Dark field methods

Dark-field methods do not help to improve the resolving power of a microscope. A small scattering particle is seen indirectly as a weak blur. Two particles must be separated by the resolution distance 8 to... 

Dark-field methods Dark field images are obtained by admitting only diffracted electrons and excluding directly transmitted electrons. Dark-field imaging selectively detects crystallites with crystallographic plane spacings within a relatively narrow range. 

Figure 7.10. The schematic diagram of the phase-object observation using the dark-field method.

STEM Scanning transmission electron miaoscope (dark-field method) Structure and composition [43]... 

WBDP weak beam dark field method... 

At this point it is worth comparing the different techniques of contrast enliancements discussed so far. They represent spatial filtering teclmiques which mostly affect the zeroth order dark field microscopy, which eliminates the zeroth order, the Schlieren method (not discussed here), which suppresses the zerotii order and one side band and, finally, phase contrast microscopy, where the phase of the zeroth order is shifted by nil and its intensity is attenuated. 

In dark-field electron microscopy it is not the transmitted beam which is used to construct an image but, rather, a beam diffracted from one facet of the object under investigation. One method for doing this is to shift the aperture of the microscope so that most of the beam is blocked and only those electrons... 

Characterization methods. The 100 kV Vacuum Generator HB-5 STEM was used to mlcroanalyze samples. The HB-5 has a KEVEX SI(LI) energy dispersive X-ray spectrometer (EDS) and micro area electron diffraction (MAED) capabilities In conjunction with simultaneous bright and dark field Imaging capabilities. A more detailed explanation of the Instrumental operation can be obtained In a publication by C. Lyman(12). 

Plant Cells and Tissues Structure-Function Relationships. Methods for the Cytochemical/Histochemical Localization of Plant Cell/Tissue Chemicals. Methods in Light Microscope Radioautography. Some Fluorescence Microscopical Methods for Use with Algal, Fungal, and Plant Cells. Fluorescence Microscopy of Aniline Blue Stained Pistils. A Short Introduction to Immunocytochemistry and a Protocol for Immunovi-sualization of Proteins with Alkaline Phosphatase. The Fixation of Chemical Forms on Nitrocellulose Membranes. Dark-Field Microscopy and Its Application to Pollen Tube Culture. Computer-Assisted Microphotometry. Isolation and Characterization of... 

Fig. 3 shows a topographic image of a Pt/y-A s catalyst. Contrast from particles is clearly separated from the substrate topography. On the other hand pores on the substrate are well defined. If the aperture includes some portion of the dark field spot then the resolution for small particles is improved. Fig. 4 shows an image of a 100% dispersed catalyst (as measur ed by chemisorption methods) in which particles of about 5 A can be seen. 

TIRF is easy to set up on a conventional upright or inverted microscope with a laser light source or, in a special configuration, with a conventional arc source. TIRF is completely compatible with standard epi-fluorescence, bright-field, dark-field, or phase contrast illumination so that these methods of illumination can be switched back and forth readily. Some practical optical arrangements for observing TIRF through a microscope are described in Section 7.4. 

In this method, four symmetries should be observed from three different diffraction patterns the Whole-Pattern (WP) symmetry, the Bright-Field (BF) symmetry, the Dark-Field (DF) symmetry and the + -g symmetries. 

The previous method has a main disadvantage. It requires the identification of four different S5mimetries observed on three different patterns (the Zone-Axis, the hkl Dark-Field and the -h-k-1 Dark-Field patterns). It is very important that these three patterns come from the same specimen area. This condition is not always easy to fulfill and to solve this problem, Tanaka proposed a method which only requires the observation of a unique diffraction pattern [4]. 

The stability of many colloidal solutions depends critically on the magnitude of the electrostatic potential ( /q) at the surface of the colloidal particles. One of the most important tasks in colloid science is therefore to obtain an estimate of V /o under a wide range of electrolyte conditions. In practice, one of the most convenient methods for obtaining /o uses the fact that a charged particle will move at some constant, limiting velocity under the influence of an applied electric field. Even quite small particles (i.e. <1 pm) can be observed using a dark-field micro-... 

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