By focusing electrons on probes with a diameter of 50 pm, aberration-corrected scanning transmission electron microscopy (STEM) is currently crossing the border to probing subatomic details. A major challenge is the measurement of atomic electric fields using differential phase contrast (DPC) microscopy, traditionally exploiting the concept of a field-induced shift of diffraction patterns. Here we present a simplified quantum theoretical interpretation of DPC. This enables us to calculate the momentum transferred to the STEM probe from diffracted intensities recorded on a pixel array instead of conventional segmented bright-field detectors. The methodical development yielding atomic electric field, charge and electron density is performed using simulations for binary GaN as an ideal model system. We then present a detailed experimental study of SrTiO3 yielding atomic electric fields, validated by comprehensive simulations. With this interpretation and upgraded instrumentation, STEM is capable of quantifying atomic electric fields and high-contrast imaging of light atoms.

We show in theory, simulation and experiment how atomic electric fields and charge densities can be measured by 4D-STEM. In this imaging mode, 2D diffraction patterns are recorded on a pixelated detector at a 2D STEM raster. In quantum mechanics, the first moment p of a diffraction pattern is related to the expectation value for the momentum transfer and provides a quantitative measure of the angular deflection of the STEM probe in electric or magnetic fields [1]. This overcomes ambiguities in conventional differential phase contrast STEM where segmented detectors record portions of the diffraction pattern and average over large angular domains [2]. Our concept is explained in figure (a), schematically showing a focused STEM probe positioned at (1) a nearly field-free region and (2) close to an atom where the projected electric field Ep is nonzero. Whereas the propagation of the wave in case (1) is preserved as illustrated by the wave fronts within the STEM illumination cone, the interaction with the electric field Ep for situation (2) causes both a distorted wave front and a deflection to the right. Assuming a Ga column in a GaN crystal with 1.3 nm thickness, we simulated the diffraction patterns (Ronchigrams) on the right and determined the first moments p as indicated. This demonstrates how the complexity of the Ronchigram condenses to a single vector with fundamental physical meaning. Due to Ehrenfest's theorem, p is proportional to the expectation value of the electric field. For sufficiently thin specimens, we found that p is also proportional to the projection of the electric field Ep, convolved with the incident probe intensity. Furthermore, its divergence directly yields the projected charge density, convolved with the probe intensity [1,3]. An early 4D STEM experiment for Strontium Titanate is shown in figure (b), where a slow-scan CCD camera was employed to raster a unit cell with 20x20 pixels. The redistributing Ronchigram intensity in the vicinity of the atomic columns is clearly seen on the left. Determining the first moments and subsequently the electric field, we obtain the atomically resolved electric field map on the right. As expected from the screened nuclear charge, atoms appear as sources of the electric field, their magnitude being determined by the atomic number. We then present recent 4D-STEM results on 2D sheets of MoS2 employing an ultrafast camera with 4kHz frame rate. Figure (c) depicts the (projected) charge density measured at a mono-/bilayer (ML/BL) edge with unit cell averages for both the ML and BL region. By comparison with DFT and 4D STEM simulations we show that the data agrees with theory quantitatively. In particular, we find a 2R-like stacking of the BL and a Mo-terminated ML/BL edge, which is discussed as to potential optical and catalytic properties. With the ability to map atomic electric fields and charge densities directly without structural input, aberration-corrected 4D-STEM can shed light on the electrical configuration of vacancies, dopant atoms, bonding or polari...

Electron vortex beams were only recently discovered and their potential as a probe for magnetism in materials was shown. Here we demonstrate a new method to produce electron vortex beams with a diameter of less than 1.2 \AA. This unique way to prepare free electrons to a state resembling atomic orbitals is fascinating from a fundamental physics point of view and opens the road for magnetic mapping with atomic resolution in an electron microscope

Landau levels and states of electrons in a magnetic field are fundamental quantum entities underlying the quantum Hall and related effects in condensed matter physics. However, the real-space properties and observation of Landau wave functions remain elusive. Here we report the real-space observation of Landau states and the internal rotational dynamics of free electrons. States with different quantum numbers are produced using nanometre-sized electron vortex beams, with a radius chosen to match the waist of the Landau states, in a quasi-uniform magnetic field. Scanning the beams along the propagation direction, we reconstruct the rotational dynamics of the Landau wave functions with angular frequency ~100 GHz. We observe that Landau modes with different azimuthal quantum numbers belong to three classes, which are characterized by rotations with zero, Larmor and cyclotron frequencies, respectively. This is in sharp contrast to the uniform cyclotron rotation of classical electrons, and in perfect agreement with recent theoretical predictions.

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We discuss the feasibility of detecting magnetic transitions with focused electron vortex probes, suggested by selection rules for the magnetic quantum number. We theoretically estimate the dichroic signal strength in the L2,3 edge of ferromagnetic d metals. It is shown that under realistic conditions, the dichroic signal is undetectable for nanoparticles larger than ∼10.25emnm. This is confirmed by a key experiment with nanometer-sized vortices.

In heterogeneous catalysis, surfaces decorated with uniformly dispersed, catalytically-active (nano)particles are a key requirement for excellent performance. Beside standard catalyst preparation routines—with limitations in controlling catalyst surface structure (i.e., particle size distribution or dispersion)—we present here a novel time efficient route to precisely tailor catalyst surface morphology and composition of perovskites. Perovskite-type oxides of nominal composition ABO3 with transition metal cations on the B-site can exsolve the B-site transition metal upon controlled reduction. In this exsolution process, the transition metal emerges from the oxide lattice and migrates to the surface where it forms catalytically active nanoparticles. Doping the B-site with reducible and catalytically highly active elements, offers the opportunity of tailoring properties of exsolution catalysts. Here, we present the synthesis of two novel perovskite catalysts Nd0.6Ca0.4FeO3-δ and Nd0.6Ca0.4Fe0.9Co0.1O3-δ with characterisation by (in situ) XRD, SEM/TEM and XPS, supported by theory (DFT+U). Fe nanoparticle formation was observed for Nd0.6Ca0.4FeO3-δ. In comparison, B site cobalt doping leads, already at lower reduction temperatures, to formation of finely dispersed Co nanoparticles on the surface. These novel perovskite-type catalysts are highly promising for applications in chemical energy conversion. First measurements revealed that exsolved Co nanoparticles significantly improve the catalytic activity for CO2 activation via reverse water gas shift reaction.

The propagation of electron beams carrying angular momentum in crystals is studied using a multislice approach for the model system Fe. It is found that the vortex beam is distorted strongly due to elastic scattering. Consequently, the expectation value of the angular momentum as well as the local vortex components change with the initial position of the vortex and the propagation depth, making numerical simulations indispensable when analyzing experiments.