Other scanning electron microscopy techniques

Additional advantages enjoyed by the dedicated STEM units are their clean, high vacuum systems in use and their very bright electron source in the field emission gun. The high vacuum (I0 torr in the gun chamber and 10 to l(E torr in the sample chamber) allows prolonged observation of the sample without contamination, and the bright source allows viewing of data collection at TV scan rates. 

The scanning tunneling electron microscope (STM), invented in 1981, allows examination of non-conductive surfaces [198] down to atomic resolution and can operate in ambient and aqueous environments [199,200]. Early references to the instrument use the acronym STEM, which produced confusion with the scanning transmission electron microscope. More recently, the consensus has been to use STM, as the acronym for the tunneling instrument. 

Since the introduction of the STM a number of variations have been devised, such as ATM (atomic force microscope). The basic concept is that piezoelectric actuators move a miniature cantilever arm (with a nm-sized tip) across the sample while a non-contact optical system measures the deflection of the cantilever caused by atomic scale features. The deflection is proportional to the normal force exerted by the sample on the probe tip and images are generated by raster scanning the sample [201]. One application of this technique was to measure the thickness and size distribution of sub-micron clay particles with diameters in the 0.1 to 1 pm size range and thickness from 0,01 to 0.12 pm [202]. 

MFM (magnetic force microscope), LFM (lateral force, or friction force microscope), etc. None of the above finds wide use for particle size determination. The AFM has however been used to determine the shape, size and types of particle on a polished silicon wafer surface [203]. 

Droplet size (jc) in microns Fig. 3.13 Presentation and interpretation of microscope data. 

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It should be noted that most of the microstructural data to be presented rely on different characterization techniques using electron microscopy in combination with other scanning probe microscopy techniques, e.g. scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The details of specimen preparation and the analysis techniques of electron microscopy are described in other chapters of this book and are therefore not further covered in this chapter. The references provided in the chapter are merely a selection, since the field of research is vivid and the number of publications is large. The goal is to show the salient features which can be further explored by use of the referenced literature. 

A. J. Bevolo. Scanning Electron Microscopy. 1985, vol. 4, p. 1449. (Scanning Electron Microscopy, Inc. Elk Grove Village, IL) Thorough exposition of the principles and applications of reflected electron energy-loss microscopy (REELM) as well as a comparison to other techniques, such as SAM, EDS and SEM. 

In contrast to many other surface analytical techniques, like e. g. scanning electron microscopy, AFM does not require vacuum. Therefore, it can be operated under ambient conditions which enables direct observation of processes at solid-gas and solid-liquid interfaces. The latter can be accomplished by means of a liquid cell which is schematically shown in Fig. 5.6. The cell is formed by the sample at the bottom, a glass cover - holding the cantilever - at the top, and a silicone o-ring seal between. Studies with such a liquid cell can also be performed under potential control which opens up valuable opportunities for electrochemistry [5.11, 5.12]. Moreover, imaging under liquids opens up the possibility to protect sensitive surfaces by in-situ preparation and imaging under an inert fluid [5.13]. 

The interface properties can usually be independently measured by a number of spectroscopic and surface analysis techniques such as secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), specular neutron reflection (SNR), forward recoil spectroscopy (FRES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), infrared (IR) and several other methods. Theoretical and computer simulation methods can also be used to evaluate H t). Thus, we assume for each interface that we have the ability to measure H t) at different times and that the function is well defined in terms of microscopic properties. 

Scanning electron microscopy is commonly used to study the particle morphology of pharmaceutical materials. Its use is somewhat limited because the information obtained is visual and descriptive, but usually not quantitative. When the scanning electron microscope is used in conjunction with other techniques, however, it becomes a powerful characterization tool for pharmaceutical materials. 

These are a few of the many examples of the uses of scanning electron microscopy. The use of this technique with other physical characterization methods results in a powerful pharmaceutical tool. 

Atomic force microscopy (AFM) is a commonly employed imaging technique for the characterization of the topography of material surfaces. In contrast to other microscopy techniques (e.g., scanning electron microscopy), AFM provides additional quantitative surface depth information and therefore yields a 3D profile of the material surface. AFM is routinely applied for the nanoscale surface characterization of materials and has been previously applied to determine surface heterogeneity of alkylsilane thin films prepared on planar surfaces [74,75,138]. 

Because of the instrumental requirements, these are usually not routine monitoring techniques. However, unlike other methods, they give detailed information on particle shapes. In addition, chemical composition information can be obtained using transmission electron microscopy (TEM) or scanning electron microscopy (SEM) combined with energy-dispersive spectrometry (EDS). The electron beam causes the sample to emit fluorescent X-rays that have energies characteristic of the elements in the sample. Thus a map showing the distribution of elements in the sample can be produced as the electron beam scans the sample. 

Transmission and scanning electron microscopy are employed for a direct study of microclusters while the distribution of sizes (or average diameter) is provided by sedimentation and other techniques. The average particle diameter is obtained by the Brunauer-Emmett-Teller (BET) surface-area method and by X-ray line broadening. 

For the detailed study of reaction-transport interactions in the porous catalytic layer, the spatially 3D model computer-reconstructed washcoat section can be employed (Koci et al., 2006, 2007a). The structure of porous catalyst support is controlled in the course of washcoat preparation on two levels (i) the level of macropores, influenced by mixing of wet supporting material particles with different sizes followed by specific thermal treatment and (ii) the level of meso-/ micropores, determined by the internal nanostructure of the used materials (e.g. alumina, zeolites) and sizes of noble metal crystallites. Information about the porous structure (pore size distribution, typical sizes of particles, etc.) on the micro- and nanoscale levels can be obtained from scanning electron microscopy (SEM), transmission electron microscopy ( ), or other high-resolution imaging techniques in combination with mercury porosimetry and BET adsorption isotherm data. This information can be used in computer reconstruction of porous catalytic medium. In the reconstructed catalyst, transport (diffusion, permeation, heat conduction) and combined reaction-transport processes can be simulated on detailed level (Kosek et al., 2005). 

In light of the fact that coal ash is measured by removal (combustion) of the organic part of the coal, there are methods for measuring the mineral matter content of coal. Such a method involves demineralization of the coal that depends on the loss of weight of a sample when it is treated with aqueous hydrofluoric acid at 55 to 60°C (130 to 140°F) (Radmacher and Mohrhauer, 1955 Given and Yarzab, 1978 ISO 602). However, pyrite is not dissolved by this treatment and must be determined separately. Other methods include the use of physical techniques such as scanning electron microscopy and x-ray diffraction (Russell and Rimmer, 1979). 

Scanning electron microscopy with an energy-dispersive x-ray system accessory has been used to identify the composition and nature of minerals in coals and to determine the associations of minerals with each other. Examinations can be made on samples resulting from ashing techniques or whole coal. With this technique it is possible to identify the elemental components and deduce the mineral types present in coal samples. Computerized systems to evaluate scanning electron microscopy images have been developed and are useful in characterizing the minerals in coal mine dusts and in coal. 

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