Search

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO

The severe capacity fade of lithium-ion cells with silicon-dominant anodes has hindered their widescale commercialization. In this work, we link cell capacity fade to the heterogeneous physicochemical evolution of silicon anodes during battery cycling. Through a multilength scale characterization approach, we demonstrate that silicon particles near the anode surface react differently from those near the copper current collector. In particular, near the anode surface we find an amorphized wispy silicon encased in a highly fluorinated matrix of electrolyte-reduction products. In contrast, closer to the current collector, the silicon retains more of its initial morphology and structure, suggesting the presence of isolated particles. The results show that the accessibility of active silicon to lithium ions varies across the anode matrix. Material and cell designs, which minimize electrode expansion resulting from the in-filling of pores with the solid electrolyte interphase (SEI), are needed to enhance anode homogeneity during the electrochemical cycling.

Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sudden crystal slips. While large crystal slips can be easily observed on the surface of deformed crystals, unraveling the underlying dislocation interaction mechanisms, however, has been a longstanding challenge in the study of single-crystal plasticity. A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al0.1CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar. Here, we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip. Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations, secondary dislocations, and dislocation pileups. The results were compared with the previously proposed slip band models. The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.

Graphene nanoribbons are of potential use in the development of electronic and optoelectronic devices. However, the preparation of narrow and long nanoribbons with smooth edges, sizeable bandgaps and high mobilities is challenging. Here we show that sub-10-nm-wide semiconducting graphene nanoribbons with atomically smooth closed edges can be produced by squashing carbon nanotubes using a high-pressure and thermal treatment. With this approach, nanoribbons as narrow as 1.4 nm can be created, and up to 54% of single- and double-walled nanotubes in a sample can be converted into edge-closed nanoribbons. We also fabricate edge-opened nanoribbons using nitric acid as the oxidant to selectively etch the edges of the squashed nanotubes under high pressure. A field-effect transistor fabricated using a 2.8-nm-wide edge-closed nanoribbon exhibits an on/off current ratio of more than 104, from which a bandgap of around 494 meV is estimated. The device also exhibits a field-effect mobility of 2,443 cm2 V−1 s−1 and an on-state channel conductivity of 7.42 mS. Narrow, long graphene nanoribbons with atomically smooth and defect-free edges can be produced by squashing carbon nanotubes, and can be used to fabricate a sub-3-nm-wide channel field-effect transistor with a mobility of 2,443 cm2 V−1 s−1.

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe$_2$As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods., Comment: 18 pages, 5 figures, plus Supplementary Material

While there is no known fundamental lower limit to the thermal conductivity of a material, the lowest thermal conductivities are typically found in amorphous and strongly disordered materials, not ...

A superconducting interface Interfaces between materials can harbor quantum states that belong to neither of the materials. A classic example is the superconducting interface between two insulating oxides, LaAlO 3 and SrTiO 3 , with a critical temperature of around 200 millikelvin. Liu et al. observed superconductivity at a different interface—formed between KTaO 3 as a substrate and an overlayer of either EuO or LaAlO 3 —at a considerably higher temperature of about 2 kelvin. Transport measurements displayed anisotropy, which may indicate an unusual superconducting state. Science , this issue p. 716

The rapid progress in the performance of today's sophisticated transmission electron microscopes also demands computational and educational tools capable of simulating the intricacy of electron image formation. The tools ideally should be widely accessible to the microscopy community and capable of covering both the breadth and depth demanded by modern materials science. Here, a cloud-based microscopy simulation platform, called cloudEMAPS and powered by cloud computing and modern server-client web programming architecture, is described. Compared to the current desktop solutions for electron microscopy simulations, cloud computing offers the unique advantages of on-demand, data sharing, high-performance computation and internet easy access using the state-of-the-art web infrastructure. This article will demonstrate these advantages using examples of interactive simulations of electron diffraction patterns and aberration-corrected electron optics.

Magnetic multilayers are a rich class of materials systems with numerous highly tunable physical parameters that determine both their magnetic and electronic properties. Here we present a comprehensive experimental study of a novel system, Pt/Co/Mn, which extends the group of Pt/Co/X ($\mathrm{X}=$ metal) multilayers that have been investigated thus far. We demonstrate that an increasing Co layer thickness changes the magnetic anisotropy from out-of-plane to in-plane, whereas the deposition of thicker Mn layers leads to a decrease in the saturation magnetization. Temperature-dependent magnetometry measurements reinforce the hypothesis of antiferromagnetic coupling at the Co/Mn interfaces being responsible for the observed Mn thickness dependence of the magnetization reversal. Moreover, magneto-optical imaging experiments indicate systematic changes in magnetic domain patterns as a function of the Co and Mn layer thickness, suggesting the existence of bubble-like domains -- potentially even magnetic skyrmions -- in the case of sufficiently thick Mn layers, which are expected to contribute to a sizeable Dzyaloshinskii-Moriya interaction in the multilayer stacks. We identify Pt/Co/Mn as a highly complex multilayer system with strong potential for further fundamental studies and possible applications., Comment: 10 pages, 9 figures

Transmission electron diffraction is a powerful and versatile structural probe for the characterization of a broad range of materials, from nanocrystalline thin films to single crystals. With recent developments in fast electron detectors and efficient computer algorithms, it now becomes possible to collect unprecedently large datasets of diffraction patterns (DPs) and process DPs to extract crystallographic information to form images or tomograms based on crystal structural properties, giving rise to data-driven electron microscopy. Critical to this kind of imaging is the type of crystallographic information being collected, which can be achieved with a judicious choice of electron diffraction techniques, and the efficiency and accuracy of DP processing, which requires the development of new algorithms. Here, we review recent progress made in data collection, new algorithms, and automated electron DP analysis. These progresses will be highlighted using application examples in materials research. Future opportunities based on smart sampling and machine learning are also discussed., 28 pages, 14 Figures and 135 references, accepted for a special issue of Microscopy (Oxford)

One hundred years after ferroelectricity was first reported [Physical Review 17, 475 (1921)], a qualitatively different phenomenon, asymmetric ferroelectricity, is reported in atomic-scale engineered crystals. Unlike the high-temperature centrosymmetric structure that prevail in ferroelectric crystals, the crystals here break inversion symmetry by design. This is achieved by sequenced stacking of three molecular components. When these tailored crystals become hysteretic below an ordering temperature, two unequal polarization states emerge. This unusual bistability points to an asymmetric energy landscape, controlled by sample architecture.

An apparent local change in chemistry in shear-bands can be observed in a Zr-based metallic glass using correlative x-ray fluorescence and electron spectroscopy. Further analysis suggests that this finding is due to the local volumetric change of the shear-band structure. These results emphasize the importance of correlated analytical methods to unambiguously reveal structural and chemical changes caused by strain localization in metallic glasses.

An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode–electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO2 particles as a model cathode material, which have effective Mg2+ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO2 particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored ch...

β-Ga2O3, with a bandgap of ∼4.6–4.9 eV and readily available bulk substrates, has attracted tremendous interest in the wide bandgap semiconductor community. Producing high aspect ratio β-Ga2O3 3D n...

The development of longer lasting and faster charging lithium-ion batteries (LIBs) remains a critical engineering priority for the U.S. Department of Energy. To enable the next-generation technology, it is essential to understand the nature and extent of materials failure during battery operation. Under fast-charging conditions, it has been shown that graphite particles, which comprise the active material of the LIB anode, suffer irreversible material degradation that limits ultimate battery performance. This damage to the graphite manifests as turbostratic lattice disorder, severe morphological roughening, and anode swelling [1]. While these findings are essential to understand the nature of damage to the anode under fast-charge conditions, it remains to be seen whether such damage occurs at slower rates or is instead accelerated by extreme rate conditions and aging. In this regard, the recent advances in high-resolution electron microscopy (HREM) and four-dimensional scanning transmission electron microscopy (4D-STEM) methods provide the essential tools to enable nanoscopic quantification of battery degradation [2], [3]. In this work, we use advanced analytical electron microscopy and diffraction strategies to analyze nanoscopic degradation of graphite anode material under different cycling rate and aging conditions. The results of this work will demonstrate novel strategies for identifying nanoscale structural disorder that arise in battery materials as well as provide key understanding of graphite material failure to aid in the development of high-performance anode materials. We study (in post-mortem) the graphite particle cross-sections harvested from LIBs at different aging conditions for both C/2 (i.e., 2 hours to full charge) and 6C (i.e., 10 minutes to full charge) rate. At the nano- to atomic-scale, we rely primarily on aberration-corrected STEM for imaging and diffraction acquisition for each sample (see Figure 1 for the results). High resolution imaging reveals the extent of lattice disorder and amorphous character localized at graphite pore edges. The 4D-STEM data helps to further quantify the extent and distribution of nanoscale structural disorder (including permanent strain and turbostratic disorder). The experimental methods employed to acquire the datasets and the strategies employed to interpret the data to understand the nature of the resulting damage, including recent progress into Cepstral STEM, will be discussed. Additional supporting evidences of graphite disorder from Raman spectroscopy studies and rate-dependent anode swelling from scanning electron microscopy will also be presented. References : [1] S. Pidaparthy, M.-T. F. Rodrigues, J.-M. Zuo, and D. P. Abraham, “Increased Disorder at Graphite Particle Edges Revealed by Multi-length Scale Characterization of Anodes from Fast-Charged Lithium-Ion Cells,” J. Electrochem. Soc., vol. 168, no. 10, p. 100509, 2021. [2] J.-M. Zuo et al., “Data-Driven Electron Microscopy: Electron Diffraction Imaging of Materials Structural Properties,” arXiv Prepr. arXiv2110.02070, 2021. [3] E. Padgett et al., “The exit-wave power-cepstrum transform for scanning nanobeam electron diffraction: robust strain mapping at subnanometer resolution and subpicometer precision,” Ultramicroscopy, vol. 214, p. 112994, 2020. Acknowledgements: SP acknowledges support from the U. S. Department of Energy Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education for the U. S. Department of Energy under contract number DE‐SC0014664. This work was carried out in part in the Materials Research Laboratory Central Research Facilities, University of Illinois. DA and MTFR acknowledge support from DOE’s Vehicle Technologies Office. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, Na(x)MO(2) (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion–based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense Na(x)CoO(2) and Na(x)MnO(2) can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na(+) ions (∼10(−12) cm(2)⋅s(−1)), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm(−2), values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks.

Orbital is one of the primary physical parameters that determine materials’ properties. Currently, experimentally revealing the electron occupancies of orbitals under the control of external field remains a big challenge due to the stringent requirements for samples such as the atomically sharp surface or defect-free large-size single crystals. Here, we developed a method with the combination of quantitative convergent-beam electron diffraction and synchrotron powder X-ray diffraction, and demonstrated the visualization of the real-space orbital occupancy by choosing LiCoO2 as a prototype. Through multipole modelling of the accurately measured structure factors, we found the opposite changes of Co t2g and eg orbital occupancies under different electrochemical states which can be well-correlated with the CoO6 octahedra distortion. This robust method provides a feasible route to quantify the real-space orbital occupancy on small-sized particles, and opens up a new avenue for exploring the orbital origin of physical properties for functional materials.

The development of four-dimensional (4D) scanning transmission electron microscopy (STEM) using fast detectors has opened-up new avenues for addressing some of long-standing challenges in electron imaging. One of these challenges is how to image severely distorted crystal lattices, such as at a dislocation core. Here we introduce a new 4D-STEM technique, called Cepstral STEM, for imaging disordered crystals using electron diffuse scattering. Local fluctuations of diffuse scattering are captured by scanning electron nanodiffraction (SEND) using a coherent probe. The harmonic signals in electron diffuse scattering are detected through Cepstral analysis and used for imaging. By integrating Cepstral analysis with 4D-STEM, we demonstrate that information about the distortive part of electron scattering potential can be separated and imaged at nm spatial resolution. We apply our technique to the analysis of a dislocation core in SiGe and lattice distortions in high entropy alloy., Comment: 21 pages, 8 figures

High entropy alloys (HEAs) belong to a new class of materials with multiple principal components that are chemically concentrated in less explored phase spaces. Since the initial discovery, HEAs have attracted tremendous interest for their remarkable structural diversity and associated properties, including high strength and high ductility. Underlining the structural diversity is the metastability of HEAs, to which a key contributor is the lattice distortion effect that emerges as a direct consequence of interplay between atomic size misfits and chemical disorder. Lattice distortion also directly contributes to alloy strengthening and ductility. Despite the recognized significance, however, the critical knowledge of lattice distortion is still missing in the study of HEAs. Here, we first report on the nature of lattice and chemical disorder in a single-phase HEA and determine its local atomic structure. Our results uncover the manifestation of disorder at three different length scales, namely, the lattice distortion at the atomic scale, the chemical disorder at the nm scale, and the emergence of nanoscopic shear at the mesoscopic scale. The multiscale disorder leads to hierarchical strengthening, unlike anything that we know before about metals. This finding provides the structural basis for theoretical understanding of structure-property relationships in HEAs, and demonstrates the randomness of disorder as a new dimension for designing future strong and ductile alloys.