Chemical contrast

Figure 8.3 STM image with chemical contrast of the Pt25Rh75(100) surface. Arrows point at brighter Rh atoms at the step edge. (Reprinted with permission from Hebenstreit et al. [1999]. Copyright 1999. Elsevier.)...

Hebenstreit ELD, Hebenstreit W, Schmid M, Varga P. 1999. Pt25Rh75(lll), (110), and (100) studied by scanning tunneUing microscopy with chemical contrast. Surf Sci 441 441 -453. 

Chemically contrasting fluids, particular fluids of contrasting redox state appear intrinsic to productive mineral systems. 

Dobson, H. E. M Groth, I. and Bergstrom, L. G. (1996). Pollen advertisement chemical contrasts between whole-flower and pollen odors. American Journal of Botany 83 877-885. 

The complex quantity, y6br = e (y(3)r) + i Im (x r), represents the nuclear response of the molecules. The induced polarization is resonantly enhanced when the Raman shift wp — ws matches the frequency Qr of a Raman-active molecular vibration (Fig. 6.1A). Therefore, y(3)r provides the intrinsic vibrational contrast mechanism in CRS-based microscopies. The nonresonant term y6bnr represents the electronic response of both the one-photon and the two-photon electronic transitions [30]. Typically, near-infrared laser pulses are used to prevent the effect of two-photon electronic resonances. With input laser pulse frequencies away from electronic resonances, y(3)nr is independent of frequency and is a real quantity. It is important to realize that the nonresonant contribution to the total nonlinear polarization is simply a source for an unspecific background signal, which provides no chemical contrast in some of the CRS microscopies. While CARS detection can be significantly effected by the nonresonant contribution y6bnr [30], SRS detection is inherently insensitive to it [27, 29]. As will be discussed in detail in Sects. 6.3 and 6.4, this has major consequences for the image contrast mechanism of CARS and SRS microscopy, respectively. 

For example, chemical contrast images were obtained by lateral force microscopy (LFM) from a topologically flat surface of a self assembled monolayer consisting of chemically different domains. In order to make the chemical adhesion the dominant contribution to the friction signal, the tip was modified by a monolayer with appropriate terminal groups [149-155]. However, since LFM operates in contact mode, the surface deformation is inevitable. 

Ade, H., Zhang, X., Cameron, S., Costello, C., Kirz, J., and Williams, S. (1992). Chemical contrast in X-ray microscopy and spatially resolved XANES spectroscopy of organic specimens. Science 258, 972-975. 

Like spontaneous Raman microscopy, CARS microscopy does not rely on natural or artificial fluorescent labels, thereby avoiding issues of toxicity and artifacts associated with staining and photobleaching of fluorophores. Instead, it depends on a chemical contrast intrinsic to the samples. However, CARS microscopy offers two distinct advantages over conventional Raman microscopy ... 

Chemical Force Microscopy. AFM experiments can be performed using probes that have been derivatized with specific chemical functional groups. These proximal probes allow for the detection and utilization of specific probe-sample interactions as a means of obtaining chemical contrast in AFM images, and this technique is commonly known as chemical force microscopy.175 Chemical force microscopy represents an extrapolation to very short (i.e., nanometer) length... 

Figure 7.17 Interface between covulcanized sheets from unfilled SBR and NR (a) Photograph, (b) ID NMR images acquired with single-point imaging and different contrast filters. Smooth line chemical contrast (right scale). Rugged line relaxation...

Ruiz et al. have prepared defect-free surfaces of self-assembled BCs from a chemical pattern prepared near the limit of current lithographic tools [154]. The pattern is obtained in two steps by using an e-beam resist layer to write a hexagonal pattern followed by use of an oxygen plasma to generate a chemical contrast on the substrate. After the self-assembly of cylinder-forming PS-b-PMMA diblock copolymer, a pattern rectification is observed compared to substrate defects leading to... 

A-B subtracting the signals enables topographical contrast to be enhanced and chemical contrast to be eliminated. 

Nevertheless, discrimination between different metal atoms at many different alloy surfaces has been observed in constant current mode, a phenomenon known as chemical contrast [22]. The basis for such images is a resolution which is high enough to probe small differences between the atoms, e.g., in electron density of states. This means that the residual noise of the STM in z... 

For PtRh(lOO) an alloying effect decreases the LDOS at the Fermi edge for the Pt atoms and therefore the Pt atoms appear darker (lower) than the Rh atoms [34]. Chemical contrast by tip-sample interaction (see next section) will also lead to Rh atoms appearing bright and may therefore enhance this effect. 

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