Scanning contrast mechanisms

Computers will be integrated more and more into commercial SEMs and there is an enormous potential for the growth of computer supported applications. At the same time, related instruments will be developed and extended, such as the scanning ion microscope, which uses liquid-metal ion sources to produce finely focused ion beams that can produce SEs and secondary ions for image generation. The contrast mechanisms that are exhibited in these instruments can provide new insights into materials analysis. 

Dale E. Newbury, Fundamentals of Scanning Electron Microscopy for Physicist Contrast Mechanisms", SEM/1977/I, p. 553-568. 

Sautet. P. Images of adsorbates with the scanning tunneling microscope Theoretical approaches to the contrast mechanism. Chem. Rev. 1997, 97, 1097. 

Microscopy of polymer surfaces and their high spatial resolution analysis is accomplished typically with the atomic force microscope (afm) used in various modes of operation. The contrast mechanism of the afm is the interatomic forces between the scanning probe and the sample s surface. The forces are measured... 

Within the numerous proximal probe techniques developed in the years following the breakthrough inventions of the STM (1) and AFM (5), scanning force microscopy (8) represents a family of scanning probe techniques that rely in their contrast mechanism on various forces between probe tip and sample (9-12) (see Atomic Force Microscopy). In order to provide a basis for an understanding and appreciation of the SFM work on polymers (13-18), as presented in this review, the basic principles of SFM, as well as selected imaging modes, are briefly discussed. 

Schwarz, U. D. Holscher, H. Wiesendanger, R., Atomic resolution in scanning force microscopy Concepts, requirements, contrast mechanisms, and image interpretation. Physical Review B 2000, 62, 13089. 

Table 5.36 shows the main features of NSOM. Optical fibre tips for NSOM with high light transmission permit surface analytical and spectroscopic applications (fiuorescence imaging, Raman) with high spatial resolution (ca. 30 nm) and high chemical information content [337], NSOM overcomes critical measurement limitations of both far-fleld vibrational microscopes (low spatial resolution) and scanned probe microscopes (lack of chemical specificity). NSOM offers conventional optical characterisation and contrast mechanisms with the resolution of SPM. The spatial resolution of this relatively new technique is almost competitive with that of SEM. Consequently, NSOM is expected to become a serious alternative for SEM, since it is non-destructive if visible light is used, and allows visualisation of specimens in air. The technique holds considerable promise for the future. At the present time it is still quite expensive. 

Abstract. This chapter is concerned with an in-depth examination of the adherend surface pretreatments used prior to structural adhesive bonding. It encompasses the various substrates encountered, particularly but not exclusively, in the aerospace industry. It compares and contrasts mechanical, chemical and electrochemical methods used for substrates comprising aluminium alloys, titanium, stainless steel, thermoplastic and thermoset fibre reinforced composites and non-metallic honeycomb. Scanning and transmission electron microscope techniques are used to analyse and characterise many of the pretreated surfaces so produced. 

If infrared absorption or Raman scattering is used as the contrast mechanism, vibrational spectra of samples can be obtained. The combination of the nanoscale spatial resolution of a scanned probe with the chemical specificity of vibrational spectroscopy allows in situ mapping of chemical functional groups with subwavelength spatial resolution. Figure 12 is a shear force image of a thin polystyrene film along with a representative near-field spectrum of the... 

As one example, in thin films of Na or K salts of PS-based ionomers cast from a nonpolar solvent, THF, shear deformation is only present when the ion content is near to or above the critical ion content of about 6 mol% and the TEM scan of Fig. 3, for a sample of 8.2 mol% demonstrates this but, for a THF-cast sample of a divalent Ca-salt of an SPS ionomer, having only an ion content of 4.1 mol%, both shear deformation zones and crazes are developed upon tensile straining in contrast to only crazing for the monovalent K-salt. This is evident from the TEM scans of Fig. 5. For the Ca-salt, one sees both an unfibrillated shear deformation zone, and, within this zone, a typical fibrillated craze. The Ca-salt also develops a much more extended rubbery plateau region than Na or K salts in storage modulus versus temperature curves and this is another indication that a stronger and more stable ionic network is present when divalent ions replace monovalent ones. Still another indication that the presence of divalent counterions can enhance mechanical properties comes from... 

Another striking example, recently discovered, is the reduction of iodoben-zene in DMF.61 The variation of the apparent transfer coefficient with the scan rate indicates that the mechanism passes from concerted to stepwise as the driving force increases (Fig. 5). As expected, the zone where the concerted mechanism prevails enlarges as one raises the temperature. In contrast, bromobenzene and 1-iodonaphthalene exhibit the characteristics of a stepwise mechanism over the whole range of scan rate. 

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