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...

The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material LiTiO (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4-9 nm, attributed to two reasons: (1) stabilization of water sensitive metal alkoxides precursor and (2) formation of volatile carboxylates from lithium nitrate evidenced by attenuated total reflection Fourier transform infrared spectroscopy and single droplet combustion experiments. In contrast, the absence of carboxylic acids resulted in larger inhomogeneous crystalline titanium dioxide (TiO) particles with significant reduction of LTO content as low as ∼34 mass %. In-depth particle characterization was performed using X-ray diffraction with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, gas adsorption, and vibrational spectroscopy. High-resolution transmission electron microscopy of the LTO product revealed excellent quality of the particles obtained at high temperature. In addition, high rate capability and efficient charge reversibility of LTO nanoparticles demonstrate the vast potential of inexpensive gas-phase synthesis for energy-storage materials.

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We present a synthesis route to fabricate submicrometer-sized colloidosomes at ambient conditions and mild pH with tailorable nanopore sizes and porosity. The capsules are formed via self-assembly of metal oxide nanoparticles on emulsion droplets in a water-in-oil emulsion. Through the adsorption of oil-soluble surfactants on emulsion droplets, which carry the same sign of charge as the colloidosome-forming particles in aqueous media, colloidosomes with positive and negative zeta potentials were synthesized. The hollow capsules are inherently rigid and necessitated no further stabilization. By varying the sizes and shapes of the nanoparticles, we were able to tailor the pore diameters and pore size distributions on the surface of the capsules. Particles of high spherical uniformity with narrow size range generated colloidosomes with a mainly hexagonal close packed surface structure and narrow pore size, while particles of elliptical and uneven shape created colloidosomes with no surface order and an inconsistent structure. Synthesizing colloidosomes on the submicrometer scale and tailoring the pore shapes and diameters is a crucial step toward their application as a versatile encapsulation and release platform of active agents in the field of life sciences.

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