Seeing into organic 2D materials: defects, dynamics, and resolution

Abstract text (incl. figure legends and references)

Probing the structure of organic 2D materials (O2DMs), ultimately down to the atomic scale, has been a long-sought goal of materials science. AC-HRTEM is capable of direct imaging atomic structures with sub-Ångström resolution ¹. However, electron irradiation damage leads to rapid disintegration of the molecular network during the imaging process, severely limiting achievable resolution regardless of TEMs' optical performance.

Here we present various experimental efforts to extract high-resolution information from O2DMs in particular imine-based 2D polymer thin films ^2,3. Two samples were synthesized using the surfactant-monolayer-assisted interfacial synthesis (SMAIS) ⁴ (Fig. 1). Using low-dose methods at room temperature, we examined 2D-PI-DhTPA ², and directly observed the grain boundaries with a resolution of 2.3 Å at 300 kV, giving insight into the formation mechanism of the 2D crystals.

Although HRTEM imaging of inorganic 2D materials can benefit substantially by restricting the electron energy below 80 keV ¹, the rare TEM studies on organic 2D crystals are still usually carried out at 300 keV. We systematically evaluated the electron accelerating voltages for AC-HRTEM imaging of O2DMs ³. We reached an image resolution of 1.9 Å at 120 kV (Fig. 2a-b). The improvement in resolution has been achieved by maximizing the efficiency of electron usage, among the acceleration voltages of 300 kV, 200 kV, 120 kV, and 80 kV. Not only could the porphyrin pores be clearly resolved, but linkers with and without hydroxyl groups could also be distinguished. We occasionally observed abnormal contrast in the vicinity of the porphyrin cores (Fig. 2c-d), conflicting with their previously reported structures. DFT calculations revealed that the additional contrast could be attributed to molecular interstitial defects (Fig. 2e) – a defect type that had not been discovered in 2D polymers before.

Recently, drawing from electron cryomicroscopy (cryoEM) in biology ⁵, we explored the potential of imaging our radiation-sensitive O2DMs at low temperature with direct electron detectors. This combination improved the information in a single micrograph such that reflections beyond a resolution of 1.3 Å are clearly resolved. We envisage that our results will bring new insights into the defect types and pore interfaces in O2DMS and promote a deeper understanding of structure-function correlations in this rising class of materials.

Fig. 1. Reaction scheme and atomic models of 2D-PI-BPDA and 2D-PI-DhTPA.

Fig. 2. a-d AC-HRTEM imaging of 2D-PI-BPDA at 120 kV, showing regions without and with TAPP interstitials. e, Plausible stacking modes of TAPP interstitials derived by DFTB calculations and corresponding simulated images.

References

Linck, M. et al. Phys. Rev. Lett. 117, 76101 (2016). Qi, H. et al. Sci. Adv. 6, eabb5976 (2020). Liang, B. et al. Nat. Commun. 13, 3948 (2022). Liu, K. et al. Nat. Chem. 11, 994–1000 (2019). Russo, C. J., & Egerton, R. MRS Bulletin 44, 935–941 (2019).

Acknowledgment

We gratefully acknowledge the funding from DFG – 492191310; 426572620; 417590517 (SFB-1415), as well as the financial support from the European Union's Horizon2020 research and innovation program under Grant Agreement No. 881603 (GrapheneCore3). This work was also supported by an Astex SIPD Fellowship (HW) and Medical Research Council grant MC_UP_120117.