Contamination layers

To minimize effects of friction and other lateral forces in the topography measurements in contact-modes AFMs and to measure topography of the soft surface, AFMs can be operated in so-called tapping mode [53,54]. It is also referred to as intermittent-contact or the more general term Dynamic Force Mode" (DFM). A stiff cantilever is oscillated closer to the sample than in the noncontact mode. Part of the oscillation extends into the repulsive regime, so the tip intermittently touches or taps" the surface. Very stiff cantilevers are typically used, as tips can get stuck" in the water contamination layer. The advantage of tapping the surface is improved lateral resolution on soft samples. Lateral forces... 

Cleaning of particle surfaces through collapse of cavity bubbles (e.g., removal of slime coatings, removal of oxidation films, and removal of collector contaminated layers on gauge particle surfaces)... 

Because of their inertia, the effect on the ions of the high frequency electric field of the electron plasma waves dealt with so far averages out to zero. However, surprisingly, light ions and/or protons have been observed in ultrahigh intensity laser pulse interaction with solid targets since the late 1990s [33-36]. Later on, these particles were found to come from contaminant layers on... 

Friction coefficients, which were near 0.2 initially, decreased with sliding distance, presumably due to the removal of contaminant layers. The friction of DLC has been found to be generally low in inert and vacuum environments, with water having a deleterious effect. ... 

The process of forest litter decomposition is one of the key processes leading to the redistribution of Cs among the ecosystem components. It is affected by two major factors the rate of decomposition, which is dependent on the composition of the litter and the time since the introduction of Cs, which determines the level of decomposition of the uppermost, most heavily contaminated layer. The distribution of i Cs between the separate litter layers is important, since Of and Oh are critical for the roots of many undergrowth plant species (such as V. myrtillus, V. vitis-idaea, V. uUginosu and L. palustre ) and fungal saprotrophs (Clitocybe,... 

An alternative technique is noncontact AFM [18]. Figure 19 illustrates the concept. The tip oscillates above the surface, and the modulation in amplitude, phase, or frequency of the oscillating cantilever in response to force gradients from the sample can be measured to indicate the surface topography. Even without contact, the amplitude, phase, or frequency can be affected by the van der Waals forces of the sample within a nanometer range, which is the theoretical resolution however, this effect can be easily blocked by the fluid contaminant layer, which is substantially thicker than... 

The contaminant layer is very thin, such that a Dirac delta pulse is apphcable. 

Pockels described in the letter her design of a rectangular tin trough with a thin tin strip laid across it. The trough was filled to the brim with water, with a thin layer of oil covering the surface of the water on one side of the tin strip and clean water on the other side. The tin strip served to vary the area of the oil-contaminated surface, and a balance measured the force necessary to lift a small disk (a button) from the surface. Pockels used this setup to study the surface tension of the oil-contaminated layer. 

Centrifugation is an important step in starch isolation that separates starch from fine fibers, insoluble or soluble protein, and gum or mucilage compounds. Centrifugation (usually 20 to 30 min at 5000 to 8000 x g, 20°C) settles the starch (a white layer) at the bottom of the centrifuge tube (Fig. E2.1.2). The water-insoluble contaminants (e.g., protein, fine fiber) form a dark or brown layer on top of the starch layer. This contaminant layer is normally removed manually by scraping with a spatula, being careful not to scrape away any of the starch layer. 

Figure 26.4(a) presents the usual AES sputter profile of the sample. Two main areas can be distinguished behind the contamination layer. First, the film area which is characterized by constant levels of W and C (40 mol% carbon). Second, the substrate area, where the signal of C decreases and the level of W increases. 

Chemical differences in the Auger peaks are also present in the a-WCl a-W2C sample. Although the usual sputter profile does not give information about the distribution of chemical states in the sample, analysis of the W and C components suggests that besides the surface component, two different carbide components are present. The behaviour of the two bulk components of W and C is the same the first is located near the surface and decreases deeper into the sample the second increases to become the most important in the deeper part of the sample. RHEED analysis indicates a-WC but the depth sensitivity of this method is about 5-10 nm. XRD analysis indicates the presence of a-WC,a-W2C and W with a depth sensitivity of a few pm. Thus, the first bulk component of W and C can be identified as a-WC and the second as a-W2C. The overall sample consists of a contamination layer, followed by a thin layer of a-WC on top of the a-W2C phase. 

In reality the value is certainly lower due to surface roughness and contamination on the surfaces. Roughness and contamination layers increase the effective contact distance. 

Surface diffraction experiments have to be done in UHV. Otherwise the surfaces are covered with a monolayer of adsorbed molecules. At this point the reader might ask why do we not have to use UHV in scanning tunneling or the atomic force microscope In both techniques the tip penetrates through the surface contamination layer. In the scanning tunneling microscope it is often just invisible because contamination layers are usually not good conductors. In... 

Note, that the surface and deformation forces are of the same order of magnitude. Therefore, surface forces should be as small as possible to minimise damaging and indentation of soft polymer samples. For example, sharp probes have a lower capillary attraction and adhesion forces, and therefore enable more gentle probing of a soft polymer than a blunt tip. A sharp tip can also be moved in and out of the contamination layer more readily than a blunt tip. This is particularly important for non-contact intermittent contact imaging described in Sect. 2.2.1. 

For many reasons, the tapping mode is one of the most versatile developments in scanning force microscopy [ 114]. It was originally introduced for stable imaging at ambient conditions as the probe penetrates the contamination layer rapidly and intermittently. Besides that, there are three other key advantages of... 

At ambient conditions the sample surface is always covered by a thin contamination layer. This layer is mainly composed of water, however, also other compounds can adsorb to the sample surface and the surface of the tip. Depending on the vapour pressure of the contaminant, the layer thickness can range from 2... 

No reaction Contaminant layer determines bond strength Properties of adhesive-contaminant mixture determine bond strength ... 

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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