Resolution spherical aberration

Urban K, Kabius B, Haider M, Rose H (1999) A way to higher resolution spherical aberration correction in a 200kV transmission electron microscope. J Electron Microsc 48 821-826 Veblen DR, Ferry JM (1983) A TEM study of the biotite-chlorite reaction and comparison with petrologic observations. Am Mineral 68 1160-1168... 

The construction of an aberration-corrected TEM proved to be teclmically more demanding the point resolution of a conventional TEM today is of the order of 1-2 A. Therefore, the aim of a corrected TEM must be to increase the resolution beyond the 1 A barrier. This unplies a great number of additional stability problems, which can only be solved by the most modem technologies. The first corrected TEM prototype was presented by Flaider and coworkers [M]- Eigure BE 17.9 shows the unprovement in image quality and interpretability gained from the correction of the spherical aberration in the case of a materials science sample. 

The specimen is immersed in the next lens encountered along the column, the objective lens. The objective lens is a magnetic lens, the design of which is the most crucial of all lenses on the instrument. Instrumental resolution is limited primarily by the spherical aberration of the objective lens. 

For the high resolution case, the phase-contrast effects are automatically introduced owing to the combined effect of defocus and spherical aberration, which gives rise to an image of a structure complicated by the fact that also the amplitude term, resulting from the propagation process, interacts in a non-linear way with the phase term [16,89,90,96]. 

The point resolution of an electron microscope is limited by the spherical aberration of the objective lens. Haider et al. (1995 1998) developed a corrector to be implemented into a standard transmission electron microscope to correct the spherical aberration. They showed that the point resolution could be improved from 2.4 A to 1.4 A. 

Figure. 4a is a [110] projected high-resolution image taken with a JEM-2010FEG electron microscope with a spherical aberration coefficient 0.5 mm. A dislocation can be seen in the framed area, of which a magnified photo is shown in Fig. 4b. It can be seen that two extra half 111 planes mnning from the top left to the bottom right and from the top right to the bottom left, respectively. Both of them end in the center of the picture.

The point resolution of a TEM, which only depends on the spherical aberration, C, and the electron wavelength. A, (which is determined by the accelerating voltage) sets the limit for a straightforward interpretation of a HRTEM image of a thin object. However, this is different from the information limit, which defines the highest frequencies that can be transferred in a microscope. 

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. 

This is the least expensive optical component, but very important and the most likely to be carelessly chosen. Many objectives are designed and marked to be used with coverslips of a certain thickness, usually 0.17 mm (or 170 5 pm), which corresponds to a thickness grade of 1.5. Ideally, for obtaining the best FISH images, it is recommended that an oil immersion lens is used and these lenses almost always require a 1.5 coverslip for optimal resolution. Any deviation from this thickness leads to spherical aberration, loss in resolution and results in larger and dimmer FISH spots. 

The overall resolution in HRTEM is governed partly by the electron wavelength and partly by the optical characteristics of the objective lens. The most important effect of the latter arises from spherical aberration. This aberration introduces a phase difference into the individual diffracted beams and when the real image is synthesized by the lens from these diffracted beams this can give rise to considerable confusion in the image contrast. 

As previously indicated, determination of an actual particle size distribution requires an expression for the volumetric sampling rate V (or frontal area Ag X the flow velocity) as a function of particle diameter. This can be achieved if the instrument parameter f I (/-number times the length of the sensitive volume) can be determined. Since f and I are the most uncertain because of spherical aberration of the lens and resolution deficiencies of the collecting optics, / I was calibrated out by determining the actual frontal area at the 1/e intensity points as described above (Figure 9). For the counter under consideration the following relation holds ... 

The overall dependency of CTF on resolution, wavelength, defocus, and spherical aberration is given by... 

K is a constant close to unity, Cs is the spherical aberration coefficient and A. is the wavelength of the electrons. Equation 4.3 simply tells us that we should decrease wavelength and spherical aberration and increase brightness of electron illumination to obtain the minimal probe size. The importance of electron gun type in determining SEM resolution becomes obvious because there are significant differences in their brightnesses. Figure 4.5 shows the difference... 

The resolution in TEM is limited by lens aberrations. In contrast to optical microscopy, where by serially ordering concave and convex lenses, aberrations can be compensated and hence the wavelength of the radiation is resolution limiting in TEM lens aberrations cannot be compensated since concave electron lenses are not feasible. The objective lens is the crucial part for image defining the microscope s resolution. The quality of the objective lens is described by the constants of spherical Cs(" 0.5-3mm) and chromatic aberration Cc ( 1-2 mm). Recently, microscopes equipped with complex correctors for the spherical aberration have become available. ... 

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