Achromatic elemental mapping beyond the nanoscale in the transmission electron microscope.

Newly developed achromatic electron optics allows the use of wide energy windows and makes feasible energy-filtered transmission electron microscopy (EFTEM) at atomic resolution. In this Letter we present EFTEM images formed using electrons that have undergone a silicon L(2,3) core-shell energy loss, exhibiting a resolution in EFTEM of 1.35 Å. This permits elemental mapping beyond the nanoscale provided that quantum mechanical calculations from first principles are done in tandem with the experiment to understand the physical information encoded in the images.

3-D reconstruction of the atomic positions in a simulated gold nanocrystal based on discrete tomography: prospects of atomic resolution electron tomography.

A novel reconstruction procedure is proposed to achieve atomic resolution in electron tomography. The method exploits the fact that crystals are discrete assemblies of atoms (atomicity). This constraint enables us to obtain a three-dimensional (3-D) reconstruction of test structures from less than 10 projections even in the presence of noise and defects. Phase contrast transmission electron microscopy (TEM) images of a gold nanocrystal were simulated in six different zone axes. The discrete number of atoms in every column is determined by application of the channelling theory to reconstructed electron exit waves. The procedure is experimentally validated by experiments with gold samples. Our results show that discrete tomography recovers the shape of the particle as well as the position of its 309 atoms from only three projections. Experiments on a nanocrystal that contains several missing atoms, both on the surface and in the core of the nanocrystal, while considering a high noise level in each simulated image were performed to prove the stability of the approach to reconstruct defects. The algorithm is well capable of handling structural defects in a highly noisy environment, even if this causes atom count "errors" in the projection data.

Electron channelling based crystallography.

Electron channelling occurs when the incident electron beam is parallel to the atom columns of an object, such as a crystal or a particular crystal defect. Then, the electrons are trapped in the electrostatic potential of an atom column in which they scatter dynamically. This picture provides physical insight and explains why a one-to-one correspondence is maintained between the exit wave and the projected structure, even in case of strong dynamical scattering. Moreover, the theory is very useful to invert the dynamical scattering, that is, to derive the projected structure from the exit wave. Finally, it can be used to determine the composition of an atom column with single atom sensitivity or to explain dynamical electron diffraction effects. In this paper, an overview of the channelling theory will be given together with some recent applications.

Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity.

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. In particular, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The concept of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80 kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are illustrated by exit wave reconstructions of TEM images of a nanocrystalline Co3O4 catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.

Advances in the environmental transmission electron microscope (ETEM) for nanoscale in situ studies of gas-solid interactions.

Although available since the early days of electron microscopy, recent technology developments of the environmental transmission electron microscope (ETEM) have enabled new research in the study of nanomaterials in gaseous environments. Significant improvements in scanning/transmission electron microscope (S/TEM) technologies, while containing a gaseous environment close to the object under investigation, enable now the atomic scale study of phenomena occurring during gas-solid interactions. A focus behind these developments is the research on nanomaterial-based technologies, for instance for efficient energy conversion, use and storage as well as for environmental protection. In situ high spatial resolution characterization provides unique information that is beneficial for understanding the relationship between the structure, properties and function of nanostructures directly on their characteristic length scales. The progress in recent research is reviewed to highlight the potential of the state-of-the-art differentially-pumped microscope platform, based on the latest microscope generation optimized for atomic scale in situ investigations. Using cases from current catalysis research, high resolution imaging reveals structural changes in nanocatalysts when being active and is instrumental in understanding deactivation processes; while spectroscopy gives additional access to reactivity. Also, imaging schemes are discussed that focus on enhancing the achievable imaging resolution, while having the effect of electron beam-solid interaction in the nanomaterial under control.

Image resolution and sensitivity in an environmental transmission electron microscope.

An environmental transmission electron microscope provides unique means for the atomic-scale exploration of nanomaterials during the exposure to a reactive gas environment. Here we examine conditions to obtain such in situ observations in the high-resolution transmission electron microscopy (HRTEM) mode with an image resolution of 0.10nm. This HRTEM image resolution threshold is mapped out under different gas conditions, including gas types and pressures, and under different electron optical settings, including electron beam energies, doses and dose-rates. The 0.10nm resolution is retainable for H(2) at 1-10mbar. Even for N(2), the 0.10nm resolution threshold is reached up to at least 10mbar. The optimal imaging conditions are determined by the electron beam energy and the dose-rate as well as an image signal-to-noise (S/N) ratio that is consistent with Rose's criterion of S/N≥5. A discussion on the electron-gas interactions responsible for gas-induced resolution deterioration is given based on interplay with complementary electron diffraction (ED), scanning transmission electron microscopy (STEM) as well as electron energy loss spectroscopy (EELS) data.