Measurement of twofold astigmatism of probe-forming lens using low-order zone-axis ronchigram.

By using a low-order zone-axis ronchigram of a crystalline sample, a simple method for measuring twofold astigmatism of a probe-forming lens is proposed. This method allows precise measurement of the value of astigmatism from only one experimental ronchigram.

An expanded approach to noise reduction from high-resolution STEM images based on the maximum entropy method.

An expanded use of the maximum entropy method (MEM) is suggested to reduce noise from an experimental high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) image. The MEM is combined with an estimate of the standard deviation of noise from an experimental HAADF STEM image and low-pass filtering using the information limit for an incoherent STEM image. Consequently, the present method has just one parameter of a Lagrange multiplier. It is demonstrated that the present method can reduce noise efficiently in high-resolution HAADF STEM images.

Precise measurement of third-order spherical aberration using low-order zone-axis Ronchigrams.

A method for the measurement of third-order spherical aberration coefficients (C(s)) is suggested, using low-order zone-axis Ronchigrams of a crystalline material. The validity of the method is confirmed using simulated and experimental Ronchigrams taken with various probe-forming lens configurations. The precision of the measured C(s) value is drastically improved compared with that obtained from the power spectrum-analysis method. In addition, a method for roughly estimating defocus values is presented.

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.

Analysis of EEL spectrum of low-loss region using the C(s)-corrected STEM-EELS method and multivariate analysis.

We analyzed a Si/SiO(2) interface using multivariate analysis and spherical aberration-corrected scanning transmission electron microscopy-electron energy loss spectroscopy which is characterized by using the electron energy loss spectrum of the low-loss region. We extracted the low-loss spectra of Si, SiO(2) and an interface state. Even if the interface is formed from materials with different dielectric functions, the present method will prove suitable for obtaining a more quantitative understanding of the dielectric characteristic.

Study of atomic resolved plasmon-loss image by spherical aberration-corrected STEM-EELS method.

To gain an understanding of a plasmon-loss image obtained with an atomic resolution scanning transmission electron microscope (STEM)-electron energy loss spectroscopy (EELS) method, the detailed analysis is experimentally and theoretically performed. In order to theoretically explain a plasmon-loss image, a dynamical simulation method of the plasmon-loss image combined with a first-principle calculation is firstly proposed. By making comparisons between simulated and experimental plasmon-loss images, we find that the experimental plasmon-loss images closely resemble the high-angle bright-field STEM images, which show the reverse contrast of the corresponding high-angle annular dark-field STEM image.

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.