Microscopy 101
Subsequent patterns were acquired by first defocusing the objective lens by a specific amount and then changing the “Z” position until a minimum contrast condition was achieved. Ten IL1 was adjusted so that the (000) spot was minimized. DiffTools was used to set the calibra- tion of the pattern from the stored value, locate the center of the pattern, do a rotational average, and find the peaks as described above. Te mea- sured Au111
d-spacing was noted and
then the DiffTools-Gold Calibrator routine run to find the calibration factor.
in Figures 4 and 5,
Te results are shown in red respectively.
Two results were so surprising that the experiment was repeated a few days later, shown in blue. First, note that
the difference from the true
Figure 4: Variation of Au(111) d-spacing value with the sample positioned at the incorrect height in the micro- scope. Red and blue curves acquired on different days.
value for the Au(111) d-spacing for the worse case was only 0.22%. Tat is not so surprising consid- ering the results from the tables. However, what is surprising is that the d-spacing values are within 1% from the true value, even when the “Z” position of the sample is a rather distant 30 μm away from where it should be. And, with a difference of 60 μm of positioning error, the calibration factors are within 1.8% of each other. However, what was really surprising and what caused the suspicion of something being wrong with the experiment were the four values of d-spacings within 5 μm of the zero-focus position. All four values had the exact same val- ues to four decimal digits, even aſter double-checking the calculations. Repeating a similar
the experiment gave result with four values
Figure 5: Variation of calibration factor with sample positioned at the incorrect height in the microscope. Red and blue curves acquired on different days.
resulting calibration factors were measured. Ten random posi- tions were first saved at the height of the original entry of the TEM holder so that the “Z” position would have to be adjusted for each pattern. Te first SAED pattern was acquired at the stan- dard position as explained above and designated the 0 nm focus.
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agreeing within four decimal dig- its, although offset from the origi- nal data, as shown in the blue curve in Figure 4. Several factors may be responsible. If Equation 4 is consid- ered for a camera length of 20 cm, the (111) ring has too low of a ring diameter in pixels for the change in diameter to have much effect. Te
difference in the diameter of the ring when the sample height is changed from the zero position is probably also compensated somewhat by the refocusing of the IL1 lens. Using the (311) ring (fourth ring) gave similar results, but some differences were seen in the values when the (420) ring (eighth ring) was used.
www.microscopy-today.com • 2020 July
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