The influence of C/C correction in analytical imaging and spectroscopy in scanning and transmission electron microscopy

Simultaneous secondary electron microscopy in the scanning transmission electron microscope with applications for in situ studies

Scanning/transmission electron microscopy (STEM) is a powerful characterization tool for a wide range of materials. Over the years, STEMs have been extensively used for in situ studies of structural evolution and dynamic processes. A limited number of STEM instruments are equipped with a secondary electron (SE) detector in addition to the conventional transmitted electron detectors, i.e. the bright-field (BF) and annular dark-field (ADF) detectors. Such instruments are capable of simultaneous BF-STEM, ADF-STEM and SE-STEM imaging. These methods can reveal the 'bulk' information from BF and ADF signals and the surface information from SE signals for materials <200 nm thick. This review first summarizes the field of in situ STEM research, followed by the generation of SE signals, SE-STEM instrumentation and applications of SE-STEM analysis. Combining with various in situ heating, gas reaction and mechanical testing stages based on microelectromechanical systems (MEMS), we show that simultaneous SE-STEM imaging has found applications in studying the dynamics and transient phenomena of surface reconstructions, exsolution of catalysts, lunar and planetary materials and mechanical properties of 2D thin films. Finally, we provide an outlook on the potential advancements in SE-STEM from the perspective of sample-related factors, instrument-related factors and data acquisition and processing.

Machine Learning Refinement of In Situ Images Acquired by Low Electron Dose LC-TEM

We have studied a machine learning (ML) technique for refining images acquired during in situ observation using liquid-cell transmission electron microscopy. Our model is constructed using a U-Net architecture and a ResNet encoder. For training our ML model, we prepared an original image dataset that contained pairs of images of samples acquired with and without a solution present. The former images were used as noisy images, and the latter images were used as corresponding ground truth images. The number of pairs of image sets was 1,204, and the image sets included images acquired at several different magnifications and electron doses. The trained model converted a noisy image into a clear image. The time necessary for the conversion was on the order of 10 ms, and we applied the model to in situ observations using the software Gatan DigitalMicrograph (DM). Even if a nanoparticle was not visible in a view window in the DM software because of the low electron dose, it was visible in a successive refined image generated by our ML model.

Transmission electron microscopy of thick polymer and biological specimens

We explore the properties of elastic and inelastic scattering in a thick organic specimen, together with the mechanisms that provide contrast in a transmission electron microscope (TEM) and scanning-transmission electron microscope (STEM). Experimental data recorded from amorphous carbon are used to predict the bright-field image intensity, mass-thickness contrast and dose-limited resolution as a function of thickness, objective-aperture size, and primary-electron energy E₀. Combining this information with estimates of chromatic aberration, objective-aperture diffraction and beam broadening in the specimen, we calculate the achievable TEM and STEM resolution to be around 4 nm at E₀ = 300 keV (or below 3 nm at MeV energies) for a 10 µm-diameter objective aperture and 1 - 2 µm thickness of hydrated biological tissue. The 3 MeV resolution for a 10-μm tissue sample is probably closer to 10 nm. We also comment on the error involved in quadrature addition of resolution factors, when one or more of the point-spread functions are non-Gaussian.

Visualizing Dynamic Environmental Processes in Liquid at Nanoscale via Liquid-Phase Electron Microscopy

Visualizing the structure and processes in liquids at the nanoscale is essential for understanding the fundamental mechanisms and underlying processes of environmental research. Cutting-edge progress of in situ liquid-phase (scanning) transmission electron microscopy (LP-S/TEM) and inferred possible applications are highlighted as a more and more indispensable tool for visualization of dynamic environmental processes in this Perspective. Advancements in nanofabrication technology, high-speed imaging, comprehensive detectors, and spectroscopy analysis have made it increasingly convenient to use LP S/TEM, thus providing an approach for visualization of direct and insightful scientific information with the exciting possibility of solving an increasing number of tricky environmental problems. This includes evaluating the transformation fate and path of contamination, assessing toxicology of nanomaterials, simulating solid surface corrosion processes in the environment, and observing water pollution control processes. Distinct nanoscale or even atomic understanding of the reaction would provide dependable and precise identification and quantification of contaminants in dynamic processes, thus facilitating trouble-tracing of environmental problems with amplifying complexity.

Liquid-Phase Transmission Electron Microscopy for Reliable In Situ Imaging of Nanomaterials

Liquid-phase transmission electron microscopy (LPTEM) is a powerful in situ visualization technique for directly characterizing nanomaterials in the liquid state. Despite its successful application in many fields, several challenges remain in achieving more accurate and reliable observations. We present LPTEM in chemical and biological applications, including studies for the morphological transformation and dynamics of nanoparticles, battery systems, catalysis, biomolecules, and organic systems. We describe the possible interactions and effects of the electron beam on specimens during observation and present sample-specific approaches to mitigate and control these electron-beam effects. We provide recent advances in achieving atomic-level resolution for liquid-phase investigation of structures anddynamics. Moreover, we discuss the development of liquid cell platforms and the introduction of machine-learning data processing for quantitative and objective LPTEM analysis.

X-ray Spectrometry in the Era of Aberration-Corrected Electron Optical Beam Lines

Aberration correction in the analytical transmission electron microscope is most closely associated with improvements in high-resolution imaging. In this paper, the combination of that technology with new system designs, which optimize both electron optics and x-ray detection, is shown to provide more than a tenfold increase in performance over the last 25 years.

The influence of chromatic aberration on the dose-limited spatial resolution of transmission electron microscopy

The effect of chromatic aberration (CC) on the spatial resolution in transmission electron microscopy (TEM) was studied in thick specimens in which the sample becomes the limiting factor in the resolution. The sample influences the energy spread of the electron beam, allows only a limited electron dose, and modulates electron scattering events. The experimental set-up consisted of a thin silicon nitride membrane and a silicon wedge containing gold nanoparticles. The resolution was measured as a function of electron dose and sample thickness for different sample configurations and for different microscopy modalities including regular TEM, energy filtered TEM (EFTEM) and CC-corrected TEM. Comparison with an analytical model aided the understanding of the experimental data applied over varied conditions. The general trend for all microscopy modalities was a transition from a noise-limited resolution at low electron dose to a CC-limited resolution at high-dose in the absence of beam blurring. EFTEM required an accurate energy slit offset and an optimal energy spread to energy-slit width ratio to surpass regular TEM. The key advantage of CC correction appeared to be the best possible resolution for larger sample thickness at low electron dose outperforming EFTEM by about fifty percent. Several hypothetical sample configurations relevant to liquid phase electron microscopy were evaluated as well to demonstrate the capabilities of the analytical model and to determine the most optimal microscopy modality for this type of experiment. The analytical model included an automated optimization of the EFTEM settings and may aid in optimizing the sample-limited resolution for experimental analysis and planning.

Graphene Liquid Cell Electron Microscopy: Progress, Applications, and Perspectives

Graphene liquid cell electron microscopy (GLC-EM), a cutting-edge liquid-phase EM technique, has become a powerful tool to directly visualize wet biological samples and the microstructural dynamics of nanomaterials in liquids. GLC uses graphene sheets with a one carbon atom thickness as a viewing window and a liquid container. As a result, GLC facilitates atomic-scale observation while sustaining intact liquids inside an ultra-high-vacuum transmission electron microscopy chamber. Using GLC-EM, diverse scientific results have been recently reported in the material, colloidal, environmental, and life science fields. Here, the developments of GLC fabrications, such as first-generation veil-type cells, second-generation well-type cells, and third-generation liquid-flowing cells, are summarized. Moreover, recent GLC-EM studies on colloidal nanoparticles, battery electrodes, mineralization, and wet biological samples are also highlighted. Finally, the considerations and future opportunities associated with GLC-EM are discussed to offer broad understanding and insight on atomic-resolution imaging in liquid-state dynamics.

Liquid-Phase Electron Microscopy for Soft Matter Science and Biology

Innovations in liquid-phase electron microscopy (LP-EM) have made it possible to perform experiments at the optimized conditions needed to examine soft matter. The main obstacle is conducting experiments in such a way that electron beam radiation can be used to obtain answers for scientific questions without changing the structure and (bio)chemical processes in the sample due to the influence of the radiation. By overcoming these experimental difficulties at least partially, LP-EM has evolved into a new microscopy method with nanometer spatial resolution and sub-second temporal resolution for analysis of soft matter in materials science and biology. Both experimental design and applications of LP-EM for soft matter materials science and biological research are reviewed, and a perspective of possible future directions is given.

Theoretical and Experimental X-Ray Peak/Background Ratios and Implications for Energy-Dispersive Spectrometry in the Next-Generation Analytical Electron Microscope

Modern analytical electron microscopes equipped with silicon drift detectors now allow for a wide range of geometrical configurations capable of performing quantitative X-ray spectrometry. Recent work has improved the collection solid angles of these detectors, however, the impact of increasing the solid angle on detection sensitivity as measured by the peak/background ratio has not been addressed. This work compares theoretical and experimental peak/background ratios for incident electron energies in the range of 20-200 keV, with X-ray detectors in both conventional orientations (on the electron entrance surface) as well as new geometries (the electron exit surface). The implications of these parameters on detectability limits for the next generations of "Lab-in-the-Gap" analytical microscope are also considered. It was found that theoretical calculations of the angular distribution of bremsstrahlung and their effects on the peak/background ratio match well with experimental measurements, and indicate that new geometries which can result in large solid angles provided an added benefit in addition to increased characteristic signal, namely increased sensitivity for the analyst.