Compositional analysis of surface layers

Basic information needed to understand the physical and chemical properties of solid surfaces and thin solid films include the atomic structures and the compositional variations across the surface and interface layers. The atomic structures can be studied with microscopies and with surface sensitive diffraction and particle scattering techniques. Compositions of surfaces and thin films can be studied with the atom-probe FIM. In general, however, compositional analyses are mostly done with surface sensitive macroscopic techniques, such as auger electron 

Sample surfaces prepared by various macroscopic techniques are not well characterized on an atomic scale over the extended area of the macroscopic surfaces. It is now well known that even the most carefully processed surfaces contain only domains of well defined atomic structures of sizes of the order of 100 to 1000 A. 

Ion sputtering induces lattice defects and atomic mixing among surface layers. Sputtering yield is different for different chemical species. Thus the composition of a sputtered surface is not necessarily the true composition of that layer. 

In most surface sensitive spectroscopies, the depth resolution is still much larger than one atomic layer. Thus even under the best conditions a true atomic layer depth resolution cannot be achieved in the compositional analysis. 

Most surface sensitive spectroscopies are insensitive to some chemical species for example AES is insensitive to hydrogen, etc. 

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Stoichiometric variations in compositions of a material and of surface layers can be revealed by AEM. Because a relatively small amount of scattering occurs through a thin HRTEM specimen, X-rays are generated from a volume that is considerably less than in the case of electron microprobe analysis (EPMA). For quantitative microanalysis, a ratio method for thin crystals (57) is used, given by the equation ... 

Consideration of Surface Analysis Concerns. The researchers in this study used a wide range of surface and other tools, taking appropriate advantage of the strengths of the various methods. Previous work had shown that AES and XPS could be used to study chromate films without unreasonable problems and provided a basis for the current study. XPS was used to obtain specific chemical information while AES was used whenever spatial resolution and electron imaging were desired. RBS and electron microprobe work was used to analyze composition structures of thicker layers. 

As is widely known, bulk species which have chromophores that absorb in the UV-Vis can be analyzed quantitatively and qualitatively by this spectroscopy. The study of electrodes or species adsorbed as thin layers by UV-Vis is more difficult, due to sensitivity problems and the availability of the appropriate chromophores [50], Another use of this type of analysis is electroreflectance [51,52], Adsorption of species on reflective electrode surfaces changes their reflectivity. Thus, this method can indicate electroadsorption processes very sensitively, in situ, although it does not provide specific information on the structure and composition of surface layers. 

The thermodynamics and dynamics of interfacial layers have gained large interest in interfacial research. An accurate description of the thermodynamics of adsorption layers at liquid interfaces is the vital prerequisite for a quantitative understandings of the equilibrium or any non-equilibrium processes going on at the surface of liquids or at the interface between two liquids. The thermodynamic analysis of adsorption layers at liquid/fluid interfaces can provide the equation of state which expresses the surface pressure as the function of surface layer composition, and the adsorption isotherm, which determines the dependence of the adsorption of each dissolved component on their bulk concentrations. From these equations, the surface tension (pressure) isotherm can also be calculated and compared with experimental data. The description of experimental data by the Langmuir adsorption isotherm or the corresponding von Szyszkowski surface tension equation often shows significant deviations. These equations can be derived for a surface layer model where the molecules of the surfactant and the solvent from which the molecules adsorb obey two conditions ... 

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