Combining images
Images can be combined to get the most information out of a sample. Here is a diffraction pattern on the left and a high resolution electron image on the right for the same material: Nd13CaO7
But more is achievable. Diffraction patterns can be combined with information from bright field and dark field images.
Here is the diffraction pattern from a cluster of minute crystals of zinc oxide (a hexagonal crystal structure).
The diffraction pattern is a mixture of information from closely positioned crystals that are all fairly similarly aligned so the patterns are being gained from three of the crystals at the same time.
In one sense it is not an ideal diffraction pattern because it contains multiple sets of information. But if we were looking at material containing different phases, or different alignments of the lattice within a material, this is the sort of image we might see.
Bright field image of the ZnO crystals.
Dark field images of the ZnO crystals (from left to right, 1 to 4, as in the diffraction pattern).
In dark field imaging, notice that only one crystal at a time produces an image and it is a ‘white’ image with a black background.
The reason only one crystal or region is seen in each dark field image is because the electron beam is lined up on a discrete intensity spot in the diffraction pattern, hence only electrons diffracted by that lattice arrangement are being imaged.
For basic information on how to align a beam for bright field or dark field imaging click here.
The process of selecting a spot in the diffraction pattern can be repeated for different intensity spots of interest and the dark field images gathered and coloured, then overlaid so we can see all the information in one image.
Notice that the fourth colour belongs to a crystal alignment that does not show up in the diffraction pattern shown (see circle number 4). The circle placement for 4 in the diffraction pattern shows where the beam has been positioned to collect the image for this fourth crystal.
To understand more about what diffraction patterns are and how they are generated, click here.
Using this approach of combining bright field, dark field and diffraction pattern imaging is useful to pin-point the presence of a phase in material because a diffraction dot that does not ‘fit’ with the overall pattern can be ‘interrogated’ by discovering exactly where that phase occurs in the material using the dark field image.
