Scanning Nanobeam Electron Diffraction Research Articles

Strain Relaxation and Activity in PtCo Fuel Cell Catalyst Nanoparticles

Interest is growing in applying fuel cells to heavy duty transportation applications, which requires the fuel cell design to prioritize efficiency and durability to provide a low lifetime cost of ownership. This motivates further development of catalysts for the oxygen reduction reaction (ORR), as ORR kinetics are a key factor in fuel cell efficiency and catalyst degradation is a major factor limiting lifetime. Pt-alloy catalysts have seen success in light-duty fuel cell applications, where they provide enhanced activity believed to originate (at least in part) from strain of the active Pt surface by the underlying Pt-alloy core. Concerns remain regarding the viability of PtCo catalysts for heavy duty applications, as they may not sustain their activity advantage over extended lifetimes and leaching of the base metal can lead to additional performance degradation. These concerns motivate investigation into the fundamental mechanisms of strain-induced activity enhancements to guide further catalyst development and understand the ultimate limits for the performance and stability of PtCo catalysts. However, practical Pt-alloy fuel cell catalysts are complex, and understanding of real strain effects in these systems remains elusive.Here we will present an investigation into strain effects in Pt-Co ORR catalysts for fuel cells, combining scanning nanobeam electron diffraction (NBED), continuum elastic strain modelling, and full-cell electrochemistry measurements. We leverage recent developments in NBED to make high-throughput strain maps in heterogeneous, carbon-supported PtCo catalysts and identify different mechanisms of strain relaxation, including dislocations and geometric effects. Supported by continuum elastic modelling, we correlate shell thickness, strain state, and activity of PtCo catalysts. Finally, we will comment on how these results provide rational guidance to optimize stable, efficient Pt-alloy catalysts.

Reconstruction of three-dimensional strain field in an asymmetrical curved core–shell hetero-nanowire

Crystal orientation and strain mapping of an individual curved and asymmetrical core–shell hetero-nanowire (NW) is performed based on transmission electron microscopy. It relies on a comprehensive analysis of scanning nanobeam electron diffraction data obtained for 1.3 nm electron probe size. The proposed approach also handles the problem of appearing twinning defects on diffraction patterns and allows for the investigation of materials with high defect densities. Based on the experimental maps and their comparison with finite element simulations, the entire core–shell geometry including full three-dimensional strain distribution within the curved core–shell NW are obtained. Our approach represents, therefore, a low-dose quasi-tomography of the strain field within a nanoobject using only a single zone axis diffraction experiment. Our approach is applicable also for electron beam-sensitive materials for which performing conventional tomography is a difficult task.

Mapping Polar Distortions using Nanobeam Electron Diffraction and a Cepstral Approach.

Measuring local polar ordering is key to understanding ferroelectricity in thin films, especially for systems with small domains or significant disorder. Scanning nanobeam electron diffraction (NBED) provides an effective local probe of lattice parameters, local fields, polarization directions, and charge densities, which can be analyzed using a relatively low beam dose over large fields of view. However, quantitatively extracting the magnitudes and directions of polarization vectors from NBED remains challenging. Here, we use a cepstral approach, similar to a pair distribution function, to determine local polar displacements that drive ferroelectricity from NBED patterns. Because polar distortions generate asymmetry in the diffraction pattern intensity, we can efficiently recover the underlying displacements from the imaginary part of the cepstrum transform. We investigate the limits of this technique using analytical and simulated data and give experimental examples, achieving the order of 1.1 pm precision and mapping of polar displacements with nanometer resolution.

Observation of Ultrathin Sc-Containing Surface Layer on Life-Tested Scandate Cathodes

To explain enhanced levels of electron emission from scandate cathodes, models invoking surface layers ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 100 nm thick) of Ba-Sc-O, or alternatively a Ba-Sc-O monolayer, have been frequently employed to describe the lowered work function that is thought to lead to these improved emission capabilities. However, limited direct experimental proof has been provided, primarily involving Auger electron spectroscopy (AES) for depth profile analyses of elemental composition. In this article, we report direct observations of an Sc-containing surface layer on scandate cathodes, as indicated by independent, complementary measurements using scanning nanobeam AES and electron energy loss spectroscopy (EELS) techniques. The ultrathin Sc-containing layer on these cathodes is no more than 3–4 nm thick and is likely even thinner than this. The surface layer may consist of amorphous or nanoscale polycrystalline materials, as suggested by scanning nanobeam electron diffraction and analytical mapping.

Structural Anisotropy Governs the Kink Formation in Cellulose Nanocrystals

Understanding the defect structure is fundamental to correlating the structure and properties of materials. However, little is known about the defects of soft matter at the nanoscale beyond their external morphology. We report here on the molecular-level structural details of kink defects of cellulose nanocrystals (CNCs) based on a combination of experimental and theoretical methods. Low-dose scanning nanobeam electron diffraction analysis allowed for correlation of the local crystallographic information and nanoscale morphology and revealed that the structural anisotropy governed the kink formation of CNCs. We identified two bending modes along different crystallographic directions with distinct disordered structures at kink points. The drying strongly affected the external morphology of the kinks, resulting in underestimating the kink population in the standard dry observation conditions. These detailed defect analyses improve our understanding of the structural heterogeneity of nanocelluloses and contribute to the future exploitation of soft matter defects.

Nanobeam electron diffraction strain mapping in monocrystalline silicon of modern trench power MOSFETs

Despite a decisive influence of residual strains in Si on its electronic properties and mechanical stability, strain distributions in vertical power transistors are still not fully investigated and understood. In this work, transmission electron microscopy (TEM) nanobeam electron diffraction (NBED) and finite element (FE) modeling were applied to reveal cross-sectional residual strain distributions in monocrystalline Si of a modern trench power MOSFET. Scanning NBED was realized in a region of interest with lateral and vertical dimensions of 0.69 μm and 1.38 μm, respectively, with a spatial resolution of 5.4 nm and a high precision better than 0.05%. The acquired results are interpreted in terms of device structure as well as process flow and are compared with the FE simulation. Complex strain distributions for the vertical, lateral and shear strain components are shown, which are not only dependent on structure and geometry but also influenced by the doping profile in Si. The comparison with the FE strain simulation shows good agreement especially for the vertical and shear strain components. However, it also reveals a limitation of the FE strain simulation with regard to the influence of doping on the Si lattice strain.

Investigation of Graphite Electrode Degradation in Lithium-Ion Cells Using 4D-STEM

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

Quantitative Strain and Topography Mapping of 2D Materials Using Nanobeam Electron Diffraction.

It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe2 monolayer together with a discussion of the accuracy of the method is provided.

Precise strain mapping of nano-twinned axial ZnTe/CdTe hetero-nanowires by scanning nanobeam electron diffraction

The occurrence of strain is inevitable for the growth of lattice mismatched heterostructures. It affects greatly the mechanical, electrical and optical properties of nano-objects. It is also the case for nanowires which are characterized by a high surface to volume ratio. Thus, the knowledge of the strain distribution in nano-objects is critically important for their implementation into devices. This paper presents an experimental data for II-VI semiconductor system. Scanning nanobeam electron diffraction strain mapping technique for hetero-nanowires characterized by a large lattice mismatch (>6% in the case of CdTe/ZnTe) and containing segments with nano-twins has been described. The spatial resolution of about 2 nm is 10 times better than obtained in synchrotron nanobeam systems. The proposed approach allows us to overcome the difficulties related to nanowire thickness variations during the acquisition of the nano-beam electron diffraction data. In addition, the choice of optimal parameters used for the acquisition of nano-beam diffraction data for strain mapping has been discussed. The knowledge of the strain distribution enables, in our particular case, the improvement of the growth model of extremely strained axial nanowires synthetized by vapor-liquid solid growth mechanism. However, our method can be applied for the strain mapping in nanowire heterostructures grown by any other method.

Fast Grain Mapping with Sub-Nanometer Resolution Using 4D-STEM with Grain Classification by Principal Component Analysis and Non-Negative Matrix Factorization.

High-throughput grain mapping with sub-nanometer spatial resolution is demonstrated using scanning nanobeam electron diffraction (also known as 4D scanning transmission electron microscopy, or 4D-STEM) combined with high-speed direct-electron detection. An electron probe size down to 0.5 nm in diameter is used and the sample investigated is a gold–palladium nanoparticle catalyst. Computational analysis of the 4D-STEM data sets is performed using a disk registration algorithm to identify the diffraction peaks followed by feature learning to map the individual grains. Two unsupervised feature learning techniques are compared: principal component analysis (PCA) and non-negative matrix factorization (NNMF). The characteristics of the PCA versus NNMF output are compared and the potential of the 4D-STEM approach for statistical analysis of grain orientations at high spatial resolution is discussed.

Investigation of the orientation relationship between nano-sized G-phase precipitates and austenite with scanning nano-beam electron diffraction using a pixelated detector

Scanning nano-beam electron diffraction with a pixelated detector was employed to investigate the orientation relationship of nanometer sized, irradiation induced G-phase (M6Ni16Si7) precipitates in an austenite matrix. Using this detector, the faint diffraction spots originating from the small G-phase particles could be resolved simultaneously as the intense matrix reflections. The diffraction patterns were analyzed using a two-stage template matching scheme, whereby the matrix is indexed first and the precipitates are indexed second after subtraction of the matrix contribution to the diffraction patterns. The results show that G-phase forms with orientation relationships relative to austenite that are characteristic of face-centered cubic (FCC) to body-centered cubic (BCC) transformations. This work demonstrates that nano-beam electron diffraction with a pixelated detector is a promising technique to investigate orientation relationships of nano-sized precipitates with complex crystal structures in other material systems with relative ease.

Characterizing transition-metal dichalcogenide thin-films using hyperspectral imaging and machine learning

Atomically thin polycrystalline transition-metal dichalcogenides (TMDs) are relevant to both fundamental science investigation and applications. TMD thin-films present uniquely difficult challenges to effective nanoscale crystalline characterization. Here we present a method to quickly characterize the nanocrystalline grain structure and texture of monolayer WS2 films using scanning nanobeam electron diffraction coupled with multivariate statistical analysis of the resulting data. Our analysis pipeline is highly generalizable and is a useful alternative to the time consuming, complex, and system-dependent methodology traditionally used to analyze spatially resolved electron diffraction measurements.

The exit-wave power-cepstrum transform for scanning nanobeam electron diffraction: robust strain mapping at subnanometer resolution and subpicometer precision

Scanning nanobeam electron diffraction (NBED) with fast pixelated detectors is a valuable technique for rapid, spatially resolved mapping of lattice structure over a wide range of length scales. However, intensity variations caused by dynamical diffraction and sample mistilts can hinder the measurement of diffracted disk centers as necessary for quantification. Robust data processing techniques are needed to provide accurate and precise measurements for complex samples and non-ideal conditions. Here we present an approach to address these challenges using a transform, called the exit wave power cepstrum (EWPC), inspired by cepstral analysis in audio signal processing. The EWPC transforms NBED patterns into real-space patterns with sharp peaks corresponding to inter-atomic spacings. We describe a simple analytical model for interpretation of these patterns that cleanly decouples lattice information from the intensity variations in NBED patterns caused by tilt and thickness. By tracking the inter-atomic spacing peaks in EWPC patterns, strain mapping is demonstrated for two practical applications: mapping of ferroelectric domains in epitaxially strained PbTiO3 films and mapping of strain profiles in arbitrarily oriented core-shell Pt-Co nanoparticle fuel-cell catalysts. The EWPC transform enables lattice structure measurement at sub-pm precision and sub-nm resolution that is robust to small sample mistilts and random orientations.

Atomic structures determined from digitally defined nanocrystalline regions.

Nanocrystallography has transformed our ability to interrogate the atomic structures of proteins, peptides, organic molecules and materials. By probing atomic level details in ordered sub-10 nm regions of nanocrystals, scanning nanobeam electron diffraction extends the reach of nanocrystallography and in principle obviates the need for diffraction from large portions of one or more crystals. Scanning nanobeam electron diffraction is now applied to determine atomic structures from digitally defined regions of beam-sensitive peptide nanocrystals. Using a direct electron detector, thousands of sparse diffraction patterns over multiple orientations of a given crystal are recorded. Each pattern is assigned to a specific location on a single nanocrystal with axial, lateral and angular coordinates. This approach yields a collection of patterns that represent a tilt series across an angular wedge of reciprocal space: a scanning nanobeam diffraction tomogram. Using this diffraction tomogram, intensities can be digitally extracted from any desired region of a scan in real or diffraction space, exclusive of all other scanned points. Intensities from multiple regions of a crystal or from multiple crystals can be merged to increase data completeness and mitigate missing wedges. It is demonstrated that merged intensities from digitally defined regions of two crystals of a segment from the OsPYL/RCAR5 protein produce fragment-based ab initio solutions that can be refined to atomic resolution, analogous to structures determined by selected-area electron diffraction. In allowing atomic structures to now be determined from digitally outlined regions of a nanocrystal, scanning nanobeam diffraction tomography breaks new ground in nanocrystallography.

Local Strain Analysis using Scanning Nanobeam Electron Diffraction during In Situ TEM Nanomechanical Testing

Local Strain Analysis using Scanning Nanobeam Electron Diffraction during In Situ TEM Nanomechanical Testing

Direct Observation of SRO effect of Ti-6Al Alloy Using Energy-filtered TEM and Scanning Nanobeam Electron Diffraction

Direct Observation of SRO effect of Ti-6Al Alloy Using Energy-filtered TEM and Scanning Nanobeam Electron Diffraction

Local nanoscale strain mapping of a metallic glass during in situ testing

The local elastic strains during tensile deformation in a CuZrAlAg metallic glass are obtained by fitting an elliptic shape function to the characteristic amorphous ring in electron diffraction patterns. Scanning nanobeam electron diffraction enables strain mapping with a resolution of a few nanometers. Here, a fast direct electron detector is used to acquire the diffraction patterns at a sufficient speed to map the local transient strain during continuous tensile loading in situ in the transmission electron microscope. The elastic strain in tensile direction was found to increase during loading. After catastrophic fracture, a residual elastic strain that relaxes over time was observed.

Optimizing disk registration algorithms for nanobeam electron diffraction strain mapping

Scanning nanobeam electron diffraction strain mapping is a technique by which the positions of diffracted disks sampled at the nanoscale over a crystalline sample can be used to reconstruct a strain map over a large area. However, it is important that the disk positions are measured accurately, as their positions relative to a reference are directly used to calculate strain. In this study, we compare several correlation methods using both simulated and experimental data in order to directly probe susceptibility to measurement error due to non-uniform diffracted disk illumination structure. We found that prefiltering the diffraction patterns with a Sobel filter before performing cross correlation or performing a square-root magnitude weighted phase correlation returned the best results when inner disk structure was present. We have tested these methods both on simulated datasets, and experimental data from unstrained silicon as well as a twin grain boundary in 304 stainless steel.

Orientation relationships of Mn0.75Ga0.25As crystallites on and within GaAs determined by scanning nano beam electron diffraction

Mn0.75Ga0.25As crystallites, partially embedded on and fully embedded within a single crystalline matrix of GaAs formed during metal organic vapor phase epitaxy (MOVPE) of Mn‐rich (Mn,Ga)As on (001) oriented GaAs wafers. Phase and orientation analysis of these crystallites were performed with scanning nano beam electron diffraction (SNBED). The investigation of plan‐view specimens using a liquid nitrogen cooling stage enabled the phase and orientation analysis of partially embedded ferromagnetic α‐phase particles. In all specimens the following two orientation relationships (O) between the α‐phase particles and the GaAs matrix were determined: urn:x-wiley:02321300:media:crat201600261:crat201600261-math-0001urn:x-wiley:02321300:media:crat201600261:crat201600261-math-0002The study of cross‐sectional specimens enabled the analysis of fully embedded crystallites. It could be unambiguously detected that a fully embedded crystallite has the structure of the atomically ordered monoclinic β´ phase. The β´ phase crystallite consists of two domains which are related in twin positions to each other. The orientation relations of the different particles are illustrated by color coded stereographic projections.

In‐depth structural and chemical characterization of engineered TiO2 films

Analytical routines for a comprehensive in‐depth morphological, structural, and chemical characterization of functionalized TiO2 films by using different state‐of‐the‐art analytical techniques are presented and discussed with the main objective to identify potential reference TiO2 coating parameters able to be certified at a later stage. TiO2 films fabricated by two different synthetic procedures as representative for two main large‐scale applications were selected: (i) pulsed d.c. magnetron sputtering for photocatalytic applications and (ii) screen printing from preformed anatase nanoparticles. The screen‐printed films were further loaded with a sensitizing dye for application as a dye‐sensitized solar cell. Film properties such as microstructure and crystallographic texture of pulsed d.c. magnetron sputtering synthesized films were systematically studied by means of scanning nanobeam electron diffraction in a transmission electron microscope and the surface and inner morphology by scanning electron microscopy. The dye distribution over the depth of screen‐printed TiO2 layers was analyzed before and after dye‐loading by means of energy dispersive X‐ray spectroscopy at scanning electron microscope, Auger electron spectroscopy and time‐of‐flight secondary ion mass spectrometry. The long‐term goal of the present study is the improvement of quality of the TiO2 film parameters as measured by using different types of reference TiO2 coatings having specific parameters certified. Copyright © 2016 John Wiley &amp; Sons, Ltd.