Carbon surface contamination

For T3250 processed wafers, ESCA shows an increase in Si and oxygen surface concentrations with a concurrent loss in carbon concentration. Therefore, carbon surface contamination on the gold is being replaced with -the chemisorbed or physisorbed primer species. HMDS-treated Au surfaces exhibited no such change in surface chemical nature, consistent with the no improvement adhesion lifting test results for that promoter. 

In the case of 308-nm irradiation with fluences above the threshold of ablation, the most complex behavior is found (Fig. 20). Only minor changes took place between 1 and 10 pulses. These changes can be attributed to removal of oxygen and carbon surface contaminations. In addition, the largest roughening of the surface (Fig. 11a) is detected for 10 pulses, which can also influence the relative atomic ratios. After more than 10 pulses the atomic ratios approach, within the error of the experiment, the starting values. This indicates that no surface modification took place and the polymer is ablated layer by layer. 

In carbon adsorption, contaminants are physically attracted or adsorbed on the surface of the carbon. Adsorption capacities are high for carbon because its porous nature provides a large surface area relative to its volume. Activated carbon is prepared from lignite, bituminous coal, coke, wood, or other organic materials such as coconut shells. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

Air pollution problems in which adsorption is considered a unit operation involve gaseous contaminants. The number of molecules present at the carbon surface is dependent on the number that reach the surface and on the residence time of these molecules on the carbon surface. If n molecules strike a unit area of a surface per unit time, and remain there for an average time, t, then a number of molecules are present per unit area of surface ... 

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

Immiscible-phase separation Transformation Processes No Fluids (such as gasoline) that are immiscible in water are a significant consideration in near-surface contamination. Deep-well injection is limited to wastestreams that are soluble in water. Well blowout from gaseous carbon dioxide formation is an example of this process that is distinct to the deep-well environment. 

Surface contamination of 2 kg of the dry salt with as little as 2 flakes of moist potassium hydroxide caused a vigorous self-sustaining fire, which was extinguished with water, but not by carbon dioxide or dry powder extinguishers. 

When the peak intensity ratio Ij/lj is measured for each of all the elements observed (carbon, oxygen, chromium and nickel), the surface atomic concentration ratio nj/nj of the elements can be given by the following equation, if surface contamination is negligible ... 

ESCA was employed to analyze membranes before and after use in the desalination cell. Wide scan ESCA spectra were obtained on the last two membranes listed in Table VII. Table IX lists the binding energies (B.E.) and the atomic fractions (A.F.) for the membranes studied. In addition to the expected carbon, oxygen, sodium, and sulfur peaks, two small peaks were attributed to nitrogen and silicon, which may be due to the contamination in the air (silicon grease). A smaller photo peak was observed at 51.3 eV and remains unasslgned. Overall, there is no significant surface contamination of the membranes. 

Since electroanalytical response ultimately depends on electron transfer between an electrode material and a solution species, any intervening layers or films are of obvious consequence. As noted earlier, carbon materials are prone to adsorption, and it is not trivial to prepare carbon surfaces that are not contaminated with adsorbed layers, usually of adventitious organic impurities. In order to avoid poor or irreproducible responses from carbon electrodes, the user must either find conditions where surface impurities have negligible or reproducible effects on the process of interest, or prepare the carbon surface in a way that avoids surface films. As is evident from the history of electroanalytical chemistry since the DME, surface cleanliness is a major issue with solid electrodes, no less so for carbon. 

A further serious factor determining the apparent gap between model studies with well-defined clean surfaces and the conditions encountered with real catalysts is the fact that the latter are never prepared in a way that the degree of surface contamination is well controlled. Elements such as sulfur or carbon might always be present in considerable concentrations on the surface so that the chemical nature of the catalytically active surface could possibly be quite different. 

The carbon surface concentration of 42.5% at 90° exit angle for water-sized fibers in Table 1 represents a high level of adventitious contamination, more typical of plant production than of fibers formed in a relatively clean laboratory environment, which typically show carbon levels of 25% to 35% on water-sized E glass [4, 7, 8]. This high level of adventitious carbon tends to obscure the... 

The initial scan shows indeed a very poor contact (Figure 17.8). At a voltage of 1V the current is below 1 nA and the conductivity is dependent on voltage and also on the location of the contact within the scanned area. After a thermal treatment of 5 min at 200°C the conductivity scan showed excellent conductivity on the whole scan area and at 0.01 V the current amplitude rises to more than 300 nA, so the conductivity increases by more than five orders of magnitude, see Figure 17.9. The reason for this behavior can be described by surface contamination. But the analytical description of the electrical behavior of this surface layer, which is mostly related to water and carbon, is hardly possible. An analytical description of the electri-... 

Most of the sputtering data are for pure materials. In fact, most investigators have attempted to avoid surface contamination in order to make measurements truly representative of the substrate material. In a Tokamak reactor the wall will be sputter cleaned and hence these data are relevant. The actual conditions in today s operating Tokamaks are more complex. For example, carbon and other materials layers have been shown to build up on the surface of walls, and hence it is desirable to have sputtering data on samples with impurity layers195. ... 

Mass analysis of the desorbing species indicates that C02 is invariably a major component regardless of the substrate involved102). Carbon, which can be present as a surface contaminant, can be oxidized by gaseous oxygen and a photon induced... 

Figure 9.15. Influence of sample cleaning on XPS scans taken on a thin-film superconductor, (a) Survey scan from an as-received surface, (b) Survey scan from surface after ion-beam (sputter) cleaning. Note the reduction in the Cls peak after cleaning, (c) Comparative Ba3d scans from both cases. Note the change in shape and size as the surface contaminant layers (probably containing carbonates and hydroxides of Ba in addition to other components) are removed. The peak shapes and intensities of other cations change, too. Initial data represent the composition and chemistry of the contaminant layer, whereas that from sputtered sample represents those of the pure underlying superconductor (possibly with sputter-induced changes that need to be accounted for).

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