STROBE Seminar, Prof. Ute Kaiser: “Atomic Defect–Mediated Phase Engineering in 2D van der Waals Structures and Nanoclusters”
Abstract: We show that targeted energy input from an electron beam in a transmission electron microscope (TEM), often combined with concurrent heating in a MEMS holder, drives atomic defect formation and phase transitions across four classes of low-dimensional materials: (1) few-layer transition metal phosphorus trichalcogenides (TMPTs), (2) graphene sandwich structures encapsulating either lithium droplets (2a) or a benzenehexathiol-based two-dimensional conjugated metal-organic framework (2b), (3) platinum nanocrystals on graphene, and (4) noble metals confined within carbon nanotubes.
In (1), controlled heating around 600 °C induces phase transitions to MnS, MnSe, and NiSe, with ab initio calculations predicting orientation- and thickness-dependent magnetic properties. In (2a), graphene vacancies nucleate polycrystalline lithium growth during lithiation, while interstitial oxygen inhibits delithiation via lithium oxide formation. Despite high beam resistance, Cu₃(BHT) (2b) transforms thermally into a crystalline CuS phase between 480 °C and 620 °C as well as under high electron flux. For (3), we resolve the solid–liquid phase transition of Pt nanoclusters and identify a mechanism where stationary Pt atoms corral molten nanodroplets, halting crystallization. Finally, in (4), we reveal contrasting bonding behaviors of Re and Kr atoms within fullerenes inside carbon nanotubes, driven by differences in electronic structure and confinement.
These findings are enabled by the Cc/Cs-corrected SALVE (Sub-Ångström Low-Voltage TEM) platform, highlighting its potential for atomic-scale in situ phase engineering.
Speaker Bio: Ute Kaiser is Senior Professor at Ulm University since 2023 and was Head of the Central Facility of Materials Science Electron Microscopy from 2004-2023. She graduated from Humboldt University Berlin, received her PhD in 1993 and her habilitation on “TEM on semiconductor quantum materials” in 2002. From 1993-2004 she was postdoc at Jena University, with extended visits at Bell Labs, Cambridge University and in Tohoku University. From 2009-2018 she was the director of the Sub-Angstrom Low-Voltage Electron Microscopy project, where she focuses on developments of low-voltage electron microscopy for application on beam-sensitive low-dimensional materials. Ute Kaiser has more than 400 publications, is „highly cited researcher “and holds several honorary adjunct positions. Since 2021 she is the Physical-Sciences Editor for Micron.