PROPERTIES OF LOW-DIMENSIONAL MATERIALS BY LOW-VOLTAGE TEM

Two-dimensional materials exhibit properties, which can differ strongly from those of the bulk counterparts and offer unique opportunities for new and miniaturized electronic and optical devices [1]. In situ electron microscopy nowadays can provide experimental data on the level of the single atom, as it has seen extremely rapid developments in recent years owing to ground-breaking advances in electron optics, electron detectors, sample preparation and manipulation, and highly versatile in situ setups can simultaneously while imaging also functionalizing the material under study. Here we present recent results using our unique chromatic and spherical aberrationcorrected SALVE instrument both in imaging and spectroscopy modes [2].

We first discuss the formation of defects in two-dimensional inorganic and organic crystals. For transition metal di-chalcogenides (TMDs) we present results at electron energies below the knock-on threshold in the range between 20-80kV and understand the role of electronic excitations. We show that the formation of vacancies is possible at electron voltages nearly half of the knock-on threshold and quantify the damage [3,4]. Further, we analyse in-situ structural and chemical modifications of different freestanding transition metal phosphorus tri-chalcogenides (TMPTs). We predict the displacement thresholds, electronic properties, and the displacement crosssection of single vacancy S and P in all materials by ab-initio calculations and using these results, the observed structural changes are understood. As the TMPTs are often very oxygen-sensitive, they were prepared with the help of a newly-developed polymer-assisted sample preparation method [5]. We also present studies on the structure of two-dimensional polymer crystals and characterize on the molecular level the defect structure [6]. Furthermore, we present in-situ studies of a miniaturized electrochemical cell, where reversibly single-crystalline bilayer graphene is lithiated and delithiated in controlled manner using an electrochemical gate confined to a device protrusion [7]. On the more fundamental base we show that differentiating between the bond nature between two metal atoms is now possible [8].

References

[1] A. Hashemi et al. (2017). J. Phys. Chem. C 121, 27207

[2] M. Linck, et al. 117 (2016) 076101. [3] S. Kretschmer, T. Lehnert et al. Nano Lett. 20 (2020), 2865-2870

[4] T. Lehnert et al. ACS Appl. Nano Mater. 2 (2019) 3262-3270

[5] J. Köster et al. (2021). Nanotechnology 32, 075704

[6] H.Qi et al. (2020), Science Advances 6, eabb5976

[7] M. Kühne et al. (2018), Nature 564, 234

[8] K. Cao, (2020), Science Advances 6, eaay5849