Image local orientation

A particular application of diffraction contrast has been in the study of the changes in local orientation which occur on annealing" thin hlms of liquid crystalline co-polyesters. Diffraction imaging, using the main equatorial rehection, was able to show the variation of director orientation as projected on to the specimen plane. On the other hand. 

Fig. 5.96 Plasma etching with argon reveals the fine internal structure of an extrudate in these SEM images. The overview (A) shows the flow lines of the polymer. Grain-like domains are observed in a region midway between the skin and core (B) with internal fine structural detail (C) which suggests local orientation. Microfibrillar structures are observed at higher magnification (D).

The structure tensor consisting of the first image derivatives was used for corner detection. It was shown to be useful in isolating chaotic regions. The chaotic area was also successfully detected by the use of a simple average of the local orientation of the image. 

Microscopy methods based on nonlinear optical phenomena that provide chemical information are a recent development. Infrared snm-frequency microscopy has been demonstrated for LB films of arachidic acid, allowing for surface-specific imaging of the lateral distribution of a selected vibrational mode, the asymmetric methyl stretch [60]. The method is sensitive to the snrface distribntion of the functional gronp as well as to lateral variations in the gronp environmental and conformation. Second-harmonic generation (SHG) microscopy has also been demonstrated for both spread monolayers and LB films of dye molecules [61,62]. The method images the molecular density and orientation field with optical resolution, and local qnantitative information can be extracted. 

Other noncontact AFM methods have also been used to study the structure of water films and droplets [27,28]. Each has its own merits and will not be discussed in detail here. Often, however, many noncontact methods involve an oscillation of the lever in or out of mechanical resonance, which brings the tip too close to the liquid surface to ensure a truly nonperturbative imaging, at least for low-viscosity liquids. A simple technique developed in 1994 in the authors laboratory not only solves most of these problems but in addition provides new information on surface properties. It has been named scanning polarization force microscopy (SPFM) [29-31]. SPFM not only provides the topographic stracture, but allows also the study of local dielectric properties and even molecular orientation of the liquid. The remainder of this paper is devoted to reviewing the use of SPFM for wetting studies. 

Figure 1.10 UHV-STM images of HtB-HBC adsorption on Cu l 1 0 showing the correlation between the orientation of the adsorbate lattice vectors and the local rotation of the molecule away from its high symmetry azimuthal orientation atop a copper atom.

Figure 4.9 illustrates time-gated imaging of rotational correlation time. Briefly, excitation by linearly polarized radiation will excite fluorophores with dipole components parallel to the excitation polarization axis and so the fluorescence emission will be anisotropically polarized immediately after excitation, with more emission polarized parallel than perpendicular to the polarization axis (r0). Subsequently, however, collisions with solvent molecules will tend to randomize the fluorophore orientations and the emission anistropy will decrease with time (r(t)). The characteristic timescale over which the fluorescence anisotropy decreases can be described (in the simplest case of a spherical molecule) by an exponential decay with a time constant, 6, which is the rotational correlation time and is approximately proportional to the local solvent viscosity and to the size of the fluorophore. Provided that... 

Figure 4. Histogram of step densities for the three Ge(OOl) surfaces shown in Fig. 3. The relative probabilities P(n) of step density n are determined by measuring the local surface orientation 6 in an area 4x4 nm centered on each image pixel and using na = tan 0. ...

When concluding on the particle size and shape from images obtained by freeze fracture TEM, the fact that the particles are fractured randomly with respect to the localization of the fracture site within the particle and to orientation toward the fracture plane has to be taken into consideration. Most of the particle remains hidden from observation. Therefore, a sufficiently large number of particles have to be investigated to obtain a realistic impression. The determination of a particle size distribution will not be possible when the particles are of anisometric shape. 

Fig. 13.3. FIM image of a W tip immediateiy after etching. The tip is etched from a single-crystal W wire with (111) orientation. The threefold symmetry is visible on a large scale. Locally, severe dislocations are observed. (Courtesy of U. Staufer.)...

Plate 5. The nascent Si(lll) surface. The Si(l 11) plane is the natural cleavage plane of the Si crystal. After cleaving, the surface immediately reconstructs into a 2X1 structure. The threefold symmetry is locally degenerated to a twofold symmetry. There are three equivalent orientations of the Si(lll)-2X1 structure. On a large scale, different orientations coexist to make a mosaic. See Feenstra and Lutz (1991) for details. Original image courtesy of R. M. Feenstra. 

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