STEM-EELS study of beam damage in polymers and extra-terrestrial organic matter using direct electron detectors

Characterizing organic compounds using STEM-EELS at high spatial resolution is crucial in materials science and geosciences, especially for organics intricately mixed with minerals at the nanoscale, as is the case in carbonaceous meteorites. However, the high spatial resolution provided by TEM comes with the challenge of electron beam sensitivity, which has long hindered the study of these fragile compounds. Here, we take advantage of direct electron detectors to revisit analytical strategies, searching for the best compromise to prevent beam damage and reach the highest spatial resolution. Our STEM-EELS parametric survey focuses on two reference polymers (PEEK and PES) which differ in their molecular structures and susceptibility to radiation-induced damage. We sequentially acquire low loss and carbon K-edge spectra at low dwell time using a multi-frame protocol, possible thanks to noiseless direct electron detectors. Results show that PES is much more sensitive than PEEK and that the main damage mechanism is radiolysis coupled to recombination. Damage rates are lower when working at an accelerating voltage of 200 keV rather than at 80 keV. Cooling the sample (- 100 °C) helps reducing mass loss and amorphization, but can also lead to the formation of undesired functional groups through recombination. The pixel size affects beam damage independently of the electron dose. Using the fastest dwell-time permitted by the detectors (80 µs) and pixel sizes of 1.5, 7.5, 15 and 30 nm, we show that PEEK resists at 15 nm pixel but is rapidly amorphized at 1 nm while PES is already unstable at 30 nm pixel size. We understand this as damage delocalization effect on successive pixels. The insoluble organic matter extracted from the Orgueil meteorite also appears to better resist damages at 200 keV, but its aliphatic groups are nevertheless affected at pixel size of 15 nm. A reasonable spectral agreement is found between STEM-EELS and synchrotron-based XANES-STXM, paving the road for investigating extra-terrestrial samples such as those returned by space mission from carbonaceous asteroids Ryugu and Bennu.