Electron microscopy
 
Inaccuracy/Artifacts in Electron Diffraction and Spurious Intensities
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The artifacts in electron diffraction patterns can be divided in different classes:
         i) Distortion of diffraction spots caused by the curvature of the Ewald sphere;
         ii) Change in the size of the pattern or a rotation due to changes in the electron lens currents or accelerating voltage;
         iii) Change of the intensity of the pattern due to change of the electron illumination;
         iv) Shift of the whole pattern due to detector drift;
         v) Inaccuracy due to specimen or beam drifts;
         vi) Parasitic stripes [1], (see page3964), which can be simulated with programs [2]. However, the parasitic stripes can be reduced by changing the acquisition time.
The parasitic intensity stripes correspond to the most intense peaks, and some parasitic oblique stripes originate mainly from the direct beam, for instance, as shown in Figure 3087. This type of artifacts especially exist when the exposure time is very short and the signal intensity in some part of the diffraction pattern is very low. These stripes are usually induced from several physical and instrumental reasons related to the sensitivity of the CCD array and to the high intensity in some of the diffracted spots [2]:
         vi.a) Smearing (blooming), caused by excess charge spilling over into vertical transfer register from a specific pixel during the CCD readout [3]. This intensity decreases from the position of the intense spot to the border of the diffraction pattern.
         vi.b) Beam blanking before aligning it to the optical axis of the TEM for diffraction acquisition. The beam is first blanked and then swiped electrically to the axis. This intensity partially overlaps to the stripes formed by the blooming effect. For this reason, the vertical stripes have an half of stripe to the middle of the pattern is almost constant, while the other half decreases due to the smearing/blooming effect only, e.g. Figure 3087(a). This intensity can be minimized by delaying the exposure time after beam centering from beam blanking.
         vi.c) Detector memory effects;
         vi.d) Blooming during the acquisition of the pattern and from the type of shutter used for data acquisition [3].

Parasitic artifacts in electron diffraction patterns: (a) Si [112], and (b) SrTiO3 [001]

Figure 3087. Parasitic artifacts in electron diffraction patterns: (a) Si [112], and (b) SrTiO3 [001]. [2] 

         vii) Irregularities in the background due to wrong online dark-count subtraction procedures;
         viii) Corrupted detector areas, which can be eliminated by replacing the malfunctioning detector;
         ix) Extra counts due to cosmic rays;
         x) Memory effects in the detector, which be cancelled by exposing the detector to an even illumination.

Spurious intensities in electron diffraction patterns originate mainly from experimental limitations related to:
         i) Shutter of the equipment;
         ii) Detector itself;
         iii) Very high intensity of the direct beam;
         iv) Very high intensity of strong diffracted beams.

Unfortunately, spurious intensities are often comparable with the weakest diffracted intensities, which present the information of the highest spatial resolution.

To eliminate the effect of spurious intensities in electron diffraction patterns , one can:
         i) Improve the diffraction signal;
         ii) Reduce the noise of a specific experimental intensity;
         iii) Remove the presence of spurious intensities. [2]

 

 

 

 

 

[1] Senninger, D. Analysis of Electron Diffraction Patterns from Carbon Nanotubes with Image Processing to Determine Structural Parameters. Ph.D. Thesis, University of Regensburg, Regensburg, Bavaria, December 2011.
[2] Francesco Scattarella, Liberato De Caro, Dritan Siliqi and Elvio Carlino, Effective Pattern Intensity Artifacts Treatment for Electron Diffractive Imaging, Crystals, 7, 186 (2017).
[3] Mollon, B.; Pan, M.; Jia, Y.; Mooney, P.; Sha, T. Development of a fast CCD camera for electron diffraction imaging in conventional TEM. Microsc. Microanal. 2009, 15, 166.

 

 

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