Modeling migration of adsorbed atoms in dense multicomponent adsorbed layers

Within the framework of a simple (i.e., ignoring lateral interactions) lattice model, surface migration of adsorbed atoms of a crystalline substance A is considered for the case in which the surface is densely covered by the adsorbed atoms of a noncrystallized substance B. Simulation of the migration, performed by means of the Monte-Carlo method, demonstrates a sharp decrease in the root-mean-square displacement of the adsorbed atoms of A with an increase in the degree of covering by adsorbed atoms of B in the case where these adsorbed atoms exhibit weak migration mobility. This effect is associated with strong correlations in the directions of individual jumps of an adsorbed atom of A. The results of the simulation are applied to estimate the influence of the degree of covering of a surface with an impurity on the temperature at which a transition is accomplished from an nascent to a stepwise-layered mechanism of growth in silicon in molecular-beam epitaxy. V. D. Kuznetsov Siberian Institute for Technical Physics. Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 89–94, June, 1998.     

Universality in structure and elasticity of polymer-nanoparticle gels

We propose a combination of polymer field theory and off-lattice computer simulations to study polymer-bridged gelation in polymer-nanoparticle mixtures. We use this method to study the structure of gels formed in attractive polymer-colloid systems. Our results indicate that such gels exhibit a universal structure with a fractal dimension d(f) approximately 2.5 characteristic of random percolation. By mapping to an affine-network model, the enhancement in elastic moduli is predicted to follow a critical exponent nu(eta) approximately 1.8 characteristic of the resistor network percolation. We analyze selected experimental results to suggest the existence of a universality class corresponding to our results.     

Concentration fluctuations in adsorbed layers

The temperature and coverage dependence of the mean square concentration fluctuations in a small open domain of an adsorbed layer is discussed for various situations. It is shown that fluctuations decrease with increasing temperature and reach a limiting value when attractive interactions predominate, but increase and reach a limiting value when repulsive interactions predominate, if a single non-ideal two -dimensional phase exists. Deviations from ideal gas behavior are strongest at half coverage. At very low coverage (low particle concentration) and very high coverage (low hole concentration) ideal behavior is approached. If the layer consists of a two phase system, for instance a two-dimensional liquid or solid in equilibrium with a two-dimensional gas, fluctuations far below the critical temperature are dominated by fluctuations in the partition between phases. As the critical temperature is approached fluctuations first decrease because the mean concentrations in the two phases approach each other, and then increase very sharply near T_c. Detailed calculations for the single phase situation are given for several approximations: a dilute gas; a mean field approximation; a lattice gas in both the Bragg-Williams and the BethePeierls-Weiss approximations. The latter which takes some account of correlations between adsorbate particle positions seems to explain reasonably the presently available experimental observations on chemisorbed layers.     

Anomalous adhesion in adsorbed polymer layers

Anomalous adhesion behaviour observed in the polydimethylsiloxane/heptane/silica system is reported. This behaviour is characterised by the infrequent appearance of one or more elastic minima which occur in addition to the ever-present primary adhesion. The resulting elastic force is attributed to loops connecting the tip to the substrate and can be described by two models; a worm-like chain, and a freely jointed chain. Both models give similar values for the characteristic segment length of around 0.24 nm and indicate that the chains have been extended by between 90 to 95% of their chain contour length before desorption. In some cases multiple adhesions were observed between tip and substrate and have been used to suggest a method for determining the distribution of adsorbed polymer loops. Received: 12 November 1997 / Accepted: 6 March 1998     

The structure of n-alkane binary mixtures adsorbed on graphite

The thermodynamics and structure of the surface adsorbed phase in binary C15-C16 and C15-C17 n-alkane mixtures confined in graphite pores have been studied by differential scanning calorimetry and small-angle X-ray scattering. The previously observed selective adsorption of the longer alkane for chain length differences greater than five carbon atoms is verified but reduced for chain length differences less than or equal to two. With a difference in chain length of one carbon atom, Vegard's law is followed for the melting points of the adsorbed mixture and the (0 2) d-spacing is a continuous function of the mole fraction x. With a two-carbon atom difference, samples aged for 1 week have a lamellar structure for which the entities A_1−xB_x try to be commensurate with the substrate. The same samples aged for 1 month show a continuous parabolic x-dependence for both the melting points and the d-spacings. An explanation in terms of selective probability of adsorption is proposed based on crystallographic considerations.     

Holographic nanocomposites for recording polymer-nanoparticle periodic structures: II. Mechanism of formation of polymer-nanoparticle bulk periodic structure and effect of parameters of forming field on structure efficiency

We studied polymerizable nanocomposites for obtaining polymer-nanoparticle periodic structures by a holographic method. A general approach to choosing components of composites is developed that ensures a maximal contrast and high efficiency of structures for different types of nanoparticles. We found that the optimal monomeric component of a nanocomposite is a combination of single- and multifunctional monomers with substantially different reactivities. In this case, the low-reactivity monomer should posses a low viscosity, be a good solvent for nanoparticles, and have a low thermodynamic affinity to the polymer network formed upon the polymerization of the high-reactivity monomer. We developed a holographic composition based on known commercially produced monomers that ensures the formation of highly efficient periodic structures for nanoparticles of different types. We described the holographic properties of obtained nanocomposites, as well as parameters of bulk gratings recorded in them.     

Determination of the structure of ordered adsorbed layers by analysis of LEED spectra

Recent work on the structure of ordered overlayers of chalcogens on nickel is reviewed, and discussed as a convincing example of the use of LEED intensity spectra for surface structure determination. Guidelines are suggested for successful application of LEED to surface structure, and the special suitability of the muffin-tin potential for the LEED calculation is brought out. The model of the complex crystal potential used for the calculation, and the systematic determination of the four parameters of the model from the data for clean Ni are described. The application to chalcogen overlayers is illustrated by the case of c(2 × 2)S and c(2 × 2)O on Ni(001) surfaces, and the correspondence between experiment and theory is exhibited for the intensity versus energy spectra of $\text{1}\text{2}\text{1}\text{2}$ beams. The correspondence is close and detailed; the complex shapes of broad features are reproduced and the average of the magnitude of the deviation between peaks in measured and calculated spectra is less than 2 eV, whereas the accidental correspondence of two spectra is shown to fluctuate around 4 eV. The observed values of the bond lengths of O, S, Se, Te on Ni are shown to be plausible on the basis of the covalent radii and the expected deviations from them. Results for other surfaces and other structures are tabulated. In all cases but one [O/Ni(110)] the chalcogen occupies sites that the next layer of Ni atoms would use.     

Density and thermodynamics of hydrogen adsorbed inside narrow carbon nanotubes

A model is proposed for calculating the thermodynamic functions and the equilibrium density of a one-dimensional chain of molecules (atoms) adsorbed inside a narrow nanotube. The model considers both the interaction between introduced atoms (molecules) and their interaction with the nanotube walls. The quantum-mechanical effects resulting in discrete energy levels of a particle and in its smeared position between neighbors are taken into account. In calculating the free energy at a nonzero temperature, the phonon contribution and the particle transitions to excited levels are considered. The model is applied to calculate the thermodynamic parameters of adsorbed hydrogen molecules inside extremely narrow single-wall carbon nanotubes of the (3,3) and (6,0) type. It is shown that external pressure gives rise to a sequence of first-order phase transitions, which change the density of adsorbed hydrogen molecules.     

Photoemission studies of adsorbed oxygen and oxide layers

A comprehensive review is given about the enormous versatility of photoelectron spectroscopy to study the especially complex interaction of oxygen with metal surfaces and the nature of the reaction products. The great variety of well definable parameters of a photoemission experiment, e.g. energy, direction of incidence and polarization of the primary photon beam as well as the detection direction of the photocurrent, yields - through the distributions of energy, momentum and spin polarization of the photoelectrons - detailed insight in the kinetic, thermodynamic, electronic and structural aspects of oxygen adsorption on metal surfaces and incipient oxidation. Characteristic electron binding energies, multiplet and satellite structures of both the oxygen and substrate emission allow a distinction between possible states of adsorbed oxygen, i.e. condensed, molecularly and atomically adsorbed, and incorporated oxygen. Even a distinction between octahedral and tetrahedral oxygen coordination of oxide cations may be possible. Analysis of peak intensities (as a measure of coverages and concentrations) as a function of time and temperature provides information about the kinetics and thermodynamics of adsorbed layer and oxide formation. Angular resolved photoemission studies have led to the determination of absolute adsorption site geometries, individual ad-orbital symmetries and two-dimensional band structure formation within the oxygen overlayer. Measurement of the photoelectron spin-polarization offers a method to study surface magnetism, e.g. of ferromagnetic oxides. The determination of local work functions through the photoemission behavior of co-adsorbed rare gas atoms establishes a uniquely important tool to characterize heterogenous surfaces, e.g. oxygenated surfaces with coexisting oxygen states. Numerous different oxygen/metal systems are chosen to demonstrate the state of the art. Results from other surface spectroscopies and theoretical model calculations are, of course, considered and still open problems are named, e.g. the ionicity of the oxygen chemisorption bond. Problems inherent in sputter profiling through surface oxides as observed with photoemission are briefly addressed. This work is rounded by a list of about 600 references in alphabetic order of the reacting metals.     

A simple model for ordering in adsorbed layers

Order-disorder phase transitions in adsorbed and confined fluids with directional interactions are studied using lattice density functional theory. A new model is developed that is capable of predicting both order-disorder and condensation phase transitions. For systems with weak interactions, the results of this model are compared with both lattice Monte Carlo simulation data and simple isotherm theories that are commonly used to fit experimental data. The results show that these simple isotherms are incapable of duplicating complex behaviour exhibited by anisotropic molecules. When sufficiently strong interactions are present, confined fluids with directional interactions may spontaneously form ordered structures. It is shown that the ordering predicted by this model can result in the assembly of long chains. Such ordering has been observed experimentally in magnetorheological fluids in a magnetic field. It is shown that the orientation of the chains predicted by this model can be controlled by adjusting the molecule-surface interaction. It may be possible to create nanoscale devices that exploit this type of molecular switching.     

Tin in silicate glasses: structure, thermodynamics and kinetics

In this work Mössbauer spectroscopy is used to investigate the oxidation states and structures of tin in silicate glasses. Thermal treatment of the glasses in atmospheres with varying oxygen partial pressure leads to the simultaneous appearance of reduction and diffusion. Experiments with varying treatment time give the opportunity to study diffusion and reduction processes in detail. Comparison of the hyperfine parameters of reference materials with measured parameter provides information about the local surroundings of the tin atoms. An octahedral surrounding for Sn^4?+? is presumed, while Sn^2?+? and three oxygen atoms form a tetrahedral coordination.     

Fluctuation forces and wetting layers in colloid-polymer mixtures

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.     

Collision-induced absorption in dense mixtures of simple fluids

Summary Collision-induced absorption (CIA) spectroscopy is a probe of the dynamical properties of dense systems. Two simple cases of CIA spectra are here discussed: the translational band observed in solutions of a noble gas in liquid Ar and the rotational lines of H₂ observed in solutions of H₂ in Ar. In the first case, the analysis of the translational band allows one to derive the characteristic frequency ω₀, associated with the rattling component of the motion of the impurity in the Ar medium. In the second case, the analysis of the rotational lines allows one to derive the parameter δ, that characterizes the density narrowing effect. In the high-density region, the parameter δ is directly related to the diffusive motion of the Ar atoms around the impurity. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Characterizing the structure of protein layers adsorbed onto functionalized surfaces by means of in-situ X-ray reflectivity

In a biological cell, proteins face a highly complex environment comprising crowding and confinement effects as well as interactions with interfaces, cosolvents, and other biomolecules. In this study, the X-ray reflectivity technique has been used for the in-situ characterization of adsorbed protein layers at solid-liquid interfaces. The adsorption of bovine serum albumin at the hydrophobic polystyrene-water interface has been investigated in the presence and in the absence of salts. The data indicate that enhanced adsorption occurs at high salt concentrations.