Cory Czarnik

Facet development during platinum nanocube growth

Watching platinum nanocube growth Size and shape drive the properties of metal nanoparticles. Understanding the factors that affect their growth is central to making use of the particles in a range of applications. Liao et al. tracked the growth of platinum nanoparticle shapes at high resolution using state-of-the-art liquid cells for in situ monitoring inside an electron microscope. The authors tracked changes in the growth rates at different crystal facets caused by differences in the mobility of the capping ligand. Science, this issue p. 916 Observation of atomic facet development during platinum nanocube growth reveals shape control. An understanding of how facets of a nanocrystal develop is critical for controlling nanocrystal shape and designing novel functional materials. However, the atomic pathways of nanocrystal facet development are mostly unknown because of the lack of direct observation. We report the imaging of platinum nanocube growth in a liquid cell using transmission electron microscopy with high spatial and temporal resolution. The growth rates of all low index facets are similar until the {100} facets stop growth. The continuous growth of the rest facets leads to a nanocube. Our calculation shows that the much lower ligand mobility on the {100} facets is responsible for the arresting of {100} growing facets. These findings shed light on nanocrystal shape-control mechanisms and future design of nanomaterials.

Unravelling surface and interfacial structures of a metal-organic framework by transmission electron microscopy

Metal-organic frameworks (MOFs) are crystalline porous materials with designable topology, porosity and functionality, having promising applications in gas storage and separation, ion conduction and catalysis. It is challenging to observe MOFs with transmission electron microscopy (TEM) due to the extreme instability of MOFs upon electron beam irradiation. Here, we use a direct-detection electron-counting camera to acquire TEM images of the MOF ZIF-8 with an ultralow dose of 4.1 electrons per square ångström to retain the structural integrity. The obtained image involves structural information transferred up to 2.1 Å, allowing the resolution of individual atomic columns of Zn and organic linkers in the framework. Furthermore, TEM reveals important local structural features of ZIF-8 crystals that cannot be identified by diffraction techniques, including armchair-type surface terminations and coherent interfaces between assembled crystals. These observations allow us to understand how ZIF-8 crystals self-assemble and the subsequent influence of interfacial cavities on mass transport of guest molecules.

Efficient linear phase contrast in scanning transmission electron microscopy with matched illumination and detector interferometry

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

Recording and Using 4D-STEM Datasets in Materials Science

Traditional scanning transmission electron microscopy (STEM) detectors are monolithic and integrate a subset of the transmitted electron beam signal scattered from each electron probe position, shown schematically in Figure 1. These convergent beam electron diffraction patterns (CBED) are extremely rich in information, containing localized information on sample structure [1], composition [2], phonon spectra [3], three-dimensional defect crystallography [4] and more. Many new imaging modes become possible if the full CBED pattern is recorded at many probe positions with millisecond dwell times. Such a four-dimensional dataset would be comprised of a 2D CBED pattern at each point in a 2D STEM raster, hence the name 4D-STEM.

In Situ Study of Fe3Pt–Fe2O3 Core–Shell Nanoparticle Formation

We report an in situ study of Fe3Pt-Fe2O3 core-shell nanoparticle growth using liquid cell transmission electron microscopy. By controlling the Fe-to-Pt ratio in the precursor solution, we achieved the growth of nanoparticles with the formation of an iron-platinum alloy core followed by an iron oxide shell in the electron beam-induced reactions. There was no substantial change in the growth kinetics of the iron oxide shell after the Fe-Pt alloy core stopped growing. The core growth was arrested by depletion of the Pt precursor. Heteroepitaxy of Fe3Pt [101] (core)||α-Fe2O3 [111] (shell) was observed in most of the nanoparticles, while a polycrystalline iron oxide shell is developed eventually for strain relaxation. Our studies suggest that Pt atoms catalyze the reduction of Fe ions to form the Fe3Pt alloy core, and when Pt is depleted, a direct precipitation of iron oxide results in the core-shell nanostructure formation.

Phase Contrast Imaging of Weakly-Scattering Samples with Matched Illumination and Detector Interferometry–Scanning Transmission Electron Microscopy (MIDI–STEM)

Aberration-corrected scanning transmission electron microscopy (STEM) has made an enormous impact on materials science, with recent examples including observations of nanometer-scale polar vortices in oxide superlattices [1], atomic-scale chemical imaging [2], atomic-resolution 3D tomography [3], and many others. However, the majority of these studies image chemical species with intermediate or high atomic numbers. Samples composed primarily of low atomic number species are more difficult to study in STEM due to their poor scattering efficiency. This is why the vast majority of biological research in electron microscopy (EM) uses phase contrast plane-wave methods, including CTF-corrected cryo-EM [4] phase-plate HRTEM [5], etc. Phase contrast methods can also be used in STEM to improve contrast in weakly-scattering samples, such as ptychography [6, 7]. These methods improve contrast, but require significantly more involved processing of the experimental data.

High Resolution Observations of Interface Dynamics Using a Direct Electron Detection Camera

Atomic-scale mechanisms of interface motion are of key interest for processes such as grain growth, deformation and phase transformations. The direct observation of such mechanisms by transmission electron microscopy is of great importance for our understanding and predictive modeling of the behavior of new materials. Recent advances in instrumentation have made it possible to extend the range of spatial and temporal resolution of such observations [1].