Atomic contamination surface

A typical model system used in tribological simulations is shown in Figure 8. In this system, two walls are separated by a fluid and shear is applied by pulling the top wall with an external device, whereas the bottom wall is held fixed. In atomic-level simulations, the two walls correspond to atomically discrete surfaces and the fluid is composed of atoms or molecules, which represent lubricants or contaminants. 

Composite interfaces exist in a variety of forms of differing materials. A convenient way to characterize composite interfaces embedded within the bulk material is to analyze the surfaces of the composite constituents before they are combined together, or the surfaces created by fracture. Surface layers represent only a small portion of the total volume of bulk material. The structure and composition of the local surface often differ from the bulk material, yet they can provide critical information in predicting the overall properties and performance. The basic unknown parameters in physico-chemical surface analysis are the chemical composition, depth, purity and the distribution of specific constituents and their atomic/microscopic structures, which constitute the interfaces. Many factors such as process variables, contaminants, surface treatments and exposure to environmental conditions must be considered in the analysis. 

This suggestion was confirmed by an experiment in which a Pd wire was exposed to a CO/Oz reaction mixture without any previous surface cleaning (173). The stationary rate of C02 formation as a function of temperature (as shown in Fig. 58) is at first rather low, but increases steeply above a certain activation temperature. In any further runs the catalyst exhibited its normal activity, as is known from atomically clean surfaces. Auger spectroscopy demonstrated that at the beginning the surface was heavily contaminated by S and C, whereas these elements were completely absent after the sample had reached its full activity. Thus the well-known break-in effect of the catalyst (210) can in this case be simply explained by the removal of the inhibitors by surface reaction with one of the reactants, and the above-stated assumption is confirmed. 

Since unknown contaminants in the form of stable chemical compounds and chemisorbed gases exist on surfaces subsequent to the most careful chemical cleaning, the production of atomically clean surfaces requires additional treatment. Methods which have been found to be eflfective are ... 

AES has been applied in three areas of semiconductor surface studies. The simplest is the assessment of amounts of contamination present on a surface, and the absence of peaks other than those associated with the substrate is used effectively to define an atomically clean surface. It should be realised, however, that there could still be up to 0.01 monolayer, or 1013 atoms cm-2 of any element (hydrogen cannot be detected) on the surface, even though no additional Auger features are present. 

A possible explanation of these results is that most of the surface of the Pd is contaminated after reduction at 773 K, with only a few sites remaining where H2 can be dissociated and then transferred to sub-surface positions. On a normal, clean Pd surface almost complete monolayer coverage occurs before any sub-surface can be formed because of the depth of the potential well for adsorbed H2 and the activation energy to diffusion. However, it is common knowledge that contaminated Pd wire or foil will not absorb H2 at room temperature, but that mechanical cleaning of the surface allows normal absorption to occur at a rate which is determined by the rate of diffusion of H atoms from surface portholes into the bulk metal. In contrast to the foregoing observations, an Exxon patent claims that Pd/Ti02 in the SMSl states does not form the 3-hydride phase. 

Sazhin OV, Borisov SF, Sharipov F (2001) Accommodation coefficient of tangential momentum on atomically clean and contaminated surfaces. J Vac Sci Technol A 19(5) 2499-2503, Erratum 20(3) 957 (2002)... 

As a fundamental basis for all STM studies, electrode-electrolyte interfaces must be prepared reproducibly, and methods must be established to observe these interfaces accurately. Well-defined single crystalline surfaces must be exposed to solution to understand surface structure-reactivity relationships on the atomic scale. Efforts have succeeded to produce extremely well-defined, atomically flat surfaces of various electrodes made of noble metals, base metals, and semiconductors without either oxidation or contamination in solution. 

Results for the clean (100), (111), and (110) germanium faces and the (100) silicon face indicate that the atomic positions in the surface planes are not the same as the corresponding positions in the bulk structure. .. It is to be noted that half-integral-order beams are present in the (Oil) azimuth... The presence of half-integral-order beams. .. requires a double spacing in (110) azimuth but a single spacing in the (100) azimuth. .. The observed structures are not due to surface contamination but are characteristic of atomically clean surfaces [15]. 

The substrate surface can be cleaned in the deposition system by several means. This in situ cleaning is intended to remove the small amount of contamination that will have developed since the external cleaning process was performed - it is not intended to replace external cleaning One technique is to cleave, fracture, or scrape the material to prepare a new surface under well controlled conditions. To obtain an atomically clean surface in ultrahigh vacuum can sometimes take weeks. 

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