Electron microscopy point resolution

Investigations based on equation (a) are indirect. Direct structural studies using diffraction techniques (X-ray or neutron), or electron microscopy, while they cannot detect the low concentrations of defects present in NiO or CoO are indispensible to the study of grossly non-stoichiometric oxides like FeO, TiOj, WOj etc., and particularly electron microscopes with a point-to-point resolution of about 0.2 nm are widely used. The first direct observation of a point defect (actually a complex of two interstitial metal atoms, and two oxygen atoms in Nb,2029) was made" using electron microscopy. 

The 1000 A column did not show any resolution between 312 nm and 57 nm particle sizes. Shown in Fig.2 are the calibration curves for the 2000 A and 3000 A columns and for their combination. The 57 nm particle standard appears to have been erroneously characterized by the supplier. This was subsequently confirmed by electron microscopy. The 2000 X column exhibited a sharp upturn in its calibration curve close to the exclusion limit. It is to be noted that while data points corresponding to 312 and 275 nm diameter particles appear on individual column calibration curves, they are not indicated for the calibration curve of the combination. This is because these larger diameter particles were completely retained in the packed colimms, generating no detector response. The percentage recovery for these particles from individual columns was considerably less than 100 resulting in their complete retention when the columns were combined in series. 

High resolution transmission electron microscopy (HRTEM) micrographies were performed with a JEOL JEM-3010 microscope operating at 300 kV (Cs= 0.6 mm, point resolution 1.7 A). Images were recorded with CCD camera (MultiScan model 794, Gatan, 1024 x 1024 pixels, pixel size 24 x 24 pm2). The powder samples were mixed in ethanol and then ultrasonicated for 10 min. A drop of the wet sample was placed on a copper grid and then allowed to dry for 10 min before TEM analysis. 

For a long time HRTEM resolution was limited by a Scherzer point resolution of about 1.8 and was the only bulk-like imaging technique with atomic resolution. Over the last decade electron microscopy made unprecedented progress that allowed breaking the 1 Angstrom barrier by HRTEM and STEM. 

Nanocrystalline iron-doped Ti02 samples (as-prepared S2 and annealed at 500°C S4 [7]) and undoped samples (Si and S3 [8]) were S5mthesised by a modified sol-gel method. The details of preparation were reported earlier [7, 8]. The X-ray diffiaction of the samples was carried out at room temperature using a Philips powder diffractometer (PW 1820) with monochromatized CuXa radiation. Transmission electron microscopy (TEM) and SAED investigations were carried out by using a JEOL JEM 2010 200 kV microscope, Cs=0.5 mm, point resolution 0.19 nm. 

The specific surface areas were determined by means of nitrogen adsorption and the metallic surface areas by using adsorption of 3-methylthiophen in liquid phase (ref. 9). The bulk composition of each sample was determined by chemical analysis and expressed by the atomic ratios Al/Ni and M/Ni. The catalysts were observed by transmission electron microscopy (JE0L 200 CX-TEM) and analysed either globally or at point level with a lateral resolution of 1.5 nm by means of a STEM (VG - HB 501) connected to an energy - dispersive X-ray analyser (EDAX). 

Catalyst surface areas were measured using the multi-point BET method on a Carlo-Erba Ins. Sorpty 1750. Before the measurements, the samples were heated under dynamic vacuum at 573 K for 1 h in order to remove adsorbed water and impurities. Measurements were made at liquid nitrogen temperature with nitrogen as the adsorbate gas. Powder X-ray diffraction measurements were performed on a Siemens Model D-500 diffractometer with Co Kc monochromatic radiation (X = 1.78901 A) and the high resolution electron microscopy was carried out on a Topcon EM-002B microscope. To prevent artefacts no solvents were used in the preparation and mounting of samples for HRTEM. 

Structural elements that interact with the microtubule surface have been identified by the effect of point mutations (Woehlke et al, 1997) and by fitting crystal structures of kinesin motor domains to low-resolution electron density maps obtained by cryo-electron microscopy of microtubules... 

The primary difference between optical and electron microscopy is that the latter uses an electron beam as the probe. Since 10- to 500-keV electron beams have much lower wavelengths than light, the resolution is greater. At the same time, the electron beam requires completely different instrumentation (source, collimator, detector, magnification control, etc.). Moreover, electrons are very readily absorbed by matter. Therefore, the entire path of the beam, from source to specimen to detector, has to be in vacuum. From the sample preparation point of view, this is of major significance. For specimens that may change in vacuum, biological tissues, for instance, this can be a major concern, and newly developed accessories such as environmental cells [8] need to be added to the microscope. 

Several ways exist to image these regions of different work function. SEM and FEM have been discussed earlier in this chapter. As an alternative, scanning photoemission microscopy is carried out by scanning a focused UV beam (beam diameter 0.5 pm) over the surface and recording the photoemission intensity point by point. This is of course a slow procedure, but much faster imaging in real time becomes available if the electrons are collected from the entire surface in parallel, as is carried out in photoemission electron microscopy (PEEM). The lateral resolution of this technique is presently around 200 nm, but by using... 

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