The lower left corner of the last darkfield image shows the edge of one square of the sample holder's copper grid. Each pair contains an area-limited diffraction image, and a brightfield picture of the area from which the diffraction image came. Breakage of the crystal also seemed to decrease the number of good diffraction sites.
To minimize this effect, extra care was required to keep the beam less focused and intense. Pieces literally and visibly popped off of the crystal as it was panned under the electron beam. In addition, apparently the high voltage of the electron beam induced such a high ferroelectric strain in the crystal that the crystal actually broke. This made the focusing and astigmatism correction processes even more iterative than usual, since every adjustment caused the sample to shift itself. Thus, moving the sample or changing the beam focus, alignment, or intensity all caused the sample to move. Imaging this sample was tricky because the piezoelectric material reacted to the high-voltage electron beam by expanding and contracting in the beam. Using darkfield illumination I was able to identify areas showing diffraction, and I took diffraction patterns from some of those regions. I hoped to find an edge region thin enough to be transparent, and I found several such regions. However, during polishing a corner of the sample was broken off, leaving a chipped edge. This was illustrated by an electron flight path simulation. Even at the very high accelerating voltage of the TEM (200 KeV), this is much too thick to be transparent. The sample was a single crystal of unpoled PMN-PT, which had been polished down to a thickness of about 10 microns. I hoped to find evidence supporting some kind of phase composition. Because the sample had (nominally) 29% PT, it fell into a range where the exact phase composition is unknown, although the presence of a rhombohedral or monoclinic structure is suggested by current literature.
The goal of this TEM work was to look for evidence of what phases were present in the sample of PMN-PT. By measuring the diffraction pattern, we can tell something about the crystal structure. In addition to looking at the transmitted image, it's possible to use electron diffraction to image the crystal structure of very small samples. The transmission electron microscope, or TEM for short, can be used to look at a very thin sample by shining electrons through it, just as you would look at a transparency or piece of film by shining light through it. Untitled Document TEM and Electron Diffraction Goal
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