Scanning TEM

In the scanning transmission electron microscope (STEM), as in the SEM, a fine electron beam or probe is formed and scanned across the specimen. But in the STEM the specimen is thin and the intensity of a transmitted signal is 

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Important ongoing developments in HRTEM that are expected to be valuable in catalysis research include the correction of spherical aberrations in electron microscope lenses and monochromatization of the electron beam for improvement of the spatial and spectral resolution. Recently, scanning-TEM (STEM) of atomically dispersed lanthanum atoms on alumina (63) has provided e.x situ aberration-corrected images, but it is noteworthy that there is no technical limitation in applying the correction devices to instruments used for making measurements of samples in reactive environments. 

In electron microscopy a sample is bombarded with a finely focused beam of monochromatic electrons. Products of the interaction of the incident electron beam with the sample are detected. If the sample is sufficiently thin—up to 200 nm thickness—the beam is transmitted after interacting with the sample, leading to the technique of transmission electron microscopy (TEM). TEM is used to probe the existence of defects in crystals and phase distributions. Scanning TEM instruments have been recently developed to obtain images over a wider area and to minimize sample degradation from the high-intensity beams. 

The same paper also reports the formation of MCI (M = Na, K, Cs) nanocrystals upon exposure of M-EMAA ionomers to chloroform. Interestingly, using scanning TEM (STEM) and energy dispersive x-ray spectroscopy, Winey and coworkers have observed the same phenomenon on thin sections of Na-EMAA exposed to the humid conditions of a Philadelphia summer for a few days (see Fig. 2). These results clearly show that aging effects in ionomers can be observed... 

Scanning TEM shows that semicrystalline Zn-EMAA neutralized between 17% and 78% contains nearly monodisperse spherical aggregates randomly distributed in the polymer matrix (Fig. The... 

Finally, two Al-neutralized copolyimides from (4,4 -hexafluoroisopropylidene) diphthalic anhydride, (4,4 -hexafluoroisopropylidene) dianiline, and 3,5-diaminobenzoic acid exhibit a variety of aggregate shapes and sizes that have not been previously observed or even postulated. Scanning TEM shows Al-rich aggregates in both copolymers, but the aggregate size and shape distributions in the ionomer with a high ionic fraction are much more heterogenous than in the ionomer with a lower ionic fraction. This is despite... 

Although there are other recent hof topics for HRTEM, e g., energy-filtered (EF) TEMs and Z-contrast imaging for scanning TEMs, few investigations with respect to silicates using these new techniques have been reported to date. 

This was confirmed by STEM-EDX (scanning TEM and energy dispersive X-ray fluorescence spectroscopy) analysis. The... 

Fig. 4.15 (Top) Images of 0.5 wt% P GNR (22) in LCP thin films under crossed polarized optical microscope. Scale bar 100 pm. Shearing direction was about 45° to the polarizer [107]. (Bottom) TEM images of 22 (n = 8) coated GNRs in LCP. a Before UV irradiation, b After UV irradiation, c After shearing, c was from scanning TEM. Copyright from ACS 2013...

In 1939 Kossel and Mollenstedt demonstrated the possibility of obtaining CBED patterns from crystals with more information than obtained from the more conventional focused patterns for special beam directions. Later, in 1940, it was demonstrated that the intensity of a reflection is a function of the excitation error, and the thickness, t, of a plane parallel crystal slab [3]. Lately, more advances in CBED became possible with the introduction of field emission guns and scanning TEM (STEM) techniques. Thus, the STEM instrument has electron optics similar to CBED for obtaining diffraction results from areas as small... 

Furthermore, high-angle annular dark-field scanning TEM (HAADF-STEM) analysis can clearly reveal the distribution of different elements in the catalysts with different colors. Usually, the core and shell of the catalysts can be distinguished from the dark shell and bright core, as shown in Figure 3.13. °... 

The successful sealing and filling by metal halides were confirmed by high-resolution TEM and scanning TEM. 

Analytical electron microscopy (AEM) scanning TEM (STEM) with EDS and electron energy loss spectroscopy (EELS) X-ray absorption spectroscopy (XAS)... 

TEM, with its many modes, and often involving ancillary materials analysis capabilities such as EDS (see EDS summary), is the mainstay of material science and analysis of small volume (areas and thickness). A fully equipped TEM laboratory will have several microscopes with differing capabilities, plus all the necessary sample preparation techniques. See also Scanning TEM (STEM), where the incident beam is focused down to almost atomic dimensions and scanned across the sample. 

TEM has two basic modes, one is conventional transmission electron microscopy (CTEM or TEM for short) mode using a broad beam, another one is scanning TEM (STEM for short) using a focused electron probe (by focusing the electron beam into a narrow spot) scanning across TEM specimen in a raster. 

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