Defocus in TEM Imaging
- Practical Electron Microscopy and Database -
- An Online Book -

http://www.globalsino.com/EM/

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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In TEM imaging, the conventional modes of high-resolution imaging are mainly determined by the objective lens defocus. There are four generally used defocus settings:
i) “Standard” defocus.
ii) Scherzer defocus maximizing the phase contrast of a weak-phase object,
iii) Lichte defocus of least confusion, minimizing contrast delocalization,
iv) Minimum phase-contrast defocus.

Except the first defocus is in-focus, underfocuses are applied to all the other defocus settings because of the existing, positive spherical aberration of the objective lens, which has to be counterbalanced in a certain way by a negative defocus aberration related to underfocus.

The defocus value (Δf) can be determined from the position of the first crossover of contrast transfer function (refer to Equation 4974b):

-------------------------- [3874]

Here, n = 0, ±1. That means different integers n give different defoci. For instance, a TEM system has a spherical aberration constant Cs 1.38 mm, is operating at 200 kV (the wavelength is 0.0025 nm), and has the first crossover of Fourier transform at u = 2.75 nm-1. Therefore, three possible defocus values are -15 nm (for n = -1), -70 nm (for n = 0), and 53 nm (for n = 1).

The values of Δf (defocus value), Cs (spherical aberration coefficient), and D (standard deviation of the Gaussian distribution of defocus due to the chromatic aberration) can be obtained by TEM image deconvolution.

Table 3874. Selections of focusing/defocusing conditions in TEM imaging.

 Best/optimum defocus condition for HRTEM imaging page4232 Optimal defocus for minimizing the delocalization in (HR)TEM imaging page1715 Minimum (phase-)contrast in (HR)TEM imaging with Gaussian defocus page4233 Maximum (phase-)contrast in (HR)TEM imaging (with Scherzer defocus) page2779

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