Electron diffraction – SAED, CBED, PED

M. Klementová

Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Husinec-Řež 1001, Czech Republic

klemari@iic.cas.cz

Keywords: electron diffraction, SAED, CBED, PED

Introduction

Due to a much stronger interaction of electrons with matter compared to X-rays, electron diffraction has non-negligible advantages over X-ray diffraction. Nano-objects that are too small for conventional X-ray diffraction experiment and would have to be taken to a synchrotron can be studied in the laboratory by electron diffraction. Electron diffraction is readily available on any TEM (transmission electron microscope) where it can be further combined with other complementary techniques such as imaging and/or spectroscopy. Moreover, electrons are scattered by light atoms relatively more strongly, and electron diffraction patterns can show reflections corresponding to a resolution beyond that available with X-rays. However, the much stronger interaction of electrons with matter as well as very small diffraction angles also cause strong dynamical diffraction effects, such as multiple diffraction, which hinder structural interpretation of electron diffraction patterns.

SAED – Selected-area electron diffraction

In SAED [1], a parallel beam (plane wave travelling in one direction) interacts with sample. An aperture is used to define the area from which the diffraction pattern is to be recorded from a thin sample. This aperture is typically located in the first image plane below the sample. Typical size of an area studied by SAED is a few hundred of nanometers. SAED diffraction patterns are either simple spot patterns corresponding to single-crystal diffraction or ring patterns corresponding to powder diffraction from multiple crystals with a variable orientation (Fig. 1).

SAED is commonly used for phase identification, determination of structural intergrowth, determination of growth directions etc. Lattice parameters from SAED have accuracy of approx. 5%, and due to multiple diffraction kinematically forbidden reflections are often present.

CBED – Convergent-beam electron diffraction

In CBED [2], the incident electron beam is a cone of incident rays impinging on sample over a range of angles. As a result, a diffraction spot will appear as a disc in the back focal plane (Fig. 2). Such disc contains information from higher-order Laue zones (HOLZ). Using convergent beam overcomes the limitation of SAED for analyzing only areas of approx. 500 nm in size. However, with CBED, the areas studied are limited by the beam size and the beam interaction volume (approx. 10 nm).

CBED yields information about specimen thickness, unit-cell parameters (accuracy of approx. 0.01%), crystal system and 3D crystal symmetry (point group and space group).

PED – Precession electron diffraction

PED is equivalent to the Buerger precession technique [3] used in X-ray diffraction where the specimen as well as the photographic plate circumscribe the surface of a cone (the precession movement) with respect to the X-ray incident beam in order to record an undistorted image of reciprocal space. In the electron precession technique, it is the electron beam that is tilted and moved along the cone surface having a common axis with the TEM optical axis and with the studied zone axis of the specimen (Fig. 3).

PED was first proposed by Vincent & Midgley [4]. The data show reduced dynamical effects because there are far fewer simultaneously excited reflections in the off-zone condition. In addition, the precession integrates the diffraction intensities through the Bragg condition, which provides data sets less subject to minor sample tilt, and makes the interpretations of pattern symmetry more reliable.

a)	b)
Figure 1. SAED: a) spot pattern – SnO2 along [001], b) ring pattern – RuO2.
a)	b)
Figure 2. CBED: a) including HOLZ – SnO2 along [001], b) simulation of centre disc of CBED by JEMS – SnO2 along [100] [5].
a)	b)
Figure 3. Electron diffraction of mayenite: a) precession off, b) precession on [6].

References

2. J.B. Poole, Philiops Tech. Runsch., 9, 33.

1. W. Kossel, G. Möllenstadt, Ann. der Phys., 36, (1939), 133.

3. M.J. Buerger, The precession method in X-ray crystallography. John Wiley and Sons, Inc. New York. 1964.

4. R. Vincent, P.A. Midgley, Ultramicroscopy, 53, (1994), 271.

5. P. Stadelmann (2009). JEMS—ems java version, CIME-EPFL,CH-1015 Lausanne, Pierre.Stadelmann@epfl.ch.

6. http://www.nanomegas.com/liveNew.php - downloaded on June 1, 2009.

Acknowledgements.

Financial support through project AV0Z40320502 is gratefully acknowledged.