Dept. Chemistry and Chemical Biology, McMaster University, Hamilton, ON, L8S4M1, Canada.
Dept. Chemistry and Chemical Biology, McMaster University, Hamilton, ON, L8S4M1, Canada.
Micron. 2019 Jun;121:8-20. doi: 10.1016/j.micron.2019.02.006. Epub 2019 Mar 7.
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 CF-CF 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 CF-CF 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 CF-CF 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/nm at 300 K (which corresponds to an absorbed dose of ∼20 MGy) amorphizes ∼10% of the CF-CF 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/nm.
扫描透射 X 射线显微镜（STXM）用于研究全氟磺酸（PFSA）旋转铸薄 膜的化学变化，这些变化是通过在透射电子显微镜（TEM）中暴露于 200kV 电子束而产生的剂量引起的。电子束通量与吸收剂量之间的关系是使用基于 PMMA 正、负光刻转换的协议的修改版本进行校准的[Leontowich 等人，J. Synchrotron Rad.19（2012）976]。STXM 用于在几种不同条件下表征和量化 PFSA 电子辐照引起的化学变化。使用 CF-CF 非晶化的临界剂量来探索样品环境对电子束损伤的影响。使用氮化硅衬底会将自由站立的 PFSA 薄膜的 CF-CF 非晶化临界剂量增加约 x2。自由站立的 PFSA 和 PMMA 薄膜在 200kV 电子束下在 100K 时受到损伤，然后在 300K（RT）下用 STXM 测量损伤。当 PMMA 薄膜在 120K 而不是 300K 下进行电子辐照时，PMMA 的光刻交越剂量发现增加了约 2x。在 120K 下辐照 PFSA 并在 300K 下加热和延迟 STXM 测量后，CF-CF 非晶化的临界剂量发现比在 300K 下增加了约 2x。为了将这些结果置于使用电子显微镜研究燃料电池系统中 PFSA 离聚物的背景下，在 300K 下每 300e/nm 的暴露量（相当于约 20MGy 的吸收剂量）会使 PFSA 中的约 10%CF-CF 键非晶化。在这个剂量水平下，如果使用具有 DQE=1 的直接电子探测器，那么 PFSA 的 TEM 成像的空间分辨率将受到辐射损伤的限制，分辨率为 3.5nm。这项工作建议在 TEM 研究中使用高于 300e/nm 的通量时，对基于 TEM 研究的 PFSA 材料的 2D 和 3D 形态信息要保持谨慎。
