Abstract
This chapter is an introduction to the transmission electron microscope (TEM), using ray diagrams to discuss its essential optical design and its component subsystems. Bright- and dark-field imaging are explained, as is selected area diffraction, convergent beam diffraction, high-angle annular dark field imaging, and high-resolution transmission electron microscopy. Some practical issues of spectroscopy, diffraction, and imaging are discussed. Both glass lenses and magnetic lenses are described, and their focusing action is explained by ray tracing and by phase distortions of the wavefront. Brightness and other issues underlying image resolution are described, and limits of resolution are presented for different high-resolution methods.
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Notes
- 1.
It is also possible to acquire a full EDX or EELS spectrum at each pixel in the raster scan. Such data are called a “spectrum image.”
- 2.
Please verify this fact by tracing back to the specimen all three rays that meet at a point in the back focal plane of Fig. 2.11.
- 3.
In practice these two requirements are nearly the same. For clarity, our ray diagrams have been expanded horizontally. Actually, the specimen arrows in Fig. 2.12 are ∼10−4 cm, the distance from the specimen to the objective lens is ∼10−1 cm, and diffraction angles are ∼10−2 radian.
- 4.
Please take a minute to trace around this outer circle of dashes to confirm the 3-fold rotational symmetry.
- 5.
A radio analogy is appropriate. The forward beam serves as the carrier, and the diffracted beams as the modulation sidebands. The music in the sideband (corresponding to the information about the specimen periodicities) cannot be heard without beating the sideband against a reference phase such as the carrier. The overall intensity in the sideband can be measured across a specimen, corresponding to conventional BF or DF imaging, but the phase information is lost without reference to the forward beam.
- 6.
The Wehnelt electrode is analogous to the “grid” in a triode electron tube (or “electron valve”).
- 7.
It is important to remember that electron diffraction involves the interference of wave crests of an individual electron (i.e., self-interference). If two or more high energy electrons are present simultaneously in the specimen, they have no wave interference with each other because electrons are fermions. We measure the intensity from individual electrons that have interacted with the specimen.
- 8.
Section 11.1 explains how the wavefront is slowed by phase delays caused by scattering in the material.
- 9.
In a paraxial imaging condition, the rays are near the optic axis, making only small angles with respect to the optic axis.
- 10.
For this third-order spherical aberration, the disk of least confusion is smaller than QQ′ at the image plane by a factor of 4. Do not confuse the radius at the image plane, MC S(α OA)3, with the circle of least confusion.
- 11.
Correctors for spherical aberration, discussed in Sect. 12.6, have changed this situation, but at a price.
- 12.
The pair are rotated 45° with respect to each other to allow different orientations for the perpendicular x and y axes.
- 13.
The other is getting the beam exactly on the optic axis of the objective lens by performing a voltage or current center adjustment (Sect. 11.5.3).
- 14.
This is strictly valid only when all broadenings are of Gaussian shape, so that convolutions of these different beam broadenings have a Gaussian form (see Sect. 9.1.3).
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© 2013 Springer-Verlag Berlin Heidelberg
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Fultz, B., Howe, J. (2013). The TEM and Its Optics. In: Transmission Electron Microscopy and Diffractometry of Materials. Graduate Texts in Physics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-29761-8_2
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DOI: https://doi.org/10.1007/978-3-642-29761-8_2
Publisher Name: Springer, Berlin, Heidelberg
Print ISBN: 978-3-642-29760-1
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