Advanced processing of differential phase contrast data: Distinction between different causes of electron phase shifts.

Differential phase contrast, in its high resolution modification also known as first moment microscopy or momentum resolved STEM [1-7] , basically measures the lateral momentum transfer to the electron probe due to the beam interaction with either electrostatic and/or magnetic fields, when the probe transmits the specimen. In other words, the result of the measurement is a vector field p→(x,y) which describes the lateral momentum transfer to the probe electrons. In the case of electric fields, this momentum transfer is easily converted to the electric field E→(x,y) causing the deflection, and from ϱ=ɛ0∇⋅E→ the local charge density can be calculated from the divergence of the electric field. However, from experimental data it is known that also the calculation of the vector field's curl ∇→×p→ in general yields non-zero results. In this paper, we use the Helmholtz decomposition (Wikipedia contributors, 2022), also known as the fundamental theorem of vector calculus, to split the measured vector fields into their curl-free and divergence-free components and to interpret the physical meaning of these components in detail. It will be shown, that non-zero curl components may be used to measure geometric phases occurring from irregularities in crystal structure such as a screw dislocation.

The differential phase contrast uncertainty relation: Connection between electron dose and field resolution.

Differential phase contrast (DPC) microscopy is a STEM imaging technique, which is used to measure magnetic and electric fields of mesoscopic and nanoscopic dimensions, i.e. interatomic distances (Chapman et al. 1978; Chapman et al. 1981; Chapman, 1984; Chapman et al. 1985; Chapman et al. 1997; Lohr et al. 2012; Shibata et al. 2015; Bauer et al. 2014; Carvalho et al. 2016; Lohr et al. 2016; Mueller-Caspary et al. 2019a,2019b; Mueller-Caspary et al. 2018; Mueller-Caspary et al. 2017; Mueller-Caspary et al. 2014; Winkler et al. 2020; Toyama et al. 2020). In this paper we will demonstrate that the electron dose per pixel deposited on the specimen is decisive to the precision and resolution of measurements of a field's local strength. Relations are given which connect a given electron dose per pixel to the fundamentally achievable precision to which the specimen's interaction with the electrons may be determined, taking into account quantum mechanical considerations. Vice versa, given a certain required precision, the required dose per pixel can be easily predicted for reliable measurements of a desired property. First, these relations are given for the case of a continuous, i.e. non-pixelated, detector followed by simulations which show that the same relations hold for pixelated detectors. Then, the achievable precision for detectors with different pixel counts in combination with different camera lengths is discussed and the maximum measurable field amplitude per set-up is determined. Finally, the effect of inhomogeneities within the diffraction disk is discussed and possible deviations from the derived relations are considered. We also demonstrate that Heisenberg's uncertainty relation determines the possible field resolution in differential phase contrast microscopy, and that the achievable local field resolution is a function of the applied electron dose per pixel.

Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst.

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z-selective semihydrogenation of alkynes under near ambient conditions (1-3 bar H2 , 30 °C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X-ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts.

Introducing a non-pixelated and fast centre of mass detector for differential phase contrast microscopy.

With the advent of probe corrected STEM machines it became possible to probe specimens on a scale of less than 50 pm resolution. This opens completely new horizons for research, as it is e.g. possible to probe the electrostatic fields between individual rows of atoms, using differential phase contrast (DPC). However, in contrast to conventional DPC, where one deals with extended fields which can be assumed constant across the electron probe, this is not possible for sub-atomic probes in DPC. For the latter case it was shown [1,2], that the strongly inhomogeneous field distribution within the probe diameter, which usually is caused by the nuclear potentials of an atomic column, leads to a complicated intensity redistribution within the diffraction disk. The task is then to determine the intensity weighted centre of the diffraction disk pattern (frequently also called centre of mass, COM), which is proportional to the average lateral momentum gained by the average electron, transmitted through the probe diameter. In first reported measurements, the determination of this COM was achieved using a pixelated detector in combination with a software-based evaluation of the COM. This suffers from two disadvantages: first, the nowadays available pixelated detectors are still not very fast (approximately 1000 fps) and quite expensive, and second, the amount of data to be processed after acquisition is comparatively huge. In this paper we report on an alternative to a pixelated detector, which is able to directly deliver the COM of a diffraction disk's intensity distribution with frequencies up to 200 kHz. We present measurements on the sensitivity of this detector as well as first results from DPC imaging. From these results we expect the detector also to serve well in sub-atomic DPC field sensing, possibly replacing today's segmented or pixelated detectors.

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations.

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr2 with LiEt3 BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol % cat., 2-10 bar H2 , 20-80 °C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism.

Entropy-limited topological protection of skyrmions.

Magnetic skyrmions are topologically protected whirls that decay through singular magnetic configurations known as Bloch points. We used Lorentz transmission electron microscopy to infer the energetics associated with the topological decay of magnetic skyrmions far from equilibrium in the chiral magnet Fe1-x Co x Si. We observed that the lifetime τ of the skyrmions depends exponentially on temperature, [Formula: see text]. The prefactor τ0 of this Arrhenius law changes by more than 30 orders of magnitude for small changes of the magnetic field, reflecting a substantial reduction of the lifetime of skyrmions by entropic effects and, thus, an extreme case of enthalpy-entropy compensation. Such compensation effects, being well known across many different scientific disciplines, affect topological transitions and, thus, topological protection on an unprecedented level.