This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Figure 3726a shows the schematic illustrations of the electric circuits of thermionic electron emission guns and thermal FEGs (field electron guns). The main difference between the two types of electron guns is green-boxed in both figures.

Figure 3726a. Schematic illustrations of the electric circuits of thermionic electron emission guns (a) and thermal FEGs (field electron guns) (b), respectively.

For the weak phase object approximation the diffractogram obtained using EFG-TEMs is the same as that using a CTEM based on W (tungsten) or LaB₆ electron guns.

FEG-TEMs with field-emission electron sources [1 - 2] offering much better spatial and temporal coherence than those offered by CTEMs with LaB₆ and W (tungsten) electron guns. Therefore, FEG-TEMs have allowed images to contain much higher spatial frequencies [3 - 4].

In images formed by elastic scattering (e.g. HRTEM images), contrast delocalization is referred to as the loss of image contrast due to lens aberrations or called blurring effect. In this case, the image information is blurred and displaced from their true locations in the TEM specimen. In other words, the contrast delocalization can also be expressed as the lateral displacement of spatial frequencies in the image. This displacement increases with spatial frequency significantly. It is rarely a problem on the microscopes with LaB₆ guns due to their limited coherence in illumination. However, it is a significant imaging artifact in HRTEM images taken from microscopes with field emission guns (FEGs) due to their high coherence that causes a strong contribution of the high spatial frequencies.

Figure 3726b shows the calculated phase contrast transfer functions (pCTF) at Scherzer defoci in 200-keV TEMs with LaB₆ and FEG sources and with different spherical aberrations (C_s).

Figure 3726b. Typical pCTFs for 200-keV TEMs: (a) At Scherzer defocus in LaB₆ TEM with large positive C_s (1.23 mm), (b) At Scherzer defocus in FEG TEM with large positive C_s (1.23 mm), (c) At Scherzer defocus in FEG TEM with small positive C_s (0.083 mm), and (d) At Scherzer defocus = Lichte defocus in FEG TEM with small negative C_s. Adapted from [5]

[1] M.T. Otten, W.M.J. Coene, Ultramicroscopy 48 (1–2) (1993) 77–91.
[2] T. Honda, T. Tomita, T. Kaneyama, Y. Ishida, Ultramicroscopy 54 (2–4) (1994) 132–144.
[3] W. Coene, G. Janssen, M. Op de Beeck, D. van Dyck, Phys. Rev. Lett. 69 (1992) 3743–3747.
[4] M. Op de Beeck, D. van Dyck, W. Coene, Ultramicroscopy 64 (1–4) (1996) 167–183.
[5] Markus Lentzen, Progress in Aberration-Corrected High-Resolution Transmission Electron Microscopy Using Hardware Aberration Correction, Microsc. Microanal. 12, 191–205, 2006.