Advanced Structural and Chemical Imaging最新文献

Scanning transmission electron microscopy (STEM) has emerged as one of the foremost techniques to analyze materials at atomic resolution. However, two practical difficulties inherent to STEM imaging are: radiation damage imparted by the electron beam, which can potentially damage or otherwise modify the specimen and slow-scan image acquisition, which limits the ability to capture dynamic changes at high temporal resolution. Furthermore, due in part to scan flyback corrections, typical raster scan methods result in an uneven distribution of dose across the scanned area. A method to allow extremely fast scanning with a uniform residence time would enable imaging at low electron doses, ameliorating radiation damage and at the same time permitting image acquisition at higher frame-rates while maintaining atomic resolution. The practical complication is that rastering the STEM probe at higher speeds causes significant image distortions. Non-square scan patterns provide a solution to this dilemma and can be tailored for low dose imaging conditions. Here, we develop a method for imaging with alternative scan patterns and investigate their performance at very high scan speeds. A general analysis for spiral scanning is presented here for the following spiral scan functions: Archimedean, Fermat, and constant linear velocity spirals, which were tested for STEM imaging. The quality of spiral scan STEM images is generally comparable with STEM images from conventional raster scans, and the dose uniformity can be improved.

Although magnetism originates at the atomic scale, the existing spectroscopic techniques sensitive to magnetic signals only produce spectra with spatial resolution on a larger scale. However, recently, it has been theoretically argued that atomic size electron probes with customized phase distributions can detect magnetic circular dichroism. Here, we report a direct experimental real-space detection of magnetic circular dichroism in aberration-corrected scanning transmission electron microscopy (STEM). Using an atomic size-aberrated electron probe with a customized phase distribution, we reveal the checkerboard antiferromagnetic ordering of Mn moments in LaMnAsO by observing a dichroic signal in the Mn L-edge. The novel experimental setup presented here, which can easily be implemented in aberration-corrected STEM, opens new paths for probing dichroic signals in materials with unprecedented spatial resolution.

The introduction of gaseous atmospheres in transmission electron microscopy offers the possibility of studying materials in situ under chemically relevant environments. The presence of a gas environment can degrade the resolution. Surprisingly, this phenomenon has been shown to depend on the electron-dose-rate. In this article, we demonstrate that both the total and areal electron-dose-rates work as descriptors for the dose-rate-dependent resolution and are related through the illumination area. Furthermore, the resolution degradation was observed to occur gradually over time after initializing the illumination of the sample and gas by the electron beam. The resolution was also observed to be sensitive to the electrical conductivity of the sample. These observations can be explained by a charge buildup over the electron-illuminated sample area, caused by the beam–gas–sample interaction, and by a subsequent sample motion induced by electrical capacitance in the sample.

The transport of intensity equation (TIE) offers a convenient method to retrieve the phase of a wave function from maps of the irradiance (images) recorded at different planes along the optic axis of an optical system. However, being a second-order partial differential equation, even for noise-free data a unique solution of the TIE requires boundary conditions to be specified which are generally not accessible experimentally, jeopardizing retrieval of the low-frequency information in particular. Here we introduce an iterative algorithm which forgoes the need for explicit boundary conditions and combines the well-known reciprocal space solution of the TIE with the charge-flipping algorithm that has originally been developed to solve the crystallographic phase problem in X-ray diffraction. Application of this algorithm to experimental data and comparison with conventionally used algorithms demonstrates an improved retrieval of the low spatial frequencies of the phase.

In situ electron microscopy allows one to monitor dynamical processes at high spatial and temporal resolution. This produces large quantities of data, and hence automated image processing algorithms are needed to extract useful quantitative measures of the observed phenomena. In this work, we outline an image processing workflow for the analysis of evolving interfaces imaged during liquid cell electron microscopy. As examples, we show metal electrodeposition at electrode surfaces; beam-induced nanocrystal formation and dissolution; and beam-induced bubble nucleation, growth, and migration. These experiments are used to demonstrate a fully automated workflow for the extraction of, among other things, interface position, roughness, lateral wavelength, local normal velocity, and the projected area of the evolving phase as functions of time. The relevant algorithms have been implemented in Mathematica and are available online.

Electron holography is employed to study variations of the electrostatic crystal potential in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells at different length scales: Long-range potential variations across the layer structure of the solar cell as well as inhomogeneities within the layers are analyzed by off-axis holography. In-line holography is applied to examine the local potential variation across a CIGS grain boundary. The phase reconstruction from a focal series is performed by a modified transport of intensity equation (TIE) which is optimized to reduce common artifacts. For comparison, three different microscopes of different optical configurations were used for in-line holography. Based on the results, the impact of the used microscope as well as further acquisition parameters on the in-line holography measurement is assessed. The measured potential variations are discussed considering the effect of different possible sources that may cause potential fluctuations. It is found that most of the variations are best explained by mean inner potential fluctuations rather than by inhomogeneities of the electronic properties. Finally, the present resolution limit of both methods is discussed regarding the feasibility of future electronic characterization of CIGS by holography.

Crystallographic image processing (CIP) techniques may be utilized in scanning probe microscopy (SPM) to glean information that has been obscured by signals from multiple probe tips. This may be of particular importance for scanning tunneling microscopy (STM) and requires images from samples that are periodic in two dimensions (2D). The image-forming current for double-tips in STM is derived with a slight modification of the independent-orbital approximation (IOA) to allow for two or more tips. Our analysis clarifies why crystallographic averaging works well in removing the effects of a blunt STM tip (that consists of multiple mini-tips) from recorded 2D periodic images and also outlines the limitations of this image-processing technique for certain spatial separations of STM double-tips. Simulations of multiple mini-tip effects in STM images (that ignore electron interference effects) may be understood as modeling multiple mini-tip (or tip shape) effects in images that were recorded with other types of SPMs as long as the lateral sample feature sizes to be imaged are much larger than the effective scanning probe tip sizes.

A study has been completed examining design issues concerning the interpretation of and dissemination of multimodal medical imaging data sets to diverse audiences. To create a model data set mouse fibrosarcoma tissue was visualised via magnetic resonance imaging (MRI), Matrix-Assisted Laser Desorption/Ionisation-Mass Spectrometry (MALDI-MSI) and histology. MRI images were acquired using the 0.25T Esaote GScan; MALDI images were acquired using a Q-Star Pulsar I mass spectrometer. Histological staining of the same tissue sections used for MALDI-MSI was then carried out. Areas assigned to hemosiderin deposits due to haemorrhaging could be visualised via MRI. In the MALDI-MSI data obtained the distribution sphingomyelin species could be used to identify regions of viable tumour. Mathematical ‘up sampling’ using hierarchical clustering-based segmentation provided a sophisticated image enhancement tool for both MRI and MALDI-MS and assisted in the correlation of images.

Chloroplasts are essential plant organelles that divide by binary fission through a coordinated ring-shaped division machinery located both on the outside and inside of the chloroplast. The first step in chloroplast division is the assembly of an internal division ring (Z-ring) that is composed of the key filamentous chloroplast division proteins FtsZ1 and FtsZ2. How the individual FtsZ filaments assemble into higher-order structures to form the dividing Z-ring is not well understood and the most detailed insights have so far been gleaned from prokaryotic FtsZ. Here, we present in situ data of chloroplast FtsZ making use of a smaller ring-like FtsZ assembly termed mini-rings that form under well-defined conditions. Structured illumination microscopy (SIM) permitted their mean diameter to be determined as 208?nm and also showed that 68?% of these rings are terminally attached to linear FtsZ filaments. A correlative microscopy-compatible specimen preparation based on freeze substitution after high-pressure freezing is presented addressing the challenges such as autofluorescence and specific fluorescence attenuation. Transmission electron microscopy (TEM) and scanning TEM (STEM) imaging of thin sections exhibited ring-like densities that matched in size with the SIM data, and TEM tomography revealed insights into the molecular architecture of mini-rings demonstrating the following key features: (1) overall, a roughly bipartite split into a more ordered/curved and less ordered/curved half is readily discernible; (2) the density distribution in individual strands matches with the X-ray data, suggesting they constitute FtsZ protofilaments; (3) in the less ordered half of the ring, the protofilaments are able to assemble into higher-order structures such as double helices and supercoiled structures. Taken together, the data suggest that the state of existence of mini-rings could be described as metastable and their possible involvement in filament storage and Z-ring assembly is discussed.