Thin films adsorbed layers

Conventionally RAIRS has been used for both qualitative and quantitative characterization of adsorbed molecules or films on mirror-like (metallic) substrates [4.265]. In the last decade the applicability of RAIRS to the quantitative analysis of adsorbates on non-metallic surfaces (e.g. semiconductors, glasses [4.267], and water [4.273]) has also been proven. The classical three-phase model for a thin isotropic adsorbate layer on a metallic surface was developed by Greenler [4.265, 4.272]. Calculations for the model have been extended to include description of anisotropic layers on dielectric substrates [4.274-4.276]. 

Reflectometry, where only the intensity of the reflected light is measured, requires a thin film adsorbed onto a reflecting substrate. The thin film has a different refractive index than both the bulk solution and the substrate so interference occurs between the rays reflected from the substrate/fllm and film/bulk solution interfaces. By model fitting the refractive index-distance behavior, it is possible to extract the volume fraction profile of the adsorbed polymer layer (15). 

Some examples of biologically relevant thin films are layers made by adsorbed proteins that are the precursors for cell attachment, bacterial films or biofilms, and coatings of minerals such as hydroxyapatite to enhance integration of the implants to bones. This section describes the evaluation of some of the biological attributes of such surface coatings. 

Atomically thin films constitute layers whose thicknesses are comparable to one or a few atomic layers. An adsorbed monolayer of gas or impurity atoms on a surface is an example of an atomically thin layer. Here the mechanical response of the thin layer is likely to be more influenced by interatomic potentials and surface energy than by macroscopic mechanical properties or by micromechanisms of deformation. 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

ADSORBED LAYERS, VERY THIN FILMS AND NUCLEI... 

The statics and dynamics of microstructures are governed by the forces that create or maintain them. Rarely can the forces be measured directly. But forces between special surfaces immersed in fluid can now be accurately gauged at separations down to 0.1 nm with the direct force measurement apparatus, an ingenious combination of a differential spring, a piezoelectric crystal, an interferometer, and crossed cyhndrical surfaces covered by atomically smooth layers of cleaved mica (Figure 9.4). This recent development is finding more and more applications in research on liquid and semiliquid microstructures, thin films, and adsorbed layers. 

Similar effects may exist at other metals. For instance, when the surface of an iron electrode is thermally reduced in hydrogen and then anodically polarized at a current density of 0.01 mA/cm in 0.1 M NaOH solution, passivation sets in after 1 to 2 min (i.e., after a charge flow of about 100 mC/cm ). This amount of charge is much smaller than that required for formation of even a thin phase film. Since prior to the experiment, oxygen had been stripped from the surface, passivation can only be due to the adsorbed layer formed as a result of polarization. 

Figure 13.1 Electrooxidation of COad and Ci adsorbate layers pie-adsorbed on a Pt/Vulcan thin-film electrode (7 JLgptCm , geometric area 0.28 cm ) in 0.5 M H2SO4 solution during a first positive-going potential scan, and subsequent response of the faradaic (a) and m/z = 44 ion current (b) to the electrode potential in the thin-layer DBMS flow cell. The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p,L s at room temperature. The respective adsorbates were adsorbed at 0.11 V for 10 minutes from CO-saturated solution (solid line), 0.1 M HCHO solution (dashed line), 0.1 M HCOOH solution (dash-dotted line), and 0.1 M CH3OH solution (dash-double-dotted line).

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

Investigation of interaction of electrons of different energies with a solid material in plasma processes may be even more intriguing and important, especially in the case of an adsorbed layer of materials contained in the reaction vessel. Provided thin semiconductor films deposited on the walls of the reaction vessel are used as solid targets, these films can be simultaneously used as targets and semiconductor sensors. This is also the case when such films are deposited on the specially manufactured quartz plates with electrodes accessible from the outside of the vessel. These sensors can be placed in any point of the vessel. 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

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