The atomic structure of a metal-supported vitreous thin silica film

Clear as glass: The atomic structure of a metal-supported vitreous thin silica film was resolved using low-temperature scanning tunneling microscopy (STM). Based on the STM image, a model was constructed and the atomic arrangement of the thin silica glass determined (see picture). The total pair correlation function of the structural model shows good agreement with diffraction experiments performed on vitreous silica.     

Kinetics of hydrogen adsorption on thin iron films with preadsorbed hydrogen sulfide

The influence of H₂S preadsorbed on thin iron film surface on the character of TD spectra and on sticking probability dependence on population for hydrogen adsorption was studied. Kisliuk's model for adsorption with the precursor state was examined.

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The effect of atomic hydrogen adsorption on single-walled carbon nanotubes properties

We investigated the adsorption of hydrogen atoms on metallic single-walled carbon nanotubes using ab initio molecular dynamics method. It was found that the geometric structures and the electronic properties of hydrogenated SWNTs can be strongly changed by varying hydrogen coverage. The circular cross sections of the CNTs were changed with different hydrogen coverage. When hydrogen is chemisorbed on the surface of the carbon nanotube, the energy gap will be appeared. This is due to the degree of the sp³ hybridization, and the hydrogen coverage can control the band gap of the carbon nanotube.     

Characterization of a circular thin film plasma source for atomic emission spectroscopy

A plasma source for analytical atomic emission spectroscopy is described based on the electrical vaporization by capacitive discharge of a thin Ag film deposited on a polycarbonate membrane filter. The source is designed for the rapid, direct analysis of solid powder samples collected by filtration from fluid media. A concentric electrode system consisting of a ring-shaped graphite electrode placed on the thin film surface and a pointed graphite rod located under the membrane substrate results in a plasma with cylindrical symmetry and a radial current path. Discharge current vs time and intensity vs time profiles are compared for the concentric electrode geometry and the linear geometry used in previous studies. Two values of tank circuit inductance also are compared. Both neutral-atom and ion line radiation from an Mn sample are more intense when the center electrode is initially cathodic. The inside diameter of the ring-shaped electrode and thus the surface area of the Ag film exposed to the plasma have relatively little effect on the intensity of continuum background and line radiation from a sample deposited near the center of the film. Particle size effects, while significant, are smaller than with the linear electrode geometry. Analytical curves are presented for several lines using both a low-inductance and a high-inductance discharge. Log-log slopes range from about 0.85 to 1.05 for ion lines. Detection limits are somewhat poorer than with the linear geometry.     

Atomic deuterium (hydrogen) adsorption on thin silver films

Atomic deuterium and hydrogen adsorption on thin silver films deposited under UHV conditions on Pyrex glass was studied by means of measurements of the resistance changes ΔR combined with thermal desorption mass spectrometry (TDMS). The roughness factor of thin Ag films of known geometry, textured as a result of sintering, was determined by means of the BET method (xenon adsorption), while their preferential crystallographic orientation (1 1 1) was estimated on the basis of XRD data. ΔR measurements were performed during various exposures of the films maintained at a constant temperature (78 or 89 K) to the flux of atomic deuterium (hydrogen) of known concentration generated on a hot tungsten filament. Every adsorption run was followed by thermal desorption. This gives a link between the ΔR measured directly in the course of adsorption and the coverage Θ determined on the basis of TDMS data, together with the BET and XRD results. It was found that at 78 K the rate of atomic deuterium (hydrogen) adsorption and recombination on the surface of sintered thin Ag films fits the Eley–Rideal (ER) mechanism, while at 89 K its overlapping with the Langmuir–Hinshelwood (LH) recombination starts to play a role. The initial sticking probability reaches 0.41 and 0.65 for D and H atoms, respectively, while the corresponding probabilities for recombination are 0.04 and 0.07. The activation energies for associative desorption of deuterium and hydrogen are 36 and 29 kJ/mol, respectively.     

Thermodynamic and kinetic properties of hydrogen adsorption on thin iron layers

Thermodynamic properties of hydrogen adsorption on iron layers evaporated under UHV conditions have been determined from calorimetric and volumetric measurements over the temperature range from 298 K to 363 K. Kinetic properties have been determined with the help of deconvolution of calorimetric curves and compared with the literature data on the sticking coefficient.

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Zusammenfassung Mittels kalorimetrischer und volumetrischer Messungen im Temperaturbereich von 298 bis 363 K wurden die thermodynamischen Eigenschaften der Wasserstoffadsorption an Eisenschichten untersucht. Die kinetischen Eigenschaften wurden mit Hilfe der Dekonvulotion kalorimetrischer Thermogramme ermittelt und mit Literaturangaben zum Haftkoeffizienten verglichen.

     

Nanocharacterization and nanofabrication of a Nafion thin film in liquids by atomic force microscopy

We demonstrated the nanocharacterization and nanofabrication of a Nafion thin film using atomic force microscopy (AFM). AFM images showed that the Nafion molecules form nanoclusters in water, in 5% methanol, and in acetic acid. Young's modulus E of a Nafion film was estimated by sequential force curve measurements in water and in 5% methanol on one sample surface. Ewater/E5% methanol was 1.75 +/- 0.40, so the film was much softer in 5% methanol than in water. Even when solvent was replaced from 5% methanol to water, Young's modulus was not recovered soon. We showed the first example of the mechanical properties of a Nafion film on the nanoscale. Furthermore, we succeeded in fabricating 3D nanostructures on a Nafion surface by AFM nanolithography in liquids. Our results showed the new potential of the AFM nanolithography of a polymer film by softening the molecules in liquids.     

The adsorption structure of nonionic surfactant on the sessile mercury electrode in a thin liquid film

In relation to a colloid stability, the adsorption structure of the Stern layer on a sessile mercury electrode in a thin liquid film of nonionic surfactant was investigated by measuring the double layer capacitance. The Stern capacitance on the electrode in the film could be detected when the measuring frequency used was low, for the resistance of the film was not extremely high but of the order of several thousand ohm. It was found that the adsorption structure of nonionic surfactant in the thin liquid film shows a stratification different from that of bulk.     

Effects of ionic surfactant adsorption on single-walled carbon nanotube thin film devices in aqueous solutions

Field-effect transistors were fabricated using high-density single-walled carbon nanotube (SWNT) thin films directly grown on suitable substrates. Such approach eliminated the variations of device behaviors in individual SWNT devices by utilizing a large number of SWNTs in each device. We have found that the behaviors of such devices are closely related to the surface charge densities around SWNTs in aqueous solutions. Adsorption of ionic surfactants on the surface could significantly modulate the device characteristics, which could be detected by measuring the conductance of the devices. The devices could be tuned to be sensitive to either anionic or cationic surfactants by tailoring the surface properties of SiO(2) substrates around SWNTs. This effect could be potentially used to design chemical and biological sensors.     

Role of the precursor state in the rate of hydrogen adsorption on thin iron films

Sticking probability for hydrogen adsorption on thin iron films deposited under ultra-high vacuum condtions was studied in dependence on the method of film preparation and the temperature of adsorption. It was observed that Kisliuk's model for a dissociative adsorption fits well the experimental data at low temperature. At high temperature Kisliuk's relation describes the adsorption as well as the Langmuir equation.

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Evaporation of a thin liquid film

Evaporation of a thin (submicrometer size) liquid film confined between two solid substrates is studied using diffuse interface hydrodynamic model supplemented by the van der Waals equation of state. The time and space evolution of the basic thermodynamic quantities such as temperature, density, entropy, chemical potential, and entropy production is presented. The values of numerical parameters chosen correspond to those of argon. The time and space scales studied range from picoseconds to microseconds and from nanometers to micrometers correspondingly.     

Dewetting of a sol-gel-derived thin film

We report on the dewetting of a thin film produced by the sol-gel method. In the early stages of dynamic morphological instability, the drying stress in the capillary wave model determines the linearly scaling behaviors of the characteristic wavelength with the initial film thickness and the square law dependence of the number density of the dewetted holes on the film thickness. These power law dependences are weaker than those observed in the case of the spinodal dewetting of a polymer thin film. The wavelength determined in the early stages also dominates the scaling behaviors of the average length of the sides and number density of the polygons and the diameter of the droplets of the dewetting pattern with the film thickness in the final stages of the dynamic instability. We also observed that further drying eventually induces wrinkles in the droplets, rim, and film, which have a characteristic wavelength that can be theoretically predicted.     

Preparation of bismuth telluride thin film by electrochemical atomic layer epitaxy (ECALE)

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Bi and Te on cold rolled silver substrate have been performed. The voltammetric analysis of underpotential shift demonstrates that the initial Te UPD on Bi-covered Ag and Bi UPD on Te-covered Ag fitted UPD dynamics mechanism. A thin film of bismuth telluride was formed by alternately depositing Te and Bi via an automated flow deposition system. X-ray diffraction indicated the deposits of Bi₂Te₃. Energy Dispersive X-ray Detector quantitative analysis gave a 2: 3 stoichiometric ratio of Bi to Te, which was consistent with X-ray Diffraction results. Electron probe microanalysis of the deposits showed a network structure that results from the surface defects of the cold rolled Ag substrate and the lattice mismatch between substrate and deposit. Translated from Chinese Journal of Applied Chemistry, 2005, 22 (11) (in Chinese)     

Selective adsorption of biopolymers on zeolites

Zeolites adsorb biopolymers on their surface and may be suitable as a new type of chromatographic carrier material for proteins, nucleic acids, and their conjugates. We report here various parameters that influence the adsorption of biopolymers on synthesized zeolites with regard to the Si/Al2 ratio and three-dimensional structure. There are three physicochemical principles that may underly the adsorption: 1) below the isoelectric point (pI), mainly Coulombic attraction similar to ion-exchange chromatography; 2) at pI, hydrophobic interactions (a kind of van der Waals attraction) plus the three-dimensional mesopore structure; and 3) above pI, the sum of the Coulombic repulsion and attraction forces, such as the hydrophobic interaction, and also substitution reaction of water on the Al molecule with a protein amino-base. At high Si/Al2 ratio in the presence of a small amount of Al and with mesopores between the zeolite particles, maximal adsorption was seen at pI and was suggested to be dependent on the number of hydrophobic interaction points on the mesopores, and their morphology. The application of zeolites to biochemistry and biotechnology is also discussed.     

Selective determination of rate constants of reactions of atomic hydrogen with various functional groups of a complex molecule

The possibility of determining absolute values of the rate constants of reactions of active intermediate species with different functional groups of molecules is demonstrated by measuring macrokinetic combustion characteristics. The Arrhenius parameters of the rate constant of the reaction between atomic hydrogen with the methylene group of ethanol and molecular oxygen within the temperature range of 830–970 K are determined. The reasons for the differences between the rate constants of reactions with the methylene and methyl groups of an ethanol molecule are discussed using thermochemical data. It is found that the obtained values of activation energies and preexponential factors of rate constants are in good agreement with the literature data on the region of lower temperatures.     

Selective protein adsorption on a phase-separated solvent-cast polymer blend

Polymer-based biomedical devices are growing increasingly sophisticated as compositions evolve toward copolymers and blends in order to satisfy complex design criteria. Such polymers afford opportunities for both micro- and macrophase separation at nano- and micro-length scales and raise questions concerning the role of heterogeneous surface morphology on protein adsorption. Adsorbed protein layers play a critical role in mediating the interaction of cells with polymer surfaces, and both understanding and controlling protein adsorption is assuming greater significance in the development of surfaces with enhanced physiological compatibility. Here we study the short-time adsorption of ferritin, a model protein highly resistant to denaturation and easily imaged in the transmission electron microscope (TEM), onto a phase-separated homopolymer blend of polycaprolactone (PCL) and a polycarbonate derived from desaminotyrosyl-tyrosine dodecyl ester (PDTD). At physiological pH, ferritin selectively adsorbs onto the PDTD phase at a surface density approximately three times greater than that on the PCL phase. By decreasing the pH below ferritin's isoelectric point so its average charge becomes positive, the selective adsorption disappears and the surface density of adsorbed ferritin becomes independent of the phase separation. We attribute the selectivity to the electrostatic repulsion between ferritin and hydrolytically charged PCL, both of which will have a net negative charge at physiological pH. To perform these experiments, we solvent-cast ultrathin polymer films onto dissolvable salt substrates, and we characterize the morphology by TEM imaging and quantitative spatially resolved electron energy-loss spectroscopy (EELS). We find that the film morphology depends strongly on such processing-related variables as the solvent evaporation rate and the nature of the surface in contact with the polymer film during casting. The adsorption of ferritin depends on whether the film is phase-separated as well as to which surface of the film the protein solution is exposed, and these findings suggest that seemingly small variations in polymer processing that influence both the bulk and surface morphology can have a profound effect on the short-time protein adsorption.