Etching of the Surface

The free surfaces of samples are studied using SEM and AFM. The smooth specimen surfaces which are required for this purpose are prepared either by ultramicrotomy or by solution-casting procedures (e.g., spin-coating). The free surface can also be etched by reactive ions via a process known as reactive ion etching (RIE) [49-51], or by using chemicals as in the case of permanganic etching [52-55]. 

Permanganic etching is frequently used to prepare surfaces for SEM examinations, notably of semicrystalline polymers and their blends and composites. In this case the etchant attacks the polymers destmctively and progressively removes the outer skin of the polymer, such that the loosely packed amorphous phase is preferentially etched out of the sample. This technique is especially useful for exposing the internal structural details of semicrystalline polymers. 

The sample surfaces as received can either be dipped directly into the etchant solution, or the fresh surface for etching can be prepared by microtomy (or ultra-microtomy). The etching period may range from a few seconds to several minutes, depending on the nature of the material under investigation and the concentration of the etchant. 

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Other means of identification sometimes used satisfactorily involve chemical etching of the surface (not to be generally recommended), or the formation of letters or numbers by means of a vibrating stylus. The former is advantageous in studies of stress-corrosion cracking in which stamped symbols could lead to regions of stress concentration. 

At the initial recording stage, for t < Ioptim, the depth of the etched groove is approximately proportional to the etching time t, and for t > roptim the relief depth tends to a limit (curve 3 in Fig. 22). The profile of the relief is quite close to the sinusoidal one but at the same time there is undesirable random etching of the surface that leads to a decrease in its reflectivity (curve 2 in Fig. 22). 

Due to the multiple desorption products, the etching of the surface with halogen appears to be quite complex. A multi-step reaction mechanism has been suggested to account for the SiCl2 desorption species. In the case of fluorine atom adsorption, F atom abstraction and dissociative chemisorption mechanisms have been suggested. In order to account for the complex surface reactions, more studies are needed. 

Sufficient (NH4)2TiF6 was added to the ammonium exchanged ZSM-5 to replace 50% of the framework A1 with Ti. The LZ-241 product, Sample C, contained 8.9 wt.% Ti02. The zeolite was 48% dealuminated after reflux for 28 hours. All of the Ti was incorporated into the zeolite. There was a small decrease in the amount of defects, a feature observed with zeolites that are not readily susceptible to acid attack and have a high silica content. This has been attributed to "etching" of the surface of the zeolite crystals by the combination of acid and fluoride, and transport of the silicon atoms in solution to fill other defect sites in the framework <1). 

Studies of interfaces (30,31) have also shown that certain substrates such as Teflon and silicone-coated sheets give featureless surfaces however, subsequent etching of the surface reveals a two-phase structure. It was further shown that the microgel size decreases with increasing amounts of catalyst. 

As biasing continues, there are ongoing adsorption of carbon and etching of the surface, with SiC etched preferentially relative to the more stable diamond nuclei. 

Adsorption of these ions (as well as some organic molecules) hampers surface recombination and/or recombination at the intergrain boundaries in the polycrystalline samples [9, 22, 23]. Incidentally, special "texturizing" etching of the surface [8, 9] is used to make surface "matt" and thus reduce its reflectivity. 

Low-temperature sublimation has been used to prepare samples for cryo-scanning electron microscopy (SEM) analysis in order to examine herbicide particles in a water suspension. The sublimation of herbicide-containing frozen water droplets provides a suitable etching of the surface for the SEM technique. 

Removal or etching of the surface was also reported ui B12212 by MaUet et al. (1996), who observed at room temperature in a helium gas atmosphere that small clusters of 100-300 A in size were cleaned by consecutive scanning on the sample. They speculate that these small clusters of different oxygen content were formed by chemical reaction on the surface. A similar observation was reported by Harmer et al. (1991). 

Extensive etching of the surface can lead to a cohesively weakened surface structure. As a result, surface-treating processes mnst be followed explicitly. Complete removal of the etching chenucals by a rinse step is essential. Highly reactive etchants can continue reacting with the surface after application of the adhesive and thereby degrade the chemical and physical stability of the surface. 

Surface Chemical Composition. The atomic composition of catalyst surfaces plays a decisive role for the catalyst properties. Electron and ion spectroscopies (48) are surface-sensitive analytical tools, which provide information on the atomic composition within the topmost atomic layers. The information depth, that is, the number of atomic layers contributing to the measured signal, depends on the method used. Concentration profiles can be obtained by sputter etching of the surface by ion bombardment. The application of these particle spectroscopies requires ultrahigh vacuum (UHV) conditions. 

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