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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The method of powder X-ray diffraction works best for micrometer-sized crystals and becomes less useful for crystals in the nanometer range [1] because the kinematic X-ray diffraction from perfect crystal lattices shows essentially delta functions for the line profiles of the individual reflections. However, the situation for smaller crystals becomes rather complex and the Bragg peaks may shift simultaneously as well as get asymmetrically or even anisotropically broadened [2], especially the powder X-ray diffraction for nanocrystals become less and less characteristic because more and more Bragg peaks overlap due to the peak broadening [3] and the intensity diminishes until they become difficult to distinguish from the background. Furthermore, higher spatial resolution is needed because the surface and near surface regions of nanocrystals are highly distorted or relaxed with respect to the bulk crystal structure. For instance, the grains of many crystalline materials, e.g. high T_c superconductors, are too small in size and are too imperfect in periodicity for an X-ray single crystal analysis to be performed, but are applicable for electron-microscopic (EM) observations.

In stead of X-ray diffraction, electron diffraction is normally used to analyze nanostructures. The atomic scattering factors for high-energy electrons are approximately three orders of magnitudes larger than for X-rays. This ensures that there will be sufficient diffracted intensity in electron diffraction patterns so that structural information can be obtained for fingerprinting purposes in the TEM even for the smallest nanocrystals. However, this strong scattering of electrons by matter may complicate the analysis due to dynamical diffraction effects.

The selected area diffraction (SAD) aperture in TEM is used to exclude parts of the area being viewed so that we can make sure the diffraction pattern originates from a small, specific area. The SAD method is ideal for areas and grains lager than ~100 nm. To be able to reduce further the area being analyzed, it is better to use CBED techniques.

[1] J. A. López Pérez, M. A. López Quintela, J. Mira, J. Rivas, and S. W. Charles. Advances in the preparation of magnetic nanoparticles by the microemulsion method. Journal of Physical Chemistry B, 101(41):8045 - 8047, 1997.
[2] Ungár, T.: Microstructure of nanocrystalline materials studied by powder diffraction. Z. Kristallogr. Suppl. 23 (2006) 313–318.
[3] Pinna, N.: X-Ray diffraction from nanocrystals. Progr. Colloid. Polym. Sci. 130 (2005) 29–32.