Ute Kaiser1
University of Ulm1
To create next generation functional low-dimensional materials, it is a growing demand in materials sciences to understand their formation mechanisms as well as their structural and electronic properties at the atomic or molecular scale. Some of the most challenging tasks nowadays are related to energy storage, nano-catalysts or battery materials.<br/>A unique type of transmission electron microscopes operating at electron energies between 80keV and 20keV was developed, which allows to undercut most of the materials knock-on damage thresholds and enables sub-Angstroem resolution in an 4000x4000 pixels sized image, single-shoot image down to 40keV by correcting not only geometrical but also the chromatic aberration of the imaging lens [1-3]. This instrument is used for the in-situ studies presented here. Before we discuss several in-situ TEM experiments, we develop a detailed understanding of the electron beam - 2D specimen interactions: By density functional theory molecular dynamics we show that excitations in the electronic system can form vacancies through ballistic energy transfer at electron energies, which are much lower than the knock-on threshold for the ground state. We propose a two-step vacancy formation process as combination of elastic and inelastic events, evaluated also by an U-net-based fully convolution neural network (FCN) [4].<br/>Based on this knowledge, we discuss the dynamics of properties of point, line and extended defects in various transition metal - dichalcogenides and –phosphor dichalcogenides [5-8]. Optical measurements are performed independent of the TEM experiment. To relate optical and TEM results we demonstrate proof-of-principle experiments in which we transfer electron-exposed defect-characterized TMD flakes from a TEM grid to arbitrary substrates and relate the subsequently performed photoluminescence and transport signals to the defect structure [9].<br/>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 [10] and report on the Li crystal nucleation and growth during lithiation and Li-O formation and amorphization during delithiation. We generate further incapsulated metal-atom-dimers within single-walled carbon nanotubes and observe the dynamics of single metallic bond formation by exact measurements of the interatomic distance [11].<br/>At the end we show that the approaches of graphene encapsulation and lowering the electron accelerating voltage (here from 300kV to 120kV) can be successfully applied for imaging 2D organic materials at the molecular level [12-14].<br/>[1] U. Kaiser et al., Ultramicroscopy, 111, 8, (2011) 1239<br/>[2] M. Linck, et al. PRL 117, (2016) 076101.<br/>[3] F. Börrnert and U. Kaiser Physical Review A 98 (2), (2018) 023861<br/>[4] S. Kretschmer, et al, Nano Lett. 20, (2020) 2865.<br/>[5] J. Köster, A. Storm et al., J. Phys. Chem., accepted<br/>[6] T. Lehnert et al., ACS Appl. Nano Mater., 2 (2019) 3262<br/>[7] J. Köster et al., Nanotechnology 32 (2021) 075704<br/>[8] A. Storm, J. Köster, et al. ACS Nano 17 (2023) 4250-4260<br/>[9] M. Quincke, ACS App. Nano Mat. 5 (2022) 11429<br/>[10] M. Kühne, F. Börrnert et al., Nature 564 (2018) 234<br/>[11] K. Cao et al., Science Advances 6 (2020) eaay5849<br/>[12] K. Liu et al., Nature Chemistry 11 (2019) 994<br/>[13] H. Qi et al., Science Advances 6, (2020) eabb5976<br/>[14] B. Liang et al. Nature Communication 13 (2022) 3948.