A test specimen with molybdenum trioxide crystals on a carbon film is good for the calibration of the image-diffraction rotation angle in TEM and STEM mode. Molybdenum trioxide is pseudo-orthorhombic (lattice parameters a 0.397nm, b 1.385nm and c 0.370nm). From a double-exposure, or the overlaying of an image and diffraction pattern, the rotation angle can be established.
Molybdenum Trioxide Crystals are most useful to determine the rotation between a diffraction pattern and the
selected area image. Select a thin crystal not overlaid by others, so that a clear Laue diffraction pattern is obtained. Starting from the diffraction pattern, change the strength of the diffraction lens until each diffraction spot shows a small image of the crystal. It will be possible to determine the sense of the rotation of the image as the magnification is increased. Check whether there is an image inversion between the diffraction position and the selected area magnification. The actual magnitude of the rotation angle between the crystal and its pattern can be determined by recording both image and diffraction pattern on a single plate. The correct rotation angle between pattern and image can then be determined by taking into account the sense of rotation and any image inversion
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