Depth -distribution information

For a solid, XPS probes 2 to 20 atomic layers deep, depending of the KE of the ejected electron, the angle w.r.t., the solid surface of detection, and the material. XPS is therefore a true surface technique. If measurement is made as a function of angle, depth distribution information is available over the depth probed. XPS is also used with sputter depth profiling to go beyond these depths. Modern laboratory XPS instruments can providepracticalldXtrA resolution capability down to the 10 to 20 pm range. Specialized systems can go lower, but acquisition time becomes very long. 

An almost equally valuable ability of AES/SAM is the ease by which sputter-depth profiles can be obtained. These and other methods to derive depth distribution information will be discussed in Section 3.3. In this procedure, the surface is slowly milled away by inert ion bombardment and AES is used to monitor the composition as a function of time. Although this can also be done with small spot XPS, AES is more commonly used because of its smaller analysis area which permits a smaller sputtered area and, hence, a faster sputter rate, its faster data-acquisition rate, and its compatibility with the higher pressures in the analysis chambers needed for sputtering with older models of ion guns. 

Depth-distribution information is available from a variety of methods, depending on the probed depth. Several of these methods are discussed below. 

The final method of determining depth-distribution information is to compare the surface-sensitive measurements with those from less surface-sensitive or thin-film techniques, such as EDS, WDS, or FTIR, or even with those from purely bulk composition techniques, such as atomic absorption, emission spectroscopy, or neutron-activation analysis. Such comparisons will tell if the constituent in question is localized on the surface, but they will not provide a distribution of constituents nor even information about the thickness of an overlayer, unless it is thick enough (or the bulk is thin enough) that the overlayer is a significant fraction of the sample, or of the detection volume for the thin-film techniques. These comparisons are useful in thick-film analysis or when the bulk of thin-film analysis is used as a screening procedure before surface analysis. 

Surface-sensitive techniques for use in the study of adhesive bonding are discussed, including X-ray photoelectron spectroscopy and auger electron spectroscopy/scanning auger microscopy. Data analysis is considered, with reference to quantification, chemical-state information, depth-distribution information and surface-behaviour diagrams. Applications to adhesive bonding are described, particularly failure analysis, hydration of phosphoric acid-anodised aluminium and adsorption of hydration inhibitors. 100 refs. 

Although the detection of heavy ions in polymers is most easily achieved by RBS, it is also possible to exploit NRA to detect lighter ions. This was illustrated by the work of Tadic et using a microbeam approach, to detect Li, F, and Mn by PICE and PIXE, respectively, and to relate the ion distribution to the properties of lithium ion batteries. Although PIXE and PICE provide only limited depth distribution information parallel to the beam direction, by rastering a... 

Because a FIXE spectrum represents the int al of all the X rays created along the particle s path, a single FIXE measurement does not provide any depth profile information. All attempts to obtain general depth profiles using FIXE have involved multiple measurements that varied either the beam energy or the angle between the beam and the target, and have compared the results to those calculated for assumed elemental distributions. Frofiles measured in a few special cases surest that the depth resolution by nondestructive FIXE is only about 100 nm and that the absolute concentration values can have errors of 10-50%. 

Figure 8.5 shows the LEIS spectra of ZnAl204 and ZnO as a characteristic example of a multicomponent system analyzed by this technique [Brongersma and Jacobs, 1994]. Since only the surface peaks of A1 and O were detected for ZnAl204, the Zn atoms must be located in the subsurface layers. The onset of the tail agrees between the spectra, indicating that Zn is present in the second and deeper layers. This example illustrates the strength of the LEIS technique, in that characteristic peaks from different elements can be used to selectively analyze the atomic composition of the topmost surface. In addition, the shape of the tails could provide information on the in-depth distribution of the elements. 

We turn now to several effects that give qualitative and sometimes quantitative information on hydrogen in specific forms or complexes but that are usually less convenient to apply to the measurement of precise depth distributions or total amounts than are the procedures discussed above. 

The information available on UV-susceptibility of different developmental stages in kelps indicate the unicellular zoospores as being most sensitive (Wiencke et al. 2000, 2004). However, zoospore UV-sensitivity varies species-specifically as a function of depth distribution of the sporophyte, i.e. kelps from shallow water are more tolerant than plants from deeper vertical positions. Consequently, any increase in UVB-induced spore mortality will result in impaired reproductive success and finally reduce fitness of the population. In addition, elevated UVB will penetrate deeper into the water column, which may result in a shift of the upper distribution limit of seaweed communities to deeper waters (Wiencke et al. 2006). 

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

It should be noted that this quantifleation approach assumes that the sample is homogeneous in depth. If this were the case however, the use of a surface analysis technique would not be Justifled. The approximation involved is applied in the absence of other information that can be used to describe the depth distribution of the elements. In particular, if a surface contamination layer (for example, atmospheric hydrocarbons) is present on the sample this will influence the intensity of the peaks to an extent which depends on the energy of the pho-toclcctron (through the dependence of X on the kinetic energy) and thus the clement. 

Chemical characteristics and the oxidation state of elements in the nearsurface layer ( 5 nm) of a sample are recorded by photoelectrons that are produced by an X-ray beam. When this technique is combined with intermittent ion sputtering, data on depth distribution can be obtained. X-ray photoelectron spectroscopy goes beyond elemental analysis to provide chemical information such as distinguishing Si-Si from Si-O bonds. Elements from Li to U may be analysed with detection levels at 0.5% under high vacuum conditions. Raster scanning techniques produce images with a spatial resolution of 26 pm and depth profiles of 1 pm thick are possible (Mossotti et al., 1987 Wilson and Bums, 1987). 

Conventional. In most of the laboratories around the world, TEM specimens are usually prepared in a conventional manner that shews only the "plan" view of the damage distribution, i.e., the plane of the foil being parallel to the inplanted or otherwise processed surface. Although much useful information concerning defect structures and their nature can be obtained, the depth distribution of defects is difficult to ascertain even from stereo microscopy. This is especially true in cases vtere either two or more discrete layers of defects separated ty a defect free region are present or vhere the defects of interest are buried under another more dense band of defects. Therefore,... 

The determination of the depth of information in zeolite materials is difficult as the termination of the oxidic surface is not equal to that of a bulk crystallographic unit cell. A film of strongly bovmd adsorbates (mostly water) and possible contamination from synthesis, catalytic appUcation or ion exchange procedures (salts) will cover the zeolite surface and compensate for the significant polarity of the surface which is an unavoidable consequence of the termination of the crystal at its surface. In quantitative elemental analysis of zeolite materials, there will always be a significant excess of oxygen and carbon and often of alkali ions. This does not mean that these excess atoms are uniformly distributed... 

Tilting the sample so that the analyser collects electrons that are emitted at a more grazing angle increases the surface sensitivity of the technique. By acquiring spectra at several different angles, which have different surface sensitivities, quantitative information about the depth distribution of the elements in the top few nanometers of the sample can be extracted, this is know as angle resolved XPS (ARXPS). 

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