Direct Imaging Mode

In practice image quality is also reduced by use of high mass resolution and energy offset. Often, therefore, mass interference cannot he avoided. Determination of element distributions is possible by use of image processing tools for classification of mappings of different masses [3.53]. 

For detection of secondary ions a laterally resolving detector is necessary. In the first step a channel plate for amplification is used secondary electrons from the output of this device are accelerated either to a fluorescent screen or to a resistive anode. If a fluorescent screen is used the image is picked up by a CCD camera and summed frame by frame by use of a computer. The principal advantage of this system is unlimited secondary ion intensities, but compared with the digital detection of the resistive anode encoder the lateral and intensity linearity is not as well-defined. 

The advantage of the imaging mode is fast data acquisition. Because all pixels are projected and detected simultaneously the measurement time for one distribution is extremely low. 

In scanning mode the sequential detection of single pixels (picture elements) and voxels (volume elements) results in long measurement times in practice, therefore, only small volumes (10 x 10 x 1 p,m ) can be measured [3.56]. 

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For measurement of local ion intensities in the direct imaging mode (see Fig. 3.19), amplification ensuring laterally uniform-single ion detection is necessary. Depending on the sensitivity of the detector a single or double channel plate is used. Two imaging devices are in use ... 

The spatial and temporal progression of individual events involved in the formation of each type of structure can be monitored directly. A combination of imaging modes can be applied, each elucidating the process at a different length scale. Millimeter-scale variations can then be explained by nanometer-scale fluctuations. After the structures are built, single-molecule imaging can be employed to study catalytic reactions inside the nanopores. 

The contrast in the image is formed from the interactions between the incident electrons and the matter. The most common operating mode in TEM is the bright field imaging mode. In this mode and in the case of non crystalline materials such as polymers, the contrast is formed directly by absorption of electrons in the sample. Actually, the image is assumed to be a simple two dimensional projection... 

Despite many advances in analytical methods in recent years, the structural characterization of materials that only occur as microcrystals less than about 30 l in diameter remains difficult and laborious. High resolution electron microscopy in the lattice imaging mode is by far the most powerful tool in giving the direct evidence of structural details essential for modelling clues, as has been demonstrated in the cases of recent zeolite structure solutions of theta-l/ZSM-23 (26) and beta (27), in addition to ECR-1. X-ray diffraction methods provide the essential confirmatory data, and sorption molecular probing and various well established spectroscopic methods are useful ancillary tools. 

The interchain periodicity of 5.6 A can typically be visualized in all three common CM-AFM imaging modes, height, deflection and friction (latter one shows highest sensitivity with scan angles of 90°, see also Chap. 2) [34—36]. In the literature, some authors have reported that even the helical structure of the PTFE chains can be resolved in some cases [37], however, this is not obvious in the images shown here. For quantitative work, it is essential that the thermal drift is minimized, hence subsequent up and down scans should match and in particular the inclination angles of the periodic structure should be invariant with this capture direction. 

The atomic force microscope can be configured in several ways, the most obvious (contact mode) merely involving scanning the tip over the sample at regular intervals, rasper fashion, and recording the deflection. Because of the proportionately very large capillary forces that arise from a contamination layer, imaging of polysaccharides in direct contact mode is carried out under a solvent... 

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