electrolyte

A refined methodology which allows attachment of submicrometer colloidal particles to atomic force microscopy cantilevers is described. The interactions between particles of this size and both mica and silica surfaces in aqueous electrolyte at pH 5.8 have been studied. The monotonic repulsive force observed was in all cases well fitted by assuming that the force was due to overlap of electrical double layers. The potential of a cleaved mica surface was found to be significantly less than that of the silica surface. However, the mica potential was found to increase, if the mica surface was immersed immediately (within seconds) in electrolyte after cleavage or after plasma cleaning. This behavior is explained by the finite aqueous solubility of adventitious carbon that spontaneously deposits on mica after cleavage in air. The surface potential of the silica spheres was obtained by fitting symmetric interaction data. The trend in zeta potential of the silica spheres with pH, determined by particle electrophoresis, was similar to that obtained from analysis of force-separation data.

Cleavage and Intercalation Properties of a Layered Compound, K4Nb6O17·3H2O, Studied with Atomic Force Microscopy

The surface structures of the title compound were studied with atomic force microscopy (AFM) after cleavage in air and after substitution of K+ with (CH3)4N+ (tetramethyl ammonium ion: TMA). Of the two interlayers, hydrated and anhydrated, cleavage in air occurred at the former when weak force was applied. Upon ion exchange at the hydrated interlayer with an aqueous solution of TMA, the niobate bilayer sheets broke free and slid against each other. The free bilayer sheets were mechanically weak enough to be scraped away by the stylus during scanning in the AFM measurement, which suggests the possibility of micromachining this material. Atom-resolved AFM images correspond to a regular arrangement of TMA with the methyl groups pointing in the directions of the crystal axes. This material can be used in constructing a two-dimensional assembly of organic molecules.

Structural characterization of MgO(100) surfaces

The structure, the microtopography and the cleanliness of MgO(100) surfaces have been investigated by a combination of thermal helium scattering, surface decoration and Auger spectroscopy. The microstructure and cleanliness are strongly dependent on the preparation of the surface. MgO surfaces obtained by cleavage under UHV are clean and present a high coefficient for coherent He reflection. MgO surfaces prepared by cleavage in air are contaminated and irreversibly damaged by water vapour. Such surfaces can be cleaned by vacuum annealing but the coefficient of coherent He reflection remains low because of the presence of a high number of defects.

Bend contours maps in titanium and mica thin films

Bend or extinction contours maps in titanium and mica are interpreted and indexed. They were constructed from a series of electron micrographs taken using beam energy of 100 keV of epitaxial titanium film about 700 Å in thickness, and thin mica (muscovite) film about 1000 Å in thickness prepared by cleavage in air.

Low-energy electron diffraction study of sodium cloride (100) surface

The (100) surface of a NaCl crystal produced by cleavage in air has been studied by low-energy electron diffraction in the electron energy range of 25–400 eV. The diffraction pattern from the crystal surface was in good agreement with that expected from an ideal NaCl (100) surface in the substrate. A surface expansion of 2.5% is found as measured from the intensity variation of the 00 spot as a function of the electron energy. This expansion is in agreement with the theoretically prediction of surface distorsion by Benson, Freeman and Dempsey 1 1 G.C. Benson, P.I. Freeman and D.E. Dempsey, Solid Surfaces and the Gas-Solid Interface, Advances in Chemistry Series 33 (1961) 26. ). Deficiency Kikuchi lines were observed in the electron energy range of 150–400 eV. These observations give evidence for multiple scattering.

Epitaxial Growth of Evaporated Cobalt Films

The characteristics of cobalt films of initial purity 99.91% and 99.999% prepared by vacuum evaporation on various substrates were studied. Variables were substrate type (KCl, NaCl, and natural graphite), substrate temperature, vacuum during evaporation, and vacuum cleavage vs cleavage in air. Epitaxial fcc cobalt films could be obtained on KCl and NaCl, and ``semiepitaxial'' hcp cobalt films could be obtained on graphite. The change in texture of the films upon annealing and their subsequent transformation behavior is discussed.