Automating ALCHEMI at the nano-scale using software compatible with PC-controlled transmission electron microscopy.

Atom location by channeling-enhanced microanalysis (ALCHEMI) is a technique to obtain atom-site-specific information on constituent elements in a crystalline sample by acquiring a set of core electron transition spectra while tilting the incident beam. This methodology has been extended to a more quantitative technique called high-angular-resolution electron-channeled X-ray/electron spectroscopy (HARECXS/HARECES). There is a growing demand for analyzing smaller areas, such as small particles and multilayers. However, the minimum size of a region of interest probed by the present hardware-assisted automated HARECXS/HARECES scheme is limited to no smaller than 1 µm, not only by the size of the electron probe and its convergence angle but also by the movement of the probe position associated with the beam tilt due to aberrations of the hardware system. Herein, QED (quantitative electron diffraction), a commercial plug-in working on an integrated software platform, Gatan Microscopy Suite, was modified to enable the calibration and control of the probe to resolve the aforementioned limitation. In addition, a more sophisticated scheme for QED was developed to realize the ALCHEMI method for energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy or both concurrently. This allows access to ALCHEMI and its derivative methods, automatically executed with any type of current PC-controlled commercial microscope on an area as small as 30 nm, without modifying the hardware system.

Quantitative Atomic-Site Analysis of Functional Dopants/Point Defects in Crystalline Materials by Electron-Channeling-Enhanced Microanalysis.

A novel elemental and chemical analysis scheme based on electron-channeling phenomena in crystalline materials is introduced, where the incident high-energy electron beam is rocked with the submicrometric pivot point fixed on a specimen. This method enables us to quantitatively derive the site occupancies and site-dependent chemical information of impurities or intentionally doped functional elements in a specimen, using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy attached to a scanning transmission electron microscope, which is of significant interest to current materials science, particularly related to nanotechnologies. This scheme is applicable to any combination of elements even when the conventional Rietveld analysis by X-ray or neutron diffraction occasionally fails to provide the desired results because of limited sample sizes and close scattering factors of neighboring elements in the periodic table. In this methodological article, we demonstrate the basic experimental procedure and analysis method of the present beam-rocking microanalysis.

Proposal for Measuring Magnetism with Patterned Apertures in a Transmission Electron Microscope.

We propose a magnetic measurement method utilizing a patterned postsample aperture in a transmission electron microscope. While utilizing electron magnetic circular dichroism, the method circumvents previous needs to shape the electron probe to an electron vortex beam or astigmatic beam. The method can be implemented in standard scanning transmission electron microscopes by replacing the spectrometer entrance aperture with a specially shaped aperture, hereafter called a ventilator aperture. The proposed setup is expected to work across the whole range of beam sizes-from wide parallel beams down to atomic resolution magnetic spectrum imaging.

Unmixing hyperspectral data by using signal subspace sampling.

This paper demonstrates how Signal Subspace Sampling (SSS) is an effective pre-processing step for Non-negative Matrix Factorization (NMF) or Vertex Component Analysis (VCA). The approach allows to uniquely extract non-negative source signals which are orthogonal in at least one observation channel, respectively. It is thus well suited for processing hyperspectral images from X-ray microscopy, or other emission spectroscopies, into its non-negative source components. The key idea is to resample the given data so as to satisfy better the necessity and sufficiency conditions for the subsequent NMF or VCA. Results obtained both on an artificial simulation study as well as based on experimental data from electron-microscopy are reported.

Quantitative determination of occupation sites of trace Co substituted for multiple Fe sites in M-type hexagonal ferrite using statistical beam-rocking TEM-EDXS analysis.

The occupation sites and the occupancies of trace dopants in La/Co co-doped Sr-M-type ferrite, SrFe12O19, were quantitatively and precisely determined by beam-rocking energy-dispersive X-ray spectroscopy (EDXS) on the basis of electron-channeling effects. Because the Co atoms, in particular, should be partially substituted for the five crystallographically inequivalent sites, which could be key parameters in improving the magneto-crystalline anisotropy, it is difficult yet intriguing to discover their occupation sites and occupancies without using the methods of large-scale facilities, such as neutron diffraction and synchrotron radiation. In the present study, we tackled this problem by applying an extended statistical atom location by channeling enhanced microanalysis method, using conventional transmission electron microscopy, EDXS and dynamical electron elastic/inelastic scattering theories. The results show that the key occupation sites of Co were the 2a, 4f1 and 12k sites. The quantified occupancies of Co were consistent with those of the previous study, which involved a combination of neutron diffraction and extended X-ray absorption fine structure analysis, as well as energetics considerations based on by first-principles calculations.

Image formation mechanisms of spherical aberration corrected BF STEM imaging methods.

In this study, we explore the formation mechanisms of different spherical-aberration (C(s))-corrected bright-field (BF) scanning transmission electron microscope (STEM) imaging methods. The C(s)-corrected BF STEM imaging modes are characterised in detail using simulated images and experimental BF STEM images obtained with several types of detectors. The Co3O4 specimen results show that the occupancy, the atomic spacing, and the atomic number of the atoms constituting the atomic columns control image formation in BF STEM imaging, which is used to detect light atomic columns. The middle-angle BF STEM image is crucial in image formation by BF STEM imaging.

Bloch wave simulations in the frozen lattice approximation.

To simulate the effects of thermal diffuse scattering in the frozen lattice (FL) approximation, we modify the conventional Bloch wave method by applying the classical scattering matrix method. The present simulation method requires that the eigenvalue operation is executed only once for each partial incident wave using the atomic positions of the thermal equilibrium configuration and thus the calculation time decreases drastically. Furthermore, from a comparison of convergent beam electron diffraction (CBED) patterns simulated by the present method with the results of CBED patterns simulated by a multislice method, we confirm that FL approximation can be simulated by the dynamical simulation based on the Bloch wave method.

Imaging of light and heavy atomic columns by spherical aberration corrected middle-angle bright-field STEM.

Simultaneous detection of both light and heavy atomic columns is theoretically and experimentally explored with spherical aberration (C(s))-corrected middle-angle bright-field (MABF) scanning transmission electron microscopy (STEM). Optimized MABF STEM visualizes both light O atomic columns and heavy Sr and Ti-O atomic columns for SrTiO3(001) as distinct bright spots and dark spots with characteristic bright rings, respectively, over practical ranges of the probe-forming lens defocus and sample thickness, although medium-heavy Ti-O atomic columns appear as blurred dark spots. The difference in contrast between heavy and light atomic columns is greater than that of annular BF STEM images. The formation of distinctive bright and dark spots is interpreted simply as the difference in the degrees of localization and inelastic absorption of channeling electrons in individual atomic columns by analyses of convergent wave fields inside the crystal in both real and reciprocal space. In addition, Bloch wave expansion of MABF STEM images suggests that bright rings are formed mainly by 2p-like convergent Bloch wave fields localized on heavy atomic columns.

Effect of convergent beam semiangle on image intensity in HAADF STEM images.

In this study, we experimentally and theoretically show that the intensities of bright spots in a spherical aberration (C(s))-uncorrected high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image of [011]-oriented Co(3)O(4), which has two different numbers of Co atoms in the projected atomic columns, are reversed with increasing sample thickness. However, C(s)-corrected HAADF STEM images produce intensities that correctly depend on the average number of atoms in the projected atomic columns. From an analysis based on the Bloch-wave theorem, it is found that an insufficient semiangle of the incident convergent beam yields intensities that do not depend on the average atomic number in the atomic columns.

Effect of chromatic aberration on atomic-resolved spherical aberration corrected STEM images.

The effect of the chromatic aberration (C(c)) coefficient in a spherical aberration (C(s))- corrected electromagnetic lens on high-resolution high-angle annular dark field (HAADF) scanning transmission electron microscope (STEM) images is explored in detail. A new method for precise determination of the C(c) coefficient is demonstrated, requiring measurement of an atomic-resolution one-frame through-focal HAADF STEM image. This method is robust with respect to instrumental drift, sample thickness, all lens parameters except C(c), and experimental noise. It is also demonstrated that semi-quantitative structural analysis on the nanometer scale can be achieved by comparing experimental C(s)- corrected HAADF STEM images with their corresponding simulated images when the effects of the C(c) coefficient and spatial incoherence are included.

Many-beam dynamical simulation for multilayer structures without a superlattice cell.

A many-beam dynamical theory for plan-view high-resolution transmission electron microscopy (HRTEM) images of multilayer systems without the limitation of a superlattice cell is proposed. The accuracy of our method is examined by comparing convergent-beam electron-diffraction calculations of Si(011) and HRTEM calculations of a system of epitaxial Al(100) on GaAs(100). Furthermore, this method is applied to CdSe clusters embedded in MgO, where it is revealed that the relative shift of their crystal-lattice planes produces moiré-like fringes.

Quantitative and easy estimation of a crystal bending effect using low-order CBED patterns.

The quantitative measurement of a crystal bending effect is performed using low-order zone-axis convergent beam electron diffraction (CBED) patterns. Although the accuracy of the present method is inferior to that of the method of using split higher order Laue zone lines, this method enables us to estimate the crystal bending effect at a region very close to the interface and to easily judge whether the crystal bending effect results in a tensile bend or a compressive bend. As an application of the present method, the crystal bending effect at a region close to the SiGe/Si interface was measured. It was found that the crystal bending effect is due to a thin-foil relaxation of almost 0.3 degrees at a region that is approximately 10 nm away from the interface.