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Register a new userBragg Coherent Modulation Imaging: Strain- and Defect- Sensitive Single Views of Extended Samples
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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.
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Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this article we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. The article also discusses the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.
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.
Bragg coherent X-ray diffraction imaging (BCDI) is a non-destructive, lensless method for 3D-resolved, nanoscale strain imaging in micro-crystals. A challenge, particularly for new users of the technique, is accurate mapping of experimental data, collected in the detector reciprocal space coordinate frame, to more convenient orthogonal coordinates, e.g. attached to the sample. This is particularly the case since different coordinate conventions are used at every BCDI beamline. The reconstruction algorithms and mapping scripts composed for individual beamlines are not readily interchangeable. To overcome this, a BCDI experiment simulation with a plugin script that converts all beamline angles to a universal, right-handed coordinate frame is introduced, making it possible to condense any beamline geometry into three rotation matrices. The simulation translates a user-specified 3D complex object to different BCDI-related coordinate frames. It also allows the generation of synthetic coherent diffraction data that can be inserted into any BCDI reconstruction algorithm to reconstruct the original user-specified object. Scripts are provided to map from sample space to detector conjugated space, detector conjugated space to sample space and detector conjugated space to detector conjugated space for a different reflection. This provides the reader with the basis for a flexible simulation tool kit that is easily adapted to different geometries. It is anticipated that this will find use in the generation of tailor-made supports for phasing of challenging data and exploration of novel geometries or data collection modalities.
By using phase retrieval, Bragg Coherent Diffractive Imaging (BCDI) allows tracking of three-dimensional displacement fields inside individual nanocrystals. Nevertheless, in the presence of significant (1% and higher) strains, such as in the process of a structural phase transformation, fails due to the Bragg peak distortions. Here we present an advanced BCDI algorithm enabling imaging three-dimensional strain fields in highly strained crystals. We test the algorithm on particles simulated to undergo a structural phase transformation. While the conventional algorithm fails in unambiguously reconstructing the phase morphology, our algorithm correctly retrieves the morphology of coexistent phases with a strain difference of 1%. The key novelty is the simultaneous reconstruction of multiple scans of the same nanoparticle at snapshots through the phase transformations. The algorithm enables visualizing phase transformations in nanoparticles of lithium-ion, sodium-ion nanoparticles, and other nanoparticulate materials in working conditions (operando).
Quantum conductance calculations on the mechanically deformed monolayers of MoS$_2$ and WS$_2$ were performed using the non-equlibrium Greens functions method combined with the Landauer-B{u}ttiker approach for ballistic transport together with the density-functional based tight binding (DFTB) method. Tensile strain and compression causes significant changes in the electronic structure of TMD single layers and eventually the transition semiconductor-metal occurs for elongations as large as ~11% for the 2D-isotropic deformations in the hexagonal structure. This transition enhances the electron transport in otherwise semiconducting materials.
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