Beyond structure and composition: Multidimensional TEM as a key for imaging electric fields, 3D shape and soft matter

In recent years, the dimensionality in transmission electron microscopy (TEM) has increased rapidly by the advent of ultrafast cameras that record at frame rates of many kHz. This development has especially paved the way for a revolution as to the versatility of scanning TEM (STEM). In particular, momentum-resolved STEM enhanced traditional Z- and phase-contrast techniques such that any conventional imaging mode is present simultaneously in a 4D data set. Most importantly, the combination of real- and reciprocal space information nowadays allows to quantify electric fields, charge densities and potentials with subatomic resolution, to measure polarisation-induced electric fields, and to solve the phase problem by ptychographic techniques.
In this presentation, a brief review of quantitative STEM is given, followed by selected works on ultrafast detectors. We demonstrate the capability of 4D-STEM using several examples such as the mapping of atomic electric fields in 2D materials and ptychographic reconstructions using different algorithms. Moreover, we report the recent development of focal series 4D STEM yielding insights into the scattering dynamics and 3D shape of specimen. The talk closes with prospects on ultrahigh time resolution for imaging (magnetic) dynamics at GHz frequencies, and opportunities for high-contrast imaging of soft matter at low doses, building a bridge between materials science methodology and life science challenges.


Biosketch
Knut Müller-Caspary received his Ph.D from Bremen University (Germany) in 2011. Between 2011 and 2016 he worked as a postdoctoral research fellow in Bremen focussing on strain, composition and electric field mapping by momentum-resolved STEM. He established cooperations with several companies to explore new detectors as to speed, dynamic range, efficiency and in-situ capability. In particular, he contributed key developments to the mapping of atomic electric fields and charge densities at subatomic scale by exploiting the full complexity of STEM diffraction patterns. In 2016 Müller-Caspary moved to the EMAT institute at the University of Antwerp (Belgium) where he applied STEM to the electrical characterisation of 2D materials. In 2018 he established a Helmholtz Young Investigator Group for momentum-resolved STEM at Forschungszentrum Jülich (Germany) and became junior professor at RWTH Aachen University in 2019. In 2021, Knut Müller-Caspary moved to the faculty of chemistry and pharmacy at Ludwig-Maximilians-University Munich as university professor for TEM.