The establishment of verifiably safe nanotechnology requires the development of assessment tools to identify hazardous nanomaterial properties that could be modified to improve nanomaterial safety. While there is a lot of debate of what constitutes appropriate safety screening methods, one approach is to use the assessment of cellular injury pathways to collect knowledge about hazardous material properties that could lead to harm to humans and the environment. We demonstrate the use of a multi-parameter cytotoxicity assay that evaluates toxic oxidative stress to compare the effects of titanium dioxide (TiO2), cerium oxide (CeO2) and zinc oxide (ZnO) nanoparticles in bronchial epithelial and macrophage cell lines. The nanoparticles were chosen based on their volume of production and likelihood of spread to the environment. Among the materials, dissolution of ZnO nanoparticles and Zn2+ release were capable of ROS generation and activation of an integrated cytotoxic pathway that includes intracellular calcium flux, mitochondrial depolarization, and plasma membrane leakage. These responses were chosen based on the compatibility of the fluorescent dyes that contemporaneously assess their response characteristics by a semi-automated epifluorescence procedure. Purposeful reduction of ZnO cytotoxicity was achieved by iron doping, which changed the material matrix to slow Zn2+ release. In summary, we demonstrate the utility of a rapid throughput, integrated biological oxidative stress response pathway to perform hazard ranking of a small batch of metal oxide nanoparticles, in addition to showing how this assay can be used to improve nanosafety by decreasing ZnO dissolution through Fe doping.

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.

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Entropic effects counteract the topological protection of magnetic skyrmions, leading to faster decay rates than expected.

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...