Imaging transient melting of a nanocrystal using an X-ray laser.

There is a fundamental interest in studying photoinduced dynamics in nanoparticles and nanostructures as it provides insight into their mechanical and thermal properties out of equilibrium and during phase transitions. Nanoparticles can display significantly different properties from the bulk, which is due to the interplay between their size, morphology, crystallinity, defect concentration, and surface properties. Particularly interesting scenarios arise when nanoparticles undergo phase transitions, such as melting induced by an optical laser. Current theoretical evidence suggests that nanoparticles can undergo reversible nonhomogenous melting with the formation of a core-shell structure consisting of a liquid outer layer. To date, studies from ensembles of nanoparticles have tentatively suggested that such mechanisms are present. Here we demonstrate imaging transient melting and softening of the acoustic phonon modes of an individual gold nanocrystal, using an X-ray free electron laser. The results demonstrate that the transient melting is reversible and nonhomogenous, consistent with a core-shell model of melting. The results have implications for understanding transient processes in nanoparticles and determining their elastic properties as they undergo phase transitions.

Micro-beam Laue alignment of multi-reflection Bragg coherent diffraction imaging measurements.

Multi-reflection Bragg coherent diffraction imaging has the potential to allow three-dimensional (3D) resolved measurements of the full lattice strain tensor in specific micro-crystals. Until now such measurements were hampered by the need for laborious, time-intensive alignment procedures. Here a different approach is demonstrated, using micro-beam Laue X-ray diffraction to first determine the lattice orientation of the micro-crystal. This information is then used to rapidly align coherent diffraction measurements of three or more reflections from the crystal. Based on these, 3D strain and stress fields in the crystal are successfully determined. This approach is demonstrated on a focused ion beam milled micro-crystal from which six reflections could be measured. Since information from more than three independent reflections is available, the reliability of the phases retrieved from the coherent diffraction data can be assessed. Our results show that rapid, reliable 3D coherent diffraction measurements of the full lattice strain tensor in specific micro-crystals are now feasible and can be successfully carried out even in heavily distorted samples.

Three-dimensional imaging of dislocation propagation during crystal growth and dissolution.

Atomic-level defects such as dislocations play key roles in determining the macroscopic properties of crystalline materials. Their effects range from increased chemical reactivity to enhanced mechanical properties. Dislocations have been widely studied using traditional techniques such as X-ray diffraction and optical imaging. Recent advances have enabled atomic force microscopy to study single dislocations in two dimensions, while transmission electron microscopy (TEM) can now visualize strain fields in three dimensions with near-atomic resolution. However, these techniques cannot offer three-dimensional imaging of the formation or movement of dislocations during dynamic processes. Here, we describe how Bragg coherent diffraction imaging (BCDI; refs 11, 12) can be used to visualize in three dimensions, the entire network of dislocations present within an individual calcite crystal during repeated growth and dissolution cycles. These investigations demonstrate the potential of BCDI for studying the mechanisms underlying the response of crystalline materials to external stimuli.

3D Imaging of Twin Domain Defects in Gold Nanoparticles.

Topological defects are ubiquitous in physics and include crystallographic imperfections such as defects in condensed matter systems. Defects can determine many of the material's properties, thus providing novel opportunities for defect engineering. However, it is difficult to track buried defects and their interfaces in three dimensions with nanoscale resolution. Here, we report three-dimensional visualization of gold nanocrystal twin domains using Bragg coherent X-ray diffractive imaging in an aqueous environment. We capture the size and location of twin domains, which appear as voids in the Bragg electron density, in addition to a component of the strain field. Twin domains can interrupt the stacking order of the parent crystal, leading to a phase offset between the separated parent crystal pieces. We utilize this phase offset to estimate the roughness of the twin boundary. We measure the diffraction signal from the crystal twin and show its Bragg electron density fits into the parent crystal void. Defect imaging will likely facilitate improvement and rational design of nanostructured materials.

Evaluation of partial coherence correction in X-ray ptychography.

Coherent X-ray Diffraction Imaging (CDI) and X-ray ptychography both heavily rely on the high degree of spatial coherence of the X-ray illumination for sufficient experimental data quality for reconstruction convergence. Nevertheless, the majority of the available synchrotron undulator sources have a limited degree of partial coherence, leading to reduced data quality and a lower speckle contrast in the coherent diffraction patterns. It is still an open question whether experimentalists should compromise the coherence properties of an X-ray source in exchange for a higher flux density at a sample, especially when some materials of scientific interest are relatively weak scatterers. A previous study has suggested that in CDI, the best strategy for the study of strong phase objects is to maintain a high degree of coherence of the illuminating X-rays because of the broadening of solution space resulting from the strong phase structures. In this article, we demonstrate the first systematic analysis of the effectiveness of partial coherence correction in ptychography as a function of the coherence properties, degree of complexity of illumination (degree of phase diversity of the probe) and sample phase complexity. We have also performed analysis of how well ptychographic algorithms refine X-ray probe and complex coherence functions when those variables are unknown at the start of reconstructions, for noise-free simulated data, in the case of both real-valued and highly-complex objects.

In situ strain evolution during a disconnection event in a battery nanoparticle.

Lithium ion batteries are the dominant form of energy storage in mobile devices, increasingly employed in transportation, and likely candidates for renewable energy storage and integration into the electrical grid. To fulfil their powerful potential, electrodes with increased capacity, faster charge rates, and longer cycle life must be developed. Understanding the mechanics and chemistry of individual nanoparticles under in situ conditions is a crucial step to improving performance and mitigating damage. Here we reveal three-dimensional strain evolution within a single nanoparticle of a promising high voltage cathode material, LiNi0.5Mn1.5O4, under in situ conditions. The particle becomes disconnected during the second charging cycle. This is attributed to the formation of a cathode electrolyte interphase layer with slow ionic conduction. The three-dimensional strain pattern within the particle is independent of cell voltage after disconnection, indicating that the particle is unable to redistribute lithium within its volume or to its neighbours. Understanding the disconnection process at the single particle level and the equilibrium or non-equilibrium state of nanoparticles is essential to improving performance of current and future electrochemical energy storage systems.

Single particle nanomechanics in operando batteries via lensless strain mapping.

We reveal three-dimensional strain evolution in situ of a single LiNi0.5Mn1.5O4 nanoparticle in a coin cell battery under operando conditions during charge/discharge cycles with coherent X-ray diffractive imaging. We report direct observation of both stripe morphologies and coherency strain at the nanoscale. Our results suggest the critical size for stripe formation is 50 nm. Surprisingly, the single nanoparticle elastic energy landscape, which we map with femtojoule precision, depends on charge versus discharge, indicating hysteresis at the single particle level. This approach opens a powerful new avenue for studying battery nanomechanics, phase transformations, and capacity fade under operando conditions at the single particle level that will enable profound insight into the nanoscale mechanisms that govern electrochemical energy storage systems.

Observations of artefacts in the x-ray ptychography method.

X-ray ptychography, a scanning coherent diffraction imaging method, was used to reconstruct images of a "Siemens star" test pattern with amplitude and phase contrast. While studying how the use of illumination with an increased bandwidth results in clear improvements in the quality of image reconstructions, we found that an artificial change in the overall distance scale factor of the algorithm leads to a systematic response in the image, which is reproduced with an incorrect number of spokes. This pathology is explained by the conflict between the length scales set by the scan and by the diffraction patterns on the detector.

Continuous scanning mode for ptychography.

We outline how ptychographic imaging can be performed without the need for discrete scan positions. Through an idealized experiment, we demonstrate how a discrete-position scan regime can be replaced with a continuously scanned one with suitable modification of the reconstruction scheme based on coherent modes. The impact of this is that acquisition times can be reduced, significantly aiding ptychographic imaging with x rays, electrons, or visible light.

Dynamic imaging using ptychography.

We demonstrate through experiment an example of "mixed state" reconstruction using x-ray ptychography. We demonstrate successful imaging of a vibrating sample that has dynamics that are of one order magnitude faster than the measurement times. We show how increased vibrational amplitude leads to an increased population of illumination modes, a characteristic of partial coherence. Implications of a vibrating sample are explored, with its possible use in manipulating coherent wave field mode shapes and coherence properties.

Dynamic sample imaging in coherent diffractive imaging.

As the resolution in coherent diffractive imaging improves, interexposure and intraexposure sample dynamics, such as motion, degrade the quality of the reconstructed image. Selecting data sets that include only exposures where tolerably little motion has occurred is an inefficient use of time and flux, especially when detector readout time is significant. We provide an experimental demonstration of an approach in which all images of a data set exhibiting sample motion are combined to improve the quality of a reconstruction. This approach is applicable to more general sample dynamics (including sample damage) that occur during measurement.

Phase-diverse coherent diffractive imaging: high sensitivity with low dose.

This Letter demonstrates that coherent diffractive imaging (CDI), in combination with phase-diversity methods, provides reliable and artefact free high-resolution images. Here, using x rays, experimental results show a threefold improvement in the available image contrast. Furthermore, in conditions requiring low imaging dose, it is demonstrated that phase-diverse CDI provides a factor of 2 improvement in comparison to previous CDI techniques.

Time-resolved in situ visualization of the structural response of zeolites during catalysis.

Zeolites are three-dimensional aluminosilicates having unique properties from the size and connectivity of their sub-nanometer pores, the Si/Al ratio of the anionic framework, and the charge-balancing cations. The inhomogeneous distribution of the cations affects their catalytic performances because it influences the intra-crystalline diffusion rates of the reactants and products. However, the structural deformation regarding inhomogeneous active regions during the catalysis is not yet observed by conventional analytical tools. Here we employ in situ X-ray free electron laser-based time-resolved coherent X-ray diffraction imaging to investigate the internal deformations originating from the inhomogeneous Cu ion distributions in Cu-exchanged ZSM-5 zeolite crystals during the deoxygenation of nitrogen oxides with propene. We show that the interactions between the reactants and the active sites lead to an unusual strain distribution, confirmed by density functional theory simulations. These observations provide insights into the role of structural inhomogeneity in zeolites during catalysis and will assist the future design of zeolites for their applications.

Visualization of the effect of additives on the nanostructures of individual bio-inspired calcite crystals.

Soluble additives provide a versatile strategy for controlling crystallization processes, enabling selection of properties including crystal sizes, morphologies, and structures. The additive species can also be incorporated within the crystal lattice, leading for example to enhanced mechanical properties. However, while many techniques are available for analyzing particle shape and structure, it remains challenging to characterize the structural inhomogeneities and defects introduced into individual crystals by these additives, where these govern many important material properties. Here, we exploit Bragg coherent diffraction imaging to visualize the effects of soluble additives on the internal structures of individual crystals on the nanoscale. Investigation of bio-inspired calcite crystals grown in the presence of lysine or magnesium ions reveals that while a single dislocation is observed in calcite crystals grown in the presence of lysine, magnesium ions generate complex strain patterns. Indeed, in addition to the expected homogeneous solid solution of Mg ions in the calcite lattice, we observe two zones comprising alternating lattice contractions and relaxation, where comparable alternating layers of high magnesium calcite have been observed in many magnesium calcite biominerals. Such insight into the structures of nanocomposite crystals will ultimately enable us to understand and control their properties.

3D lattice distortions and defect structures in ion-implanted nano-crystals.

Focussed Ion Beam (FIB) milling is a mainstay of nano-scale machining. By manipulating a tightly focussed beam of energetic ions, often gallium (Ga+), FIB can sculpt nanostructures via localised sputtering. This ability to cut solid matter on the nano-scale revolutionised sample preparation across the life, earth and materials sciences. Despite its widespread usage, detailed understanding of the FIB-induced structural damage, intrinsic to the technique, remains elusive. Here we examine the defects caused by FIB in initially pristine objects. Using Bragg Coherent X-ray Diffraction Imaging (BCDI), we are able to spatially-resolve the full lattice strain tensor in FIB-milled gold nano-crystals. We find that every use of FIB causes large lattice distortions. Even very low ion doses, typical of FIB imaging and previously thought negligible, have a dramatic effect. Our results are consistent with a damage microstructure dominated by vacancies, highlighting the importance of free-surfaces in determining which defects are retained. At larger ion fluences, used during FIB-milling, we observe an extended dislocation network that causes stresses far beyond the bulk tensile strength of gold. These observations provide new fundamental insight into the nature of the damage created and the defects that lead to a surprisingly inhomogeneous morphology.

In Situ 3D Imaging of Catalysis Induced Strain in Gold Nanoparticles.

Multielectron transfer processes are crucially important in energy and biological science but require favorable catalysts to achieve fast kinetics. Nanostructuring catalysts can dramatically improve their properties, which can be difficult to understand due to strain- and size-dependent thermodynamics, the influence of defects, and substrate-dependent activities. Here, we report three-dimensional (3D) imaging of single gold nanoparticles during catalysis of ascorbic acid decomposition using Bragg coherent diffractive imaging (BCDI). Local strains were measured in single nanoparticles and modeled using reactive molecular dynamics (RMD) simulations and finite element analysis (FEA) simulations. RMD reveals the pathway for local strain generation in the gold lattice: chemisorption of hydroxyl ions. FEA reveals that the RMD results are transferable to the nanocrystal sizes studied in the experiment. Our study probes the strain-activity connection and opens a powerful avenue for theoretical and experimental studies of nanocrystal catalysis.

Strain-relief by single dislocation loops in calcite crystals grown on self-assembled monolayers.

Most of our knowledge of dislocation-mediated stress relaxation during epitaxial crystal growth comes from the study of inorganic heterostructures. Here we use Bragg coherent diffraction imaging to investigate a contrasting system, the epitaxial growth of calcite (CaCO3) crystals on organic self-assembled monolayers, where these are widely used as a model for biomineralization processes. The calcite crystals are imaged to simultaneously visualize the crystal morphology and internal strain fields. Our data reveal that each crystal possesses a single dislocation loop that occupies a common position in every crystal. The loops exhibit entirely different geometries to misfit dislocations generated in conventional epitaxial thin films and are suggested to form in response to the stress field, arising from interfacial defects and the nanoscale roughness of the substrate. This work provides unique insight into how self-assembled monolayers control the growth of inorganic crystals and demonstrates important differences as compared with inorganic substrates.

Fly-scan ptychography.

We report an experimental ptychography measurement performed in fly-scan mode. With a visible-light laser source, we demonstrate a 5-fold reduction of data acquisition time. By including multiple mutually incoherent modes into the incident illumination, high quality images were successfully reconstructed from blurry diffraction patterns. This approach significantly increases the throughput of ptychography, especially for three-dimensional applications and the visualization of dynamic systems.

Atomic diffusion within individual gold nanocrystal.

Due to their excess surface free energy and structural instabilities, nanoparticles exhibit interesting physical and chemical properties. There has been an ever-growing interest in investigating these properties, driven by the desire to further miniaturize electronic devices, develop new functional materials and catalysts. Here, the intriguing question of how diffusion evolves in a single nanoparticle is investigated by measuring the spatial and temporal variations of the diffracted coherent X-ray intensity during copper diffusion into a gold nanocrystal. Dislocation loops formed from the insertion of single layer of extra atoms between neighbouring gold host lattice planes are detected. Au-Cu alloy channels are found to penetrate the nanocrystal due to the differential diffusion rate along different directions. With the advent of higher brilliance sources and free-electron-lasers, Bragg Coherent X-ray Diffraction Imaging can play an important role in unveiling atomic behaviours in three dimensions for nanomaterials during various fundamental processes.