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SAED Indexing

Figure1

Figure 1 SAED pattern indexing.

The toolbar contains tool buttons to:

The controls and options are placed in 8 panes to:

Figure3a Figure3b

Figure 3a Align rotation.

Figure 3b Crystals.

The indexing crystals are added to or deleted from the table by the tool buttons:

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Figure3c Figure3d

Figure 3c Mask.

Figure 3d Microscope.

The Mask drawing has the zone axis geometry (Figures 4a, 4b). The mask is enabled in the Options pane. The mask size can be adjusted and adapted to the SAED pattern. It allows to separate reflections from overlapping crystals. It is rotated using the Align pane.

The Microscope pane sets the accelerating voltage and camera length as well as the measurements errors of angles and distances.

Figure3e Figure3f

Figure 3e Options.

Figure 3f Processing.

The Options pane contains checkboxes to change the background color, to show the Mask and to snap the arrows at the spots position (when the FindPeak has been done). It also allows to reflect the image and to extend the indexing search to all possible zone axes, to use 3 spots (default is 2) and to filter the indexing by likelihood.

Figure3g Figure3h Figure3i

Figure 3g Scale.

Figure 3h Spots angles.

Figure 3i Spots distances.

Scaling the arrows and mask drawing requires to point the Double arrow on known features of the pattern (SAED spots of known distance or angle, scale marker, ...). This is necessary when the camera length is not clearly defined as is very often the case. A precise scaling restricts the indexing to the sets of (hkl) reflections in a range of distances to the (000) transmitted beam defined by the accelerating voltage and camera length. When not precisely known an extended search option can be set (Fig. 3e). When the indexing window is loaded for the first time, the default scale is either mm or nm-1 per pixel.

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Indexing procedure

To index SAED (spots) patterns follow these steps:

Figure2a

Figure 2a GaN 200KV.

Figure2b

Figure2c

Figure 2b Si [110].

Figure 2c Diffractogram Si [110].

Figure4

Figure 4 SAED pattern after image processing (contrast reversal and peak finding).

Figure5

Figure 5 After defining the order of the arrows, the ratio and angle is displayed.

Figure6

Figure 6 The pattern is scaled with the blue double arrow.

Figure7a

Figure7b

Figure 7a The Magnifier glass helps to check that the arrow points to a peak.

Figure 7b Magnifier glass.

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Indexing solutions

Figure8a

Figure8b

Figure 8a Stereogram with indexing solutions.

Figure 8b Table with indexing solutions and camera length.

The stereogram provides the possible solutions. Its toolbar contains tool buttons to:

The camera length of a possible indexing zone axis should be closed to the camera length setting (982 mm is within the error setting).

Figure9

Figure 9 The calculated pattern is superposed on the SAED image.

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Note

The success of the indexing routine depends critically on the accelerating voltage, camera length, error bars and spots order settings. When no solution is found cross check them. When the camera length is unknown use the Extended search option (Fig. 3e). This option will display many solutions where the ratio of the arrows and their angle is within the measurement range, but mainly with incorrect camera length (Figures 10a, 10b).

Figure10a

Figure10b

Figure 10a Stereogram with indexing solutions (extended search).

Figure 10b Table with indexing solutions and camera length (extended search).

Figure9

Figure 11 A (wrong) solution with the correct arrows ratio and angle.


PDF documentation: SAED indexing.