A couple of weeks ago, I obtained my first set of TEM
electron diffraction data with the help of another lab member, Thinh Le. Much
like how electrons passing through a grating will produce an interference
pattern due to particle-wave duality, electrons accelerated through a TEM
sample’s periodic structure will result in a diffraction pattern. As seen in
the examples below, a crystalline specimen will produce a spot pattern whereas
a semi-crystalline or amorphous specimen will produce diffraction rings. Ring
formation occurs because each randomly oriented domain produces its own
diffraction pattern, and the superposition of these patterns forms rings.
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http://www.ammrf.org.au/myscope/tem/background/concepts/imagegeneration/diffractionimages.php
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Since the samples I was studying were polymers (P3HT and
P3HT-b-PFTBT), their electron diffraction patterns were rings. However, they are
very sensitive to electron beam damage – in fact, every time I moved the
electron beam to a new location on the sample, I could see the diffraction ring
fading away right before my eyes the longer it was exposed to the beam. For
this very reason, the first milestone that I hope to achieve is to calculate a
critical dose Dc at which soft materials such as P3HT and P3HT-b-PFTBT can be
imaged with maximum contrast without destroying their structure.
To characterize this radiation damage, I took 10 consecutive
electron diffraction images at one sample location at regular time intervals
for both P3HT and P3HT-b-PFTBT (with increasing time, the electron dose also
increases proportionally). By extracting the intensity of the diffraction rings
using the software Digital Micrograph, I was then able to plot the intensity of
the rings against the electron dose. The resulting data can be fitted to an
exponential curve and Dc can be calculated as the inverse of the decay rate.
Unfortunately, the values for Dc that I calculated did not
agree with values previously calculated by my lab. Possibilities for this
discrepancy could be an inaccurate electron dose calibration or imprecise
intensities. The first of these issues is out of my hand, but I will attempt to
remedy the second by calculating radially integrated intensities using
Mathematica.
