Imaging ideal microscopes

Figure 41. Image formation in an ideal microscope a) Specimen b) Objective lens c) Back-focal plane, objective aperture d) Image plane...

In many ways the nanocrystal characterization problem is an ideal one for transmission electron microscopy (TEM). Here, an electron beam is used to image a thin sample in transmission mode [119]. The resolution is a sensitive fimction of the beam voltage and electron optics a low-resolution microscope operating at 100 kV might... 

Computer image simulation may be achieved by several different techniques (12), but the most common method is the multislice calculation. In this method, input consists of a structural model, the projection direction through the structure, and parameters describing the electron microscope. Slight misorientations of the structure from the ideal projection direction may also be simulated. 

Linked with its qualities, assessed above, as an imaging and structural tool, the STEM assumes prime importance when considered as a microanalytical instrument. As pointed out in the introduction, the interaction of the fine probe in STEM with, potentially, only a small volume of the sample suggests the possibility of microanalysis on a scale hitherto unattainable. Two main areas will be considered here -the emission of characteristic A -rays by the sample, and the loss of energy from the primary beam in traversing the latter. Ideally, a fully equipped analytical electron microscope will utilize both techniques, since, as a result of the relative positions of A"-ray detector and the energy loss spectrometer in the electron optical column, simultaneous measurements are possible. However, for the sake of convenience we will consider the methods separately. 

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