A.J. D'Alfonso

Probe integrated scattering cross sections in the analysis of atomic resolution HAADF STEM images

The physical basis for using a probe-position integrated cross section (PICS) for a single column of atoms as an effective way to compare simulation and experiment in high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) is described, and the use of PICS in order to make quantitative use of image intensities is evaluated. It is based upon the calibration of the detector and the measurement of scattered intensities. Due to the predominantly incoherent nature of HAADF STEM, it is found to be robust to parameters that affect probe size and shape such as defocus and source coherence. The main imaging parameter dependencies are on detector angle and accelerating voltage, which are well known. The robustness to variation in other parameters allows for a quantitative comparison of experimental data and simulation without the need to fit parameters. By demonstrating the application of the PICS to the chemical identification of single atoms in a heterogeneous catalyst and in thin, layered-materials, we explore some of the experimental considerations when using this approach.

Energy dispersive X-ray analysis on an absolute scale in scanning transmission electron microscopy.

We demonstrate absolute scale agreement between the number of X-ray counts in energy dispersive X-ray spectroscopy using an atomic-scale coherent electron probe and first-principles simulations. Scan-averaged spectra were collected across a range of thicknesses with precisely determined and controlled microscope parameters. Ionization cross-sections were calculated using the quantum excitation of phonons model, incorporating dynamical (multiple) electron scattering, which is seen to be important even for very thin specimens.

Quantitative atomic resolution elemental mapping via absolute-scale energy dispersive X-ray spectroscopy.

Quantitative agreement on an absolute scale is demonstrated between experiment and simulation for two-dimensional, atomic-resolution elemental mapping via energy dispersive X-ray spectroscopy. This requires all experimental parameters to be carefully characterized. The agreement is good, but some discrepancies remain. The most likely contributing factors are identified and discussed. Previous predictions that increasing the probe forming aperture helps to suppress the channelling enhancement in the average signal are confirmed experimentally. It is emphasized that simple column-by-column analysis requires a choice of sample thickness that compromises between being thick enough to yield a good signal-to-noise ratio while being thin enough that the overwhelming majority of the EDX signal derives from the column on which the probe is placed, despite strong electron scattering effects.

Chemical Imaging at Atomic Resolution as a Technique To Refine the Local Structure of Nanocrystals

The challenging problem of mapping the chemical composition of cation columns in individual nanocrystals at atomic resolution is addressed by using a method based on aberration-corrected electron microscopy, core-loss electron energyloss spectroscopy, and simulations. The potential of this novel approach to provide unique structural information, which is the key to rationalizing macroscopic behavior, is illustrated with the analysis of ceria–zirconia mixed oxides, which are nanomaterials with substantial technological impact. Metal nanoparticles supported on this family of oxides are currently materials of interest as catalysts in a variety of chemical transformations in the area of environmental protection, such as low-temperature water-gas shift, selective oxidation of CO in the presence of large amounts of hydrogen, or three-way catalysis. Strong variations in the chemistry of ceria–zirconia mixed oxide catalysts have been observed after they have undergone redox cycles involving reduction treatments at high temperatures ( 1173 K) then oxidation at mild temperatures ( 823 K). In particular their reducibility is significantly enhanced after such aging treatments. Scanning transmission electron microscopy (STEM) techniques have provided crucial information to account for these changes in the redox behavior. High-resolution electron microscopy (HREM) combined with high-angle annular dark-field (HAADF) imaging and tomography have revealed the occurrence of a disorder–order transformation in the cationic sublattice of these oxides, which tend to rearrange into a distribution characteristic of the so called pyrochlore phase. This phase is an archetype structure for A2B2O7 (A= + 3 cation, B=+ 4 cation) compounds and can be considered a fluorite superstructure. The structural transformation takes place during the reduction step of the cycle, in which the fully reduced mixed oxide with Ce/Zr molar ratio 1:1 adopts the Ce2Zr2O7 stoichiometry. Nevertheless, HAADF studies have clearly shown that, in the case of ceria–zirconia mixed oxides, this cation-ordered arrangement is preserved even after full reoxidation, that is, in the oxide with Ce2Zr2O8 stoichiometry, whenever the oxidation temperature does not exceed 823 K. Electron-microscopy studies have also revealed another remarkable feature of the ceria–zirconia aged oxides with the pyrochlore-type cation sublattice: the occurrence of compositional heterogeneities at the nanometer scale. Taking these observations into account and also considering that the disorder–order transition may not be completed in the time scale and under the temperature conditions used in the redox-cycling treatments, the important question arises whether these heterogeneities are in fact occurring on a finer scale, that is, at the atomic level. Such heterogeneities, compatible with the HREM and HAADF observations, will strongly influence the details of the counterpart oxygen sublattice and, consequently, the chemical and catalytic response of these oxides. To date, the atomic-column by atomic-column compositional analysis of the oxidized pyrochlore required to justify such a possibility has not been accomplished. Herein, using the capabilities of an aberration-corrected Nion UltraSTEM microscope (operated at 100 kV) we not only provide the first direct chemical evidence of the cationic order present in the Ce2Zr2O8 oxidized pyrochlore but we also show how atomicresolution electron energy-loss spectroscopy (EELS) mapping, based on core–shell ionization, can be combined with EELS image simulation to detect quite subtle local deviations in the cation sublattice from the completely ordered structure. This information provides a much more accurate structural description of the active catalyst nanocrystals, which must be considered to model both their oxygen-exchange capabilities and, eventually, their catalytic performance. [*] Dr. S. Trasobares, Dr. M. L pez-Haro, Dr. J. A. Perez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016286 E-mail: susana.trasobares@uca.es Homepage: http://www.uca.es/tem-uca

Thermal diffuse scattering in transmission electron microscopy.

In conventional transmission electron microscopy, thermal scattering significantly affects the image contrast. It has been suggested that not accounting for this correctly is the main cause of the Stobbs factor, the ubiquitous, large contrast mismatch found between theory and experiment. In the case where a hard aperture is applied, we show that previous conclusions drawn from work using bright field scanning transmission electron microscopy and invoking the principle of reciprocity are reliable in the presence of thermal scattering. In the aperture-free case it has been suggested that even the most sophisticated mathematical models for thermal diffuse scattering lack in their numerical implementation, specifically that there may be issues in sampling, including that of the contrast transfer function of the objective lens. We show that these concerns can be satisfactorily overcome with modest computing resources; thermal scattering can be modelled accurately enough for the purpose of making quantitative comparison between simulation and experiment. Spatial incoherence of the source is also investigated. Neglect or inadequate handling of thermal scattering in simulation can have an appreciable effect on the predicted contrast and can be a significant contribution to the Stobbs factor problem.

Surface determination through atomically resolved secondary-electron imaging

Unique determination of the atomic structure of technologically relevant surfaces is often limited by both a need for homogeneous crystals and ambiguity of registration between the surface and bulk. Atomically resolved secondary-electron imaging is extremely sensitive to this registration and is compatible with faceted nanomaterials, but has not been previously utilized for surface structure determination. Here we report a detailed experimental atomic-resolution secondary-electron microscopy analysis of the c(6 × 2) reconstruction on strontium titanate (001) coupled with careful simulation of secondary-electron images, density functional theory calculations and surface monolayer-sensitive aberration-corrected plan-view high-resolution transmission electron microscopy. Our work reveals several unexpected findings, including an amended registry of the surface on the bulk and strontium atoms with unusual seven-fold coordination within a typically high surface coverage of square pyramidal TiO5 units. Dielectric screening is found to play a critical role in attenuating secondary-electron generation processes from valence orbitals.

X-ray holography with a customizable reference.

In X-ray Fourier-transform holography, images are formed by exploiting the interference pattern between the X-rays scattered from the sample and a known reference wave. To date, this technique has only been possible with a limited set of special reference waves. We demonstrate X-ray Fourier-transform holography with an almost unrestricted choice for the reference wave, permitting experimental geometries to be designed according to the needs of each experiment and opening up new avenues to optimize signal-to-noise and resolution. The optimization of holographic references can aid the development of holographic techniques to meet the demands of resolution and fidelity required for single-shot imaging applications with X-ray lasers.

Elemental mapping in scanning transmission electron microscopy

We discuss atomic resolution chemical mapping in scanning transmission electron microscopy (STEM) based on core-loss electron energy loss spectroscopy (EELS) and also on energy dispersive X-ray (EDX) imaging. Chemical mapping using EELS can yield counterintuitive results which, however, can be understood using first principles calculations. Experimental chemical maps based on EDX bear out the thesis that such maps are always likely to be directly interpretable. This can be explained in terms of the local nature of the effective optical potential for ionization under those imaging conditions. This is followed by an excursion into the complementary technique of elemental mapping using energy-filtered transmission electron microscopy (EFTEM) in a conventional transmission electron microscope. We will then consider the widely used technique of Z-contrast or high-angle annular dark field (HAADF) imaging, which is based on phonon excitation, where it has recently been shown that intensity variations can be placed on an absolute scale by normalizing the measured intensities to the incident beam. Results, showing excellent agreement between theory and experiment to within a few percent, are shown for Z-contrast imaging from a sample of PbWO4 .

Direct exit-wave reconstruction from a single defocused image

We propose a direct, non-iterative method for the exact recovery of the complex wave in the exit-surface plane of a coherently illuminated object from a single defocused image. The method is applicable for a wide range of illumination conditions. The defocus range is subject to certain conditions, which if satisfied allow the complex exit-surface wave to be directly recovered by solving a set of linear equations. These linear equations, whose coefficients depend on the incident illumination, are obtained by analyzing the autocorrelation function of an auxiliary wave which is related to the exit-surface wave in a simple way. This autocorrelation is constructed by taking the inverse Fourier transform of the defocused image. We present an experimental proof of concept by recovering the exit-surface wave of a microfiber illuminated by a plane wave formed using a HeNe laser.

Image Contrast in Aberration-Corrected Scanning Confocal Electron Microscopy

Abstract The larger objective lens numerical aperture allowed by spherical aberration correction in electron optics leads to a reduced depth of focus, which typically becomes less than the thickness of the sample. Although this may complicate image interpretation, it leads to an opportunity to measure three‐dimensional information though the technique of optical sectioning. This chapter presents a theoretical analysis of transfer functions and image contrast for aberration‐corrected scanning confocal electron microscopy (SCEM). A comparison is made to optical sectioning using conventional scanning transmission electron microscopy (STEM). It is shown that for bright‐field SCEM there is little contrast, and a missing cone in the transfer function that is also seen for STEM. Energy‐filtered SCEM is seen to have strong transfer and no missing cone in the transfer function.