Aberration corrected

The first corrected electron-optical SEM was developed by Zach [10]. Eor low-voltage SEM (LVSEM, down to 500 eV electron energy instead of the conventional energies of up to 30 keV) the spot size is extremely large without aberration correction. Combining and correction and a electrostatic objective lens, Zach showed that a substantial improvement in spot size and resolution is possible. The achievable resolution in a LVSEM is now of the order of 1-2 mn. More recently, Krivanek and colleagues succeeded in building a corrected STEM [53,M]. 

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. 

Figure Bl.17.9. A CoSi grain boundary as visualized in a spherical-aberration-corrected TEM (Haider et a/ 1998). (a) Individual images recorded at different defocus with and without correction of C(b) CTFs in the case of the uncorrected TEM at higher defocus (c) CTF for the corrected TEM at only 14 nm underfocus. Pictures by courtesy of M Haider and Elsevier.

Krivanek O L, Deiiby N, Spence A J and Brown L M 1998 Sphericai aberration correction in dedicated STEM Electron Microscopy 1998 14th Int. Oonf. on Electron Microscopy (Oancun) voi 1 (Bristoi institute of Physics Pubiishing) pp 55-6... 

Haider M, Rose H, Uhiemann S, Schwab E, Kabius B and Urban K 1998 A sphericai-aberration-corrected 200 keV transmission eiectron microscope Ultramicroscopy 75 53-60... 

We have also not mentioned the last breakthrough in electron microscopy, where the spherical aberration problem has been finally solved by means of multipole lenses [117,118], and new aberration corrected electron microscopes are entering the market. 

To improve the resolution, one, therefore, minimizes Cs (with aberration correction almost to zero) and increases the electron energy. However, electron energy spread and stability issues are also critical as the resolution is improved. 

Aberration-corrected ETEM/STEM (130) is expected to offer superior (subatomic) resolution under catalytic reaction conditions furthermore, it will provide improved flexibility for tilting the sample to different crystallographic orientations to allow understanding of the geometry of surface structural changes, enable the use of complex sample stages, and perhaps higher gas pressures. 

Key words Electron diffraction, crystallographic image processing, 3D reconstruction. Structure factors, ultrafast electron crystallography, aberration corrected electron microscope, oversampling phasing method... 

The only practical difficulty comes from determining the appropriate aberration correction. In Truelmage, this has been automated, e.g. for the defocus and the 2-fold astigmatism. For a weak phase object, the real part of exit wave function should be constant (Equation 2). Thus, the defocus and 2-fold astigmatism can be determined by minimizing the contrast of the real part of the exit-wave function. 

Figure 10. Reconstructed exit wave function (phase) of the same area as shown in Figure 5. The left image shows the phase of the exit wave function before aberration correction and the right image shows the phase after correction for residual 2-, 3-fold astigmatism, and coma. The difference clearly illustrates that numerical correction of residual aberrations is crucial for...

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. 

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. 

Su DS, et al. Surface chemistry of Ag particles identification of oxide species by aberration-corrected TEM and by DFT calculations. Angew Chem Int Ed. 2008 47(27) 5005-8. 

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