Microstructural investigations of tin-molybdenum oxides by high-resolution electron microscopy

Tin-molybdenum oxides, formed by the calcination of precipitates in air, have been examined by high-resolution electron microscopy. Low-temperature calcination gives rise to the formation of small tin(IV) oxide-type crystals amidst an amorphous material whereas higher-temperature treatment results in the development of a highly crystalline rutile-related phase composed of larger particles. High concentrations of molybdenum in the initial precipitates inhibits the thermally induced crystal growth. The common occurrence of superficial disorder in the larger particles is associated with surface damage resulting from the volatilization of excess molybdenum as molybdenum(VI) oxide. Planar faults were frequently observed within the particles and, in some cases, these defects were identified as twin boundaries enriched with molybdenum. The formation of these planar faults is discussed in terms of the preparative procedure.     

TOF-SIMS investigations on weathered silver surfaces

Silver-coated quartz crystal microbalance (QCM) disks were treated under different environmental conditions (including changes in parameters such as relative humidity (%RH) and SO₂/H₂S content) in atmospheres of synthetic air and pure N₂ for 24 h in a weathering chamber. The corroded surfaces were subjected to depth profiling by a time of flight (TOF) secondary ion mass spectrometry (SIMS) instrument, equipped with a Bi⁺ analysis gun and Cs⁺ sputter gun. The evaluation of the in-depth distribution of several elements and species provides evidence for the formation of a corrosion layer containing Ag₂SO₃, even in the absence of oxidizing agents, such as H₂O₂ or NO₂. Furthermore it could be elucidated that the thickness of the formed Ag₂SO₃ layer does not depend on the SO₂ concentration but rather on the humidity and oxygen content of the ambient atmosphere. In weathering experiments in atmospheres composed of synthetic air, humidity, and H₂S, the presence of different oxygen species (surface and bulk) and silver sulfide could be detected by TOF-SIMS depth profiling experiments. The obtained results for both acidifying gases are in good correlation with the corresponding tapping mode atomic force microscopy (TM-AFM) investigations and in situ QCM measurements.     

Scanning tunneling microscopic investigations of the adsorption and segregation of carbon and sulfur on nickel single crystal surfaces

The adsorption and segregation of carbon or sulfur on Ni single crystal surfaces have been investigated by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Different adsorbate(segregate)-induced surface modifications have been detected in dependence on the original Ni surface orientation and the kind of nonmetal atoms: i) Adsorption of carbon from ethylene on Ni(111) at 6.7×10^–4 Pa and 1000K leads to the epitaxial growth of a graphitic carbon monolayer which exhibits the structure of the hexagonal basal plane of graphite. However, as is found for highly oriented pyrolytic graphite (HOPG), only three of six carbon atoms of the (0001) graphite plane are imaged by STM. In contrast, on Ni(771) at 663 to 1000 K carbon islands have been formed but no graphite monolayer formation is detected. This behavior can be understood by considering the aspects that no large-area epitaxy between the graphite basal plane and the Ni(110) terraces exists and that the surface carbon activity was too low to initiate substrate restructuring. ii) Segregation of sulfur (from the Ni bulk containing 5 to 7 ppm S) on Ni(110) at 1043K and _s0.4 ML initiates the growth of sulfur islands which show a c(2×2)-S overlayer structure, whereas on Ni(111) at 823K and _s0.2 ML (average value) a reconstructed surface phase is forming which can be described as an adsorbed two-dimensional sulfide Ni₂S.     

XPS and SIMS investigations on plasma-treated glass surfaces

Commercially available float-glass samples were exposed to ion bombardment in an HF-plasma. This should form an SiO₂-rich layer close to the surface of the samples. From XPS-investigations it was found that treatment times between 2 and 4 min in Ar plasma lead to a pronounced depletion of alkalis. Further FAB-SIMS depth profiles gave additional information about the extension of the layers with altered stoichiometry.     

Structural landscape investigations on bendable plastic crystals of isonicotinamide polymorphs

Three polymorphs (forms I, II and V) of isonicotinamide (INA) were mechanically flexible and exhibited one-dimensional (1D) plasticity. Anisotropic intermolecular interactions contribute to the plasticity of single crystals: weak dispersive interactions between slip planes such as 1D columns in forms I and II or 2D layers in form V were stabilized by strong hydrogen bonds, allowing the layer or column's surface to glide smoothly without hindrance. The disparity of intermolecular interactions on plastic properties of INA polymorphic crystals was confirmed by energy framework analysis, nanoindentation tests and micro-Raman spectroscopy. The crystal which exhibits plastic property provides a promising application in pharmaceuticals and material sciences.     

Potential energy surfaces of carbon dioxide

Several bent valence states of CO₂ are characterized by means of full-valence-space MCSCF calculations. The ground state potential energy surface exhibits a double well corresponding to a ring minimum, with C_2vsymmetry (¹A₁) and a 73.1° OCO angle, in addition to the linear (¹σ) global minimum. The transition state for the ring opening process, which has a barrier of 12.1 kcal/mole with respect to the ring minimum, is however found to have C_s symmetry. Double minima are also shown to exist for the ¹A₂, ¹B₁ and ¹B₂ excited states. However, in these cases all minima are bent. Cross sections through the ground state potential energy surface corresponding to the two collinear exchange reactions O(¹D) + CO(¹σ⁺) → OC(¹σ⁺) + O(¹D) C(³P) + O₂(³σ) → CO(¹σ⁺) + O(¹D) are also calculated and their energy contour maps are reported. The latter reveals the existence of a stable linear intermediate with the structure COO. © 1994 John Wiley & Sons, Inc.     

Electrochemical capacitances of well-defined carbon surfaces

Reported is the capacitive behavior of homogeneous and well-defined surfaces of pristine carbon nanofibers (CNFs) and surface-modified CNFs. The capacitances of the well-defined CNFs were measured with cyclic voltammetry to correlate the surface structure with capacitance. Among the studied pristine CNFs, the edge surfaces of platelet CNFs (PCNF) and herringbone CNFs were more effective in capacitive charging than the basal plane surface of tubular CNF by a factor of 3-5. Graphitization of PCNF (GPCNF) changed the edge surface of PCNF into a domelike basal plane surface, and the corresponding capacitances decreased from 12.5 to 3.2 F/g. A chemical oxidation of the GPCNF, however, recovered a clear edge surface by removal of the curved basal planes to increase the capacitance to 5.6 F/g. The difference in the contribution of the edge surface and basal-plane surface to the capacitance of CNF was discussed in terms of the anisotropic conductivity of graphitic materials.     

Density functional theoretical investigations on various nanostructural zeolite surfaces

Through density functional calculations, the Br?nsted acidities on various nanostructural ZSM-5 zeolite surfaces were studied as well as the hydrogen exchanging processes with adsorbed H(2)O monomer or dimer. The Br?nsted acidities on the four nanostructural surfaces show differences, although slightly, with their strengths increasing as (100) < (210) < (410) < (001). For hydrogen exchanging processes with H(2)O monomer or dimer, the reaction rate increases in the order (210) < (100) < (001) < (410) or (210) < (410) < (001). No transition-state structure is present on H(2)O dimer/(100) surface system. The introduction of a second H(2)O molecule accelerates the hydrogen exchanging processes and meanwhile influences the nanostructural geometries such that they are more evident. Besides the activation barrier, the adsorption energy and reaction heat display differences from one surface to another, which results in the preference of catalytic reactions to a specific nanostructural zeolite surface, such as the hydrogen exchanging processes studied in this paper.     

Activation of carbon dioxide and carbon monoxide at aluminium surfaces

Dynamic photoelectron spectroscopy has shown that the adsorption of carbon dioxide at aluminium surfaces is followed by a dissociative reaction leading to the formation of a metastable surface carbonate in the temperature range 80-120 K. The carbonate is subsequently reduced (120–475 K) (deoxygenated) to generate two different forms of surface carbon, one carbidic C^δ- (a) and the other less ionic C⁰(a) possibly graphitic. Quantification of the C(ls) and O(ls) spectra enable each of the species O^2-(a), CO₃^2-(a), C^δ-(a) and C⁰ (a) to be distinguished and their surface concentrations calculated over a wide temperature range. The temperature and pressure dependences of CO₂ reduction suggest the participation of a precursor dimer state (CO₂^---CO₂)(a) which then disproportionates. Furthermore studies of the coadsorption of ammonia and carbon dioxide in analogous systems indicate that a discrete and specifically reactive species, O^- (s), is formed during carbonate formation. The results are discussed in the context of recent theoretical studies of FREUND and MESSMER and also comparisons made with metal-CO₂ complexes.The facile surface reduction of CO₂ via a surface carbonate suggested that a possible route to carbon-oxygen bond cleavage in carbon monoxide interaction with an sp-metal surface (aluminium) was a step-wise oxidation to CO₂ leading to surface carbonate which was then readily deoxygenated. Studies of carbon monoxide: dioxygen mixtures (100: I) confirmed that this indeed occurred. A modified ELEY-RIDEAL type mechanism involving a hopping "non-adsorbed" CO molecule and a short-lived surface O^- (s) species is suggested.     

Reflectance FTIR investigations of the reactions of silanes on silica surfaces

CIR sampling has been used to demonstrate that the antimicrobial silane SiQAC is stable hydrolysis in aqueous solution at near neutral pH values. However, rapid hydrolysis occurs in mildly acidic solutions, but not accompanied by condensation of the silanol groups. After hydrolysis, condensation to form siloxane bonds is rapid in basic solution. The degree of hydrolysis is increased in the presence of silica gel. The silane on silica gel is quite durable towards desorption when contacted with water.     

AES investigations of fracture surfaces of aluminium doped sintered molybdenum rods

The addition of aluminium to K, Si-doped molybdenum leads to higher potassium and silicon contents in the final sintered rods. The increase is proportional to the Al-concentration. The increase of potassium and silicon is caused by the formation of potassium-alumosilicates and potassium aluminates which were identified by AES. The addition of A1³⁺ surprisingly shows a strong gettering effect on grain boundaryoxygen. The long interconnected system of pores in the sintered rod, causing splitting etc., is destroyed by addition of aluminium.     

Preferential destruction of metallic single-walled carbon nanotubes by laser irradiation

Upon laser irradiation in air, metallic single-walled carbon nanotubes (SWNTs) in carbon nanotube thin film can be destroyed in preference to their semiconducting counterparts when the wavelength and power intensity of the irradiation are appropriate and the carbon nanotubes are not heavily bundled. Our method takes advantage of these two species' different rates of photolysis-assisted oxidation, creating the possibility of defining the semiconducting portions of carbon nanotube (CNT) networks using optical lithography, particularly when constructing all-CNT FETs (without metal electrodes) in the future.     

Quantitative investigations of atomic processes on metal surfaces at atomic resolution

Using the atomic resolution of the field-ion microscope (FIM) and the single-atom identification capability of the time-of-flight atom-probe field-ion microscope, fundamental properties of solid surfaces can be investigated at atomic level. Atomic structures of many surface planes are fully resolved. The chemical composition of a surface can be determined with a single atomic layer spatial resolution. Several interactions of single surface atoms, which are basic to the understanding of many surface phenomena, such as the atom to substrate surface interaction, the adatom-adatom interaction, the adatom-impurity atom interaction, the adatom-plane edge interaction, etc. can all be studied in atomic details. The charge distribution of a single atom, as manifest in its dipole moment and polarizability, can also be studied. Great advantages of FIM investigations include the availability of atomically perfect surface planes, which can be prepared by field evaporation process, and the capability of a direct observation of the atomic images. Although some electronegative atoms cannot be imaged, the surface conditions and the state of the atoms can be characterized by the atom-probe. Basic principles and results of these single-atom FIM studies are presented and discussed.     

Microstructural,magnetic and crystal field investigations of nanocrystalline Dy3+ doped zinc oxide

Dy³⁺ doped zinc oxide was prepared by co-precipitation method. The as-prepared samples were annealed at different temperatures to obtain the samples with different particle sizes. The crystallographic phases of all the samples were confirmed by X-ray diffraction (XRD) patterns. Rietveld analysis of the XRD pattern of the sample annealed at 80 °C showed that most of the Dy³⁺ ions were substituted in the Zn²⁺ site of the hexagonal ZnO lattice. But in case of samples annealed at higher temperatures, a fraction of Dy³⁺ ions comes out from the ZnO lattice and this fraction increases with the increase of annealing temperature. The sizes of nanoparticles and the lattice strains of all the samples were obtained from the Hall–Williamson plot. High resolution transmission electron microscopy showed that ZnO nanoparticles are more or less spherical. Magnetic susceptibilities (χ) of some selected samples measured in the temperature range of 300–14 K indicate that the samples are paramagnetic. Values of χ were successfully fitted by Curie–Weiss law. A good theoretical simulation of χ of the sample annealed at 80 °C has been achieved using the one-electron crystal field interaction of the Dy³⁺ ions with its diamagnetic neighbors in the hexagonal single crystal.     

Time resolution investigations for general purpose plastic scintillation detectors in different thicknesses

Time spectra of ⁹⁰Sr, ¹³⁷Cs and ²⁰⁷Bi radioactive sources were obtained by using a BC 400 type plastic scintillation detectors in different thicknesses. Time resolution values through the time spectra were obtained by means of leading edge and constant fraction timing methods. The obtained time resolution values from leading edge method were compared with those of constant fraction timing method.     

Phenobarbital interactions with derivatized activated carbon surfaces

The interactions between phenobarbital and activated carbon surfaces were studied in detail in this work. This was accomplished by utilizing different reagents to manipulate the surface polar functional group compositions of different activated carbons, and determining how those modifications changed phenobarbital adsorption. Oxidation of an activated carbon surface caused a systematic decrease in the basal carbon surface, resulting in a concurrent systematic decrease in the non-specific adsorption of phenobarbital. Even more interesting, it was shown for the first time that chemical reduction of some of the carbonyl-containing functional groups on the activated carbon surface caused a significant increase in the specific adsorption of phenobarbital without any significant effect on the non-specific adsorption. These results support the notion that the OH groups on activated carbon surfaces are the specific adsorption sites for phenobarbital from aqueous solutions, and that the basal carbon surface is the region where non-specific adsorption takes place.     

k-Restoring processes at carbon depleted ultralow-k surfaces

In this study we investigate the silylation of OH groups with different silazanes. In particular we use density functional theory and the nudged elastic band method to study the different reaction mechanisms. For the silylation reaction of hexamethyldisilazane and trimethylaminosilane with silanol, the minimum energy paths as well as the activation and reaction energies are discussed in detail. From minimum energy reaction paths we found that all studied silazanes react exothermically. Bis(dimethylamino)dimethylsilane shows the most exothermic silylation reaction with the lowest activation energies. Therefore, it is a good candidate for the chemical repair of porous films in the semiconductor k-restoring process.     

X-ray reflectivity investigations of glass surfaces produced by float and draw techniques

Surfaces of soda-lime glass and borosilicate glass have been investigated by grazing incidence X-ray reflectivity (GIXR). Characteristic differences are obtained in dependence on the fabrication procedure, the composition and the cleaning procedure. Strong variation is recorded between the two soda-lime float glass surfaces while minor differences are analysed between the top and bottom side of borosilicate float glass. This is attributed to the reduced amount of tin diffused into the bottom side of the borosilicate glass surface. Different cleaning procedures generate characteristic changes on the glass surfaces which can be verified by GIXR. The results indicate that borosilicate float glass combines the merits of the good surface quality of float glass with the high chemical resistivity of borosilicate glass.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Controlled immobilization of membrane proteins to surfaces for fourier transform infrared investigations

We show that it is possible to immobilize membrane proteins uniformly and reversibly as self-assembled (sub)monolayers on nitrilotriacetic acid-covered sensor surfaces via hexahistidine sequences present either in the protein or in lipid membranes. Fourier transform infrared spectra of such self-assembled (sub)monolayers deliver important structural information of the membrane proteins and are suited to screen the function of cellular receptors.     

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Summary Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.²⁹Si and¹³C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.