STEM imaging of thick specimens with off-axis detectors

For scanning transmission electron microscopy (STEM) images obtained with relatively small objective aperture sizes, the contrast of small objects contained within thick specimens may be considerably enhanced by using an off-axis detector aperture situated on the edge of the central beam spot. The effect is demonstrated for both crystalline and amorphous specimens. The effect arises because the detector collects part of the small angle inelastic scattering and is modified by refraction effects for specimens of rapidly changing thickness.     

Calculation of Bohmian quantum trajectories for STEM

We present in this work the calculation of Bohmian quantum trajectories representing the wave function propagation in a crystal for a focused electron probe in a scanning transmission electron microscope (STEM). The wave function and quantum trajectories are obtained from the calculation of time‐dependent Schrödinger equation by fast Fourier transformation multislice algorithm. In our work, the Bohmian quantum trajectories of a scanning probe penetrating a Cu crystal are studied as an example of this calculation scheme. The results help us to better understand the electron diffraction process in a microscopic imaging from a trajectory‐based point of view. This Bohmian quantum trajectory method can be used to extend the application of classical Monte Carlo method from the study of electron interaction with amorphous solid to crystalline structure.     

Quantitative STEM mass measurement of biological macromolecules in a 300 kV TEM

For almost four decades, the scanning transmission electron microscope (STEM) has made significant contributions to structural biology by providing accurate determinations of the molecular masses of large protein assemblies that have arbitrary shapes and sizes. Nevertheless, STEM mass mapping has been implemented in very few laboratories, most of which have employed cold field‐emission gun (FEG) electron sources operating at acceleration voltages of 100 kV and lower. Here we show that a 300 kV commercial transmission electron microscope (TEM) equipped with a thermally assisted Shottky FEG can also provide accurate STEM mass measurements. Using the recently published database of elastic‐scattering cross sections from the National Institute of Standards and Technology, we show that the measured absolute mass values for tobacco mosaic virus and limpet hemocyanin didecamers agree with the known values to within better than 10%. Applying the established approach, whereby tobacco mosaic virus is added to a specimen as a calibration standard, we find that the measured molecular weight of the hemocyanin assemblies agrees with the known value to within 3%. This accuracy is achievable although only a very small fraction (∼0.002) of the incident probe current of 300 kV electrons is scattered onto the annular dark‐field STEM detector. FEG TEMs operating at intermediate voltages (200–400 kV) are becoming common tools for determining the structure of frozen hydrated protein assemblies. The ability to perform mass determination with the same instrument can provide important complementary information about the numbers of subunits comprising the protein assemblies whose structure is being studied.     

STEM study of Li4Ti5O12 anode material modified with Ag nanoparticles

Comprehensive scanning transmission electron microscopy (STEM) analysis of Li₄Ti₅O₁₂ (LTO) powder modified by deposited Ag nanoparticles was performed. Nanocomposite powders with Ag content of 1 wt.%, 4 wt.%, 10 wt.% were fabricated in a chemical process from suspensions of Ag and LTO. Apart from the STEM results, the presence of pure silver on the surface of the ceramic powder was confirmed by XRD and XPS analyses. The silver particles deposited on the LTO particles were characterized using the EDS mapping technique. The quantified results of the EDS mapping showed a relatively homogenous distribution of silver nanoparticles on the powder surface for every metal content. The mean diameter of the nanoparticles deposited on the LTO powder was about 4 nm in all cases. An increase in the Ag content during chemical surface modification did not cause changes in the microstructure. Focusing on an analysis of the metallic nanoparticles on the ceramic powder, electron tomography was used as an investigative technique. A very precise analysis of three‐dimensional nanostructures is desirable for a comprehensive analysis of complex materials. The quantified analysis of the Ag nanoparticles visualized using electron tomography confirmed the results of the size measurements taken from the two‐dimensional EDS maps.     

Aberration‐corrected STEM for atomic‐resolution imaging and analysis

Aberration‐corrected scanning transmission electron microscopes are able to form electron beams smaller than 100 pm, which is about half the size of an average atom. Probing materials with such beams leads to atomic‐resolution images, electron energy loss and energy‐dispersive X‐ray spectra obtained from single atomic columns and even single atoms, and atomic‐resolution elemental maps. We review briefly how such electron beams came about, and show examples of applications. We also summarize recent developments that are propelling aberration‐corrected scanning transmission electron microscopes in new directions, such as complete control of geometric aberration up to fifth order, and ultra‐high‐energy resolution EELS that is allowing vibrational spectroscopy to be carried out in the electron microscope.     

The effect of vacancies on the annular dark field image contrast of grain boundaries: a SrTiO(3) case study

The analysis of grain boundary structure in high resolution electron microscopy is often hindered by contrast variation within the grain boundary region which is not explained by simple models of the grain boundary structure. Recent work suggests that structural disorder along the beam direction and the presence of vacancies contribute significantly to this effect. One might expect a significant reduction in contrast in a Z-contrast image of a grain boundary would imply that vacancies present must result from the absence of heavier elements. Using a [001](210) Σ5 grain boundary in SrTiO₃ as a test case and first principles structure relaxation to calculate stable defect structures, we show that the reduction in the intensity from fully occupied Sr columns due to the structural distortion resulting from a nearby O vacancy can be as great as that due to introducing a Sr vacancy in the column itself. The effect on energy dispersive X-ray spectroscopy signals is also considered, but found to be smaller than that on Z-contrast images.     

Elemental mapping with elastically scattered electrons

We describe a technique for efficient, quantitative, standardless elemental mapping using a high-angle annular detector in a scanning transmission electron microscope (STEM) to collect elastically scattered electrons. With a single crystal specimen, contrast due to thickness variations, diffraction, and channelling effects can be avoided, so that the resulting image contrast quantitatively reflects variations in impurity concentration. We compare a number of simple analytical approximations to the elastic scattering cross sections and show that a standardless analysis is possible over a wide range of atomic number and inner detector angle to an absolute accuracy of better than 20%.     

Miniature magnetic electron lenses with permanent magnets

New permanent magnet materials offer the possibility of designing lenses for applications at lower voltages. This paper briefly presents the finite-element method (FEM) program for computation of lenses based on permanent magnets. The design of a lens unit for a low-voltage TEM, realized in Brno, is described together with some design proposals of SEM and STEM lenses and computed results for a rotation-free projector proposed by Le Poole in 1972.