arXiv subjects

W. Cha

Publications and source records attributed to W. Cha.

5 recordsLinked to original sources

arXiv2022

A differentiable forward model for the concurrent, multi-peak Bragg coherent x-ray diffraction imaging problem

We present a general analytic approach to spatially resolve the nano-scale lattice distortion field of strained and defected compact crystals with Bragg coherent x-ray diffraction imaging (BCDI). Our approach relies on fitting a differentiable forward model simultaneously to multiple BCDI datasets corresponding to independent Bragg reflections from the same single crystal. It is designed to be faithful to heterogeneities that potentially manifest as phase discontinuities in the coherently diffracted wave, such as lattice dislocations in an imperfect crystal. We retain fidelity to such small features in the reconstruction process through a Fourier transform -based resampling algorithm designed to largely avoid the point spread tendencies of commonly employed interpolation methods. The reconstruction model defined in this manner brings BCDI reconstruction into the scope of explicit optimization driven by automatic differentiation. With results from simulations and experimental diffraction data, we demonstrate significant improvement in the final image quality compared to conventional phase retrieval, enabled by explicitly coupling multiple BCDI datasets into the reconstruction loss function.

cond-mat.mtrl-sci↗

arXiv2021

Interaction and transformation of metastable defects in intercalation materials

Non-equilibrium defects often dictate macroscopic functional properties of materials. In intercalation hosts, widely used in rechargeable batteries, high-dimensional defects largely define reversibility and kinetics1,2,3,4. However, transient defects briefly appearing during ionic transport have been challenging to capture, limiting the understanding of their life cycle and impact. Here, we overcome this challenge and track operando the interaction and impact of metastable defects within NaxNi1-xMnyO2 intercalation hosts in a charging sodium-ion battery. Three-dimensional coherent X-ray imaging3,4,5 reveals transformation and self-healing of a metastable domain boundary, glissile dislocation loop, and stacking fault. A local strain gradient suggests a quantifiable difference in ion diffusion, coincident with the macroscopic change in diffusion coefficient. Analysis of the unexpected4,6 defect anisotropy highlights the importance of mesostructure, suggesting a possible control approach and disputing the rigidity of the framework layers. The shared nature of oxygen framework layers makes our results applicable to a wide range of intercalation materials.

cond-mat.mtrl-sci↗

arXiv2018

Bragg Coherent Modulation Imaging: Strain- and Defect- Sensitive Single Views of Extended Samples

Nanoscale heterogeneity (including size, shape, strain, and defects) significantly impacts material properties and how they function. Bragg coherent x-ray imaging methods have emerged as a powerful tool to investigate, in three-dimensional detail, the local material response to external stimuli in reactive environments, thereby enabling explorations of the structure-defect-function relationship at the nanoscale. Although progress has been made in understanding this relationship, coherent imaging of extended samples is relatively slow (typically requiring many minutes) due to the experimental constraints required to solve the phase problem. Here, we develop Bragg coherent modulation imaging (BCMI), which uses a modulator to solve the phase problem thereby enabling fast, local imaging of an extended sample. Because a known modulator is essential to the technique, we first demonstrate experimentally that an unknown modulator structure can be recovered by using the exit wave diversity that exists in a standard Bragg coherent diffraction imaging (BCDI) experiment. We then show with simulations that a known modulator constraint is sufficient to solve the phase problem and enable a single view of an extended sample that is sensitive to defects and dislocations. Our results pave the way for BCMI investigations of strain and defect dynamics in complex extended crystals with temporal resolution limited by the exposure time of a single diffraction pattern.

cond-mat.mtrl-sci↗

arXiv2016

Stability Limits and Surface Chemistry of Ag Nanoparticles in Non-Adsorbing Electrolytes Probed by Bragg Coherent Diffractive Imaging

Surface chemistry is important across diverse fields such as corrosion and nanostructure synthesis. Unfortunately, many as-synthesized nanomaterials, including partially dealloyed nanoparticle catalysts for fuel cells, with highly active surfaces are not stable in their reactive environments, preventing widespread application. Thus, understanding instability by focusing on the structure-stability and defect-stability relationship at the nanoscale is crucial and will likely play an important role in meeting grand challenges. To this end, recent advances in imaging nanostructure stability have come via both electron, x-ray, and other techniques such as atomic force microscopy, but tend to be limited to specific sample environments and/or two-dimensional images. Here, we report investigations into the defect-stability relationship of silver nanoparticles to voltage-induced electrochemical dissolution imaged in-situ in three-dimensional (3D) detail by Bragg Coherent Diffractive Imaging (BCDI). We first determine the average dissolution kinetics by Stationary Probe Rotating Disk Electrode (SPRDE) in combination with inductively coupled plasma mass spectrometry (ICP-MS), which allows real-time in-situ measurement of Ag+ ions formation and the corresponding electrochemical current. We then observe the dissolution and redeposition processes in 3D with BCDI in single nanocrystals, providing unique insight about the role of surface strain, defects, and their coupling to the dissolution chemistry. The methods developed and the knowledge gained go well beyond a "simple" silver electrochemistry and are applicable to all electrocatalytic reactions where functional links between activity and stability are controlled by structure and defect dynamics.

cond-mat.mtrl-sci↗

arXiv2016

Chrono CDI: Coherent diffractive imaging of time-evolving samples

Bragg coherent x-ray diffractive imaging is a powerful technique for investigating dynamic nanoscale processes in nanoparticles immersed in reactive, realistic environments. Its temporal resolution is limited, however, by the oversampling requirements of 3D phase retrieval. Here we show that incorporating the entire measurement time series, which is typically a continuous physical process, into phase retrieval allows the oversampling requirement at each time step to be reduced leading to a subsequent improvement in the temporal resolution by a factor of 2 to 20 times. The increased time resolution will allow imaging of faster dynamics and of radiation dose sensitive samples. This approach, which we call "chrono CDI", may find use in improving time resolution in other imaging techniques.

cond-mat.mtrl-sci↗