Kazuo Ishizuka

Element-selective imaging of atomic columns in a crystal using STEM and EELS

Microstructure characterization has become indispensable to the study of complex materials, such as strongly correlated oxides, and can obtain useful information about the origin of their physical properties. Although atomically resolved measurements have long been possible, an important goal in microstructure characterization is to achieve element-selective imaging at atomic resolution. A combination of scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) is a promising technique for atomic-column analysis. However, two-dimensional analysis has not yet been performed owing to several difficulties, such as delocalization in inelastic scattering or instrumentation instabilities. Here we demonstrate atomic-column imaging of a crystal specimen using localized inelastic scattering and a stabilized scanning transmission electron microscope. The atomic columns of La, Mn and O in the layered manganite La1.2Sr1.8Mn2O7 are visualized as two-dimensional images.

Direct observation of single dopant atom in light-emitting phosphor of β-SiAlON:Eu2+

Rare-earth doped nitride attracts considerable attention because of its application as a light-emitting phosphor. The atomic site of dopants in a crystal is important for the development of advanced materials. Here, we directly observe a single Eu dopant atom in phosphor β-SiAlON using scanning transmission electron microscopy (STEM). A STEM annular dark-field image reveals that a Eu dopant exists in a continuous atomic channel in a β-Si3N4 structure. The image contrast of the single Eu dopant is confirmed based on the comparison of experimental and simulation results.

Changes of magnetic domain structure induced by temperature-variation and electron-beam irradiation in Pr0.5Sr0.5CoO3

In situ observation of magnetic domain structures of Pr0.5Sr0.5CoO3 was made by Lorentz electron microscopy to reveal an unusual magnetic behavior around 120K below the Curie temperature (TC∼220K). The observations in a cooling run clearly showed that the magnetic domain structure changes below about 90K and that the transformed domain structure remains unchanged down to 20K. Furthermore, it was found that an electron-beam irradiation in an electron microscope induces a reversible transformation of the magnetic domain structure at 20K. Possible mechanisms of such magnetic-domain structural changes are discussed.

FFT Multislice Method—The Silver Anniversary

The first paper on the FFT multislice method was published in 1977, a quarter of a century ago. The formula was extended in 1982 to include a large tilt of an incident beam relative to the specimen surface. Since then, with advances of computing power, the FFT multislice method has been successfully applied to coherent CBED and HAADF-STEM simulations. However, because the multislice formula is built on some physical approximations and approximations in numerical procedure, there seem to be controversial conclusions in the literature on the multislice method. In this report, the physical implication of the multislice method is reviewed based on the formula for the tilted illumination. Then, some results on the coherent CBED and the HAADF-STEM simulations are presented.

Assessment of lower-voltage TEM performance using 3D Fourier transform of through-focus series

We assess the imaging performance of a transmission electron microscopy (TEM) system operated at a relatively low acceleration voltage using the three-dimensional (3D) Fourier transform of through-focus images. Although a single diffractogram and the Thon diagram cannot distinguish between the linear and non-linear TEM imaging terms, the 3D Fourier transform allows us to evaluate linear imaging terms, resulting in a conclusive assessment of TEM performance. Using this method, information transfer up to 98 pm is demonstrated for an 80 kV TEM system equipped with a spherical aberration corrector and a monochromator. We also revisit the Young fringe method in the light of the 3D Fourier transform, and have found a considerable amount of non-linear terms in Young fringes at 80 kV even from a typical standard specimen, such as an amorphous Ge thin film.

Quantitative annular dark-field imaging of single-layer graphene

A quantification procedure for annular dark-field (ADF) imaging, in which a quantitative contrast is given as a scattering intensity normalized by an incident probe current, is presented. The obtained ADF images are converted to quantitative ADF images using an empirical equation, which is a function of an ADF imaging system setting. The quantification procedure fully implements the nonlinear response of the ADF imaging system, which is critical in high-sensitivity observation. We applied the procedure for observation of a graphene specimen with 1-4 layers. The inner and outer angles of an ADF detector, which are important parameters in quantitative analyses, were precisely measured. The quantitative contrast of ADF images was in agreement with that of simulated images, and the quantitative ADF imaging allowed us to directly count the number of graphene layers.

Possible origins of the magnetoresistance gain in colossal magnetoresistive oxide La0.69Ca0.31MnO3: Structure fluctuation and pinning effect on magnetic domain walls

The spatial fluctuation of the magnetic domain (MD) and charge/orbital ordering (CO/OO) structure at around the Curie temperature (TC) was directly observed in a colossal magnetoresistance (CMR) compound, La0.69Ca0.31MnO3, in which extraordinary anisotropic magnetoresistance (AMR) has also been observed. It was found that the long range MD structure collapses upon the emergence of short range CO/OO in a narrow temperature regime, which provides abundant evidence in support of a gain in magnetoresistance at around TC. Moreover, the pinning effect on the MD wall was discerned and it may contribute to the CMR as well as to the extraordinary AMR effect.

Quantitative annular dark-field imaging of single-layer graphene-II: atomic-resolution image contrast.

We have investigated how accurately atomic-resolution annular dark-field (ADF) images match between experiments and simulations to conduct more reliable crystal structure analyses. Quantitative ADF imaging, in which the ADF intensity at each pixel represents the fraction of the incident probe current, allows us to perform direct comparisons with simulations without the use of fitting parameters. Although the conventional comparison suffers from experimental uncertainties such as an amorphous surface layer and specimen thickness, in this study we eliminated such uncertainties by using a single-layer graphene as a specimen. Furthermore, to reduce image distortion and shot noises in experimental images, multiple acquisitions with drift correction were performed, and the atomic ADF contrast was quantitatively acquired. To reproduce the experimental ADF contrast, we used three distribution functions as the effective source distribution in simulations. The optimum distribution function and its full-width at half-maximum were evaluated by measuring the residuals between the experimental and simulated images. It was found that the experimental images could be explained well by a linear combination of a Gaussian function and a Lorentzian function with a longer tail than the Gaussian function.

Quantitative evaluation of temporal partial coherence using 3D Fourier transforms of through-focus TEM images

We evaluate the temporal partial coherence of transmission electron microscopy (TEM) using the three-dimensional (3D) Fourier transform (FT) of through-focus images. Youngs fringe method often indicates the unexpected high-frequency information due to non-linear imaging terms. We have already used the 3D FT of axial (non-tilted) through-focus images to reduce the effect of non-linear terms on the linear imaging term, and demonstrated the improvement of monochromated lower-voltage TEM performance [Kimoto et al., Ultramicroscopy 121 (2012) 31-39]. Here we apply the 3D FT method with intentionally tilted incidence to normalize various factors associated with a TEM specimen and an imaging device. The temporal partial coherence of two microscopes operated at 30, 60 and 80 kV is evaluated. Our method is applicable to such cases where the non-linear terms become more significant in lower acceleration voltage or aberration-corrected high spatial resolution TEM.