Electron beam damage of perfluorosulfonic acid studied by soft X-ray spectromicroscopy

Lis G A Melo; Adam P Hitchcock

doi:10.1016/j.micron.2019.02.006

Electron beam damage of perfluorosulfonic acid studied by soft X-ray spectromicroscopy

Micron. 2019 Jun:121:8-20. doi: 10.1016/j.micron.2019.02.006. Epub 2019 Mar 7.

Authors

Lis G A Melo¹, Adam P Hitchcock²

Affiliations

¹ Dept. Chemistry and Chemical Biology, McMaster University, Hamilton, ON, L8S4M1, Canada. Electronic address: lismellow@gmail.com.
² Dept. Chemistry and Chemical Biology, McMaster University, Hamilton, ON, L8S4M1, Canada.

PMID: 30875488
DOI: 10.1016/j.micron.2019.02.006

Abstract

Scanning transmission X-ray microscopy (STXM) was used to study chemical changes to perfluorosulfonic acid (PFSA) spun cast thin films as a function of dose imparted by exposure of a 200 kV electron beam in a Transmission Electron Microscope (TEM). The relationship between electron beam fluence and absorbed dose was calibrated using a modified version of a protocol based on the positive to negative lithography transition in PMMA [Leontowich et al, J. Synchrotron Rad. 19 (2012) 976]. STXM was used to characterize and quantify the chemical changes caused by electron irradiation of PFSA under several different conditions. The critical dose for CF₂-CF₂ amorphization was used to explore the effects of the sample environment on electron beam damage. Use of a silicon nitride substrate was found to increase the CF₂-CF₂ amorphization critical dose by ∼x2 from that for free-standing PFSA films. Freestanding PFSA and PMMA films were damaged by 200 kV electrons at ∼100 K and then the damage was measured by STXM at 300 K (RT). The lithography cross-over dose for PMMA was found to be ∼2x higher when the PMMA thin film was electron irradiated at 120 K rather than at 300 K. The critical dose for CF₂-CF₂ amorphization in PFSA irradiated at 120 K followed by warming and delayed measurement by STXM at 300 K was found to be ∼2x larger than at 300 K. To place these results in the context of the use of electron microscopy to study PFSA ionomer in fuel cell systems, an exposure of 300 e^-/nm² at 300 K (which corresponds to an absorbed dose of ∼20 MGy) amorphizes ∼10% of the CF₂-CF₂ bonds in PFSA. At this dose level, the spatial resolution for TEM imaging of PFSA is limited to 3.5 nm by radiation damage, if one is using a direct electron detector with DQE = 1. This work recommends caution about 2D and 3D morphological information of PFSA materials based on TEM studies which use fluences higher than 300 e^-/nm².

Keywords: Electron beam damage; Perfluorosulfonic acid; Radiation damage; STXM; Soft materials; Transmission electron microscope.

Publication types

Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, Non-P.H.S.