Abstract text (incl. figure legends and references)
AC-HRTEM imaging is capable of atomically precise observation of two-dimensional materials. It has also proven itself to be a very powerful tool for probing the structures of 2D polymers, covalent and metal organic frameworks1. However, for these organic materials the achievable resolution is often limited by the low number of electrons the material under study can accept before it is destroyed. In extreme cases the total dose for amorphization can be as low as a single electron per square Ångström. Thus, increasing this critical dose of these sensitive materials is of highest importance. This can be achieved by various techniques developed in the past. For layer-stacked materials especially, the thickness dependency of the critical dose is a key feature, due to the easily controllable thickness. However, the effects an increase of the thickness has on the electron resilience is not completely understood2,3. The current assumption is, that additional layers build a cage around the inner parts, holding in produced radicals2. Through this recombination is made possible and the material can self-heal. Aimed at gathering a better understanding of this effect, in our study the thickness dependency of the critical dose was examined in more detail. To achieve this, the critical dose of a triazine-based 2D polymer4 was measured for a wide thickness range. The polymer samples, obtained by mechanical exfoliation, ranged from 15 nm to 85 nm thickness. The thickness of the polymer was obtained by combining light microscopy and atomic force microscopy. In order to find the critical dose for the polymer, sequences of electron diffraction patterns with a dose of only 0.5 electrons per Å2 were obtained and the fading of diffraction spots was monitored (Fig 1). The measurements revealed that the critical dose for amorphization of this polymer is only 1-2 e-/Å2 (Fig. 2), surprisingly independent of sample thickness and HRTEM imaging is made extremely challenging if not impossible. The lack of increase in critical dose with growing thickness is attributed to the porous structure of the polymer, leading to escape paths for the produced radicals. With that the caging effect is circumvented. This assumption is further strengthened by the application of graphene layers on top and bottom, preventing the escape of atoms from the polymer layers. Through that the critical dose is increased, independent of specimen thickness.
References
Haoyuan, Q. et al. Sci. Adv. 6, eabb5976 (2020). Egerton, R. F. Ultramicroscopy 127, 100–108 (2013). Fryer, J. R. Ultramicroscopy 14, 227–236 (1984). Hu, F. et al. J. Am. Chem. Soc. 143, 5636–5642 (2021).Acknowledgement
We acknowledge the funding from Deutsche Forschungsgemeinschaft (DFG) – 492191310; 426572620; 417590517 (SFB-1415), and European Union GrapheneCore3 (881603). We thank Yingjie Zhao from Qingdao University for providing the polymer.
Fig 1. a) Model of the trianzine-based polymer. b) Selected area electron diffraction pattern of the polymer. c) Series of diffraction patterns. e) With the measured intensity in every image d), the relation between intensity and accumulated dose can be obtained. The critical dose is reached, if the intensity falls below 1/e of its starting value.
Fig. 2. Diffraction patterns obtained on a) a (15.8 ± 0.9) and b) a (85 ± 4) nm thick area. c) Thickness dependent first order peak intensity of the first image of the dose series. d) Thickness dependent critical dose of the 2DP.