Unveiling Core-Shell Structure Formation in a Ni3Fe Nanoparticle with In Situ Multi-Bragg Coherent Diffraction Imaging.

Solid-state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e., its morphology (facets) as well as its deformation field, has been followed by applying multireflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600 °C) under an Ar atmosphere. This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a difference of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell due to preferential iron oxidation. Providing in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study─with the support of atomistic simulations─not only showcases the power of in situ multireflection BCDI but also provides valuable insights into the mechanisms at work during a solid-state reaction characterized by a core-shell transition.

Imaging the strain evolution of a platinum nanoparticle under electrochemical control.

Surface strain is widely employed in gas phase catalysis and electrocatalysis to control the binding energies of adsorbates on active sites. However, in situ or operando strain measurements are experimentally challenging, especially on nanomaterials. Here we exploit coherent diffraction at the new fourth-generation Extremely Brilliant Source of the European Synchrotron Radiation Facility to map and quantify strain within individual Pt catalyst nanoparticles under electrochemical control. Three-dimensional nanoresolution strain microscopy, together with density functional theory and atomistic simulations, show evidence of heterogeneous and potential-dependent strain distribution between highly coordinated ({100} and {111} facets) and undercoordinated atoms (edges and corners), as well as evidence of strain propagation from the surface to the bulk of the nanoparticle. These dynamic structural relationships directly inform the design of strain-engineered nanocatalysts for energy storage and conversion applications.

Anomalous Glide Plane in Platinum Nano- and Microcrystals.

At the nanoscale, the properties of materials depend critically on the presence of crystal defects. However, imaging and characterizing the structure of defects in three dimensions inside a crystal remain a challenge. Here, by using Bragg coherent diffraction imaging, we observe an unexpected anomalous {110} glide plane in two Pt submicrometer crystals grown by very different processes and having very different morphologies. The structure of the defects (type, associated glide plane, and lattice displacement) is imaged in these faceted Pt crystals. Using this noninvasive technique, both plasticity and unusual defect behavior can be probed at the nanoscale.

Gwaihir: Jupyter Notebook graphical user interface for Bragg coherent diffraction imaging.

Bragg coherent X-ray diffraction is a nondestructive method for probing material structure in three dimensions at the nanoscale, with unprecedented resolution in displacement and strain fields. This work presents Gwaihir, a user-friendly and open-source tool to process and analyze Bragg coherent X-ray diffraction data. It integrates the functionalities of the existing packages bcdi and PyNX in the same toolbox, creating a natural workflow and promoting data reproducibility. Its graphical interface, based on Jupyter Notebook widgets, combines an interactive approach for data analysis with a powerful environment designed to link large-scale facilities and scientists.

Imaging the facet surface strain state of supported multi-faceted Pt nanoparticles during reaction.

Nanostructures with specific crystallographic planes display distinctive physico-chemical properties because of their unique atomic arrangements, resulting in widespread applications in catalysis, energy conversion or sensing. Understanding strain dynamics and their relationship with crystallographic facets have been largely unexplored. Here, we reveal in situ, in three-dimensions and at the nanoscale, the volume, surface and interface strain evolution of single supported platinum nanocrystals during reaction using coherent x-ray diffractive imaging. Interestingly, identical {hkl} facets show equivalent catalytic response during non-stoichiometric cycles. Periodic strain variations are rationalised in terms of O2 adsorption or desorption during O2 exposure or CO oxidation under reducing conditions, respectively. During stoichiometric CO oxidation, the strain evolution is, however, no longer facet dependent. Large strain variations are observed in localised areas, in particular in the vicinity of the substrate/particle interface, suggesting a significant influence of the substrate on the reactivity. These findings will improve the understanding of dynamic properties in catalysis and related fields.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation.

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge. We report here an unusual twin boundary migration process in a single platinum nanoparticle during CO oxidation using Bragg coherent diffraction imaging as the characterisation tool. Density functional theory calculations show that twin migration can be correlated with the relative change in the interfacial energies of the free surfaces exposed to CO. The x-ray technique also reveals particle reshaping during the reaction. In situ and non-invasive structural characterisation of defects during reaction opens new avenues for understanding defect behaviour in confined crystals and paves the way for strain and defect engineering.

Deciphering the interactions between single arm dislocation sources and coherent twin boundary in nickel bi-crystal.

The introduction of a well-controlled population of coherent twin boundaries (CTBs) is an attractive route to improve the strength ductility product in face centered cubic (FCC) metals. However, the elementary mechanisms controlling the interaction between single arm dislocation sources (SASs), often present in nanotwinned FCC metals, and CTB are still not well understood. Here, quantitative in-situ transmission electron microscopy (TEM) observations of these mechanisms under tensile loading are performed on submicron Ni bi-crystal. We report that the absorption of curved screw dislocations at the CTB leads to the formation of constriction nodes connecting pairs of twinning dislocations at the CTB plane in agreement with large scale 3D atomistic simulations. The coordinated motion of the twinning dislocation pairs due to the presence of the nodes leads to a unique CTB sliding mechanism, which plays an important role in initiating the fracture process at a CTB ledge. TEM observations of the interactions between non-screw dislocations and the CTB highlight the importance of the synergy between the repulsive force of the CTB and the back stress from SASs when the interactions occur in small volumes.

Author Correction: Continuous scanning for Bragg coherent X-ray imaging.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Mapping Inversion Domain Boundaries along Single GaN Wires with Bragg Coherent X-ray Imaging.

Gallium nitride (GaN) is of technological importance for a wide variety of optoelectronic applications. Defects in GaN, like inversion domain boundaries (IDBs), significantly affect the electrical and optical properties of the material. We report, here, on the structural configurations of planar inversion domain boundaries inside n-doped GaN wires measured by Bragg coherent X-ray diffraction imaging. Different complex domain configurations are revealed along the wires with a 9 nm in-plane spatial resolution. We demonstrate that the IDBs change their direction of propagation along the wires, promoting Ga-terminated domains and stabilizing into {11̅00}, that is, m-planes. The atomic phase shift between the Ga- and N-terminated domains was extracted using phase-retrieval algorithms, revealing an evolution of the out-of-plane displacement (∼5 pm, at maximum) between inversion domains along the wires. This work provides an accurate inner view of planar defects inside small crystals.

Continuous scanning for Bragg coherent X-ray imaging.

We explore the use of continuous scanning during data acquisition for Bragg coherent diffraction imaging, i.e., where the sample is in continuous motion. The fidelity of continuous scanning Bragg coherent diffraction imaging is demonstrated on a single Pt nanoparticle in a flow reactor at [Formula: see text] in an Ar-based gas flowed at 50 ml/min. We show a reduction of 30% in total scan time compared to conventional step-by-step scanning. The reconstructed Bragg electron density, phase, displacement and strain fields are in excellent agreement with the results obtained from conventional step-by-step scanning. Continuous scanning will allow to minimise sample instability under the beam and will become increasingly important at diffraction-limited storage ring light sources.

Inversion Domain Boundaries in GaN Wires Revealed by Coherent Bragg Imaging.

Interfaces between polarity domains in nitride semiconductors, the so-called Inversion Domain Boundaries (IDB), have been widely described, both theoretically and experimentally, as perfect interfaces (without dislocations and vacancies). Although ideal planar IDBs are well documented, the understanding of their configurations and interactions inside crystals relies on perfect-interface assumptions. Here, we report on the microscopic configuration of IDBs inside n-doped gallium nitride wires revealed by coherent X-ray Bragg imaging. Complex IDB configurations are evidenced with 6 nm resolution and the absolute polarity of each domain is unambiguously identified. Picoscale displacements along and across the wire are directly extracted from several Bragg reflections using phase retrieval algorithms, revealing rigid relative displacements of the domains and the absence of microscopic strain away from the IDBs. More generally, this method offers an accurate inner view of the displacements and strain of interacting defects inside small crystals that may alter optoelectronic properties of semiconductor devices.

Signature of dislocations and stacking faults of face-centred cubic nanocrystals in coherent X-ray diffraction patterns: a numerical study.

Crystal defects induce strong distortions in diffraction patterns. A single defect alone can yield strong and fine features that are observed in high-resolution diffraction experiments such as coherent X-ray diffraction. The case of face-centred cubic nanocrystals is studied numerically and the signatures of typical defects close to Bragg positions are identified. Crystals of a few tens of nanometres are modelled with realistic atomic potentials and 'relaxed' after introduction of well defined defects such as pure screw or edge dislocations, or Frank or prismatic loops. Diffraction patterns calculated in the kinematic approximation reveal various signatures of the defects depending on the Miller indices. They are strongly modified by the dissociation of the dislocations. Selection rules on the Miller indices are provided, to observe the maximum effect of given crystal defects in the initial and relaxed configurations. The effect of several physical and geometrical parameters such as stacking fault energy, crystal shape and defect position are discussed. The method is illustrated on a complex structure resulting from the simulated nanoindentation of a gold nanocrystal.

Scanning force microscope for in situ nanofocused X-ray diffraction studies.

A compact scanning force microscope has been developed for in situ combination with nanofocused X-ray diffraction techniques at third-generation synchrotron beamlines. Its capabilities are demonstrated on Au nano-islands grown on a sapphire substrate. The new in situ device allows for in situ imaging the sample topography and the crystallinity by recording simultaneously an atomic force microscope (AFM) image and a scanning X-ray diffraction map of the same area. Moreover, a selected Au island can be mechanically deformed using the AFM tip while monitoring the deformation of the atomic lattice by nanofocused X-ray diffraction. This in situ approach gives access to the mechanical behavior of nanomaterials.