Coverage profiles

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

Figure 6.20. Typical coverage profiles by species with different Sc [29] (a) Sc = 0.001 for TEOS and (b) Sc = 0.5 for SiH4-02...

To reduce axial coverage profiles in die case of the fixed bed TS-PF-SSR used for studying desorption alone, the reactor tube might also be made as short as possible. As mentioned in Chapter 5 this brings up the problem of maintaining an even carrier gas flow through a thin wide bed. 

The systems discussed above are, in many ways, ideal in that adsorption is very site-specific or limited to the surface layer. Many systems are known to absorb as well as adsorb. This effect is sometimes reflected in sticking probability versus coverage profiles. These may show an increase in s because of a sudden freeing of surface sites due to the absorption process. One example is 02 on A1 111] [434]. However, the adsorbed species may form at all coverages and the s versus N profiles look like those of a typical mobile precursor-trapping model. Fromm [435] has proposed a model to fit this absorption—adsorption mechanism. His... 

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Etch Profiles. The final profile of a wet etch can be strongly influenced by the crystalline orientation of the semiconductor sample. Many wet etches have different etch rates for various exposed crystal planes. In contrast, several etches are available for specific materials which show Httle dependence on the crystal plane, resulting in a nearly perfect isotropic profile. The different profiles that can be achieved in GaAs etching, as well as InP-based materials, have been discussed (130—132). Similar behavior can be expected for other crystalline semiconductors. It can be important to control the etch profile if a subsequent metallisation step has to pass over the etched step. For reflable metal step coverage it is desirable to have a sloped etched step or at worst a vertical profile. If the profile is re-entrant (concave) then it is possible to have a break in the metal film, causing an open defect. 

Phase transitions in overlayers or surfaces. The structure of surface layers may undergo a transition with temperature or coverage. Observation of changes in the diffraction pattern gives a qualitative analysis of a phase transition. Measurement of the intensity and the shape of the profile gives a quantitative analysis of phase boundaries and the influence of finite sizes on the transition. ... 

Fig. 9. Comparison of the analytical SCF model [56] with the full numerical SCF calculation [53] for the segment density profile in flat, grafted layers at various surface densities (o is the fraction of the maximum possible surface coverage of grafted ends). The analytical profile is parabolic to its tip, while the numerical calculation shows that the density at the periphery of the layer drops off exponentially...

Figure 8.13 In situ electrochemical SXS characterization of PtsNi) 11) and Pt(l 11) surfaces (a)XRV measurements forPtsNitlll) at the (0, 0, 2.7) (filled squares) andPt(lll)at (1, 0, 3.6) (open triangles) (b) surface coverage by underpotentially deposited hydrogen (Hupd) and hydroxyl species (OHad) calculated from the cyclic voltammograms (c) segregation profile ascertained from the SXS measurements. (Reprinted with permission from Stamenkovic et al. [2007a]. Copyright 2007. American Association for the Advancement in Science.)...

It is clear that one of the major limitations of this analysis is the assumption of constant excited-state coverage. Deviations from the behavior described by Eq. (45) in the low frequency range have been observed at photocurrent densities higher than 10 Acm [50]. These deviations are expected to be connected to excited-state diffusion profiles similar to those considered by Dryfe et al. [see Eq. (38)] [127]. A more general expression for IMPS responses is undoubtedly required for a better understanding of the dynamics involved in back electron transfer as well as separation of the photoproducts. 

Figure 16.5. CV profiles for the Pt(lll) electrode in 0.05 M HCIO4 (main graph) and evolution of the CeHe surface coverage (inset) as a function of C6H<5 concentration electrode cycling in the 0.05-0.42 V range . s =50 mV s T= 298 K.

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