Radiation damage study of P3HT and P3HT-b-PFTBT

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.

http://www.ammrf.org.au/myscope/tem/background/concepts/imagegeneration/diffractionimages.php

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 D_c 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 D_c can be calculated as the inverse of the decay rate.

Unfortunately, the values for D_c 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.