Elemental mapping in achromatic atomic-resolution energy-filtered transmission electron microscopy

Current opinion on the prospect of mapping electronic orbitals in the transmission electron microscope: State of the art, challenges and perspectives

The concept of electronic orbitals has enabled the understanding of a wide range of physical and chemical properties of solids through the definition of, for example, chemical bonding between atoms. In the transmission electron microscope, which is one of the most used and powerful analytical tools for high-spatial-resolution analysis of solids, the accessible quantity is the local distribution of electronic states. However, the interpretation of electronic state maps at atomic resolution in terms of electronic orbitals is far from obvious, not always possible, and often remains a major hurdle preventing a better understanding of the properties of the system of interest. In this review, the current state of the art of the experimental aspects for electronic state mapping and its interpretation as electronic orbitals is presented, considering approaches that rely on elastic and inelastic scattering, in real and reciprocal spaces. This work goes beyond resolving spectral variations between adjacent atomic columns, as it aims at providing deeper information about, for example, the spatial or momentum distributions of the states involved. The advantages and disadvantages of existing experimental approaches are discussed, while the challenges to overcome and future perspectives are explored in an effort to establish the current state of knowledge in this field. The aims of this review are also to foster the interest of the scientific community and to trigger a global effort to further enhance the current analytical capabilities of transmission electron microscopy for chemical bonding and electronic structure analysis.

Lattice contrast in the core-loss EFTEM signal of graphene

The realization of chromatic aberration correction enables energy-filtered transmission electron microscopy (EFTEM) at atomic resolution even for large energy windows. Previous works have demonstrated lattice contrast from ionization-edge signals such as the L_2,3 edges of silicon or titanium. However, the direct interpretation as chemical information was found to be hampered by contributions from elastic contrast with dynamic scattering, especially for thick samples. Here we demonstrate that even for thin samples with light atoms, the interpretation of the ionization-edge signal is complicated by inversions from bright-atom to dark-atom contrast. Our EFTEM experiments for graphene show lattice contrast in the carbon K-edge signal, and we find bright-atom and dark-atom contrast for different defoci.

Guest Transition Metals in Host Inorganic Nanocapsules: Single Sites, Discrete Electron Transfer, and Atomic Scale Structure

Host-guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate-type molybdenum oxide-based porous, spherical clusters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here, we demonstrate the synthetically controlled encapsulation of first-row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate anions, {Mo132PO4}. The resulting M2+x⊂{Mo132PO4} host-guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encapsulation of up to 10 equiv showed little magnetic interaction between the encapsulated metals, which indicated that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal were possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light-induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future, such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photoinitiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.

CryoEM at 100 keV: a demonstration and prospects

100 kV is investigated as the operating voltage for single-particle electron cryomicroscopy (cryoEM). Reducing the electron energy from the current standard of 300 or 200 keV offers both cost savings and potentially improved imaging. The latter follows from recent measurements of radiation damage to biological specimens by high-energy electrons, which show that at lower energies there is an increased amount of information available per unit damage. For frozen hydrated specimens around 300 Å in thickness, the predicted optimal electron energy for imaging is 100 keV. Currently available electron cryomicroscopes in the 100-120 keV range are not optimized for cryoEM as they lack both the spatially coherent illumination needed for the high defocus used in cryoEM and imaging detectors optimized for 100 keV electrons. To demonstrate the potential of imaging at 100 kV, the voltage of a standard, commercial 200 kV field-emission gun (FEG) microscope was reduced to 100 kV and a side-entry cryoholder was used. As high-efficiency, large-area cameras are not currently available for 100 keV electrons, a commercial hybrid pixel camera designed for X-ray detection was attached to the camera chamber and was used for low-dose data collection. Using this configuration, five single-particle specimens were imaged: hepatitis B virus capsid, bacterial 70S ribosome, catalase, DNA protection during starvation protein and haemoglobin, ranging in size from 4.5 MDa to 64 kDa with corresponding diameters from 320 to 72 Å. These five data sets were used to reconstruct 3D structures with resolutions between 8.4 and 3.4 Å. Based on this work, the practical advantages and current technological limitations to single-particle cryoEM at 100 keV are considered. These results are also discussed in the context of future microscope development towards the goal of rapid, simple and widely available structure determination of any purified biological specimen.

The energy dependence of contrast and damage in electron cryomicroscopy of biological molecules

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Carbon elastic and inelastic electron scattering cross sections are measured vs. energy.

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Elastic scattering is compared to energy deposition and radiation damage.

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An optimal energy for cryoEM of a given biological specimen thickness is determined.

We have measured the dependence on electron energy of elastic and inelastic scattering cross-sections from carbon, over the energy range that includes 100 keV to 300 keV. We also compared quantitatively the radiation damage to bacteriorhodopsin and paraffin (C₄₄H₉₀) at 100 keV and 300 keV by observing the fading of the diffraction spots from two-dimensional crystals as a function of electron fluence. The elastic cross-section is 2.01 - fold greater at 100 keV than at 300 keV, whereas the radiation damage increased by only 1.57. This implies that the amount of useful information from diffraction patterns or images of most biological structures should be 25% greater using 100 keV rather than 300 keV electrons. Using these measurements, we calculate the energy dependence of the available information per unit damage for a specimen of a particular thickness, which we call the “information coefficient.” This allows us to determine the optimal energy for imaging a biological specimen of a given thickness. We find that for most single particle cryoEM specimens, 100 keV provides not only the highest potential for information per unit damage, but would also simplify the instrument while retaining the potential to reach high resolution with a minimum of data. These measurements will help guide the development and use of electron cryomicroscopes for biology.

Atomic scale imaging of magnetic circular dichroism by achromatic electron microscopy

In order to obtain a fundamental understanding of the interplay between charge, spin, orbital and lattice degrees of freedom in magnetic materials and to predict and control their physical properties^1-3, experimental techniques are required that are capable of accessing local magnetic information with atomic-scale spatial resolution. Here, we show that a combination of electron energy-loss magnetic chiral dichroism ⁴ and chromatic-aberration-corrected transmission electron microscopy, which reduces the focal spread of inelastically scattered electrons by orders of magnitude when compared with the use of spherical aberration correction alone, can achieve atomic-scale imaging of magnetic circular dichroism and provide element-selective orbital and spin magnetic moments atomic plane by atomic plane. This unique capability, which we demonstrate for Sr₂FeMoO₆, opens the door to local atomic-level studies of spin configurations in a multitude of materials that exhibit different types of magnetic coupling, thereby contributing to a detailed understanding of the physical origins of magnetic properties of materials at the highest spatial resolution.

Atomic resolution elemental mapping using energy-filtered imaging scanning transmission electron microscopy with chromatic aberration correction

This paper addresses a novel approach to atomic resolution elemental mapping, demonstrating a method that produces elemental maps with a similar resolution to the established method of electron energy-loss spectroscopy in scanning transmission electron microscopy. Dubbed energy-filtered imaging scanning transmission electron microscopy (EFISTEM) this mode of imaging is, by the quantum mechanical principle of reciprocity, equivalent to tilting the probe in energy-filtered transmission electron microscopy (EFTEM) through a cone and incoherently averaging the results. In this paper we present a proof-of-principle EFISTEM experimental study on strontium titanate. The present approach, made possible by chromatic aberration correction, has the advantage that it provides elemental maps which are immune to spatial incoherence in the electron source, coherent aberrations in the probe-forming lens and probe jitter. The veracity of the experiment is supported by quantum mechanical image simulations, which provide an insight into the image-forming process. Elemental maps obtained in EFTEM suffer from the effect known as preservation of elastic contrast, which, for example, can lead to a given atomic species appearing to be in atomic columns where it is not to be found. EFISTEM very substantially reduces the preservation of elastic contrast and yields images which show stability of contrast with changing thickness. The experimental application is demonstrated in a proof-of-principle study on strontium titanate.

Simulation in elemental mapping using aberration-corrected electron microscopy

Elemental mapping at the atomic scale in aberration-corrected electron microscopes is becoming increasingly widely used. In this paper we describe the essential role of simulation in understanding the underlying physics and thus in correctly interpreting these maps, both qualitatively and quantitatively.

Modified automatic term selection v2: A faster algorithm to calculate inelastic scattering cross-sections

We present a new algorithm for calculating inelastic scattering cross-section for fast electrons. Compared to the previous Modified Automatic Term Selection (mats) algorithm (Rusz et al. [18]), it has far better convergence properties in zone axis calculations and it allows to identify contributions of individual atoms. One can think of it as a blend of mats algorithm and a method described by Weickenmeier and Kohl [10].

Deciphering the physics and chemistry of perovskites with transmission electron microscopy

Perovskite oxides exhibit rich structural complexity and a broad range of functional properties, including ferroelectricity, ferromagnetism, and superconductivity. The development of aberration correction for the transmission electron microscope and concurrent progress in electron spectroscopy, electron holography, and other techniques has fueled rapid progress in the understanding of the physics and chemistry of these materials. New techniques based on the transmission electron microscope are first surveyed, and the applications of these techniques for the study of the structure, chemistry, electrostatics, and dynamics of perovskite oxides are then explored in detail, with a particular focus on ferroelectric materials.

Addressing preservation of elastic contrast in energy-filtered transmission electron microscopy

Energy-filtered transmission electron microscopy (EFTEM) images with resolutions of the order of an Ångström can be obtained using modern microscopes corrected for chromatic aberration. However, the delocalized nature of the transition potentials for atomic ionization often confounds direct interpretation of EFTEM images, leading to what is known as "preservation of elastic contrast". In this paper we demonstrate how more interpretable images might be obtained by scanning with a focused coherent probe and incoherently averaging the energy-filtered images over probe position. We dub this new imaging technique energy-filtered imaging scanning transmission electron microscopy (EFISTEM). We develop a theoretical framework for EFISTEM and show that it is in fact equivalent to precession EFTEM, where the plane wave illumination is precessed through a range of tilts spanning the same range of angles as the probe forming aperture in EFISTEM. It is demonstrated that EFISTEM delivers similar results to scanning transmission electron microscopy with an electron energy-loss spectrometer but has the advantage that it is immune to coherent aberrations and spatial incoherence of the probe and is also more resilient to scan distortions.

The role of symmetry in the theory of inelastic high-energy electron scattering and its application to atomic-resolution core-loss imaging

The inelastic scattering of a high-energy electron in a solid constitutes a bipartite quantum system with an intrinsically large number of excitations, posing a considerable challenge for theorists. It is demonstrated how and why the utilization of symmetries, or approximate symmetries, can lead to significant improvements in both the description of the scattering physics and the efficiency of numerical computations. These ideas are explored thoroughly for the case of core-loss excitations, where it is shown that the coupled angular momentum basis leads to dramatic improvements over the bases employed in previous work. The resulting gains in efficiency are demonstrated explicitly for K-, L- and M-shell excitations, including such excitations in the context of atomic-resolution imaging in the scanning transmission electron microscope. The utilization of other symmetries is also discussed.