Prof. Dr. Ute Kaiser, Electron Microscopy Group of Materials Science (Ulm University)

"Insights in the happy marriage of low-dimensional materials and low voltage transmission electron microscopy to see atoms at work"

Two-dimensional materials exhibit properties, which can differ strongly from those of their bulk counterparts. They offer unique opportunities for new and miniaturized electronic and optical devices [1], which properties may even alter strongly with changes of even single atom’s positions at surfaces and interfaces. The world is made of atoms as we definitely know since Demokrit but can we perceive whatever holds the world together in its inmost folds as Goethe urged to know and can we see those little particles that move around in perpetual motion distantly as Feynman asked directly, finally today?

In situ transmission 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 sample manipulation. In this talk we will focus on our own development performed within the frame of the SALVE project and present recent results using the unique chromatic and spherical aberration-corrected SALVE instrument [2]. Although not planned to image inorganic two-dimensional materials, as this group of materials was almost unknown when the project idea developed, we intend to convince you that the two, both now in the prime of their years, fit perfectly together.

We first try to understand the beam electron – specimen interaction on the atomic level [3] and discuss then the formation of atomic defects in two-dimensional inorganic and organic crystals accompanied by quantum-mechanical calculations and how this changes their properties also in dependence of the layer number [4-7]. Moreover, we start to see and understand the signature of amorphous matter from a single image [8], which is essential for proton and Lithium permeation [9]. On the fundamental base we show that differentiating of the bond nature between two metal atoms is now possible and vibrations of atoms can be directly observed [10]. Furthermore, we present in-situ studies of a miniaturized electrochemical cell, a mini-battery, where reversibly single-crystalline bilayer graphene is lithiated and delithiated in controlled manner using an electrochemical gate confined to a device protrusion [11]. Thus, we see atoms at work and understand the nature of lithiation and delithiation, a fundament for next generation batteries.

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

[2] M. Linck, et al. 117 (2016) 076101,

[3] S. Kretschmer, et al. (2020) Nano Lett. 20 2865.

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

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

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

[8] P.Y. Huang et al.(2013) Science 342, 224.

[9] E. Griffin, et al. (2020) ACS Nano 14, 7280.

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

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

Prof. Dr. Arno Rauschenbeutel, Fundamentals of Optics and Photonics (HU Berlin)

"Atoms and light - a pairing that will surprise you"

The interaction of a single-mode light field with a single atom or an ensemble of atoms is governed by conceptually simple equations and has been extensively studied. Still, the vectorial properties of light combined with the multilevel structure of real atoms and their collective response yield rich and surprising physics. In our group, we are investigating this topic using nanophotonic components, such as subwavelength-diameter optical fibers and whispering-gallery-mode resonators, to couple light and atoms. I will present three effects that we have recently observed in experiments with these systems and that go beyond the standard description of light-matter coupling. First, light which is tightly confined can locally carry transverse spin angular momentum which leads to propagation direction-dependent emission and absorption of light. Second, when imaging an elliptically polarized emitter with a perfectly focused, aberration-free imaging system, its apparent position differs significantly from the actual position. Third, an ensemble of atoms can change the photon statistics of laser light transmitted through the ensemble, yielding pronounced bunching or anti-bunching. Interestingly, these effects are not limited to a nanophotonic setting and even occur for freely propagating light fields.