Imaging in STEM

The most useful imaging device in STEM is the annular detector, particularly the high-angle annular detector pioneered by Crewe [3.13] and by Howie 

Still in the development stage are the various forms of electron holography which allow the phase information of the sample to be recovered [3.18]. The STEM has the built-in advantage of a highly coherent electron source because such machines usually have FEG tips. It is mentioned here because of the potential implications for superconductors in terms of imaging the flux dis- 

---

Figure Bl.14.8. Time course study of the arrival and accumulation of labelled sucrose in the stem of a castor bean seedling. The labelled tracer was chemically, selectively edited using CYCLCROP (cyclic cross polarization). The first image in the upper left comer was taken before the incubation of the seedlmg with enriched hexoses. The time given in each image represents the time elapsed between tire start of the incubation and the acquisition. The spectmm in the lower right comer of each image shows the total intensity...

Except for the phase-contrast detector in STEM [9], STEM and SEM detectors do not track the position of the recorded electron. The spatial information of an image is fonned instead by assigning the measured electron current to the known position of the scaimed incident electron beam. This infomiation is then mapped into a 2D pixel array, which is depicted either on a TV screen or digitalized in a computer. 

Another specialized application of EM image contrast is mass measurement. Using the elastic dark-field image in the STEM or the inelastic image in the EETEM, a direct measurement of the scattering mass can be performed. Eor reviews on this teclmique see [60.61]. 

There are three types of instruments that provide STEM imaging and analysis to various degrees the TEM/STEM, in which a TEM instrument is modified to operate in STEM mode the SEM/STEM, which is a SEM instrument with STEM imaging capabilities and dedicated STEM instruments that are built expressly for STEM operation. The STEM modes of TEM/STEM and SEM/STEM instruments provide useful information to supplement the main TEM and SEM modes, but only the dedicated STEM with a field emission electron source can provide the highest resolution and elemental sensitivity. 

Figure 8 X-ray elemental imaging in a field-emission STEM (a) EDS data of Pd /Ce /alumina catalyst particle poisoned with SO2 and (b) 128 X 128 digital STEM images formed using X-ray counts collected at each image pixel for aluminum, palladium, cerium, and sulfur. (Courtesy of North-Holland Publishers) ...

In STEM applications the highest recorded image Fourier coefficient is usually taken as a direct proof for resolution. Contributions of this component to the separation of atom columns are often not considered or compared with noise levels. Further, unisotropic signal transfer is disregarded. P. Batson considers intensity measurements on single atoms for probing resolution ... 

Characterization techniques continue to develop and will impact their application to zeolitic systems. Aberration corrected electron microscopes are now being used to improve our understanding of catalysts and other nano-materials and will do the same for zeolites. For example, individual Pt atoms dispersed on a catalyst support are now able to be imaged in the STEM mode [252]. The application of this technique for imaging the location of rare-earth or other high atomic number cations in a zeolite would be expected to follow. Combining this with tomography... 

Blom, D.A., Bradley, S.A., Sinkler, W., and Allard, LF. (2006) Observation of Pt atoms, clusters and rafts on oxide supports, by sub-Angstrom Z-contrast imaging in an aberration-corrected STEM/TEM. Proc. Microsc. Microanal, 12, 50-51. 

---